JP2012203013A - Electromagnetic wave generating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic wave generating device capable of generating an electromagnetic wave of a frequency variable terahertz band with a simple device configuration without needing control of a crossing angle between two pump lights due to frequency change and without providing a complicated optical system at a terahertz wave output side.SOLUTION: The electromagnetic wave generating device includes a first pump light outgoing part 24 for outgoing a first pump light, a second pump light outgoing part 25 for outgoing a second pump light having wavelength different from that of the first pump light with wavelength variable, a pseudo phase matching element 19 having a periodical polarization reversal structure to generate an electromagnetic wave of a difference frequency between the first pump light and the second pump light, and an optical system (M, 18) for adjusting the crossing angle between the first pump light and the second pump light to a phase matching range angle to fix the phase matching range angle and making the first pump light and the second pump light incident on the pseudo phase matching element 19, and generates a terahertz electromagnetic wave of variable frequency from the pseudo phase matching element 19 by changing the frequency of the second pump light.

Description

  The present invention relates to a frequency variable terahertz band electromagnetic wave generator.

  In recent years, the possibility of applying terahertz technology has been proposed one after another, so that a high-power and durable terahertz radiation source has been demanded. In the last few years, two main sources of terahertz radiation have been studied. One is terahertz radiation from a semiconductor by femtosecond laser excitation, and the other is difference frequency generation using a nonlinear optical crystal. In the case of femtosecond laser excitation, an excessive surge current is required by optical rectification to generate a picosecond pulse width. For difference frequency generation, in most cases two pump lasers are required. Difference frequency generation can be expected to have a relatively large output, but this requires accurate beam alignment and synchronization adjustment of various optical elements in order to satisfy phase matching.

Conventionally, a terahertz electromagnetic wave generator using a polariton mode in a lithium niobate (LiNbO ₃ ) crystal as a dielectric as shown in FIG. 26 has been known as a tunable terahertz electromagnetic wave generator using difference frequency generation. In the conventional electromagnetic wave generator as shown in FIG. 26, the first pump light hν ₁ and the second pump light hν ₂ are incident on the LiNbO ₃ crystal 101, and the terahertz electromagnetic wave hν ₃ = hν _{1 is} generated by the difference frequency generation. to generate a -hν _2. The frequency band of the terahertz electromagnetic wave hν ₃ thus obtained is coherent light in the range of approximately 0.7 THz to 2.5 THz. However, for the purpose of discriminating various biological materials using the difference in terahertz electromagnetic spectrum, the above frequency band as a spectrum variable range is too narrow in the conventional electromagnetic wave generator as shown in FIG. It was. In the conventional electromagnetic wave generator shown in FIG. 26, the terahertz electromagnetic wave hν ₃ is transmitted through a prism array including a plurality of silicon (Si) prisms 105a, 105b, 105c, and 105d arranged on the surface of the LiNbO ₃ crystal 101. The terahertz electromagnetic wave hν ₃ is extracted at a larger angle. Therefore, the overlap of the terahertz electromagnetic wave hν ₃ and the pump light hν ₁ and hν ₂ is poor, which causes a decrease in both frequency band and efficiency.

On the other hand, it is also possible to generate a terahertz electromagnetic wave hν ₃ by parametric oscillation by making one pump light incident on the LiNbO ₃ crystal. In such a parametric oscillation type LiNbO ₃ terahertz generator, the spectral line width of the generated terahertz electromagnetic wave hν ₃ is reduced to about 100 MHz by performing injection seeding with a continuous wave (CW) laser diode (semiconductor laser). Narrow line width. In other words, the LiNbO ₃ terahertz generator is a slave laser (host laser), and a CW laser diode with a narrow spectral width is used as a seed laser (master oscillator), and these two lasers are combined to combine the properties of both a slave laser and a slave laser. By doing so, it is possible to generate a terahertz electromagnetic wave hν ₃ having a narrow spectrum width. However, because of the mode hopping of the CW laser diode, it is difficult to continuously sweep the frequency over a wide range. On the other hand, if injection seeding is not performed, the spectral line width is extremely large and exceeds 100 GHz (0.1 THz), so that the resolution is remarkably lowered and the performance of substance identification is not sufficient in combination with the narrow frequency band.

Therefore, as shown in FIG. 27, a first pump-light emitting unit 24 for emitting a first pump light hv ₁ of the first wavelength, the first second pump wavelength different from the pump light hv ₁ A second pump light emitting unit 25 that emits light hν ₂ with a variable wavelength, and a nonlinear optical crystal 109 that generates an electromagnetic wave hν ₃ having a difference frequency between the first pump light hν ₁ and the second pump light hν _2. And the external crossing angle θ _in ^ext at the difference frequency of 1 THz between the first pump light hν ₁ and the second pump light hν ₂ is adjusted to within 0.5 °, and the first pump light and the second pump light are adjusted. Has been proposed (see Patent Document 1), which includes an optical system (M ₆ , 18) for causing the light to enter the nonlinear optical crystal 109. Here, the external crossing angle θ _in ^ext is the crossing angle between the first pump light hν ₁ and the second pump light hν ₂ outside the nonlinear optical crystal 109, and is defined inside the nonlinear optical crystal 109. The internal crossing angle θ _in is approximately:

θ _in ^ext = n _L θ _in ...... (1)

Have the relationship Since n _L is the refractive index of the nonlinear optical crystal 109 at the pump light frequency, when the nonlinear optical crystal 109 is GaP, the refractive index n _L is about 3.1.

In the conventional electromagnetic wave generating device using difference frequency generation described in Patent Document 1, the optical system (M ₆ , 18) includes a mirror M ₆ and a polarization beam splitter 18, and the mirror M ₆ is the first pump light. The angle at which the first pump light hν ₁ emitted from the emission unit 24 is reflected and the first pump light hν ₁ enters the polarization beam splitter 18 is adjusted. As shown in FIG. 27, by arranging the optical system (M ₆ , 18), the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is transmitted through the polarization beam splitter 18. The pump light (first pump light) hν ₁ emitted from the first pump light emitting unit 24 is incident from the vertical direction using the mirror M ₆ and reflected by the polarization plane of the polarization beam splitter 18 to be parallel. The pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ are coupled in the nonlinear optical crystal 109 at a minute external crossing angle θ _in ^ext close to.

When n _I is the refractive index of the nonlinear optical crystal 109 at the frequency of the terahertz electromagnetic wave hν ₃ , and n _L is the refractive index of the nonlinear optical crystal 109 at the frequencies of the pump lights hν ₁ and hν ₂ , the two pump lights hν ₁ , When the internal crossing angle θ _in formed by hν ₂ in the crystal is sufficiently small, the condition shown in the following formula (2) is necessary from the geometrical relationship of the wave vector shown in FIG.

θ _in = (2 Δq / q) ^1/2 (n _I / n _L ) (ν ₃ / ν ₁ ) (2)

That is, the external crossing angle θ _in ^ext increases in proportion to the difference frequency ν ₃ as shown in the equation (2) in a region sufficiently smaller than the transverse wave lattice vibration frequency of the nonlinear optical crystal 109. When the difference frequency ν ₃ becomes close to the transverse wave lattice vibration frequency, the external crossing angle θ _in ^ext begins to deviate from the proportionality shown by the equation (2) and changes along a curve (curve) indicating the frequency dispersion of the nonlinear optical crystal 109. .

For this reason, in the conventional electromagnetic wave generator described in Patent Document 1, as shown in FIG. 27, the rotation of the rotary stage 55 on which the polarization beam splitter 18 is mounted or the linear stage for the beam splitter is received by a signal from the personal computer 11. It is necessary to control the position of 54 using the stage controller 35 and to sweep the wavelength of the pump light source of the second pump light emitting unit 25 using the laser controller 31. The two pump lights hν ₁ and hν ₂ are always superposed at the same point on the surface of the nonlinear optical crystal 109 arranged approximately 60 mm away from the polarizing beam splitter 18 at an arbitrary minute external crossing angle θ _in ^ext . As shown in FIG. 27, the polarization beam splitter 18 requires a complicated drive system that is controlled by the stage controller 35 so as to move on the beam splitter linear stage 54 in conjunction with the rotation by the rotary stage 55. It was.

Furthermore, in the electromagnetic wave generator described in Patent Document 1, as shown in FIG. 28, the terahertz electromagnetic wave hν ₃ emitted from the nonlinear optical crystal 109 is emitted at an emission angle (pump light hν ₁ , hν) by its frequency ν _{3. 2} and the angle formed by the terahertz electromagnetic wave hν ₃ ) θ _I was changed. For example, at the frequency ν ₃ = 0.4 to 6 THz, the emission angle θ _I is 30 to 60 °.

In order to allow the outgoing angle θ _I to pass through a fixed position or a fixed line regardless of the frequency ν ₃ , it is necessary to use a pair of non-axial parabolic mirrors 7a and 7b as shown in FIG. 1 has a problem that the system configuration of the electromagnetic wave generator described in 1 is complicated. That is, the first non-axial parabolic mirror 7a is arranged in the vicinity of the nonlinear optical crystal 109 so as to capture the terahertz electromagnetic wave hν ₃ in the above frequency range. At this time, the focal point of the first non-axial parabolic mirror 7a in the position in the optical arrangement as emission point of the terahertz electromagnetic wave hv ₃ of the nonlinear optical crystal 109 comes, regardless of the direction of emission of THz radiation hv _3, so that a beam of THz radiation hv ₃ may be the same optical path There is a problem that an optical system necessary for a complicated drive system is required.

International Publication No. WO2005 / 073795

As described above, in the conventional electromagnetic wave generator described in Patent Document 1, the rotation in which the polarization beam splitter 18 for controlling the minute external crossing angle θ _in ^ext of the incident pump lights hν ₁ and hν ₂ is mounted. The stage 55 and the linear stage for beam splitter 54 are driven in conjunction with the laser controller 31 to automatically control the external crossing angle θ _in ^ext between the _two pump lights hν ₁ and hν ₂ , and further to a paraboloid The movement of the mirror linear stage 53 is also controlled by the personal computer 11 so that the stage controller 35 can control the movement of the mirror linear stage 53. That is, in the conventional electromagnetic wave generator described in Patent Document 1, continuously swept frequency [nu ₃ automatically, and in order to obtain the maximum output at each frequency [nu _3, the second pump light emitted It is necessary to control the wavelength of the unit 25 and the external crossing angle θ _in ^ext between the _two pump lights hν ₁ and hν _{2 and} the position of the second non-axial parabolic mirror 7b at the same time. there were.

  In view of the above problems, the present invention does not require control of the crossing angle between two pump lights accompanying a frequency change, and does not provide a complicated optical system on the output side of the terahertz wave, and has a simple apparatus configuration and a frequency. An object of the present invention is to provide an electromagnetic wave generator capable of generating a variable terahertz band electromagnetic wave.

  In order to achieve the above object, an aspect of the present invention includes: (a) a first pump light emitting unit that emits first pump light; and (b) a second pump light having a wavelength different from that of the first pump light. A second pump light emitting section that emits pump light with a variable wavelength; and (c) a periodic polarization inversion structure that generates an electromagnetic wave having a difference frequency between the first pump light and the second pump light. And (d) adjusting and fixing the cross angle between the first pump light and the second pump light to the phase matching range angle, and simulating the first pump light and the second pump light. The gist of the present invention is an electromagnetic wave generation device including an optical system that is incident on a phase matching element. The electromagnetic wave generator according to this aspect of the present invention is characterized in that a terahertz electromagnetic wave having a variable wavelength is generated from the quasi phase matching element by changing the frequency of the second pump light in a state where the crossing angle is fixed. To do. Here, the “crossing angle” is an “external crossing angle δ” or “internal crossing angle θ” related to each other, as will be described later.

  According to the present invention, it is not necessary to control the crossing angle between the two pump lights according to the frequency change, and the frequency variable terahertz can be achieved with a simple apparatus configuration without providing a complicated optical system on the output side of the terahertz wave. It is possible to provide an electromagnetic wave generator capable of generating a band electromagnetic wave.

It is a schematic diagram explaining the outline of the whole structure of the electromagnetic wave generator which concerns on the basic concept of this invention. It is a typical bird's-eye view explaining the outline of the structural example of the pseudo phase matching element used for the electromagnetic wave generator which concerns on the basic concept of this invention. In the quasi phase matching element, the phases that cancel each other out are the schematic diagrams for explaining that the phase of the SHG wavelength converted light is reversed and the intensity is increased by reversing the polarization of the crystal. It is a typical bird's-eye view explaining the outline of the manufacturing method of the quasi phase matching element shown in FIG. It is a typical bird's-eye view which illustrates roughly another example of the manufacturing method of a quasi phase matching element. It is typical sectional drawing explaining the outline of the other structural example of the pseudo phase matching element used for the electromagnetic wave generator which concerns on the basic concept of this invention. It is a two-dimensional vector diagram (vector composition diagram) for explaining phase matching in the electromagnetic wave generator according to the basic concept of the present invention. In the two-dimensional vector diagram for explaining phase matching in the electromagnetic wave generator according to the first embodiment of the present invention, the intersection of two circles having radii k _La and k _Lb and radii k _Ta and k _Tb is generally FIG. 4 is a diagram for explaining the existence of two. For one of the intersections of the two circles in FIG. 8, one vector k _La derived from the polarization inversion structure of the electromagnetic wave generator according to the first embodiment defined in FIG. 8 and the corresponding wave vector of the terahertz wave each k _Ta, the slope of the quasi-phase matching element, a diagram shown with wave vector k _p2 of the first wave vector k _p1 of the pump light, and a second pump light. Other vectors k different from those shown in FIG. 9 derived from the polarization inversion structure of the electromagnetic wave generator according to the first embodiment defined in FIG. 8 with respect to the other of the intersection points of the two circles in FIG. A diagram showing _Lb and the corresponding wave vector k _Tb of another terahertz wave together with the inclination of the pseudo phase matching element, the wave vector k _p1 of the first pump light, and the wave vector k _p2 of the second pump light It is. For the case of vector k _p2 <vector k _p1 , a two-dimensional vector showing a plurality of circles indicating the trajectories of a plurality of terahertz wave numbers and explaining the phase matching in the electromagnetic wave generator according to the first embodiment FIG. For the case of vector k _p2 > vector k _p1 , a two-dimensional vector illustrating a plurality of circles indicating the trajectories of a plurality of terahertz wave vectors and explaining phase matching in the electromagnetic wave generator according to the first embodiment FIG. It is a figure which shows the terahertz wave transmission characteristic of PPKTP crystal used for the electromagnetic wave generator which concerns on 1st Embodiment. In the case where the value of the internal crossing angle exceeds the phase matching range angle, a plurality of circles indicating the trajectories of the wave number vectors of the plurality of terahertz waves are shown, and the phase matching in the electromagnetic wave generator according to the first embodiment FIG. In the case where the value of the internal crossing angle corresponds to the phase matching range angle, a plurality of circles indicating the trajectories of a plurality of terahertz waves are shown, and the phase matching in the electromagnetic wave generator according to the first embodiment is performed. It is a two-dimensional vector diagram to explain. When the value of the internal crossing angle corresponds to the phase matching range angle, the horizontal axis represents the inclination angle (k _L angle) of the vector k _L and the vertical axis represents the frequency of the terahertz wave. And angle of the wave vector k _T of the terahertz wave is a schematic diagram for explaining the definition of the angle of inclination of the vector k _L (k _L angles). When FIG. 17 is an enlarged view showing a portion of an inner matching condition plotted with a square in the center of FIG. 16 when the value of the internal crossing angle corresponds to the phase matching range angle. In the case of vector k _p2 > vector k _p1 , the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure of the electromagnetic wave generator according to the first embodiment and the wave vector k _T of the terahertz wave The value of the direction angle is a 3D plot. In the case of vector k _p2 <vector k _p1 , the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure of the electromagnetic wave generator according to the first embodiment and the wave vector k _T of the terahertz wave The value of the direction angle is a 3D plot. In the case where the value of the internal crossing angle corresponds to the phase matching range angle, the inclination angle (k _L angle) of the vector k _L of the electromagnetic wave generation device according to the second embodiment of the present invention and the frequency of the terahertz wave It is a figure which shows a relationship. In the case of vector k _p2 <vector k _p1 , the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure of the electromagnetic wave generator according to the second embodiment and the wave vector k _T of the terahertz wave The value of the direction angle is a 3D plot. It is a schematic diagram explaining the outline of the whole structure of the electromagnetic wave generator which concerns on the 3rd Embodiment of this invention. It is a schematic diagram explaining the outline of the whole structure of the electromagnetic wave generator which concerns on the 4th Embodiment of this invention. It is a schematic diagram explaining the outline of the whole structure of the electromagnetic wave generator which concerns on the 5th Embodiment of this invention. It is typical sectional drawing explaining a part of electromagnetic wave generator which concerns on a prior art. It is a schematic diagram explaining the outline of the whole structure of the electromagnetic wave generator which concerns on another prior art. It is a two-dimensional vector diagram explaining phase matching in the electromagnetic wave generator according to the prior art.

  Next, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thickness of each layer, and the like are different from the actual ones. Therefore, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

(Basic concept)
As shown in FIG. 1, the electromagnetic wave generator according to the basic concept of the present invention has a first pump light emitting section 24 that emits the first pump light hν1 and a first pump light having a wavelength different from that of the first pump light hν1. The second pump light emitting section 25 that emits the second pump light hν2 with a variable wavelength, and a periodically domain-inverted structure with a difference frequency between the first pump light hν1 and the second pump light hν2. The external crossing angle δ and the internal crossing angle θ of the quasi-phase matching element 19 that generates the electromagnetic wave, the first pump light hν1 and the second pump light hν2 are adjusted and fixed, and the first pump light hν1 and the first pump light hν1 And an optical system (M6, 18) for causing the two pump lights hν2 to enter the quasi phase matching element 19. Where the external crossing angle δ Is the crossing angle between the _first pump light hν ₁ and the second pump light hν ₂ outside the quasi phase matching element 19, and the internal crossing angle θ, which is the crossing angle inside the quasi phase matching element 19, Almost:

δ = n _L θ _in ...... (3)

This relationship is the same as in the formula (1). Equation (3) is expressed as follows: wave number vectors such as the first pump light hν1 and the second pump light hν2 propagating in the air, the first pump light hν1 and the second pump propagating in the quasi phase matching element 19 This means that the wave vector for the light hν2 etc. is different, and caution is required. Since n _{L in} Equation (3) is the refractive index of the pseudo phase matching element 19 at the pump light frequency, if the frequency dispersion can be ignored, the refractive index n _THz of the pseudo phase matching element 19 with respect to the terahertz wave hν _T is used.

n _L = N _THz (4)

And can be approximated.

  As shown in FIG. 1, the electromagnetic wave generator according to the basic concept further includes a terahertz band electromagnetic wave detector 34 such as a silicon bolometer or a DTGS infrared light detector, and the terahertz band emitted from the pseudo phase matching element 19. Are detected by the electromagnetic wave detector 34. The electromagnetic wave detector 34 is connected to a voltmeter 33, and the voltmeter 33 is connected to a personal computer (PC) 11, and performs arithmetic processing on an electric signal based on the terahertz band electromagnetic wave detected by the electromagnetic wave detector 34. Is configured to be possible. Since the terahertz band electromagnetic wave emitted from the quasi phase matching element 19 is emitted in a fixed direction, the electromagnetic wave detector 34 can be arranged at a fixed position.

The optical system (M ₆ , 18) includes a mirror M ₆ and a polarization beam splitter 18 as shown in FIG. The mirror M ₆ is rotatable so as to reflect the first pump light hν ₁ emitted from the first pump light emitting unit 24 and adjust the angle at which the first pump light hν ₁ enters the polarization beam splitter 18. It is a mirror. With the arrangement of the optical systems (M ₆ , 18) shown in FIG. 1, the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is transmitted through the polarization beam splitter 18. Further, the pump light (first pump light) hν ₁ emitted from the first pump light emitting unit 24 is incident from the vertical direction using the mirror M ₆ and reflected by the polarization plane of the polarization beam splitter 18. An optical system that couples the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ within the quasi phase matching element 19 with a minute external crossing angle δ that is almost parallel is constructed. ing.

  As shown in FIG. 1, in the electromagnetic wave generator according to the basic concept, the quasi phase matching element 19 is mounted on a rotary stage 57, and the rotary stage 57 is driven by a stage controller 32. The stage controller 32 is connected to be controlled by the personal computer 11. Specifically, the rotary stage 57 is driven by, for example, a stepping motor, and the stage controller 32 can be constituted by a motor controller that controls the stepping motor. The stage controller 32 can be controlled from the personal computer 11 by GPIB communication, USB communication, RS-232C communication, or the like. However, as will be described later, in the electromagnetic wave generator according to the first to fourth embodiments of the present invention, a terahertz wave radiation source of 0.3 to 2 THz can be designed without substantially rotating the quasi phase matching element 19. Therefore, the rotary stage 57 can be omitted.

Further, the first pump light emitting unit 24 and the second pump light emitting unit 25 are connected to the laser controller 31, and the output intensity and frequency are controlled by the laser controller 31. The laser controller 31 is connected to the personal computer 11 and controlled by GPIB communication, USB communication, RS-232C communication, or the like. In particular, the wavelength of the second pump light emitting unit 25 is controlled by the personal computer 11 via the laser controller 31, but automatically and continuously by driving and controlling the stage controller 32 in conjunction with the laser controller 31. to sweep the frequency [nu _3, in order to obtain the maximum output with and each frequency [nu _3, the rotation angle of the wavelength and the quasi-phase matching element 19 of the second pump-light emitting unit 25 is controlled.

As described at the beginning, in the conventional electromagnetic wave generator described in Patent Document 1, the external crossing angle θ _in ^ext is expressed by the formula (2) in a region sufficiently smaller than the transverse wave lattice vibration frequency of the nonlinear optical crystal 109. Thus, when the difference frequency ν ₃ increases in proportion to the difference frequency ν ₃ , and the difference frequency ν ₃ approaches the transverse wave lattice vibration frequency, the external crossing angle θ _in ^ext changes along a curve indicating the frequency dispersion of the nonlinear optical crystal 109. Therefore, conventionally, it has been necessary to control the rotation of the rotary stage 55 on which the polarization beam splitter 18 is mounted and the position of the linear stage 54 for the beam splitter using the stage controller 32 by a signal from the personal computer 11. In the electromagnetic wave generator according to the concept, as shown in FIG. Does not depend on the difference frequency ν ₃ , and therefore can be configured to be able to emit light in a certain direction. Therefore, the rotating stage 55, the beam splitter linear stage 54, and the stage controller 32 for driving and controlling them as shown in FIG. 27 are unnecessary. It is.

Here, the first pump light emitting unit 24 includes, for example, a chromium (Cr) -added forsterite laser, an ytterbium (Yb) -added YLF (yttrium-lithium fluoride) laser, and Yb-added that emits the first pump light hν1. The second pump light emitting unit 25 includes a first pump light hν1 source composed of any one of fiber lasers, and includes, for example, a Cr-doped forsterite laser, a Yb-doped YLF laser, and Yb that emits the second pump light hν2. It is possible to provide a second pump light source consisting of any of the doped fiber lasers. Here, the Cr-added forsterite laser uses the Cr level in the forsterite (SiO ₂ · 2MgO = Mg ₂ SiO ₄ ), which is the main component of olivine, and is therefore suitable for OPO without injection seeding. In comparison, the spectral line width is extremely narrow. The Cr-added forsterite laser is excited by using an excitation light source (YAG laser) 16 having a wavelength of 1.064 μm, and does not use the third harmonic of YAG unlike OPO, so that the efficiency is high. An excitation light source is further provided for exciting the first pump light hν1 and the second pump light source, respectively, and emitting the first pump light hν1 and the second pump light hν2 from the first and second pump light sources. Also good. Alternatively, the output of the YAG laser having a wavelength of 1.064 μm is emitted from the first pump light emitting unit 24 as the first pump light hν1, and the second pump light emitting unit 25 is an optical parametric oscillator having an injection seeding mechanism. , And the optical parametric oscillator may be excited by the output of the YAG laser so that the output of the optical parametric oscillator is emitted from the second pump light emitting unit 25 as the second pump light hν2.

As the quasi-phase matching element 19 having a periodic polarization reversal structure, for example, a structure in which the polarization direction of the crystal is periodically reversed for each coherence length lc as shown in FIG. 2 can be employed. In FIG. 2, the portions 191 _M polarized in the direction M with the thickness lc and the portions 191 _N polarized in the direction N opposite to the direction _M with the thickness lc are alternately and periodically stacked. . Historically, a periodic domain-inverted structure as shown in FIG. 2 was originally used mainly for phase matching when generating a second harmonic. The coherence length lc for generating the second harmonic is such that k ₁ is the wave number of the fundamental wave (incident light) and k ₂ is the wave number of the second harmonic wave (emitted light).

lc = π / (k ₂ -2k ₁ ) (5)

When the sum frequency is generated, the coherence length l c is obtained when k ₃ and k ₄ are wave numbers of incident light and k ₅ is wave number of emitted light.

lc = π / {k ₅ − (k ₃ + k ₄ )} (6)

It is represented by

The period of polarization inversion (lattice constant) Λ is expressed by Λ = 2 mlc (where m is a positive integer representing the order). As shown in FIG. 2, Λ is the length of one period of polarization reversal. When m is an odd number, the length lc (M of the portion 191 _M polarized in the reference direction M in the m is an odd number. ) And the length lc (N) of the portion 191 _N polarized in the direction N opposite to the direction M is Λ = lc (M) + lc (N) where m = 1. At this time, a relationship of lc (M): lc (N) = 1: 1 is desirable. On the other hand, when m is an even number, the length t (M) of the portion 191 _M polarized in the reference direction M and the length t of the portion 191 _N polarized in the opposite direction N of the direction M The relationship (N) is also Λ = t (M) + t (N), but it is desirable that m = 2 and t (M): t (N) = 1: 3.

  As shown in FIG. 2, the quasi phase matching element 19 has a periodically poled structure in which the polarization (M, N) of the crystal is periodically reversed. As can be seen from the principle of quasi phase matching. The polarization reversal period Λ is twice as long as the coherence length lc with the pair of positive and negative polarization regions as a pair. One of the major features of the quasi phase matching element 19 is that the corresponding wavelength and conversion method (harmonic generation, optical parametric oscillation, etc.) can be customized by adjusting the polarization inversion period. The quasi phase matching element 19 can use a nonlinear constant having a high orientation that cannot be realized by conventional birefringence phase matching by selecting the orientation in which the domain-inverted structure is formed. Good wavelength conversion becomes possible.

  In general, when phase matching is not achieved, there is a difference in phase velocity between the fundamental wave light and the generated wavelength converted light, so that the wavelength converted light generated one after another as the fundamental wave propagates in the crystal is The phase shifts little by little. The generated wavelength-converted lights are gradually increased in intensity as they are added, but when the phase difference between the wavelength-converted lights generated at two points separated by a distance lc becomes π, they cancel each other and conversely the intensity. It decays. As a result, the intensity of the wavelength-converted light periodically repeats the intensity at the polarization inversion period Λ, as indicated by the dotted line in FIG. In the quasi phase matching element 19, as shown by the solid line in FIG. 3 (b), the phases of the wavelength-converted light generated in this phase are reversed and stabilized in the phases that cancel each other, by reversing the polarization of the crystals. The strength is increased. As indicated by the solid line in FIG. 3 (b), even if the phases cancel each other out, they can be shifted to a strengthening phase, and the strength can always be increased.

The quasi-phase matching element 19 used in the electromagnetic wave generator according to the basic concept is transparent to the wavelengths of the first pump light hν1, the second pump light hν2, and the terahertz wave hν _T , and has mechanical / optical characteristics. A dielectric or semiconductor that is excellent and has the largest nonlinear optical constant and is inexpensive is suitable.

The quasi-phase matching element 19 used in the electromagnetic wave generator according to the basic concept shown in FIG. 2 includes, for example, as shown in FIG. 4 lithium lithium niobate (LiNbO ₃ ) crystal (LN crystal), lithium tantalate (LiNbO ₃ ). Thin plates 191 _M , 191 _N , 191 _M , 191 from several μm to 50 μm from a dielectric crystal such as a crystal (LN crystal) or potassium phosphate titanate (KTiOPO ₄ ) crystal (KTP crystal) by ultra-precision machining and polishing. _N , 191 _M , 191 _N ,... Are prepared, and the thin plates 191 _M , 191 _N , 191 _M , 191 _N , 191 _M , 191 _N ,. 4 (a) indicates the directions of polarization M and N.) By joining, the quasi phase matching element 19 can be manufactured as shown in FIG. 4 (b). As a bonding method in FIG. 4B, diffusion bonding or room temperature bonding can be employed, and optical / mechanically stable bonding can be performed by using diffusion bonding or room temperature bonding. In FIG. 4A, for convenience, the case of joining 8 sheets is illustrated, but the thin plates 191 _M , 191 _N , 191 _M , 191 _N used for the quasi phase matching element 19 used in the electromagnetic wave generator according to the basic concept. , 191 _M , 191 _N ,... Are not limited to the number illustrated in FIG. Further, since the thickness Λ / 2 of the thin plates 191 _M , 191 _N , 191 _M , 191 _N , 191 _M , 191 _N ,... Can be increased with a high-order period (by increasing the order m), the processing accuracy is increased. It is sufficient to set the optimal order according to.

  Further, the quasi phase matching element 19 used in the electromagnetic wave generator according to the basic concept shown in FIG. 2 can be manufactured by a manufacturing method as shown in FIG. When the quasi phase matching element 19 is made by stacking and joining the necessary number of thin plates by the method as shown in FIG. 4, the number of the sheets becomes very large, and all of them are ground / polished and joined. It takes time and price. Therefore, as shown in FIG. 3, first, two comb-shaped substrates 19a and 19b are formed from a dielectric crystal or a semiconductor crystal as a material by X-ray lithography, ultraprecision processing, or the like. At this time, the length direction of the comb-shaped tooth portion is set to the polarization (M, N) direction (the arrow indicates the polarization (M, N) direction). Then, the comb-shaped substrates 19a and 19b may be fitted and joined so that the polarizations (M, N) of the comb-shaped tooth portions are reversed from each other. As the bonding method shown in FIG. 5, diffusion bonding or room temperature bonding can be used. At this time, the structure of the comb-shaped portion is formed by precision processing or anisotropic etching so that the interval between the grooves becomes narrower as the grooves become deeper so that the two comb-shaped substrates 19a and 19b can be easily fitted to each other. May be. If it can be used simply by fitting without using bonding, it is processed so that there is a little gap between the comb teeth of the comb-shaped substrates 19a and 19b, and the gap is filled with matching oil or the like. May be.

Furthermore, as shown in FIG. 6, the quasi phase matching element 19 used in the electromagnetic wave generator according to the basic concept has the surface of the substrate 192 of polarization M having a circular cross section in the direction perpendicular to the surface, and the polarization direction N However, the periodically poled structure may be configured by a structure in which the region 193 in the direction opposite to the polarization M is embedded in a stripe pattern periodically in the plate surface direction with a period Λ. That is, the quasi phase matching element 19 used in the electromagnetic wave generator according to the basic concept can adopt various periodic polarization inversion structures as long as the polarization (M, N) of the crystal is periodically inverted. Various methods can be adopted as the manufacturing method.

(First embodiment)
In the following description of the electromagnetic wave generator according to the first embodiment of the present invention, in the configuration of the electromagnetic wave generator according to the basic concept shown in FIG. "PPKTP crystal") is used as an example, and a differential frequency signal is generated in the quasi-phase matching element 19 under the quasi-phase matching condition using the quasi-phase matching element 19, thereby generating a terahertz wave. The structure to perform is demonstrated. In the description of the basic concept of FIG. 1, the configuration in which the quasi phase matching element 19 is mounted on the rotary stage 57 and the rotary stage 57 is driven by the stage controller 32 has been described as an example for convenience. In the electromagnetic wave generation device according to the embodiment, it is possible to emit a terahertz wave of 0.3 to 2 THz from the pseudo phase matching element 19 without almost rotating the pseudo phase matching element 19.

  Therefore, in the electromagnetic wave generation device according to the first embodiment, the stage controller 32 used in the description of the electromagnetic wave generation device according to the basic concept shown in FIG. 1 is controlled by the personal computer 11 or shown in FIG. The rotary stage 57 itself described in the basic concept may be omitted. In the description of the electromagnetic wave generator according to the first embodiment below, in the range of 0.3 to 2 THz, the range of dispersion of the terahertz wave emitted from the pseudo phase matching element 19 is 3.0 ° to 3 °. Since it can be set to a small value of about 5 ° or less, the position of the electromagnetic wave detector 34 can be fixed at a predetermined position in the configuration of the electromagnetic wave generator according to the basic concept shown in FIG. It is not necessary to use the non-axial parabolic mirrors 7a and 7b as shown in FIG.

  First, FIG. 13 shows the result of examining the terahertz wave transmission characteristics of a PPKTP crystal used as the quasi-phase matching element 19 of the electromagnetic wave generator according to the first embodiment. In the measurement of FIG. 13, transmission characteristics were measured for a 465 μm thick KTP substrate using a terahertz wave radiation source capable of broadband output. As shown in FIG. 13, a relatively high permeability was observed in the region of 0.5 to 2 THz. In the frequency range below that, it could not be measured due to the limitation of the light source, but the KTP substrate is estimated to be transparent up to at least 0.3 THz.

The frequency width of the terahertz wave generated by the difference frequency generation in the electromagnetic wave generation device according to the first embodiment may be determined in consideration of the terahertz wave transmission characteristics of the pseudo phase matching element 19. Therefore, as exemplified in the first embodiment, if the quasi phase matching element 19 is a PPKTP crystal, the target frequency region can be set to 0.3 to 2 THz. Note that the refractive index n _THz = 3.393 of the terahertz wave PPKTP is calculated from the Fabry-Perot interference fringes of the transmission spectrum shown in FIG. The data shown in FIGS. 14 to 20 are calculated using the value of the refractive index n _THz = 3.393 of PPKTP. However, as can be understood from the explanation of the electromagnetic wave generator according to the basic concept described above, by using the quasi phase matching element 19 having a smaller absorption rate and refractive index n _THz , it is possible to realize difference frequency generation that is more feasible and easy to handle. The electromagnetic wave generator used can be realized.

  As shown in FIG. 1, in the electromagnetic wave generator according to the first embodiment, the first pump light emitting unit 24 that emits the first pump light hν1 and the first pump light hν1 have different wavelengths. Two pump lights hν 1... With the second pump light emitting section 25 that emits the second pump light hν 2 with variable wavelength. A difference frequency is generated by the interaction of hν2. The wavelength of the first pump light hν1 is fixed at 1.21 μm, and the second pump light hν2 is adjusted to 1.199 μm to 1.221 μm. The Cr-added forsterite laser completely covers the wavelength band of the laser used in the electromagnetic wave generator according to the first embodiment. The quasi phase matching element 19 having a periodic polarization reversal structure has a polarization reversal period Λ of 6 μm.

The photon momentum derived from the periodic polarization reversal structure of the quasi-phase matching element 19 is represented by a vector k _L , the terahertz wave vector is represented by a vector k _T, and the first pump light hν 1 is represented by a vector k _p1 . If the wave vector of the second pump light hν2 is expressed by the vector k _p2 , the momentum conservation law in the quasi-phase matching is expressed by Equation (7):

Vector k _{p1 −vector} k _p2 = vector k _T + vector k _L (7)

The relationship of equation (7) is shown in FIG.

The vector k _p1 of the first pump light hν 1 is calculated with the vector k _p1 = 2nπ / λ _p1 as the refractive index n _THz = 1.745 in this wavelength region, and is fixed at the wavelength 1.21 μm. It is fixed at .06 μm ⁻¹ . On the other hand, the vector k _p2 of the second pump light hν 2 changes from the wavelength of 1.199 μm to 1.221 μm by controlling the wavelength of the second pump light emitting unit 25 by the personal computer 11 via the laser controller 31. As a result, the refractive index n _THz = 1.745 in this wavelength region is calculated by the vector k _p2 = 2nπ / λ _p2 , and thus changes from 8.987 μm ⁻¹ to 9.145 μm ⁻¹ . As shown in FIG. 8, the momentum vector vector k _p1 is on a parallel line.

The internal crossing angles θ handled by the electromagnetic wave generator according to the first embodiment are all internal angles viewed from the vector k _p1 , and the angles are the internal crossing angles θ in the quasi phase matching element 19. Thereafter, unless otherwise indicated, the internal crossing angle θ obtained by calculation means an angle in the quasi-phase matching element 19. The value (˜1 μm ⁻¹ ) of the vector k _{L on} the right side of Equation (7) is derived as the vector k _L = 2π / Λ using the value of the polarization inversion period Λ = 6 μm of the quasi phase matching element 19. . On the other hand, the wave number k _{T of the} terahertz wave is the absolute value of the momentum vector vector of the difference frequency, and can be expressed as the following equation (8):

k _T ≈ ABS (vector k _{p1 −vector} k _p2 ) (8)

Here, ABS (vector k _{p1 -vector} k _p2 ) represents the absolute value of (vector k _{p1 -vector} k _p2 ).

The locus of the vector k _{T in} equation (8) is depicted in FIG. 8 as a circle whose radius is vector k _Ta = vector k _Tb . It is clear that the arrangement of the quasi phase matching element 19 is easy. Vector k _Ta, the radius vector k _Tb is determined by Equation (8), the vector k _Ta, each arrow root (i.e. radius k _Ta, circle k _Tb center) of the k _Tb is the Vector k _p2 Located at the end point. On the other hand, the roots of the circles representing the vectors k _La and k _Lb derived from the domain-inverted structure (that is, the centers of the circles having the radii k _La and k _Lb ) are located at the end points of the vector k _p1 .

Unlike the terahertz wave number vectors k _Ta and k _Tb , the vectors k _La and k _Lb representing the photon momentum derived from the periodic polarization inversion structure of the quasi phase matching element 19 are different from the polarization inversion of the quasi phase matching element 19. Since it is determined by the period Λ = 6 μm, it is a constant having a value of 1 μm ⁻¹ , and the intersection of two circles is a point that satisfies the phase matching condition. From FIG. 8, there are generally two intersections except for the case where the two circles having the radii k _La and k _Lb and the radii k _Ta and k _Tb are in contact with each other to form one intersection. Therefore, as shown in FIG. 8, two kinds of intersections are defined, the vector k _La and the vector k _Ta are defined at one intersection, and the vector k _Lb and the vector k _Tb are defined at the other intersection, respectively. . 8 that the upper limit of the period Λ of polarization inversion of the quasi phase matching element 19 is preferably about the wavelength of the terahertz wave, that is, about 1 mm (0.3 THz), for example. When the polarization inversion period Λ of the quasi-phase matching element 19 is further increased beyond the wavelength of the terahertz wave, the vectors k _La and k _Lb shown in FIG. 8 become smaller and the conventional quasi-phase matching element 19 is not used. The difference from the vector diagram of the difference frequency generation disappears, and the feature of the present invention is lost. On the other hand, the lower limit of the period Λ of polarization inversion is limited by the manufacturing technique of the quasi phase matching element 19, but it is not preferable to make it too small. When the value of the period Λ of polarization inversion is reduced to about 0.7 μm, the vector k _La , has the same magnitude as the wave vector k _{p1 of} the first pump light hν 1 and the wave vector k _{p2 of} the second pump light hν 2. The absolute value of k _Lb increases. In this case, as can be understood from FIG. 8, in order to preserve the momentum, it becomes a vector composition diagram of a nearly equilateral triangle, and the angle between the wave vector k _p1 and the wave vector k _p2 of the two excitation waves is about 60 °. , The overlap of the beam will be small, not convenient. Therefore, the lower limit of the polarization inversion period Λ of the quasi phase matching element 19 is preferably selected to be larger than the wavelength of the excitation light.

FIG. 9 shows a vector k _La derived from the polarization inversion structure of the electromagnetic wave generator according to the first embodiment defined in FIG. 8 and a wave number vector k _Ta of the terahertz wave, respectively, for the pseudo phase matching element 19 and the FIG. 10 is a diagram showing the wave number vector k _p1 of the first pump light hν 1 and the wave number vector k _{p2 of} the second pump light hν 2, and FIG. 10 is an electromagnetic wave generator according to the first embodiment defined in FIG. The vector k _Lb and the wave vector k _Tb of the terahertz wave derived from the domain-inverted structure are respectively represented by the quasi phase matching element 19, the wave vector k _{p1 of} the first pump light hν1, and the wave vector of the second pump light hν2. It is a figure shown with _kp2 . As shown in FIGS. 9 and 10, in the electromagnetic wave generator according to the first embodiment, there are two patterns of course directions of the wave number vectors k _Ta and k _Tb of the terahertz wave generated from the pseudo phase matching element 19. . Therefore, in the design of the electromagnetic wave generation device according to the first embodiment, the direction of the wave vector k _Ta of the terahertz wave or the direction of the wave vector k _Tb of the terahertz wave according to the specifications required by the system. Either of these can be selected.

The personal computer 11 controls the wavelength of the second pump light emitting unit 25 via the laser controller 31, and the wavelength of the second pump light hν2 changes from 1.199 μm to 1.221 μm. The value of the wave number vector k _p2 of the second pump light hν 2 changes from 8.977 μm ⁻¹ to 9.145 μm ⁻¹ . As the value of the wave vector k _p2 of the second pump light hν 2 changes from 8.977 μm ⁻¹ to 9.145 μm ⁻¹ , as shown in FIGS. 11 and 12, each of the vectors of different wave vector k _p2 Circles with different radii k _T are respectively drawn so that the centers of the circles are located at the end points of, respectively.

As shown in FIG. 11, when the vector k _p1 is larger than the vector k _p2 (vector k _p2 <vector k _p1 ), the wave vector k _p2 that changes in the range of 8.977 μm ⁻¹ to 9.145 μm ⁻¹ each of the end point of the vector, so that the center of the circle are positioned respectively drawn circle of different radii k _T, respectively, a circle of different radii k _T, respectively, and the circle of the vector k _L derived from the domain-inverted structure Intersect at different points. On the other hand, the root of the circle representing the vector k _L derived from the domain-inverted structure (that is, the center of the circle having the constant radius k _L ) is located at the end point of the vector k _p1 . This is an absolute value and direction of the terahertz wave wave vector k _T generated by difference frequency generation, the second value of the wave vector k _p2 of the pump light Etchinyu2, suggesting that changes. Further, as the value of the wave vector k _p2 of the second pump light hν2 approaches the wave vector vector k _p1 of the first pump light hν1, the radius of the circle of the wave vector k _T of the terahertz wave, that is, the vector k _T The absolute value is expected to gradually decrease. Therefore, when vector k _p1 @ vector k _p2 (when vector k _T is close to 0 as much as possible), depending on the value of internal crossing angle θ between vector k _p1 and vector k _p2 , phase matching as shown in FIG. May not be satisfied.

As shown in FIG. 12, when the vector k _p1 is smaller than the vector k _p2 (vector k _p2 > vector k _p1 ), as in FIG. 11, the center of the circle is placed at each end point of the vector of the wave vector k _p2. Are circled with different radii k _T so that the circles with different radii k _T intersect with the circles with vector k _L derived from the domain-inverted structure at different points. Also in FIG. 12, the root of the circle representing the vector k _L derived from the domain-inverted structure is located at the end point of the vector k _p1 . When the vector k _p1 shown in FIG. 12 is smaller than the vector k _p2 , the absolute value and direction of the wave vector k _T of the terahertz wave generated by the difference frequency generation are the same as those in the second pump light hν 2, as in FIG. It can be seen that it varies depending on the value of the wave vector k _p2 . As in the case of FIG. 11, as the value of the wave vector k _p2 of the second pump light hν2 approaches the wave vector vector k _p1 of the first pump light hν1, the radius of the circle of the wave vector k _T of the terahertz wave Is predicted to gradually decrease, so that when vector k _p1 @ vector k _p2 , phase matching is not satisfied depending on the value of internal crossing angle θ between vector k _p1 and vector k _p2 Can happen.

FIG. 14 is an enlarged view of FIGS. 11 and 12 when the PPKTP crystal is used for the quasi phase matching element 19 and the internal crossing angle θ = ˜7.44 °. However, FIG. 14, if the vector k _p1 that vector k _p1 shown in FIG. 11 is shown in FIG. 12 and greater than the vector k _p2 (vector k _p2 <vector k _p1) is less than the vector k _p2 (vector This corresponds to an enlarged view of a diagram obtained by combining two diagrams of k _p2 > vector k _p1 ). As shown in FIG. 14, when the internal cross angle θ = ˜7.44 °, the vector k _L derived from the polarization inversion structure of the quasi phase matching element 19 does not contact the vector k _{p2 of} the second pump light hν 2. Therefore, this corresponds to the case where the value of the internal crossing angle θ exceeds the phase matching range angle. The circle with the largest radius shown by the broken line on the left side of FIG. 14 represents the wave vector k _T when the frequency of the terahertz wave is 8.81 THz. Further, the circle with the second largest radius indicated by the two-dot chain line inside the circle indicated by the broken line on the left side of FIG. 14 indicates the wave vector k _T when the frequency of the terahertz wave is 6.15 THz, The circle with the third largest radius indicated by the one-dot chain line inside the circle indicated by the two-dot chain line indicates the wave vector k _T when the frequency of the terahertz wave is 3.37 THz. The circles indicating the three wave number vectors k _T whose frequencies of the terahertz waves are 8.81 THz, 6.15 THz, and 3.37 THz intersect with the circle representing the vector k _L derived from the polarization inversion structure, and the quasi phase matching condition is satisfied. Exists. However, on the left side of FIG. 14, the circle with the smallest radius indicated by the solid line on the inner side of the circle indicated by the alternate long and short dash line has no intersection with the circle indicating the vector k _L derived from the domain-inverted structure, and the pseudo It can be seen that the phase matching condition is not satisfied.

Similarly, the circle with the largest radius shown by the broken line on the right side of FIG. 14 represents the wave vector k _T when the frequency of the terahertz wave is 12.74 THz. Also, the second largest radius circle indicated by a two-dot chain line inside the circle indicated by the broken line on the right side of FIG. 14 indicates the wave vector k _T when the frequency of the terahertz wave is 9.19 THz, A circle indicated by a one-dot chain line having the third largest radius inside a circle indicated by a two-dot chain line indicates a wave vector k _T when the frequency of the terahertz wave is 5.82 THz. The circles indicating the three wave number vectors k _T having the terahertz wave frequencies of 12.74 THz, 9.19 THz, and 5.82 THz intersect with a circle representing the vector k _L derived from the polarization inversion structure, and the quasi phase matching condition is satisfied. Exists. However, on the left side of FIG. 14, the circle with the smallest radius indicated by the solid line on the inner side of the circle indicated by the alternate long and short dash line has no intersection with the circle indicating the vector k _L derived from the domain-inverted structure, and the pseudo It can be seen that the phase matching condition is not satisfied. In this way, when the value of the internal crossing angle θ becomes large and exceeds the phase matching range angle, the quasi phase matching condition is satisfied particularly when the radius of the circle indicating the wave vector k _T of the terahertz wave is smaller than a certain value. It turns out that it will not be satisfied.

Similarly, FIG. 15 is an enlarged view of FIG. 11 and FIG. 12 for a case where the internal crossing angle θ = ˜6.34 ° when a PPKTP crystal is used for the quasi phase matching element 19 for easy understanding. It is a thing. Similar to FIG 14, FIG 15, than vector k _p1 vector k _p2 the vector k _p1 shown in FIG. 11 is shown in FIG. 12 and greater than the vector k _p2 (vector k _p2 <vector k _p1) This is equivalent to an enlarged view of a figure obtained by combining two figures in a small case (vector k _p2 > vector k _p1 ). Unlike the case shown in FIG. 14, when the internal cross angle θ = ˜6.34 ° as shown in FIG. 15, the vector k _L derived from the polarization inversion structure of the quasi phase matching element 19 is the second pump. Since it is in contact with the vector k _{p2 of the} light hν2, this corresponds to the case where the value of the internal crossing angle θ is within the range of the phase matching range angle. That is, when the wave vector k _{p1 of} the first pump light hν 1 is 9.06 μm ⁻¹ , the value of the internal crossing angle θ when the vector k _L derived from the polarization inversion structure and the wave vector k _{T of the} terahertz wave just touch each other. Is ˜6.34 ° as shown in FIG. The circle with the largest radius indicated by the broken line on the left side of FIG. 15 indicates the wave number vector k _T when the frequency of the terahertz wave is 8.81 THz, and is the second indicated by the two-dot chain line inside the circle indicated by the broken line. The circle with the larger radius indicates the wave vector k _T when the frequency of the terahertz wave is 6.15 THz, and the circle with the third largest radius indicated by the one-dot chain line inside the circle indicated by the two-dot chain line is The wave vector k _T when the frequency of the terahertz wave is 3.37 THz, and the circle with the smallest radius indicated by the solid line inside the circle indicated by the one-dot chain line has a frequency of the terahertz wave of 0.45 THz. Shows the wave vector k _T. The circles indicating the four wave number vectors k _T with the terahertz wave frequencies on the left side of FIG. 15 having the frequencies of 8.81 THz, 6.15 THz, 3.37 THz, and 0.45 THz are all vectors k _L derived from the domain-inverted structure. And a quasi-phase matching condition exists.

On the other hand, the circle with the largest radius indicated by the broken line on the right side of FIG. 15 indicates the wave vector k _T when the frequency of the terahertz wave is 12.74 THz, and is indicated by a two-dot chain line inside the circle indicated by the broken line. The circle with the second largest radius indicates the wave vector k _T when the frequency of the terahertz wave is 9.19 THz, and the circle with the third largest radius indicated by the one-dot chain line inside the circle indicated by the two-dot chain line. Indicates the wave vector k _T when the frequency of the terahertz wave is 5.82 THz, and the circle with the smallest radius indicated by the solid line inside the circle indicated by the one-dot chain line has a frequency of terahertz wave of 2. A wave vector k _T in the case of 61 THz is shown. The circles indicating the four wave number vectors k _T having the terahertz wave frequencies of 12.74 THz, 9.19 THz, 5.82 THz, and 2.61 THz on the right side of FIG. 15 are all vectors k _L derived from the domain-inverted structure. It can be seen that there is a quasi-phase matching condition. Thus, when the value of the internal crossing angle θ is within the range of the phase matching range angle, even if the radius of the circle indicating the wave vector k _T of the terahertz wave is smaller than a certain value, the pseudo phase is always set. It can be seen that the matching condition is satisfied.

Similarly, when the value of the internal crossing angle θ becomes smaller and smaller than the optimum value, it can be seen that the same situation as in FIG. 14 occurs and the quasi phase matching condition is not satisfied. In any case, as shown in FIG. 14 and FIG. 15, except that the circle indicating the trajectory of the wave vector k _T of the terahertz wave is in contact with the circle indicating the trajectory of the vector k _L derived from the polarization inversion structure. There are two points that can satisfy the phase matching in the case of vector k _p2 <vector k _{p1 and in} the case of vector k _p2 > vector k _p1 , so two pseudo phase matching conditions for a certain vector k _p2 value It can be seen that exists.

Furthermore, as can be seen from FIGS. 14 and 15, shown <the circle on the left indicating the trajectory of the vector k _T where the vector k _p1, vector k _p2> vector k _p2 a locus of vector k _T where the vector k _p1 With reference to the position where the right side is circumscribed, the pseudo phase of the vector k _T far from the circumscribed position in each of the two cases of vector k _p2 <vector k _p1 and vector k _p2 > vector k _p1 and matching condition, in the quasi-phase matching conditions of the vector k _T of the near side to the OS position, quasi-phase matching condition of the two ways are classified. That is, in FIG. 8, in the case of the vector k _p2 <vector k _p1 , the vector on the left side corresponds to two intersections of the circle having the radius k _Ta = k _{Tb and} the circle representing the radius k _La = k _Lb. k _La and vector k _Ta are defined, and vector k _Lb and vector k _Tb are defined on the right side. Similarly, in the case of vector k _p2 > vector k _p1 , the intersection of vector k _p1 and vector k _p2 From the above, it is possible to classify the pseudo phase matching condition of the wave vector k _T of the terahertz wave on the far side and the pseudo phase matching condition of the wave vector k _T of the terahertz wave on the near side. In FIG. 14 and FIG. 15, in both cases of vector k _p2 <vector k _p1 and vector k _p2 > vector k _p1 , the outer sides are almost symmetrical as viewed from the circumscribed positions of the two circles. It is possible to define a quasi-phase matching condition (outside matching condition) at an intersection point (both sides) and a quasi-phase matching condition (inner matching condition) at an intersection point (in the vicinity of the circumscribed position).

FIG. 17A shows a direction in which the angle of the wave vector k _T of the terahertz wave is defined with reference to a vector k _p1 parallel to the positive direction of the x-axis, and FIG. The direction in which the angle φ of the inclination of the vector k _L derived from the polarization inversion structure of the quasi phase matching element 19 is defined with reference to the vector k _p1 parallel to the positive direction of the axis. Here, the direction of the vector k _L itself is defined as the direction in which the domain-inverted structures are periodically arranged. That is, the direction of the vector k _L is defined as a direction perpendicular to the stripe that defines the domain-inverted structure illustrated in FIGS. In the case where the angle of inclination of the vector k _L defined in FIG. 17 (k _L angle) is on the horizontal axis and the optimized internal crossing angle θ = 6.34 °, the vertical axis in FIG. 16 satisfies the phase matching condition. The frequency of the terahertz wave in the 0.3-2 THz range is shown (refractive index n _THz = 3.93 in FIG. 16). As shown in FIG. 16, the inclination angle (k _L angle) of the vector k _L Dispersion (angle change) is much greater in the entire region from 0.3 to 2 THz, with the outer matching condition plotted with Δ marks on both sides compared to the dispersion of the inner matching condition plotted with □ marks at the center. It can be seen that it is big.

The outer matching conditions plotted with Δ marks close to left and right symmetry in FIG. 16 correspond to the pseudo phase matching conditions when the vector k _p2 > vector k _{p1 on} the left side, and the pseudo conditions when the vector k _p2 <vector k _p1 on the right side. It corresponds to the phase matching condition. When vector k _p2 > vector k _p1, the k _L angle decreases as the frequency of the terahertz wave increases, and when vector k _p2 <vector k _p1 , the k _L angle increases as the frequency of the terahertz wave increases. It can also be understood from FIG. Incidentally, the symmetry point in FIG. 16 is the location of the vector k _{p1 @vector} k _p2 .

As can be seen from FIG. 16, in the electromagnetic wave generator according to the first embodiment, the frequency region of the terahertz wave that satisfies the quasi-phase matching condition in the outer matching condition of the two-dimensional vector diagram of FIG. The inner alignment condition of the dimension vector diagram can be satisfied similarly. Therefore, in the electromagnetic wave generator according to the first embodiment, a certain frequency region of the terahertz wave generated as the difference frequency between the first pump light hν1 and the second pump light hν2 is not wasted. However, the system may be designed in consideration of only the inner alignment condition of the two-dimensional vector diagram of FIG. Furthermore, in the electromagnetic wave generator according to the first embodiment, a simpler and feasible design can be expected by considering only the inner matching conditions of the two-dimensional vector diagram of FIG. That is, since the angular dispersion of the vector k _L derived from the periodic polarization inversion structure of the quasi phase matching element 19 can be minimized, in the frequency region of the terahertz wave considered, the stage shown in FIG. Driving by the rotary stage 57 via the controller 32 can provide an advantageous effect that the rotation of the pseudo phase matching element 19 can be minimized. The beam divergence angle of the laser used for the first pump light hν1 and the second pump light hν2 is generally about 1 mrad (≈0.0573 °). On the other hand, it can be seen from FIG. 16 that the rotation angle required for the rotary stage 57 illustrated in FIG. 1 is a value smaller than the beam spread, so it is necessary to rotate the quasi phase matching element 19 by the rotary stage 57. Since it is eliminated, the rotary stage 57 may be omitted. Therefore, in the electromagnetic wave generator according to the first embodiment, there is an advantageous effect that the stage controller 32 and the rotary stage 57 shown in FIG. 1 can be omitted depending on circumstances.

FIG. 18 is an enlarged view of the inner matching condition portion plotted with a square in the center of FIG. FIG. 16 plots only up to 2 THz in consideration of the transmittance of PPKTP shown in FIG. 13, but in FIG. 18, in a generalized case, when the internal cross angle θ = 6.34 °, the pseudo phase The case where the matching element 19 is transparent in a high frequency region of 2 THz or more is plotted. From FIG. 18, it can be seen that the range of the inclination angle (k _L angle) of the vector k _L that satisfies the quasi-phase matching condition in the inner matching condition of the two-dimensional vector diagram of FIG. 15 is about 0.007 °. .

According to FIG. 18, the inclination angle (k _L angle) of the vector k _L in the frequency range of 273.1627 ° to 273.1636 ° indicates a blank band gap in the frequency region of the terahertz wave caused by the difference frequency generation. As can be seen, this band gap indicates that quasi-phase matching is not established in the region of k _L angle of 273.1627 ° to 273.1636 °. The region in which the k _L angle serving as the band gap is 273.1627 ° to 273.1636 ° corresponds to the wave vector k _L near the wave vector k _p1 @wave vector k _p2 . In FIG. 18, the inner matching conditions plotted with □ marks that are almost symmetrical are corresponding to the quasi-phase matching condition in the case where the vector k _p2 > vector k _p1 on the right side and the vector k _p2 <vector k _p1 on the left side. This corresponds to the quasi phase matching condition. If the right vector k _p2 <vector k _p1 , the terahertz wave frequency decreases and the k _L angle increases, and if the left vector k _p2 > vector k _p1 , the k _L angle increases as the terahertz wave frequency increases. The increase can also be understood from FIG.

From FIG. 18, when designing the electromagnetic wave generator according to the first embodiment, only one of the two cases of the right vector k _p2 > vector k _p1 and the left vector k _p2 <vector k _p1 is used. It can be seen that the obtained terahertz wave frequency band does not change even if is selected. Therefore, by selecting only one of them, the range of the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure of the pseudo phase matching element 19 can be further reduced to 0.0035 °. It becomes. In view of the change limited to the extremely narrow k _L angle range in FIG. 18, it is necessary to greatly rotate the quasi phase matching element 19 in the design of the electromagnetic wave generator according to the first embodiment from the viewpoint of the system configuration. It can be understood that the stage controller 32 and the rotary stage 57 shown in FIG. 1 can be omitted. That is, the beam divergence angle of the laser used for the first pump light hν1 and the second pump light hν2 is generally about 1 mrad (≈0.0573 °) as described above. On the other hand, it can be seen from FIG. 18 that the rotation angle required for the rotary stage 57 illustrated in FIG. 1 can be much smaller than the beam spread. This is because it is not necessary to rotate the matching element 19. From FIG. 18, it can be seen that when the internal crossing angle θ = 6.34 °, the phase matching condition is satisfied up to about 12 THz. However, if the internal crossing angle θ is set to a larger angle θ, the phase matching is performed up to 12 THz or more. You can meet the conditions.

As can be seen from FIG. 8, FIG. 11, FIG. 12, FIG. 14, FIG. 15, and the like, the wave number of the terahertz wave having the direction corresponding to the vector k _L derived from the polarization inversion structure of the quasi phase matching element 19 is obtained. vector k _T is present in one-to-one. The range of the value of the wave number vector k _T of the terahertz wave is as important as the vector k _L derived from the polarization inversion structure. This is because, when considering the wave vector k _T of the terahertz wave and terahertz wave generation source, the angle distribution of the generated terahertz wave is because preferably as small as possible. Conventionally, when performing frequency adjustment using an electromagnetic wave generator as shown in FIG. 27, the wavelength of the second pump light hν ₂ , the external crossing angle θ _in ^ext between the _two pump lights hν ₁ and hν ₂ , It is necessary to control the position of the second non-axial parabolic mirror 7b at the same time, and a very complicated system configuration is required. In practice, it is difficult to always satisfy the difference frequency generation condition. FIG. 19 is a 3D plot of the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure and the direction angle value of the wave vector k _T of the terahertz wave in the case of vector k _p2 > vector k _p1. However, only the inner matching condition of the two-dimensional vector diagram of FIG. 15 is targeted. On the other hand, FIG. 20 shows the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure and the terahertz wave when the vector k _p2 <vector k _p1 in the inner matching condition of the two-dimensional vector diagram of FIG. values of direction angles of the wave vector k _T is obtained by the 3D plots.

From FIG. 19, the range of the value of the direction angle of the vector k _T corresponding to the slight change of the range of the direction angle of the vector k _L from 273.1636 ° to 273.1664 ° is less than 3.5 °. Thus, it can be seen that the range of the value of the direction angle of the vector k _T corresponding to the slight change of the range of the direction angle of the vector k _L from 273.164 ° to 273.187 ° is less than 3.0 °. As described above, the inclination angle (k _L angle) of the vector k _L and the direction angle value of the wave number vector k _T of the terahertz wave are values defined for the terahertz wave propagation in the quasi-phase matching element 19. The refraction effect is not taken into account. However, since the range of the inclination angle (k _L angle) of the vector k _L and the direction angle value of the wave vector k _T of the terahertz wave is very small, even in free space, the refractive index effect is a terahertz wave. It is sufficient to apply a slight correction to the direction angle in the direction of the occurrence of.

In the electromagnetic wave generator according to the first embodiment, the spectral purity of the generated terahertz wave is limited only by the adjustment resolution of the vector k _p2 , the line width of the excitation laser, and the angular resolution of the quasi phase matching element 19. As shown in the equation (3), the external crossing angle δ of the quasi phase matching element 19 between the two laser beams of the first pump light hν 1 and the second pump light hν 2 is set in the quasi phase matching element 19. This is different in relation to the internal crossing angle θ and the refractive index. Further, the internal crossing angle θ changes as the quasi phase matching element 19 rotates, and this change occurs when the quasi phase matching element 19 is rotated by 5 °, and the value of the internal crossing angle θ changes by 0.02 ° or more. It is confirmed by calculation that it does not. Such a small change in the value of the internal crossing angle θ can be covered by a circular focus adjustment of the beam diameter of one of the first pump light hν1 and the second pump light hν2.

As described above, according to the electromagnetic wave generation device according to the first embodiment, when a PPKTP crystal is used for the quasi phase matching element 19, two lasers of the first pump light hν1 and the second pump light hν2 are used. If the beam is incident on the quasi phase matching element 19 at an internal crossing angle θ of ˜6.34 °, it is possible to generate a narrowband terahertz wave electromagnetic wave in a frequency range of 0.3-2 THz. is there. That is, in the electromagnetic wave generator according to the first embodiment, by changing the frequency of the second pump light hν2 while fixing the external crossing angle δ or the internal crossing angle θ related to each other in the equation (3). A terahertz electromagnetic wave having a variable wavelength can be generated from the pseudo phase matching element 19. According to the electromagnetic wave generator according to the first embodiment, the quasi-phase matching element 19 having a periodic domain-inverted structure is adjusted to have a terahertz wave frequency within a frequency band of 0.3 to 2 THz terahertz wave. In order to sweep, only 0.0035 ° needs to be rotated. That is, as described above, the beam divergence angle of the laser used for the first pump light hν1 and the second pump light hν2 is generally about 1 mrad (≈0.0573 °), but is 0.0035 °. Since the angle is much smaller than the beam spread, it is not necessary to rotate the quasi phase matching element 19 by the rotary stage 57 illustrated in FIG. Therefore, as an actual configuration of the electromagnetic wave generator according to the first embodiment, the rotary stage 57 is not necessary. Further, as the difference frequency, the terahertz wave generated from the quasi-phase matching element 19 spreads only by about 3.0 ° to 3.5 °. Therefore, in the configuration of the electromagnetic wave generator according to the basic concept shown in FIG. In the state where the position of the detector 34 can be fixed and the position of the electromagnetic wave detector 34 is fixed, it is used in the prior art as shown in FIG. 27 between the pseudo phase matching element 19 and the electromagnetic wave detector 34. It is not necessary to use the non-axial parabolic mirrors 7a and 7b. Therefore, according to the electromagnetic wave generator according to the first embodiment, it is possible to reduce the number of parts used and to form a very simple and inexpensive configuration. Therefore, the electromagnetic wave generator according to the first embodiment The industrial applicability of is extremely high.

(Second Embodiment)
In the following description of the electromagnetic wave generator according to the second embodiment of the present invention, in the configuration of the electromagnetic wave generator according to the basic concept shown in FIG. An example of using “PPGaP crystal”) is illustrated. That is, although the illustration of the device configuration is omitted, the electromagnetic wave generating device according to the second embodiment of the present invention is the same as that shown in FIG. 1, and the first pump that emits the first pump light hν1. A light emitting section 24, a second pump light emitting section 25 for emitting a second pump light hν2 having a wavelength different from that of the first pump light hν1, with a variable wavelength, and a periodic polarization inversion structure, The external cross angle between the quasi phase matching element 19 made of PPGaP that generates an electromagnetic wave having a difference frequency between the first pump light hν1 and the second pump light hν2, and the first pump light hν1 and the second pump light hν2. An optical system (M6, 18) is provided that adjusts and fixes δ and the internal crossing angle θ and makes the first pump light hν1 and the second pump light hν2 enter the quasi-phase matching element 19 made of PPGaP. Where the external crossing angle δ Is the crossing angle between the _first pump light hν ₁ and the second pump light hν ₂ outside the quasi-phase matching element 19 made of PPGaP, and is the crossing angle inside the quasi-phase matching element 19 made of PPGaP. The internal crossing angle θ has the relationship shown in the already described formula (3). Terahertz wave PPGaP refractive index n _THz = 3.4, and PPGaP refractive index n _L at the pump light frequency (λ = 1.55 μm). = 3.055.

  The quasi-phase matching element 19 made of PPGaP has a polarization inversion period Λ of 6 μm, for example, a structure in which the polarization direction of the GaP crystal is periodically reversed for each coherence length lc as shown in FIG. Two comb-shaped GaP substrates 19a and 19b as shown in FIG. 5 (b) may be bonded to each other, or the surface of the polarization M GaP substrate 192 as shown in FIG. Periodic polarization with a structure in which a GaP region 193 in which the cross-sectional shape in the vertical direction is an arc shape and the polarization direction N is the direction opposite to the polarization M is periodically embedded in the plate surface direction with a period Λ An inversion structure may be configured. That is, the quasi-phase matching element 19 made of PPGaP used in the electromagnetic wave generator according to the second embodiment has various periodic polarization inversion structures as long as the polarization (M, N) of the GaP crystal is periodically inverted. In addition, various methods can be adopted as the manufacturing method.

  As shown in FIG. 1, the electromagnetic wave generator according to the second embodiment further includes a terahertz band electromagnetic wave detector 34 such as a silicon bolometer or a DTGS infrared light detector. In the state where 34 is fixed, the electromagnetic wave in the terahertz band emitted from the quasi-phase matching element 19 made of PPGaP can be detected by the electromagnetic wave detector 34. The electromagnetic wave detector 34 is connected to a voltmeter 33, and the voltmeter 33 is connected to a personal computer (PC) 11, and performs arithmetic processing on an electric signal based on the terahertz band electromagnetic wave detected by the electromagnetic wave detector 34. Is configured to be possible. Since the terahertz band electromagnetic wave emitted from the quasi-phase matching element 19 made of PPGaP is emitted in a fixed direction, it is used in the conventional technique as shown in FIG. 27 between the quasi-phase matching element 19 and the electromagnetic wave detector 34. It is not necessary to use the non-axial parabolic mirrors 7a and 7b.

The optical system (M ₆ , 18) can be composed of the mirror M ₆ and the polarization beam splitter 18 as shown in FIG. With the arrangement of the optical system (M ₆ , 18) shown in FIG. 1, the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is transmitted through the polarization beam splitter 18, and the first The pump light (first pump light) hν ₁ emitted from the pump light emitting unit 24 is incident from the vertical direction using the mirror M ₆ , reflected by the polarization plane of the polarization beam splitter 18, and a minute amount close to parallel. The pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ can be combined in the quasi phase matching element 19 made of PPGaP at the external crossing angle δ.

  In the electromagnetic wave generator according to the second embodiment, the quasi phase matching element 19 is mounted on the rotary stage 57, and the rotary stage 57 is driven by the stage controller 32, as shown in FIG. Although it is possible to design a terahertz wave radiation source of 0.3 to 7 THz without almost rotating the quasi phase matching element 19 made of PPGaP, the rotation stage 57 is omitted. Is also possible. As described in the electromagnetic wave generator according to the first embodiment, the beam divergence angle of the laser used for the first pump light hν1 and the second pump light hν2 is generally about 1 mrad (≈0.0573 °). On the other hand, since the rotation angle required by the rotary stage 57 is smaller than the beam spread, the electromagnetic wave generator according to the second embodiment similarly uses the rotary stage 57. This is because it is not necessary to rotate the pseudo phase matching element 19.

  The first pump light emitting unit 24 used in the electromagnetic wave generator according to the second embodiment is, for example, a Cr-doped forsterite laser, Yb-doped YLF laser, or Yb-doped fiber laser that emits the first pump light hν1. The first pump light hν1 source comprising either of them, and the second pump light emitting section 25 is, for example, a Cr-doped forsterite laser, Yb-doped YLF laser, or Yb-doped fiber laser that emits the second pump light hν2. It is possible to provide the 2nd pump light source which consists of either. At this time, an excitation light source is further provided for exciting the first pump light hν1 and the second pump light source, and emitting the first pump light hν1 and the second pump light hν2 from the first and second pump light sources, respectively. You may do it. Alternatively, the output of the YAG laser having a wavelength of 1.064 μm is emitted from the first pump light emitting unit 24 as the first pump light hν1, and the second pump light emitting unit 25 is an optical parametric oscillator having an injection seeding mechanism. , And the optical parametric oscillator may be excited by the output of the YAG laser so that the output of the optical parametric oscillator is emitted from the second pump light emitting unit 25 as the second pump light hν2.

Similar to the first embodiment, the photon momentum derived from the periodic polarization inversion structure of the quasi-phase matching element 19 made of PPGaP is represented by a vector k _L , the wave vector of the terahertz wave is a vector k _T, and the first If the wave vector of the pump light hν1 is represented by the vector k _p1 and the wave vector of the second pump light hν2 is represented by the vector k _p2 , the law of conservation of momentum in the quasi-phase matching is given by the equation (7) already described. Indicated. The value (˜1 μm ⁻¹ ) of the vector k _{L on} the right side of the equation (7) is expressed as the vector k _L = 2π / Λ using the value of the polarization inversion period Λ = 6 μm of the quasi phase matching element 19 made of PPGaP. The wave number k _{T of the} derived terahertz wave can also be expressed by Expression (8) already described.

Although not shown in the two-dimensional vector diagram, from equation (7) and (8), in the same manner as shown in FIG. 15, if the vector k _p1 is larger than the vector k _p2 (vector k _p2 <vector k _p1 ) Are drawn with circles having different radii k _T so that the centers of the circles are located at the respective end points of the vector of the wave vector k _p2 that changes in the range of 7.083 μm ⁻¹ to 7.329 μm ^−1. Thus, the circles having different radii k _T intersect with the circle of the vector k _L derived from the polarization inversion structure, respectively. On the other hand, the root of the circle representing the vector k _L derived from the PPGaP domain-inverted structure (that is, the center of the circle having the constant radius k _L ) is located at the end point of the vector k _p1 .

In the electromagnetic wave generation device according to the second embodiment, when the internal crossing angle θ = ˜4.85 °, the vector k _L derived from the polarization inversion structure of PPGaP is the vector k _{p2 of} the second pump light hν ₂ . This can correspond to the case where the value of the internal crossing angle θ is within the range of the phase matching range angle. As described with reference to FIGS. 14 and 15, the outer alignment condition and the inner alignment condition can be defined. In the electromagnetic wave generation device according to the second embodiment, the frequency region of the terahertz wave that satisfies the quasi phase matching condition in the outer matching condition can be satisfied in the inner matching condition as well. Therefore, in the electromagnetic wave generator according to the second embodiment, a certain frequency region of the terahertz wave generated as the difference frequency between the first pump light hν1 and the second pump light hν2 is not wasted. It is only necessary to design the system considering only the inner alignment conditions. Furthermore, in the electromagnetic wave generator according to the second embodiment, a simpler and feasible design can be expected by considering only the inner matching conditions.

FIG. 21 shows the angle of inclination of the vector k _L (k _L angle) derived from the PPGaP domain-inverted structure and the pseudo phase matching element 19 in the inner matching condition of the electromagnetic wave generator according to the second embodiment. It is a figure which shows the relationship of the frequency of the terahertz wave to do. According to FIG. 21, the inclination angle (k _L angle) of the vector k _L of 272.423 ° to 272.4234 ° indicates a blank band gap in the frequency region of the terahertz wave generated by the difference frequency. I understand. This band gap indicates that the quasi phase matching is not established in the k _L angle region of 272.423 ° to 272.4234 °. The region where the band gap k _L angle is 272.4423 ° to 272.4234 ° corresponds to the wave vector k _L near the wave vector k _p1 @wave vector k _p2 . In FIG. 21, the inner matching conditions plotted with □ marks that are almost symmetrical to the left and right correspond to the quasi-phase matching conditions when the vector k _p2 > vector k _p1 on the right side and the vector k _p2 <vector k _p1 on the left side. This corresponds to the quasi phase matching condition. If the right vector k _p2 <vector k _p1 , the terahertz wave frequency decreases and the k _L angle increases, and if the left vector k _p2 > vector k _p1 , the k _L angle increases as the terahertz wave frequency increases. The increase can be understood from equations (7) and (8).

From FIG. 21, when designing the electromagnetic wave generator according to the second embodiment, only one of the two cases of the right vector k _p2 > vector k _p1 and the left vector k _p2 <vector k _p1 is used. It can be seen that the obtained terahertz wave frequency band does not change even if is selected. Therefore, by selecting only one of them, the range of the inclination angle (k _L angle) of the vector k _L derived from the PPGaP domain-inverted structure can be further reduced to 0.001 °. In view of the change limited to the extremely narrow k _L angle range in FIG. 21, in the design of the electromagnetic wave generator according to the second embodiment, the pseudo phase matching element 19 made of PPGaP is greatly increased from the viewpoint of the system configuration. It can be understood that the stage controller 32 and the rotary stage 57 shown in FIG. In FIG. 21, the frequency of the terahertz wave generated from the quasi phase matching element 19 is plotted only up to 2 THz. However, when the internal cross angle θ = 4.85 °, the phase matching condition is satisfied up to about 7 THz.

As can be seen from equation (7) and (8), to the vector k _L derived from the poled structure of pPGAP, wave vector k _T of the terahertz wave having a direction corresponding thereto are present in one-to-one. The range of the value of the wave number vector k _T of the terahertz wave is as important as the vector k _L derived from the polarization inversion structure of PPGaP. This is because, when considering the wave vector k _T of the terahertz wave and terahertz wave generation source, the angle distribution of the generated terahertz wave is because preferably as small as possible. As already mentioned, conventionally, when performing frequency adjustment by using the electromagnetic wave generator shown in FIG. 27, a second wavelength of the pump light hv _2, 2 one pump light hv _1, the external cross between hv ₂ It is necessary to control the angle θ _in ^ext and the position of the second non-axis parabolic mirror 7b at the same time, and a very complicated system configuration is required. In practice, it is difficult to always satisfy the difference frequency generation condition. Met. FIG. 22 shows the inclination angle (k _L angle) of the vector k _L derived from the polarization inversion structure of PPGaP and the direction angle of the wave vector k _T of the terahertz wave when the vector k _p2 <the vector k _p1 in the inner matching condition Is a 3D plot.

From Figure 22, it can be seen that the range of values of direction angles of the vector k _T corresponding to changes in the scope of the direction angles of slight vector k _L of 274.4234 ° ~272.4223 ° is less than 2.2 ° . As described above, the inclination angle (k _L angle) of the vector k _L and the direction angle value of the wave number vector k _T of the terahertz wave are defined for the terahertz wave propagation in the quasi-phase matching element 19 made of PPGaP. The refraction effect is not taken into account. However, since the range of the inclination angle (k _L angle) of the vector k _L and the direction angle value of the wave vector k _T of the terahertz wave is very small, even in free space, the refractive index effect is a terahertz wave. It is sufficient to apply a slight correction to the direction angle in the direction of the occurrence of.

As described above, according to the electromagnetic wave generator according to the second embodiment, when a PPGaP crystal is used for the quasi phase matching element 19, two lasers of the first pump light hν1 and the second pump light hν2 are used. If the beam is incident on the quasi phase matching element 19 made of PPGaP at an internal crossing angle θ of ˜4.85 °, an electromagnetic wave of a narrow band terahertz wave in a frequency region of 0.3 to 7 THz is generated. Is possible. That is, in the electromagnetic wave generator according to the second embodiment, by changing the frequency of the second pump light hν2 in a state where the external crossing angle δ or the internal crossing angle θ related to each other in the equation (3) is fixed. A terahertz electromagnetic wave having a variable wavelength can be generated from the pseudo phase matching element 19. According to the electromagnetic wave generator according to the second embodiment, the quasi-phase matching element 19 made of PPGaP having a periodic polarization reversal structure is placed in the terahertz wave frequency band of 0.3 to 7 THz. It is only necessary to rotate 0.001 ° to sweep the frequency. In addition, the terahertz wave generated as the difference frequency spreads only about 0.85 ° within a frequency range of 0.34 to 7.2 THz and limited to within 2.2 ° and 0.8 to 7.2 THz. Industrial applicability is extremely high.

(Third embodiment)
Electromagnetic wave generator according to a third embodiment of the present invention, as shown in FIG. 23, a first pump-light emitting unit 24 for emitting a first pump light hv _1, the first pump light hv ₁ The second pump light emitting section 25 that emits the second pump light hν ₂ having a wavelength different from that of the first pump light hν ₂ with a variable wavelength, and the electromagnetic wave hν having a difference frequency between the first pump light hν ₁ and the second pump light hν _{2. 3} , the internal crossing angle θ of the first pump light hν ₁ and the second pump light hν ₂ is adjusted and fixed to the phase matching range angle, and the first pump light and An optical system (M ₆ , 18) for causing the second pump light to enter the quasi phase matching element 19 is provided. Where the internal crossing angle θ Is the crossing angle between the _first pump light hν ₁ and the second pump light hν ₂ inside the quasi phase matching element 19, and the external crossing angle δ defined outside the quasi phase matching element 19 is , Which has a relationship of approximately equation (3). For example, when a PPKTP crystal is used for the quasi phase matching element 19, the value of the internal cross angle θ = 6.34 ° is a phase matching range angle, and when a PPGaP crystal is used for the quasi phase matching element 19, The crossing angle θ = about 4.85 ° is the phase matching range angle. In the electromagnetic wave generator according to the third embodiment, the pseudo frequency is changed by changing the frequency of the second pump light hν2 while the external crossing angle δ or the internal crossing angle θ related to each other in Expression (3) is fixed. A variable wavelength terahertz electromagnetic wave is generated from the phase matching element 19.

In the electromagnetic wave generator according to the third embodiment, as shown in FIG. 23, the YAG laser 6 that excites the first YAG rod 62 and the second YAG rod 63 with one flash lamp 61, that is, the “double pulse YAG laser 6”. Is used as an excitation light source, and two Cr-added forsterite lasers are excited by causing the double pulse YAG laser 6 to function as a timing control mechanism (means). The double pulse YAG laser 6 excites the first YAG rod 62 and the second YAG rod 63 with one flash lamp 61 to obtain _two output pulse lights hν ₁ and hν ₂ having a wavelength of 1064 nm.

  Since the first YAG rod 62 and the second YAG rod 63 are excited by the flash lamp 61, the YAG laser is oscillated by a Q-switch pulse with a considerable delay, so that the timing of the Q switch pulses of the first YAG rod 62 and the second YAG rod 63 is changed. Each can be changed with an accuracy of 1 nanosecond. As a result, the excitation light pulse from the first YAG rod 62 excites the first pump light source (Cr-added forsterite laser), and the excitation light pulse from the second YAG rod 63 changes to the second pump light source (Cr-added forsterite laser). By adjusting the time difference between the excitation time of the stellite laser) and the pulse light of the pump light from the first pump light source (first pump light) and the signal light from the second pump light source (second The timing is controlled so that the pulse of the pump light) reaches the quasi phase matching element 19 almost simultaneously. The term “substantially simultaneously” means “in a time sufficiently shorter than the pulse width of the output light of the Cr-doped forsterite laser”. For example, the pulse width of the Cr-doped forsterite laser is generally 20 to 30 nanometers. In such a case, a time difference of 1 nanosecond or less may be used.

The optical system (M ₆ , 18) includes a reflecting mirror (mirror) M ₆ and a polarizing beam splitter 18. The reflecting mirror (mirror) M ₆ reflects the first pump light hν ₁ emitted from the first pump light emitting unit 24 and adjusts the angle at which the first pump light hν ₁ is incident on the polarization beam splitter 18. It is a rotatable mirror. With the arrangement of the optical system (M ₆ , 18) shown in FIG. 23, the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is transmitted through the polarization beam splitter 18. Further, the pump light (first pump light) hν ₁ emitted from the first pump light emitting unit 24 is incident from the vertical direction by using a reflecting mirror (mirror) M _6, and the polarization plane of the polarization beam splitter 18. The pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ are coupled to the quasi phase matching element 19 so that the internal crossing angle θ is the phase matching range angle. An optical system is configured.

However, the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ need only be superposed in parallel by the polarization beam splitter 18, so FIG. 9 of the fourth embodiment. As shown in FIG. 5, the pump light (first pump light) hν ₁ is transmitted through the polarization beam splitter 18, and the signal light (second pump) whose polarization plane is orthogonal to the pump light (first pump light) hν _1. Light) hν ₂ is incident from the vertical direction and reflected by the polarization plane of the polarization beam splitter 18, and the first pump light and the second pump light are pseudo-phased so that the internal crossing angle θ is the phase matching range angle. An optical system to be coupled in the matching element 19 may be configured.

The optical system (M ₆ , 18) shown in FIG. 23 is an example, and in addition to the reflecting mirror (mirror) M ₆ and the polarizing beam splitter 18, other optical elements such as a mirror may be added. instead of the reflecting mirror (mirror) M _6, it is of course may be replaced with an optical element having a reflecting mirror such as a prism (mirror) M ₆ and equivalent function.

  The pulse of the pump light (first pump light) from the first pump light source and the pulse of the signal light (second pump light) from the second pump light source reach the pseudo phase matching element 19 almost simultaneously. This means that the outputs (pulses) from the first and second pump light sources are made substantially simultaneously. In order to make the outputs (pulses) of the two Cr-added forsterite lasers as the first and second pump light sources substantially coincide with each other, a program is previously made so as to change the timing of the Q switch pulse of the double pulse YAG laser 6. You should do it. Alternatively, the pump light from the first pump light source (first pump light) and the second pump light source are formed by forming a loop that feedback-controls the timing of Q switch pulse generation so that the terahertz electromagnetic wave output is maximized. The timing of the signal light from the second light (second pump light) can be matched.

The double pulse YAG laser 6 is driven by a laser controller 31 shown in FIG. That is, the laser controller 31 generates the flash lamp excitation pulse P ₁ and the YAG laser Q-switch pulses P ₂ and P ₃ , and the signal light (second pump light) is generated by the laser controller 31 and the double pulse YAG laser 6. ) And a pump light (first pump light) pulse reach the quasi-phase matching element 19 almost simultaneously. The timing control mechanism (means) (6, 31) is configured.

In FIG. 23, a Cr-added forsterite laser is used as the first pump light source of the first pump light emitting unit 24 and the second pump light source of the second pump light emitting unit 25, respectively. The delay time of the Cr-added forsterite laser is mainly determined by the excitation light intensity from the double-pulse YAG laser 6 and the output wavelength of the Cr-added forsterite laser, so that two Cr additions as the first and second pump light sources are added. A forsterite laser does not necessarily generate an output at the same timing. To sweep the frequency [nu ₃ of THz radiation hv _3, for example, the wavelength of one of Cr added forsterite laser constituting the first pump-light emitting unit 24 (first pump light source) is fixed, the second The wavelength of the other Cr-added forsterite laser (second pump light source) constituting the pump light emitting section 25 is swept (reversely, the Cr-added forsterite laser (second pump) constituting the second pump light emitting section 25. (The pump light source) may be fixed, and the wavelength of the Cr-added forsterite laser (first pump light source) constituting the first pump light emitting section 24 may be swept.

Since the wavelength and intensity of the Cr-added forsterite laser (second pump light source) constituting the second pump light emitting section 25 change with sweeping, two Cr-added forsterite as a function of these two parameters. The timing difference t between the _two Q-switch pulses P ₂ and P ₃ for simultaneous laser output may be programmed in advance, and the two Q-switch pulses P ₂ and P ₃ may be generated according to the program. Since the output pulse width of the Cr-added forsterite laser is about 10 nanoseconds, it is sufficient to control the timing difference t between the Q-switch pulses P ₂ and P ₃ with an accuracy of 1 nanosecond.

The pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ emitted from the first pump light emitting portion 24 and the second pump light emitting portion 25 respectively are polarized beam splitter 18. Therefore, the beams are overlapped so that the internal crossing angle θ is set to the phase matching range angle, and enter the pseudo phase matching element 19. The pseudo phase matching element 19 outputs a terahertz electromagnetic wave hν ₃ having a frequency that is the difference between the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ .
When two Cr-added forsterite lasers are used, the delay time may be 10 nanoseconds or more depending on the oscillation wavelength. In such a case, in a variable delay line configured by using a plurality of reflecting mirrors, the optical path becomes long, and the configuration of the apparatus may be too large. Therefore, when the delay time to be adjusted becomes long, the method using the double pulse YAG laser 6 as in the electromagnetic wave generator according to the third embodiment is effective for downsizing the electromagnetic wave generator. The Cr-doped forsterite laser as the first and second pump light sources used in the first pump light emitting section 24 or the second pump light emitting section 25 is replaced with a YLF-doped YLF laser or Yb-doped fiber laser. It is also possible to do this as in the fourth embodiment and the fifth example.

  As described above, according to the electromagnetic wave generator according to the third embodiment, when a PPKTP crystal is used for the quasi phase matching element 19, two lasers of the first pump light hν1 and the second pump light hν2 are used. If the beam is incident on the quasi phase matching element 19 at an internal crossing angle θ of about 6.34 °, a narrow band terahertz wave electromagnetic wave in the frequency range of 0.3 to 2 THz is generated from the PPKTP crystal. Is possible. Further, when a PPGaP crystal is used for the quasi phase matching element 19, the two laser beams of the first pump light hν1 and the second pump light hν2 are made of PPGaP with an internal crossing angle θ of about 4.85 °. If incident on the phase matching element 19, it is possible to generate a narrow band terahertz wave electromagnetic wave from a PPGaP crystal in a frequency range of 0.3 to 7 THz.

When the PPKTP crystal is used as the quasi-phase matching element 19, the quasi-phase matching element 19 is only 0.0035 ° in order to sweep the terahertz wave frequency within the frequency band of 0.3 to 2 THz terahertz wave. There is no need to rotate. Further, as the difference frequency, the terahertz wave generated from the PPKTP crystal spreads only by about 3.0 ° to 3.5 °. On the other hand, when the quasi-phase matching element 19 made of PPGaP crystal is used, the quasi-phase is only 0.001 ° in order to sweep the terahertz wave frequency within the terahertz wave frequency band of 0.3 to 7 THz. There is no need to rotate the matching element 19, and the terahertz wave generated from the PPGaP crystal as the difference frequency is limited to within 2.2 ° and 0.8 to 7.2 THz within the frequency range of 0.34 to 7.2 THz. For example, it spreads only about 0.85 °. Therefore, according to the electromagnetic wave generator according to the third embodiment, in the configuration shown in FIG. 1, the position of the electromagnetic wave detector 34 can be fixed and the position of the electromagnetic wave detector 34 is fixed. It is not necessary to use the non-axis parabolic mirrors 7a and 7b used in the prior art as shown in FIG. 27 between the phase matching element 19 and the electromagnetic wave detector 34. For this reason, according to the electromagnetic wave generator according to the third embodiment, it is possible to reduce the number of parts used and to form an extremely simple and inexpensive configuration. The electromagnetic wave generator according to the third embodiment The industrial applicability of is extremely high.

(Fourth embodiment)
As shown in FIG. 24, the electromagnetic wave generator according to a fourth embodiment of the present invention includes a first pumping light outputting section 24 for emitting a first pump light hv _1, the first pump light hv ₁ The second pump light hν ₂ having a wavelength different from that of the second pump light emitting section 25 that emits the wavelength tunably, and the electromagnetic wave having a difference frequency between the first pump light hν ₁ and the second pump light hν _2. The quasi-phase matching element 19 that generates hν ₃ and the internal crossing angle θ between the _first pump light hν ₁ and the second pump light hν ₂ are adjusted and fixed to the phase matching range angle, and the first pump light And an optical system (M ₃ , M ₆ , M ₅ , 18) for causing the second pump light to enter the quasi phase matching element 19. Where the internal crossing angle θ Is the crossing angle between the _first pump light hν ₁ and the second pump light hν ₂ inside the quasi phase matching element 19, and the external crossing angle δ defined outside the quasi phase matching element 19 is , Which has a relationship of approximately equation (3). For example, when a PPKTP crystal is used for the quasi phase matching element 19, the value of the internal cross angle θ = 6.34 ° is a phase matching range angle, and when a PPGaP crystal is used for the quasi phase matching element 19, The crossing angle θ = about 4.85 ° is the phase matching range angle. In the electromagnetic wave generator according to the fourth embodiment, the pseudo frequency is changed by changing the frequency of the second pump light hν2 while fixing the external crossing angle δ or the internal crossing angle θ related to each other in the equation (3). A variable wavelength terahertz electromagnetic wave is generated from the phase matching element 19.

The optical system (M ₃ , M ₆ , M ₅ , 18) includes a half mirror M ₃ , a first mirror M ₅ , a second mirror M _6, and a polarization beam splitter 18. The half mirror M ₃ transmits a part of the excitation light emitted from the YAG laser as the excitation light source 16 and enters the first pump light emission unit 24, and in addition to the excitation light emitted from the excitation light source (YAG laser) 16. This is a mirror that reflects a part of the light and makes it incident on the second pump light emitting unit 25. The first mirror M ₅ further reflects the excitation light reflected by the half mirror M ₃ and causes the other part of the excitation light emitted from the excitation light source (YAG laser) 16 to enter the second pump light emitting unit 25. It is a mirror. The second mirror M ₆ reflects the first pump light hν ₁ emitted from the first pump light emitting unit 24 and adjusts the angle at which the first pump light hν ₁ enters the polarization beam splitter 18. It is a rotatable mirror. With the arrangement of the optical systems (M ₃ , M ₆ , M ₅ , 18) shown in FIG. 24, the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is passed through the polarization beam splitter 18. Make it transparent. The pump light (first pump light) hν ₁ emitted from the first pump light emitting unit 24 is incident from the vertical direction using the second mirror M ₆ and reflected by the polarization plane of the polarization beam splitter 18. Then, the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ are combined in the quasi phase matching element 19 so that the internal crossing angle θ is the phase matching range angle. An optical system is configured.

However, the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ may be set to a phase matching range angle by the polarization beam splitter 18. Pump light) hν ₁ is transmitted through the polarization beam splitter 18, and signal light (second pump light) hν ₂ whose polarization plane is orthogonal to the pump light (first pump light) hν _{1 is} incident from the vertical direction. And an optical system that couples the first pump light and the second pump light in the quasi phase matching element 19 so that the internal crossing angle θ is the phase matching range angle. It may be configured. The optical system (M ₃ , M ₆ , M ₅ , 18) shown in FIG. 24 is an example, and in addition to the half mirror M ₃ , the first mirror M ₅ , the second mirror M _6, and the polarization beam splitter 18. to, may be added to optical elements such as other mirrors, the first mirror M _5, in place of such second mirror M _6, a first mirror M ₅ and the second mirror M ₆ such equivalent function such as a prism Of course, it may be replaced by an optical element having

  A chromium (Cr) -added forsterite laser pumped by a pumping light source (YAG laser) 16 as the first and second pump light sources provided in the first pump light emitting unit 24 and the second pump light emitting unit 25, respectively. use. As described above, the Cr-added forsterite laser uses the Cr level in the forsterite, and therefore has a very narrow spectral line width. The Cr-added forsterite laser can be pumped by using a pumping light source (YAG laser) 16 having a wavelength of 1.064 μm, and the YAG third harmonic is not used unlike OPO, so that the efficiency can be increased. is there.

That is, in the electromagnetic wave generating device according to the fourth embodiment, the first and second pump light emitting units 25 and 25 are provided with two Cr-added forsterite lasers, respectively. Spectral line width of the terahertz electromagnetic wave hν ₃ used as a pump light source, the first pump light emitting part 24 as a fixed wavelength, the second pump light emitting part 25 as a tunable pump light source, and as a difference frequency without injection seeding Can be narrowed. The wavelength of the pump light (first pump light) hν ₁ is fixed at, for example, 1.20 μm. The first pump light source provided in the first pump light emitting unit 24 can narrow the spectral line width to about 1 GHz at a fixed wavelength by installing an etalon inside. The second pump light source provided in the second pump light emitting unit 25 uses the same kind of Cr-added forsterite laser and sweeps the wavelength. Since the second pump light emitting unit 25 is a continuous wavelength sweep and does not insert an etalon, the spectral line width is 10 GHz to 15 GHz. Further, the Cr-added forsterite laser does not require the use of YAG harmonics unlike OPO, and is characterized in that it can be excited by a fundamental wave of 1.064 μm, has high output and is inexpensive. Incidentally, a Cr-doped forsterite laser as the first and second pump light sources used for the first pump light emitting section 24 and the second pump light emitting section 25 is a YLF laser doped with Yb or a fiber doped with Yb. It can be replaced with a laser. Even when using a Yb-doped YLF laser or a Yb-doped fiber laser, a relatively narrow line width can be obtained without injection seeding, which has the advantage of being extremely inexpensive. However, in the case of a Yb-doped YLF laser or a Yb-doped fiber laser, the wavelength is slightly shorter than the wavelength of 1.064 μm of the YAG laser 16, so that it is necessary to excite with the second harmonic of the YAG laser 16 (wavelength 530 nm).

  As described above, according to the electromagnetic wave generator according to the fourth embodiment, when a PPKTP crystal is used for the quasi phase matching element 19, two lasers of the first pump light hν1 and the second pump light hν2 are used. If the beam is incident on the quasi phase matching element 19 at an internal crossing angle θ of about 6.34 °, a narrow band terahertz wave electromagnetic wave in the frequency range of 0.3 to 2 THz is generated from the PPKTP crystal. Is possible. Further, when a PPGaP crystal is used for the quasi phase matching element 19, the two laser beams of the first pump light hν1 and the second pump light hν2 are made of PPGaP with an internal crossing angle θ of about 4.85 °. If incident on the phase matching element 19, it is possible to generate a narrow band terahertz wave electromagnetic wave from a PPGaP crystal in a frequency range of 0.3 to 7 THz.

When the PPKTP crystal is used as the quasi-phase matching element 19, the quasi-phase matching element 19 is only 0.0035 ° in order to sweep the terahertz wave frequency within the frequency band of 0.3 to 2 THz terahertz wave. There is no need to rotate. Further, as the difference frequency, the terahertz wave generated from the PPKTP crystal spreads only by about 3.0 ° to 3.5 °. On the other hand, when the quasi-phase matching element 19 made of PPGaP crystal is used, the quasi-phase is only 0.001 ° in order to sweep the terahertz wave frequency within the terahertz wave frequency band of 0.3 to 7 THz. There is no need to rotate the matching element 19, and the terahertz wave generated from the PPGaP crystal as the difference frequency is limited to within 2.2 ° and 0.8 to 7.2 THz within the frequency range of 0.34 to 7.2 THz. For example, it spreads only about 0.85 °. Therefore, according to the electromagnetic wave generator according to the fourth embodiment, in the configuration shown in FIG. 1, the position of the electromagnetic wave detector 34 can be fixed and the position of the electromagnetic wave detector 34 is fixed. It is not necessary to use the non-axis parabolic mirrors 7a and 7b used in the prior art as shown in FIG. 27 between the phase matching element 19 and the electromagnetic wave detector 34. For this reason, according to the electromagnetic wave generator according to the fourth embodiment, it is possible to reduce the number of parts used and to form an extremely simple and inexpensive configuration. The electromagnetic wave generator according to the fourth embodiment The industrial applicability of is extremely high.

(Fifth embodiment)
As shown in FIG. 25, the electromagnetic wave generator according to a fifth embodiment of the present invention includes a first pumping light outputting section 24 for emitting a first pump light hv _1, the first pump light hv ₁ The second pump light hν ₂ having a wavelength different from that of the second pump light emitting section 25 that emits the wavelength tunably, and the electromagnetic wave having a difference frequency between the first pump light hν ₁ and the second pump light hν _2. The quasi phase matching element 19 that generates hν ₃ and the internal crossing angle θ at the difference frequency ν ₃ = 1 THz between the first pump light hν ₁ and the second pump light hν ₂ are adjusted and fixed to the phase matching range angle. And an optical system (M ₁ , M ₆ , 18) for causing the first pump light and the second pump light to enter the quasi phase matching element 19. Where the internal crossing angle θ Is the crossing angle between the _first pump light hν ₁ and the second pump light hν ₂ inside the quasi phase matching element 19, and the external crossing angle δ defined outside the quasi phase matching element 19 is , Which has a relationship of approximately equation (3). For example, when a PPKTP crystal is used for the quasi phase matching element 19, the value of the internal cross angle θ = 6.34 ° is a phase matching range angle, and when a PPGaP crystal is used for the quasi phase matching element 19, The crossing angle θ = about 4.85 ° is the phase matching range angle. In the electromagnetic wave generation device according to the fifth embodiment, the pseudo frequency is changed by changing the frequency of the second pump light hν2 while the external crossing angle δ or the internal crossing angle θ related to each other in Expression (3) is fixed. A variable wavelength terahertz electromagnetic wave is generated from the phase matching element 19.

The optical system (M ₁ , M ₆ , 18) that adjusts and fixes the internal crossing angle θ to the phase matching range angle as described above and makes the first and second pump lights enter the quasi phase matching element 19 includes: It consists of a first mirror M ₁ , a second mirror M ₆ and a polarizing beam splitter 18. The first mirror M ₁ is the first reflects the pump light hv _1, mirror for incident first pump light hv ₁ to the second pump-light emitting unit 25 which is emitted from the first pumping light exit portion 24 is there. The second mirror M ₆ reflects the first pump light hν ₁ emitted from the first pump light emitting unit 24 and adjusts the angle at which the first pump light hν ₁ enters the polarization beam splitter 18. It is a rotatable mirror. With the arrangement of the optical systems (M ₁ , M ₆ , 18) shown in FIG. 25, the signal light (second pump light) hν ₂ emitted from the second pump light emitting unit 25 is transmitted through the polarization beam splitter 18. The pump light (first pump light) hν ₁ emitted from the first pump light emitting unit 24 is incident from the vertical direction via the second mirror M ₆ and reflected by the polarization plane of the polarization beam splitter 18. An optical system that couples the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ in the pseudo phase matching element 19 so that the internal crossing angle θ is the phase matching range angle. Is configured.

However, the pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ need only be superposed in parallel by the polarization beam splitter 18, so that the pump light (first pump light) Light) hν ₁ is transmitted through the polarization beam splitter 18, and signal light (second pump light) hν ₂ whose polarization plane is orthogonal to the pump light (first pump light) hν _{1 is} incident from the vertical direction to be polarized. An optical system is configured to couple the first pump light and the second pump light in the quasi phase matching element 19 so that the light is reflected by the polarization plane of the beam splitter 18 and the internal crossing angle θ is the phase matching range angle. May be.

The optical system (M ₁ , M ₆ , 18) shown in FIG. 25 is an example, and in addition to the first mirror M ₁ , the second mirror M _6, and the polarization beam splitter 18, the optical system such as other mirrors is used. may be added an element, a first mirror M _1, instead of the second mirror M _6, it is replaced with the optical element having a first mirror M ₁ and the second mirror M ₆ such as a prism equivalent functions, internal Of course, the first and second pump lights can be incident on the quasi phase matching element 19 by adjusting and fixing the crossing angle θ to the phase matching range angle.

Specifically, a YAG laser 16 that oscillates a pulse having a wavelength of 1.064 μm is used as the first pump light emitting unit 24. A wavelength variable parametric oscillator (OPO) 17 is used as the second pump light emitting unit 25. The wavelength variable OPO 17 is pumped by light of 355 nm obtained by multiplying the fundamental wave of the pump light (first pump light) hν ₁ from the YAG laser as the pumping light source 16 by three.

In order to operate the terahertz electromagnetic wave hν ₃ with a narrow spectral line width and without a mode hop over a wide frequency, the wavelength tunable OPO 17 includes an injection seeding mechanism (means). As an injection seeding mechanism (means), a seed laser (master oscillator) is provided in the wavelength variable OPO 17. As a seed laser (master oscillator), for example, a resonator having a high Q value may be used for a barium borate (BaB ₂ O ₄ ) crystal to narrow the spectrum width. That is, the parametric effect of the BaB ₂ O ₄ crystal is used for both the seed laser (master oscillator) and the slave laser (host laser). Then, the resonator length of the wavelength tunable OPO 17 that is a slave laser (host laser) may be controlled so that the longitudinal modes of the seed laser and the slave laser match. Since the resonator lock that controls the resonator length of the slave laser requires accurate control at least in the order of (wavelength) / (cavity finesse), for example, controlling the resonator length with a piezo driver Is preferred.

That is, the output of the wavelength variable OPO 17 is detected by a photodetector, and the signal is input to the lock-in amplifier via the blanking circuit. Further, the output signal of the lock-in amplifier is added to the modulation signal from the function generator by a piezo driver, converted into a piezo drive voltage, and the modulation mirror is moved. At this time, the tuning signal of the function generator may be used as a reference signal for the lock-in amplifier. Such a wavelength tunable OPO 17 avoids wavelength degeneration that causes the spectral line width to be broadened by being excited by the third harmonic of the excitation light source (YAG laser) 16, that is, wavelength 355 nm light, and further by performing injection seeding. Since the spectral line width of the wavelength variable OPO 17 can be narrowed, the spectral line width of the terahertz electromagnetic wave hν ₃ generated as the difference frequency is similarly narrowed.

If the output light wavelength of the wavelength variable OPO 17 is selected in the range of 1.038 μm to 1.0635 μm, the difference frequency ν ₃ will be in the range of 0.15 THz to 7 THz. Further, the wavelength of the wavelength variable OPO 17 may be selected in the range of 1.0646 μm to 1.091 μm. Similarly, the pumping light source (YAG laser) 16 uses a resonator having a high Q value as a seed laser (master oscillator), narrows the spectral width, and performs injection seeding to reduce the line width. It is preferable to use an excitation light source.

The pump light (first pump light) hν ₁ and the signal light (second pump light) hν ₂ have orthogonal polarization directions, and the polarization beam splitter 18 causes the beam to have an internal crossing angle θ that is a phase matching range angle. Overlapping. The beam splitter 18 or the second mirror M ₆ is slightly rotated, and the internal crossing angle θ that becomes the phase matching range angle according to the frequency ν ₃ of the terahertz electromagnetic wave hν _3. The beam direction of the pump light (first pump light) hν ₁ or the signal light (second pump light) hν ₂ is adjusted as follows.
Since the spectral line width of the excitation light source (YAG laser) 16 that is the first pump light emitting unit 24 is sufficiently narrow, the spectral line width of the terahertz electromagnetic wave hν ₃ that is the difference frequency is that of the OPO 17 of the second pump light emitting unit 25. It is determined by the spectral line width. Since the OPO 17 is injection seeded, the spectral line width is about 4 GHz, and the spectral line width of the terahertz electromagnetic wave hν ₃ , which is the difference frequency, is also about 4 GHz. Since the spectral line width of a solid or liquid substance in the terahertz band is generally 50 GHz or more, the electromagnetic wave generator according to the fifth embodiment enables sufficiently high-resolution spectroscopic measurement.

  As described above, according to the electromagnetic wave generator according to the fifth embodiment, when a PPKTP crystal is used for the quasi phase matching element 19, two lasers of the first pump light hν1 and the second pump light hν2 are used. If the beam is incident on the quasi phase matching element 19 at an internal crossing angle θ of about 6.34 °, a narrow band terahertz wave electromagnetic wave in the frequency range of 0.3 to 2 THz is generated from the PPKTP crystal. Is possible. Further, when a PPGaP crystal is used for the quasi phase matching element 19, the two laser beams of the first pump light hν1 and the second pump light hν2 are made of PPGaP with an internal crossing angle θ of about 4.85 °. If incident on the phase matching element 19, it is possible to generate a narrow band terahertz wave electromagnetic wave from a PPGaP crystal in a frequency range of 0.3 to 7 THz.

When the PPKTP crystal is used as the quasi-phase matching element 19, the quasi-phase matching element 19 is only 0.0035 ° in order to sweep the terahertz wave frequency within the frequency band of 0.3 to 2 THz terahertz wave. There is no need to rotate. Further, as the difference frequency, the terahertz wave generated from the PPKTP crystal spreads only by about 3.0 ° to 3.5 °. On the other hand, when the quasi-phase matching element 19 made of PPGaP crystal is used, the quasi-phase is only 0.001 ° in order to sweep the terahertz wave frequency within the terahertz wave frequency band of 0.3 to 7 THz. There is no need to rotate the matching element 19, and the terahertz wave generated from the PPGaP crystal as the difference frequency is limited to within 2.2 ° and 0.8 to 7.2 THz within the frequency range of 0.34 to 7.2 THz. For example, it spreads only about 0.85 °. Therefore, according to the electromagnetic wave generator according to the fifth embodiment, in the configuration shown in FIG. 1, the position of the electromagnetic wave detector 34 can be fixed and the position of the electromagnetic wave detector 34 is fixed. It is not necessary to use the non-axis parabolic mirrors 7a and 7b used in the prior art as shown in FIG. 27 between the phase matching element 19 and the electromagnetic wave detector 34. For this reason, according to the electromagnetic wave generator according to the fifth embodiment, it is possible to reduce the number of parts used, and to form an extremely simple and inexpensive configuration. The electromagnetic wave generator according to the fifth embodiment The industrial applicability of is extremely high.

(Other embodiments)
As described above, the present invention has been described according to the first embodiment of the present invention. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.

  For example, in the above description of the third embodiment of the present invention, the first pump light emitting section 24 is a Cr-doped forsterite laser, Yb-doped YLF laser, or Yb-doped beam that emits the first pump light hν 1. For example, a Cr-doped forsterite laser, a Yb-doped YLF laser, or a Yb laser that emits the second pump light hν2 is provided as the second pump-light emitting portion 25, which includes a first pump light hν1 source that is formed of any one of fiber lasers. It has been described that a second pump light source composed of any one of the additive fiber lasers can be provided, but at least one of the first pump light emitting unit 24 and the second pump light emitting unit 25 is selected. If one of them is a semiconductor laser, the device configuration can be further reduced.

  As described above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

6 ... YAG laser 7a ... first non-axis parabolic mirror 7b ... second non-axis parabolic mirror 11 ... personal computer 16 ... excitation light source (YAG laser)
17 ... OPO
DESCRIPTION OF SYMBOLS 18 ... Polarizing beam splitter 19 ... Pseudo phase matching element 19a, 19b ... Comb substrate (GaP substrate)
DESCRIPTION OF SYMBOLS 24 ... 1st pump light emission part 25 ... 2nd pump light emission part 31 ... Laser controller 32 ... Stage controller 33 ... Voltmeter 34 ... Electromagnetic wave detector 35 ... Stage controller 53 ... Linear stage 54 for parabolic mirrors 54 ... beam splitter for linear stage 55 ... rotary stage 57 ... rotary stage 61 ... flash lamp 62 ... first 1YAG rod 63 ... first 2YAG rod 101 ... crystals 105a, 105b, 105c, 105d ... prism 109 ... non-linear optical crystal 191 _M, 191 _N, ... Thin plate 192 ... GaP substrate 193 ... GaP region M ₁ ... First mirror M ₃ ... Half mirror M ₅ ... First mirror M ₆ ... Mirror (second mirror)
P ₁ ... Pulse for flash lamp excitation P ₂ , P ₃ ... Switch pulse

Claims (6)

A first pump light emitting unit that emits first pump light;
A second pump light emitting unit that emits a second pump light having a wavelength different from that of the first pump light in a variable wavelength;
A quasi-phase matching element having a periodic polarization reversal structure and generating an electromagnetic wave having a difference frequency between the first pump light and the second pump light;
The crossing angle between the first pump light and the second pump light is adjusted and fixed to a phase matching range angle, and the first pump light and the second pump light are incident on the pseudo phase matching element. An electromagnetic wave generator comprising: a variable wavelength terahertz electromagnetic wave generated from the pseudo phase matching element by changing a frequency of the second pump light.   The crossing angle is an internal crossing angle defined as a crossing angle between the wave number vector of the first pump light and the wave number vector of the second pump light inside the quasi phase matching element. A first circle drawn as a vector trajectory derived from the polarization inversion structure of the quasi-phase matching element, the angle of which is centered on the end point of the wave vector of the first pump light, and the second pump light 2. The electromagnetic wave generator according to claim 1, wherein the second circle drawn as a trajectory of the wave vector of the terahertz electromagnetic wave centering on an end point of the wave vector is set to intersect each other.   3. The electromagnetic wave generator according to claim 1, wherein the internal intersection angle is set such that a wave vector of the second pump light circumscribes the first circle. 4. The output of the YAG laser is emitted from the first pump light emitting unit as the first pump light,
The second pump light emitting unit includes an optical parametric oscillator having an injection seeding mechanism, and the optical parametric oscillator is excited by the output of the YAG laser, thereby causing the optical parametric oscillator to emit light from the second pump light emitting unit. The electromagnetic wave generator according to any one of claims 1 to 3, wherein an output of an oscillator is emitted as the second pump light. The first pump light emitting section includes a first pump light source comprising any one of a chromium-doped forsterite laser, an ytterbium-doped yttrium / lithium / fluoride laser, and an ytterbium-doped fiber laser that emits the first pump light. Prepared,
The second pump light emitting unit includes a second pump light source made of any one of a chromium-doped forsterite laser, an ytterbium-doped yttrium / lithium / fluoride laser, and an ytterbium-doped fiber laser that emits the second pump light. The electromagnetic wave generator according to any one of claims 1 to 3, wherein the electromagnetic wave generator is provided. 6. The pump light source according to claim 5, further comprising an excitation light source for exciting the first and second pump light sources, respectively, and emitting the first and second pump light from the first and second pump light sources. The electromagnetic wave generator of description.