JP4468842B2 - Electromagnetic wave generating element - Google Patents

Description

  The present invention relates to an electromagnetic wave generating element, and in particular, can generate an electromagnetic wave of 0.1 to 10 THz, which is indispensable for noninvasive medical diagnosis, nondestructive inspection, or spectroscopic measurement of chemical substances, over a wide wavelength range with high efficiency. The present invention relates to an electromagnetic wave generating element that can be used.

  In recent years, research on electromagnetic waves (light) in the optical communication wavelength band, millimeter waves, and electromagnetic waves (radio waves) in the submillimeter wave band has progressed steadily. Has been successful. However, research and development of electromagnetic wave generation of 0.1 to 10 THz in the middle region between light and radio waves is in the process of development, and various studies are being conducted today. In addition, the search for application fields of electromagnetic waves in that band is very active. Light with a frequency about 10 times larger (up to 100 THz) is well-known as mid-infrared light, and combines a readily available dielectric material (or semiconductor nonlinear optical material) with a semiconductor light source for communication, providing a highly efficient difference. It is now possible to generate frequency light. Moreover, by performing spectrum analysis using the difference frequency light, it is proceeding to interesting applications such as identification of the type of atmospheric gas and measurement of moisture content in the gas. On the other hand, an electromagnetic wave having a frequency of 0.1 to 10 THz has a longer wavelength than that of mid-infrared light, and therefore has a high material permeability and a wavelength resolution higher than that of a radio wave. Application to non-destructive inspection is highly expected. In addition, since many toxic and explosive constituent molecules have characteristic vibration spectra in the 0.1 to 10 THz band, it is considered to be very useful for detecting dangerous substances by spectral analysis. Thus, electromagnetic waves in the 0.1 to 10 THz band are considered to be extremely important in fields such as non-invasive inspection and dangerous substance detection.

  Several methods have already been proposed for generating electromagnetic waves in the frequency band of 0.1 to 10 THz. In particular, as shown in FIG. 11, a plurality of semiconductor wafers that are nonlinear optical materials are periodically crystallized in crystal orientation. Using the structure 9 bonded by the semiconductor direct bonding technology while being inverted, two light waves having different wavelengths are made incident in a direction with periodicity, and a target electromagnetic wave is generated by the pseudo phase matching method and the difference frequency generation method. It is overwhelmingly superior to other methods in terms of output efficiency, and is considered promising. The high efficiency of the difference frequency light using the structure of FIG. 11 has already been realized in the mid-infrared region (Non-Patent Document 1), but the technology for realizing this in the frequency band of 0.1 to 10 THz is industrial. It is considered extremely important from the viewpoint of application.

In the above difference frequency generation method, the electric polarizability Ρ is ε = ε ₀ (χ ⁽¹⁾ Ε + χ ⁽²⁾ Ε ² + χ ^{(3) ３ 3} + ...) with respect to the electric field 入射 of the incident light. Among the responding nonlinear optical materials (ε ₀ : dielectric constant in vacuum), light having a frequency ω ₃ and ω ₂ is made incident using a material having a large secondary response χ ⁽²⁾ , and the frequency ω ₁ = ω ₃ − This is a method of generating converted light having ω ₂ (see FIG. 11). However, the refractive index n of a well-known substance generally has frequency dependence (this is called material dispersion). Therefore, the wave number k is k = n (ω) ω / c (c: light velocity), and the phase mismatch amount Δk = k (ω ₃ ) −k (ω ₂ ) −k (ω ₁ ) of the difference frequency light is It is not 0 except in a vacuum. When Δk ≠ 0, it is a big problem that the intensity of the difference frequency light decreases by sin ² (LΔk / 2) / (LΔk / 2) ² . Here, L is the length of the substance. In order to solve this problem, crystal momentum can be imparted to Δk by producing a structure 9 (periodic orientation inversion structure) 9 in which the polarizability χ ⁽²⁾ is periodically inverted as shown in FIG. Can be realized. The effect of this structure 9 has been proven in experiments. This method is known as a quasi phase matching method. By this method, highly efficient generation of difference frequency light can be realized.

D. Zheng, A. Gordon, YSWu, RSFeigelson, MMFejer, RLByer, K, L. Vodopyanov, “16-μm infrared generation by difference-frequency mixing in diffusion-bonded-stacked GaAs", Optics Letters, Vol .23, Issue 13, pp.1010-1012, 1998.

  However, when the difference frequency light is generated by the conventional quasi phase matching method as described above, Δk = 0 can be realized only for a certain wavelength, and the wavelength range where high output can be obtained is Usually very narrow.

  In other words, high-efficiency difference frequency light generation in a periodically inverted semiconductor structure using a conventional quasi-phase matching method is effective only for a fixed wavelength, and a spectrum is obtained by sweeping the wavelength over a wide range. It cannot be used for purposes such as collecting.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an electromagnetic wave generating element whose wavelength is variable over a wide range while maintaining a high output.

  In order to solve the above-mentioned problem, the present inventor uses phonon-polariton dispersion to generate a wavelength (or frequency) -independent portion of the phase mismatch amount, and performs quasi-phase matching on the portion. Thus, it was discovered that a wavelength-tunable output can be obtained over a wide range, and a material that exhibits such an effect was selected. Here, the occurrence of the wavelength-independent portion in the phase mismatch amount is equivalent to the fact that the group velocities of the input signal light and the output difference frequency light are equal.

  The electromagnetic wave generation element according to the first aspect of the present invention has a pseudo phase matching type in which the frequency difference between two incident light waves is in the range of 0.1 to 10 THz, and an electromagnetic wave of 0.1 to 10 THz is generated by the difference frequency generation method. The wavelength conversion element is a group III-V or II-VI group compound semiconductor having a zinc blende structure as a non-linear optical material, and the {110} plane is a laminated surface. The [111] axial directions (or crystallographically equivalent axial directions) are periodically arranged so as to be antiparallel to each other, and the input signal is obtained using the phonon-polariton dispersion of the compound semiconductor. By setting the frequency of the input pump light so that the group velocities of the light and the output difference frequency light coincide in a wide frequency range, the difference frequency light is generated in the frequency range of 0.1 to 10 THz without reducing the output efficiency. It is characterized by doing.

  Here, the III-V group compound semiconductor may be an electromagnetic wave generating element made of GaAs, GaP, or InP.

Further, the group III-V compound semiconductor is _{In x1 Al x2 Ga 1-x1} -x2 N y1 As y2 P 1-y1-y2 (0 ≦ x 1, x 2, y 1, y 2 ≦ 1,0 ≦ 1 -X ₁ -x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1) The electromagnetic wave generating element can be characterized.

  The II-VI group compound semiconductor may be an electromagnetic wave generating element made of ZnSe or CdTe.

The II-VI group compound semiconductor is Zn _x1 Cd _x2 Hg _{1 -x1-x2 Sy1} Se _y2 Te _{1 -y1-y2} (0 ≦ x ₁ , x ₂ , y ₁ , y ₂ ≦ 1, 0 ≦ 1 -X ₁ -x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1) The electromagnetic wave generating element can be characterized.

  The input pump light may be an electromagnetic wave generating element having a wavelength λ of 850 nm <λ <950 nm.

  The input pump light may be an electromagnetic wave generating element having a wavelength λ of 1250 nm <λ <1350 nm.

  The electromagnetic wave generating element of the second aspect of the present invention has a quasi phase matching type in which the frequency difference between two incident light waves is in the range of 0.1 to 10 THz, and an electromagnetic wave of 0.1 to 10 THz is generated by the difference frequency generation method. The wavelength conversion element is a wurtzite group III-V or II-VI group compound semiconductor as a non-linear optical material, which is made into a wafer.

The structure has a structure in which the [0001] axial directions (or crystallographically equivalent axial directions) are periodically arranged so as to be antiparallel to each other, and the phonon-polariton dispersion of the compound semiconductor. The frequency of the input pump light is set so that the group velocities of the input signal light and the output difference frequency light coincide in a wide frequency range, so that the frequency of 0.1 to 10 THz can be obtained without reducing the output efficiency. A difference frequency light is generated in a range.

  Here, an electromagnetic wave generating element in which the III-V group compound semiconductor is GaN can be characterized.

Further, the group III-V compound semiconductor is _{In x1 Al x2 Ga 1-x1} -x2 N y1 As y2 P 1-y1-y2 (0 ≦ x 1, x 2, y 1, y 2 ≦ 1,0 ≦ 1 -X ₁ -x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1) The electromagnetic wave generating element can be characterized.

  Further, the electromagnetic wave generating element may be characterized in that the II-VI group compound semiconductor is ZnS.

Further, the group II-VI compound semiconductor is _{Zn x1 Cd x2 Hg 1-x1} -x2 S y1 Se y2 Te 1-y1-y2 (0 ≦ x 1, x 2, y 1, y 2 ≦ 1,0 ≦ 1 -X ₁ -x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1) The electromagnetic wave generating element can be characterized.

  Furthermore, in both the first and second aspects, a waveguide structure may be provided for the input signal light, the input pump light, or the output difference frequency light.

  In the present invention, in a quasi-phase matching type wavelength conversion element using a zinc-blende or wurtzite group III-V or II-VI compound semiconductor, phonon-polariton dispersion is used, and the input signal light and the output difference Since the frequency of the input pump light is set so that the group velocities of the frequency light coincide in a wide frequency range, highly efficient difference frequency light can be generated in the frequency range of 0.1 to 10 THz. Thereby, according to this invention, the electromagnetic wave generating element which can be utilized also for uses, such as sweeping a wavelength and collecting a spectrum, over a wide range, can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The electromagnetic wave generating element of the present invention can be applied to, for example, an electromagnetic wave generating apparatus including a laser generator 1, a wavelength tunable device 2, and an electromagnetic wave generating element 3 as shown in FIG. As the laser generator 1, a Q switch YAG laser or the like can be used. As the wavelength tunable device 2, an optical parametric oscillator or the like can be used. The light from the laser generator 1 is separated into two by the beam splitter 4. One light passes through the wavelength tunable device 2, changes the wavelength, and is then combined with the other light by the beam splitter 4. Incident on the electromagnetic wave generating element 3. As a result, excitation light having two different wavelengths necessary for the difference frequency generation can be generated and input to the electromagnetic wave generating element 3. In addition, 5 is a mirror and 6 is an optical path.

  When the wavelength of the light of the laser generator 1 deviates from a desired value, it can be adjusted by preparing two wavelength variable devices 2 as shown in FIG.

  If two laser generators 1 are prepared as shown in FIG. 3, the wavelength tunable device 2 can be omitted. However, in this case, the wavelength variability is determined by the variable range of the laser generator 1, and thus becomes smaller than the former apparatus shown in FIGS. Reference numeral 7 denotes a coupler (optical multiplexer), and 8 denotes an optical fiber.

  The electromagnetic wave generating element 3 is produced by the following method. A zinc-blende (or wurtzite) III-V or II-VI compound semiconductor wafer is bonded by a direct semiconductor bonding technique without using an adhesive. Thus, an integrated element having a constant refractive index can be manufactured. In the case where the wafer is a compound semiconductor having a zinc blende structure, the [111] planes are laminated so that the [111] axis directions (or crystallographically equivalent axis directions) are antiparallel to each other. Or in the case of wurtzite structure,

Any of the surfaces is laminated so that the [0001] axial directions (or crystallographically equivalent axial directions) are antiparallel to each other. As a result, the maximum component of the tensor of the second-order nonlinear optical constant can be used. In addition, the periodic structure is formed by forming a pair of two orientation reversal structures bonded antiparallel to each other and repeatedly bonding them.

Examples of the III-V compound semiconductor include GaAs, GaP, AlAs, AlP, InAs, InP, GaN, InP, and mixed crystals thereof, that is, In _x1 AlIn _x2 Ga _1-x1-x2 N _y1 As _y2 P _1. It is used _-y1-y2 a _{(0 ≦ x 1, x 2} , y 1, y 2 ≦ 1,0 ≦ 1-x 1 -x 2 ≦ 1,0 ≦ 1-y 1 -y 2 ≦ 1) it can. Examples of the II-VI group compound semiconductor include ZnSe, CdTe, HgTe, ZnS, and mixed crystals thereof, that is, Zn _x1 Cd _x2 Hg _1-x1-x2 S _y1 Se _y2 Te _1-y1-y2 (0 ≦ x ₁ , x ₂ , y ₁ , y ₂ ≦ 1, 0 ≦ 1-x ₁ −x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1) can be used. As a result, the second-order nonlinear optical constant d ₁₄ is much larger than dielectric materials such as LiNbO ₃ and LiTaO ₃ that are usually used in nonlinear optical wavelength conversion elements, and light absorption in the 0.1 to 10 THz band is also small. High-efficiency electromagnetic waves can be generated in that frequency band.

The above III-V or II-VI compound semiconductors excite TO and LO phonons from the long wave side when light of 20 to 50 μm is incident, respectively, and excite wavelengths λ _TO and λ _LO (frequency ω _TO and ω _LO). ) Since anomalous dispersion occurs in the vicinity, the dispersion k (ω) = n (ω) ω / c is as shown in FIG. 4 (n (ω): refractive index). Such dispersion is known as phonon-polariton dispersion. In the quasi-phase matching (QPM) type wavelength conversion element, the wavelength of the light output by the difference frequency generation is 20 μm or less, and so far such dispersion characteristics have not been utilized. Therefore, the present inventor has invented a broadband QPM method utilizing such characteristics. The details are shown below.

Consider a case in which signal light having a frequency ω and pump light having a frequency ω ₀ are incident and an electromagnetic wave having a frequency Δω = ω−ω ₀ is emitted as difference frequency light. At this time, if the phase mismatch amount Δk (ω) = k (ω) −k (ω ₀ ) −k (ω) is replaced with ω → ω ′ + δω and developed around ω ′.

It becomes. That is, as shown in FIG. 4, when the slopes of the curves at ω ′ = ω and Δω are equal (that is, the group velocities are equal).

Thus, a portion having no dependence on ω ′ is formed in Δk (ω ′) (FIG. 5, W is a frequency-independent portion width). When the phase mismatch amount of the flat portion is Δk ₀ , the azimuth inversion period L is L = | 2π / Δk ₀ |. In the case of Δk ₀ → 0, QPM can be achieved over a wide frequency range (indicated by a thick line in FIG. 5). At this time, the band of the difference frequency output light can be expanded as shown in FIG. 6 with high efficiency (η is the output light efficiency). The broken lines in FIG. 5 and FIG. 6 illustrate Δk (ω ′) and η (ω ′) when Δω> ω _LO , respectively, and it can be seen that the effect is effective only at a fixed frequency.

As an example of this embodiment, FIG. 7 shows the relationship between the azimuth inversion period L = | 2π / Δk ₀ (λ _out ) | and the wavelength λ _out of the output light when the compound semiconductor is GaAs. FIG. 7 shows a change in output wavelength obtained by changing the wavelength λ ₀ of at least one of the two input light waves to 1270 nm and changing the other. As shown by the dashed circle in FIG. 7, a flat portion where the value of the azimuth inversion period L is constant is formed in the vicinity of λ _out = 105 μm, and the azimuth inversion period at λ _out is L = 1444 μm. Assuming that the wafer thickness is L / 2 = 722 μm and the number of wafers is 10 (total element length D = 7.22 mm), the output light efficiency η is as shown in FIG. 8, and the bandwidth is wide near λ _out = 90 to 140 μm. It can be seen that η is realized.

The broken lines in FIGS. 7 and 8 correspond to the case where λ ₀ = 1550 nm, and no flat portion is generated in the L-λ _out graph of FIG. 7, so that even when QPM is performed at λ _out = 105 μm, as shown in FIG. In addition, the band of η is narrow. It can be seen that the 3 dB width of the former η is 53.5 μm, which is 6.1 times larger than the latter 8.79 μm (the same D is used for comparison). Further, in order to implement the above-described wideband method at 100 μm <λ _out <300 μm, it is necessary that 1250 nm <λ ₀ <1350 nm. When the frequency corresponding to λ ₀ is f ₀ and converted to frequency, 222 THz <f ₀ <240 THz.

As another example of this embodiment, when the compound semiconductor is GaP, graphs of L-λ _out and η-λ _out are shown in FIGS. Here, λ ₀ = 900 nm, λ _out = 91 μm, a = 705.7 μm, and D = 7.057 mm. It can be seen that the 3 dB band of η is 44.4 μm, which is 12.3 times larger than 3.62 μm in the case of λ ₀ = 1550 nm. Further, in order to implement the above-described broadband method at 100 μm <λ _out <300 μm, it is necessary that 850 nm <λ ₀ <950 nm. When this is converted into a frequency, 316 THz <f ₀ <353 THz. In the group III-V and group II-VI compound semiconductors, the values of ω _TO and ω _LO of the phonon-polariton dispersion are slightly different within the range of 6 to 15 THz. Therefore, the f ₀ of the compound semiconductor is 222 THz. If it is in the range of <f ₀ <353 THz, the same effect as that of the above embodiment can be obtained.

  In addition, since the above semiconductor wafer laminated structure has a waveguide structure for input signal light, input pump light, or output difference frequency light, the optical power density can be improved, so that more efficient output light can be obtained. Can be obtained.

  According to the present invention, high-efficiency and wide-band difference frequency light can be generated with the above principle and element configuration.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention.

It is a block diagram which shows an example of a structure of the electromagnetic wave generator to which the electromagnetic wave generator of this invention is applied. It is a block diagram which shows the other example of a structure of the electromagnetic wave generator to which the electromagnetic wave generating element of this invention is applied. It is a block diagram which shows the further another example of the structure of the electromagnetic wave generator to which the electromagnetic wave generator of this invention is applied. It is a characteristic view which shows the relationship between wave number k ((omega)) and frequency (omega) in embodiment of this invention. FIG. 6 is a characteristic diagram showing a relationship between a phase mismatch amount Δk (ω) and a frequency ω in the embodiment of the present invention. It is a characteristic view which shows the relationship between optical conversion efficiency (eta) and frequency (omega) in embodiment of this invention. FIG. 6 is a characteristic diagram showing a relationship between an azimuth inversion period L of GaAs and an output wavelength λ _{out in} an embodiment of the present invention. FIG. 6 is a characteristic diagram showing the relationship between GaAs light conversion efficiency η and output wavelength λ _{out in} an embodiment of the present invention. It is a characteristic view which shows the relationship between the azimuth | direction inversion period L of GaP and output wavelength (lambda) _out in embodiment of this invention. It is a characteristic view which shows the relationship between the optical conversion efficiency (eta) of GaP and output wavelength (lambda) _out in embodiment of this invention. It is a schematic diagram which shows a well-known periodic orientation inversion structure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser generator 2 Wavelength variable device 3 Electromagnetic wave generating element 4 Beam splitter 5 Mirror 6 Optical path 7 Coupler 8 Optical fiber 9 Periodic direction inversion structure

Claims (11)

  An electromagnetic wave generation element using a quasi-phase matching type wavelength conversion element that has a frequency difference between two incident light waves in a range of 0.1 to 10 THz and generates an electromagnetic wave of 0.1 to 10 THz by a difference frequency generation method. The wavelength conversion element is a III-V or II-VI group compound semiconductor having a zinc blende structure as a non-linear optical material, and a [111] axis with a {110} plane being a laminated surface formed as a wafer. Having a structure in which the directions (or crystallographically equivalent axial directions) are periodically arranged so as to be anti-parallel to each other, and using the phonon-polariton dispersion of the compound semiconductor, the input signal light and the output difference frequency light By setting the frequency of the input pump light so that the group velocities coincide with each other in a wide frequency range, the difference frequency light is generated in the frequency range of 0.1 to 10 THz without reducing the output efficiency. An electromagnetic wave generating element characterized. The electromagnetic wave generating element has a frequency difference between two incident light waves in a range of 0.1 to 10 THz, and an electromagnetic wave using a quasi phase matching type wavelength conversion element that generates an electromagnetic wave of 0.1 to 10 THz by a difference frequency generating method. A generating element, wherein the wavelength converting element is a wurtzite group III-V or II-VI group compound semiconductor as a nonlinear optical material, which is formed into a wafer.

The structure has a structure in which the [0001] axial directions (or crystallographically equivalent axial directions) are periodically arranged so as to be antiparallel to each other, and the phonon-polariton dispersion of the compound semiconductor. , The frequency of the input pump light is set so that the group velocities of the input signal light and the output difference frequency light coincide with each other in a wide frequency range, so that the output efficiency of 0.1 to 10 THz can be reduced without lowering the output efficiency. An electromagnetic wave generating element that generates difference frequency light in a frequency range.   2. The electromagnetic wave generating element according to claim 1, wherein the III-V group compound semiconductor is GaAs, GaP, or InP.   The electromagnetic wave generating element according to claim 2, wherein the III-V group compound semiconductor is GaN. The group III-V compound semiconductor is _{In x1 Al x2 Ga 1-x1} -x2 N y1 As y2 P 1-y1-y2 (0 ≦ x 1, x 2, y 1, y 2 ≦ 1,0 ≦ 1-x The electromagnetic wave generating element according to claim _{1, wherein 1} −x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1).   2. The electromagnetic wave generating element according to claim 1, wherein the II-VI group compound semiconductor is ZnSe or CdTe.   The electromagnetic wave generating element according to claim 2, wherein the II-VI group compound semiconductor is ZnS. The Group II-VI compound semiconductor is _{Zn x1 Cd x2 Hg 1-x1} -x2 S y1 Se y2 Te 1-y1-y2 (0 ≦ x 1, x 2, y 1, y 2 ≦ 1,0 ≦ 1-x The electromagnetic wave generating element according to claim _{1, wherein 1} −x ₂ ≦ 1, 0 ≦ 1−y ₁ −y ₂ ≦ 1).   2. The electromagnetic wave generating element according to claim 1, wherein a wavelength λ of the input pump light is 850 nm <λ <950 nm.   2. The electromagnetic wave generating element according to claim 1, wherein the wavelength λ of the input pump light is 1250 nm <λ <1350 nm. 11. The electromagnetic wave generating element according to claim 1, wherein the electromagnetic wave generating element has a waveguide structure with respect to the input signal light, the input pump light, or the output difference frequency light.

Images