JP5380357B2 - Terahertz wave generator - Google Patents

Description

  The present invention relates to a terahertz wave generator.

  The terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 100 THz corresponding to an intermediate region between the light wave and the radio wave, and has an intermediate property between the light wave and the radio wave. As an application of such a terahertz wave, a technique for acquiring information on the measurement object by measuring a time waveform of the electric field amplitude of the terahertz wave transmitted or reflected by the measurement object has been studied.

  As a terahertz wave generation technique, a technique for generating a terahertz wave by making a pump light pulse incident on a nonlinear optical crystal is known. The pump light pulse is ultrashort pulse light output from, for example, a femtosecond laser light source. For example, when a pump light pulse having a center wavelength of 800 nm is used, ZnTe crystals are widely used as satisfying the phase matching condition. When the pump light pulse is incident on the nonlinear optical crystal that satisfies the phase matching condition, a terahertz wave is generated coaxially with the pump light pulse (Non-Patent Documents 1 and 2).

L. Xu, et al., Appl. Phys. Lett. 61, 1784 (1992). A. Nahata, et al., Appl. Phys. Lett. 69, 2321 (1996).

  However, when a pump light pulse is incident on a nonlinear optical crystal to generate a terahertz wave on the same axis, the pump light pulse generally has a Gaussian-shaped intensity distribution in the cross section, so that the generated terahertz wave is also in the cross section. A Gaussian-shaped intensity distribution results, and there arises a problem that the intensity distribution becomes non-uniform in the cross section of the terahertz wave.

  The present invention has been made to solve the above-described problems. When a pump light pulse having a nonuniform intensity distribution in a cross section is incident on a nonlinear optical crystal to generate a terahertz wave, the intensity distribution of the terahertz wave is reduced. An object of the present invention is to provide a terahertz wave generator that can be made uniform.

  The terahertz wave generation device of the present invention includes a light source that outputs a pump light pulse, and a terahertz wave generation unit that generates a terahertz wave when the pump light pulse output from the light source is input. The terahertz wave generation unit includes a first crystal that generates a terahertz wave by propagation of the pump light pulse, and the intensity of the pump light pulse is higher than the thickness of the first crystal at a position where the intensity of the pump light pulse is strong. The first crystal at a weak position is thick.

  The terahertz wave generation unit of the terahertz wave generation device of the present invention may further include a second crystal that does not generate a terahertz wave by propagation of a pump light pulse, and the first crystal and the second crystal may be bonded together.

  Alternatively, the terahertz wave generation unit of the terahertz wave generation device of the present invention further includes a second crystal that generates a terahertz wave having a polarization direction different from the terahertz wave by propagation of the pump light pulse, and the first crystal and the second crystal The terahertz waves generated in the first crystal out of the terahertz waves generated from the first crystal and the second crystal and emitted from the emission surface may be selectively output by the polarizer.

  In the terahertz wave generation device of the present invention, the first crystal may be thickened stepwise from the optical axis in the radial direction, or may be thickened linearly from the optical axis in the radial direction. In addition, it may be thicker in a curve from the optical axis toward the radial direction.

  According to the present invention, when a terahertz wave is generated by making a pump light pulse having a nonuniform intensity distribution in a cross section incident on a nonlinear optical crystal, the intensity distribution of the terahertz wave can be made uniform.

It is a figure explaining the incidence of the pump light pulse and the emission of the terahertz wave in the terahertz wave generation unit. 1 is a configuration diagram of a terahertz wave imaging apparatus 2. FIG. It is sectional drawing of 21 A of terahertz wave generation parts. It is a perspective view of terahertz wave generation part 21A. It is sectional drawing of the terahertz wave generation part 21B. It is sectional drawing of the terahertz wave generation part 21C. It is sectional drawing of terahertz wave generation part 21D. It is sectional drawing of the terahertz wave generation part 21E. It is sectional drawing of the terahertz wave generation part 21F.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

FIG. 1 is a diagram for explaining pump light pulse incidence and terahertz wave emission in the terahertz wave generation unit 21. The terahertz wave generation unit 21 has an incident surface 21a on which the pump light pulse P is vertically incident and an emission surface 21b on which the terahertz wave T is emitted vertically. Each of the entrance surface 21a and the exit surface 21b is a flat surface, and both are parallel to each other. The terahertz wave generation unit 21 is made of, for example, a non-linear optical crystal such as a LiNbO ₃ crystal (sLN crystal), a LiTaO ₃ crystal (sLT crystal), ZnTe, or GaP having a stoichiometric composition. In the figure, the intensity distribution of the pump light pulse P incident on the incident surface 21a is indicated by a plurality of arrows, and the intensity distribution of the terahertz wave T emitted from the emission surface 21b is indicated by a plurality of arrows.

  The light source that outputs the pump light pulse P is preferably a pulse laser light source that outputs an ultrashort pulse laser beam, for example, a femtosecond laser light source. As shown in this figure, the pump light pulse P incident on the incident surface 21a has an intensity distribution such that the cross-section has the strongest intensity at the center and becomes weaker as the distance from the center decreases. In many cases, it has a Gaussian intensity distribution. Therefore, the terahertz wave T emitted from the emission surface 21b also has an intensity distribution in which the intensity is the strongest at the center in the cross section, and the intensity becomes weaker as the distance from the center increases. The intensity distribution will be as follows.

  FIG. 2 is a configuration diagram of the terahertz wave imaging apparatus 2. The terahertz wave imaging device 2 shown in this figure includes the terahertz wave generation device including the terahertz wave generation unit 21 and the light source 11 shown in FIG. When the terahertz wave generator 1 has the nonuniformity of the intensity distribution in the cross section of the terahertz wave as described above, the terahertz wave imaging device 2 has the following problems.

  In the terahertz wave imaging apparatus 2 shown in this figure, the light pulse output from the light source 11 is bifurcated by the branching unit 12 into a pump light pulse and a probe light pulse. The pump light pulses output from the branching unit 12 are sequentially reflected by the mirrors M <b> 1 to M <b> 3 and input to the terahertz wave generation unit 21. The terahertz wave generation unit 21 generates and outputs a terahertz wave on the same axis according to the input of the pump light pulse. The terahertz wave output from the terahertz wave generation unit 21 passes through the measurement target unit 9 to acquire information (for example, an absorption coefficient and a refractive index) of the measurement target 9, and then passes through the lens L1 and the lens L2. Are input to the multiplexing unit 41. On the other hand, the probe light pulses output from the branching unit 12 are sequentially reflected by the mirrors M <b> 4 to M <b> 8, converted into linearly polarized light by the polarizer 31, and input to the multiplexing unit 41.

  The terahertz wave and the probe light pulse input to the combining unit 41 are combined so as to be coaxial with each other by the combining unit 41 and input to the electro-optic crystal 42 at substantially the same timing. In the electro-optic crystal 42 to which the terahertz wave and the probe light pulse are input, birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse changes due to the birefringence. The probe light pulse output from the electro-optic crystal 42 is imaged by the imaging unit 44 via the analyzer 43, the lens L3, and the lens L4.

  The distribution of the change in the polarization state of the probe light pulse in the electro-optic crystal 42 includes a polarizer 31 provided on the optical path of the probe optical system, an analyzer 43 provided on the output side of the electro-optic crystal 42, and the analyzer. An image is picked up as a light intensity distribution by the image pickup unit 44 that picks up the probe light pulse that has passed through 43. In this way, the distribution of the polarization state change of the probe light pulse in the electro-optic crystal 42 is detected, and as a result, an image of the terahertz wave transmitted through the measuring object 9 is obtained.

  The three mirrors M <b> 1 to M <b> 3 constitute an optical path length difference adjusting unit 13. That is, the optical path length of the pump optical system from the branching section 12 to the terahertz wave generation section 21 is adjusted by moving the mirrors M1 and M2. As a result, the optical path length difference adjusting unit 13 includes the optical path of the pump optical system and the terahertz wave optical system from the branching unit 12 to the multiplexing unit 41 and the probe from the branching unit 12 to the multiplexing unit 41. The optical path length difference ΔL from the optical path of the optical system can be adjusted.

  In such a terahertz wave imaging apparatus 2, if there is non-uniformity in the intensity distribution in the cross section of the terahertz wave output from the terahertz wave generation unit 21, the measurement object in the image obtained by the imaging unit 44 is captured. It is impossible to distinguish whether the intensity distribution is derived from 9 or the intensity distribution in the cross section of the terahertz wave output from the terahertz wave generation unit 21. Therefore, the non-uniformity of the intensity distribution in the cross section of the terahertz wave hinders application to imaging.

  Using the image acquired by the imaging unit 44 when the measurement target 9 does not exist, and correcting the image acquired by the imaging unit 44 when the measurement target 9 is present by signal processing, the cross section of the terahertz wave It is also conceivable to eliminate the influence of the unevenness of the intensity distribution. However, since the light source 11 used in the terahertz wave imaging apparatus 2 as shown in FIG. 2 is, for example, a femtosecond laser light source, the intensity fluctuation between optical pulses is large, and therefore the intensity distribution in the cross section of the terahertz wave is large. The effect of non-uniformity cannot be removed.

  In addition, it is conceivable to make the intensity distribution uniform by signal processing. In this case, a portion with a low intensity (a portion close to the noise level) becomes a signal with a very large noise, which is not suitable for practical use. Furthermore, since the intensity distribution is relatively uniform in the central region of the terahertz wave cross section, it is possible to use only the central region of the terahertz wave cross section for measurement. In this case, the peripheral area portion is wasted, and the measurement range of the measurement object 9 is narrowed. Therefore, it is important to make the intensity distribution uniform when terahertz waves are generated.

  Hereinafter, the terahertz wave generation units 21A to 21F of the terahertz wave generation apparatus according to the present embodiment will be described with reference to FIGS. Each of these terahertz wave generation units 21A to 21F is used in place of the terahertz wave generation unit 21 in FIG. 1 or FIG.

  FIG. 3 is a cross-sectional view of the terahertz wave generation unit 21A. In the figure, the intensity distribution of the pump light pulse P incident on the incident surface 21a is indicated by a plurality of arrows, and the intensity distribution of the terahertz wave T emitted from the emission surface 21b is indicated by a plurality of arrows. FIG. 4 is a perspective view of the terahertz wave generation unit 21A.

  The terahertz wave generating unit 21 </ b> A includes the first crystal 111 that generates the terahertz wave T by the propagation of the pump light pulse P. The first crystal 111 is, for example, ZnTe, GaAs, GaSe, sLN, CdTe, GaP, DAST, DASC, DSTMS, BNA, or the like. Compared to the thickness of the first crystal 111 at a position where the intensity of the pump light pulse P is strong (position near the center), the first crystal 111 at a position where the intensity of the pump light pulse P is weak (position far from the center) is thick. In particular, the first crystal 111 of the terahertz wave generating portion 21A is thicker in a step shape from the optical axis toward the radial direction.

  The terahertz wave generating unit 21A configured in this manner has a terahertz wave that is emitted from the emission surface 21b when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface 21a as it is closer to the center in the cross section. The intensity distribution of T can be made uniform.

  FIG. 5 is a cross-sectional view of the terahertz wave generation unit 21B. The terahertz wave generation unit 21B includes a first crystal 121 that generates a terahertz wave T by propagation of the pump light pulse P, and a second crystal 122 that does not generate a terahertz wave, and the first crystal 121 and the second crystal 122 include They are bonded together.

  Adhesion between the first crystal 121 and the second crystal 122 is performed by any method such as an adhesive (for example, wax), optical contact, direct bonding, or the like that absorbs less terahertz waves. The refractive indexes of the first crystal 121 and the second crystal 122 are preferably equal to each other. For example, the first crystal 121 is a ZnTe crystal with an orientation (110), and the second crystal 122 is a ZnTe crystal with an orientation (100).

  The first crystal 121 is the same as the first crystal 111 shown in FIGS. The second crystal 122 is thin stepwise from the optical axis in the radial direction. The entire terahertz wave generation unit 21B formed by bonding the first crystal 121 and the second crystal 122 has a uniform thickness.

  The terahertz wave generation unit 21B configured as described above is configured to generate a terahertz wave T emitted from the emission surface when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface. The intensity distribution can be made uniform. Further, the terahertz wave generating unit 21B has a flat entrance surface and an exit surface, so that diffraction of the terahertz wave T at the time of emission can be suppressed.

  FIG. 6 is a cross-sectional view of the terahertz wave generation unit 21C. The terahertz wave generation unit 21C includes a first crystal 131 that generates a terahertz wave T by propagation of the pump light pulse P, and a second crystal 132 that does not generate a terahertz wave, and the first crystal 131 and the second crystal 132 include They are bonded together.

  The first crystal 131 of the terahertz wave generating unit 21C is linearly thicker in the radial direction from the optical axis. The second crystal 132 is linearly thinner from the optical axis toward the radial direction. The entire terahertz wave generating portion 21C formed by bonding the first crystal 131 and the second crystal 132 has a uniform thickness.

  The terahertz wave generating unit 21C configured as described above is configured to generate a terahertz wave T emitted from the emission surface when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface. The intensity distribution can be made uniform. Further, the terahertz wave generation unit 21C has a flat entrance surface and an exit surface, and refraction of the terahertz wave T at the time of emission can be suppressed.

  FIG. 7 is a cross-sectional view of the terahertz wave generation unit 21D. The terahertz wave generation unit 21D includes a first crystal 141 that generates a terahertz wave T by propagation of the pump light pulse P, and a second crystal 142 that does not generate a terahertz wave, and the first crystal 141 and the second crystal 142 include They are bonded together.

  The first crystal 141 of the terahertz wave generation unit 21D is thicker in a curve from the optical axis toward the radial direction. The second crystal 142 is curvilinearly thinner from the optical axis toward the radial direction. The entire terahertz wave generation unit 21D formed by bonding the first crystal 141 and the second crystal 142 has a flat entrance surface and an exit surface, and a uniform thickness.

  The terahertz wave generation unit 21D configured in this manner has a terahertz wave T emitted from the emission surface when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface as it is closer to the center in the cross section. The intensity distribution can be made uniform. Further, the terahertz wave generation unit 21D can suppress the refraction of the terahertz wave T at the time of emission.

  FIG. 8 is a cross-sectional view of the terahertz wave generation unit 21E. The terahertz wave generation unit 21E includes a first crystal 151 that generates a terahertz wave T by propagation of the pump light pulse P, and second crystals 152 and 153 that do not generate a terahertz wave, and the second crystal 152 and the first crystal 151 And the 2nd crystal | crystallization 152 is adhere | attached in order.

  The first crystal 151 of the terahertz wave generation unit 21E is thicker in a curve from the optical axis toward the radial direction. Further, the whole of the second crystal 152 and the second crystal 153 is curvilinearly thin from the optical axis toward the radial direction. The entire terahertz wave generation unit 21E formed by bonding the first crystal 151 and the second crystals 152 and 153 has a flat entrance surface and an exit surface, and a uniform thickness.

  The terahertz wave generating unit 21E configured as described above is configured to generate a terahertz wave T emitted from the emission surface when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface. The intensity distribution can be made uniform. Further, the terahertz wave generation unit 21E can suppress the refraction of the terahertz wave T at the time of emission.

  FIG. 9 is a cross-sectional view of the terahertz wave generation unit 21F. The terahertz wave generation unit 21 </ b> F includes first crystals 161 and 162 that generate a terahertz wave T by propagation of the pump light pulse P, and second crystals 163 that do not generate a terahertz wave, and the first crystal 161 and the second crystal 163. And the 1st crystal | crystallization 162 is adhere | attached in order and is comprised.

  The whole of the first crystal 161 and the first crystal 162 of the terahertz wave generation unit 21F is thicker in a curve from the optical axis toward the radial direction. The second crystal 163 is curvilinearly thinner from the optical axis toward the radial direction. The entire terahertz wave generation unit 21F formed by bonding the first crystals 161, 162 and the second crystal 163 has a flat entrance surface and an exit surface, and a uniform thickness.

  The terahertz wave generating unit 21F configured as described above is configured to generate a terahertz wave T emitted from the emission surface when the pump light pulse P having a stronger intensity and a non-uniform intensity distribution is incident on the incident surface. The intensity distribution can be made uniform. Further, the terahertz wave generation unit 21F can suppress the refraction of the terahertz wave T at the time of emission.

  In the terahertz wave generation units 21B to 21F, the second crystal may generate a terahertz wave having a polarization orientation different from that of the terahertz wave generated in the first crystal. In this case, the terahertz wave generated in the first crystal among the terahertz waves generated in the first crystal and the second crystal and emitted from the emission surface is selectively output by a polarizer such as a wire grid polarizer. Can do.

  The terahertz wave generation apparatus of the present embodiment can irradiate a measurement target with a large area terahertz wave when performing terahertz wave imaging, and the application for nondestructive inspection and the like spreads.

DESCRIPTION OF SYMBOLS 1 ... Terahertz wave generator, 2 ... Terahertz wave imaging device, 11 ... Light source, 12 ... Branch part, 13 ... Optical path length difference adjustment part, 21, 21A-21F ... Terahertz wave generator, 31 ... Polarizer, 41 ... Wave part 42 ... electro-optic crystal 43 ... analyzer 44 ... imaging part 111 ... first crystal 121 ... first crystal 122 ... second crystal 131 ... first crystal 132 ... second crystal 141 ... 1st crystal, 142 ... 2nd crystal, 151 ... 1st crystal, 152, 153 ... 2nd crystal, 161, 162 ... 1st crystal, 163 ... 2nd crystal, L1-L4 ... Lens, M1-M8 ... Mirror .

Claims (6)

A light source that outputs a pump light pulse, and a terahertz wave generation unit that generates a terahertz wave by inputting the pump light pulse output from the light source,
The terahertz wave generation unit includes a first crystal that generates the terahertz wave by propagation of the pump light pulse, and the pump light is compared with a thickness of the first crystal at a position where the intensity of the pump light pulse is strong. The first crystal is thick at a position where the pulse intensity is weak,
The terahertz wave generator characterized by the above-mentioned. The terahertz wave generation unit further includes a second crystal that does not generate a terahertz wave by propagation of the pump light pulse, and the first crystal and the second crystal are bonded together,
The terahertz wave generator according to claim 1. The terahertz wave generation unit further includes a second crystal that generates a terahertz wave having a polarization orientation different from the terahertz wave by propagation of the pump light pulse, and the first crystal and the second crystal are bonded together, A terahertz wave generated in the first crystal out of the terahertz waves generated in the first crystal and the second crystal and emitted from the emission surface is selectively output by a polarizer,
The terahertz wave generator according to claim 1.   The terahertz wave generation device according to any one of claims 1 to 3, wherein the first crystal is thicker in a stepped shape from the optical axis toward the radial direction.   4. The terahertz wave generation device according to claim 1, wherein the first crystal is linearly thicker in a radial direction from the optical axis. The terahertz wave generation device according to any one of claims 1 to 3, wherein the first crystal is curvedly thicker in a radial direction from the optical axis.

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