JP5244671B2 - Terahertz wave generator and generation system - Google Patents

Description

  The present invention relates to a wavelength-converted light generation device and a wavelength-converted light generation system that generate wavelength-converted light such as terahertz waves by the incidence of pulsed excitation light.

  An electromagnetic wave region around a frequency of 1 THz (terahertz) (a terahertz wave region, for example, approximately 0.01 THz to 100 THz or a wide frequency region including the surrounding region) is a frequency region located at a boundary between a light wave and a radio wave. is there. Such terahertz waves are expected to be applied to various fields such as nondestructive inspection, imaging, and communication.

  In the generation of terahertz waves using short pulse light as excitation light, a method using the optical rectification effect in a nonlinear optical crystal has been widely used. When pulse excitation light from a laser light source is incident on a nonlinear optical crystal, an optical rectification effect occurs in the crystal. At this time, a terahertz wave corresponding to the difference frequency component is generated by each frequency component of the pulse excitation light having a predetermined spectrum, whereby a terahertz wave generator can be realized.

Y.-S. Lee et al., "Generation of narrow-band terahertz radiation via optical rectificationof femtosecond pulses in periodically poled lithium niobate", Appl. Phys. Lett. Vol. 76 No. 18 (2000) pp. 2505-2507 Masaru Minamiguchi et al., “Enhancement of Quasi-Phase Matched Second Harmonics in 1D Photonic Crystals of Multilayered Multilayers”, Proceedings of the 66th Japan Society of Applied Physics Academic Lecture (Autumn 2005, Tokushima University) 9p-H-20

  As described above, in a terahertz wave generator using a nonlinear optical crystal, the pulse excitation light and the terahertz wave are caused by the difference between the group velocity of the excitation light such as a short pulse laser beam and the phase velocity of the terahertz wave generated in the crystal. There is a problem that the output of the terahertz wave decreases as the crystal proceeds in the crystal. On the other hand, a method for improving the generation and output efficiency of terahertz waves using quasi phase matching (QPM) has been proposed.

For example, Non-Patent Document 1 discloses that a terahertz wave is realized by forming a periodic inversion structure of a nonlinear optical constant in a nonlinear optical crystal (for example, LiNbO ₃ ) used for generating a terahertz wave, thereby realizing pseudo phase matching. A method for improving the generation efficiency of the is shown. In Non-Patent Document 2, pseudo-phase matching can be realized by a laminated structure in which a nonlinear optical crystal and a normal crystal having no nonlinear optical effect are alternately stacked, instead of a nonlinear optical constant positive / negative inversion structure. It is shown that there is.

  In the terahertz wave generator having the above-described structure, the period of the inversion structure of the nonlinear optical constant or the period of the laminated structure of the nonlinear optical crystal and the normal crystal, and the phase matching condition therefor are the pulse width of the pulse excitation light, the group refraction. The period of the inversion structure and the laminated structure needs to be set in accordance with conditions such as the frequency (wavelength) of the generated terahertz wave. For this reason, once the period is determined and a periodic structure for generating a terahertz wave is formed, there is a problem that it is difficult to adjust and change the wavelength of the terahertz wave generated by the structure. Such a problem also occurs in the generation of wavelength-converted light using a nonlinear optical crystal in a wavelength region other than the terahertz wave.

  The present invention has been made to solve the above problems, and provides a wavelength-converted light generation apparatus and a generation system capable of variably controlling the wavelength of wavelength-converted light generated by incidence of pulsed excitation light. The purpose is to provide.

In order to achieve such an object, a terahertz wave generator according to the present invention is composed of a nonlinear optical crystal, generates a terahertz wave that is wavelength-converted light by incidence of pulsed excitation light, and is adjacent to each other at a distance from each other. A plurality of crystal layers arranged in order so as to meet each other and a predetermined material, and alternately arranged with a plurality of crystal layers so as to be interposed between two adjacent crystal layers, and a phase Physical distance adjusting means for variably adjusting the physical distance between two adjacent crystal layers via the adjustment layer with respect to at least one adjustment layer used for adjustment of the matching condition and a plurality of crystal layers; provided, adjustment layer, filling the space between the crystalline layers of the two layers adjacent with is constituted by substantially no liquid material nonlinear optical effect, the physical distance adjusting means, a plurality In the crystal layer, characterized Rukoto to have a resilient member connecting the crystal layer of two layers adjacent to each other.

The terahertz wave generation system according to the present invention includes the terahertz wave generation device having the above-described configuration , and excitation light supply means that supplies the terahertz wave generation device with pulse excitation light used to generate terahertz waves. It is characterized by.

  In the wavelength-converted light generator and the generation system described above, an element that becomes a wavelength conversion medium is configured by a laminated structure of a plurality of crystal layers formed by nonlinear optical crystals and at least one adjustment layer interposed therebetween. In the laminated structure, the physical distance adjustment means adjusts the phase matching condition in the laminated structure by variably adjusting the physical distance between the two crystal layers. According to such a configuration, by adjusting the physical distance between the crystal layers corresponding to the thickness of the adjustment layer, the period of the laminated structure used for generating the wavelength-converted light and the phase matching condition thereby are set. Alternatively, it is possible to variably control the wavelength of wavelength-converted light such as a terahertz wave that is generated.

  Here, regarding the material of the adjustment layer interposed between the crystal layers, the adjustment layer is preferably made of a liquid material that fills between two adjacent crystal layers. The adjustment layer is preferably made of a material that has substantially no nonlinear optical effect (a material whose nonlinear optical constant is substantially 0). Thereby, the laminated structure in which the physical distance between the crystal layers is variably adjusted as described above can be suitably realized.

  Regarding the specific configuration of the adjusting means for adjusting the physical distance, the physical distance adjusting means variably adjusts the physical distance by moving each of the plurality of crystal layers in the material constituting the adjusting layer. It is possible to use a configuration having crystal layer driving means. In addition, the physical distance adjusting means may use a configuration having an elastic member that connects two adjacent crystal layers in a plurality of crystal layers.

  In addition, regarding the laminated structure of the crystallized layer and the adjustment layer in the wavelength conversion light generator, n layers (n is an integer of 3 or more) are provided as a plurality of crystal layers. It is preferable that the physical distance is variably adjusted in a state where n−1 physical distances between two crystal layers adjacent to each other through the layers are substantially equal to each other. According to such a configuration, the phase matching condition can be suitably controlled in the entire stacked structure including three or more crystal layers.

  In addition, the wavelength-converted light generation system controls the phase matching condition in the wavelength-converted light generator by controlling the operation of the physical distance adjusting means in addition to the wavelength-converted light generator and the excitation light supply means. It is good also as a structure provided with a control means. Thereby, the phase matching conditions in the laminated structure used for the generation of wavelength-converted light and the wavelength of the generated wavelength-converted light can be suitably controlled to desired conditions and wavelengths.

  According to the wavelength-converted light generation apparatus and the generation system of the present invention, a wavelength-converted medium for generating wavelength-converted light by a laminated structure of a plurality of crystal layers made of nonlinear optical crystals and at least one adjustment layer interposed therebetween. In addition, the physical distance between the two crystal layers is variably adjusted by the physical distance adjusting means, and the phase matching condition in the laminated structure is adjusted, thereby changing the wavelength of the wavelength converted light to be generated. It becomes possible to control to.

It is a figure shown about an example of generation | occurrence | production of the terahertz wave by pseudo phase matching. It is a figure shown about the other example of generation | occurrence | production of the terahertz wave by pseudo phase matching. It is a block diagram which shows the structure of one Embodiment of a terahertz wave generation system. It is a schematic diagram shown about adjustment of the phase matching conditions in a terahertz wave generator. It is a figure shown about the structure of the conventional wavelength conversion light generation element. It is a figure which shows typically the 1st structural example of a terahertz wave generator. It is a figure which shows the structure of the physical distance adjustment part in the generator shown in FIG. It is a figure which shows typically the 2nd structural example of a terahertz wave generator. It is a figure which shows typically the 3rd structural example of a terahertz wave generator.

  Hereinafter, preferred embodiments of a wavelength-converted light generation apparatus and a wavelength-converted light generation system according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  In the following, in the generation of wavelength converted light using a nonlinear optical crystal, a terahertz wave is assumed as wavelength converted light generated by incidence of pulsed excitation light, and the terahertz wave generator and generation system are taken as an example. The configurations of the wavelength-converted light generator and the generation system according to the invention will be described.

  First, a terahertz wave generation method using quasi phase matching (QPM), which is a premise of the terahertz wave generation apparatus according to the present invention, will be described.

  FIG. 1 is a diagram illustrating an example of generation of terahertz waves by quasi phase matching. The terahertz wave generating element 80 shown in FIG. 1 includes a first crystal region A1, A2, A3 having a thickness Da made of a nonlinear optical crystal having a predetermined nonlinear optical constant, and a nonlinearity in which the first crystal region is inverted between positive and negative. The second crystal regions B1, B2, and B3 having a thickness Db and made of a nonlinear optical crystal having an optical constant have an inverted structure that is alternately arranged along the traveling direction of the pulse excitation light L. Further, in the generating element 80, the end surface on the first crystal region A1 side is an excitation light incident surface 81, and the end surface on the second crystal region B3 side is a terahertz wave output surface 82.

Such a configuration can be realized, for example, by forming a domain-inverted structure with a predetermined period in a nonlinear optical crystal. For example, in Non-Patent Document 1, LiNbO ₃ is used as a nonlinear optical material, and by poling, the polarization is reversed in the direction opposite to the spontaneous polarization, and a periodic positive / negative inversion structure with a nonlinear optical constant as shown in FIG. Forming. The poling of LiNbO ₃ is performed by setting the material to a high temperature (about 80 ° C. to 200 ° C.) and applying an electric field (about 20 kV / mm).

  In the configuration shown in FIG. 1, different nonlinear optical materials can be used for the first crystal region A and the second crystal region B. However, in this case, in order to avoid light reflection loss at the interface between the first and second crystal regions, materials having the same refractive index are used in the first crystal region A and the second crystal region B. Is preferred.

  When pulse excitation light L having a predetermined wavelength, intensity, and pulse width is incident on the terahertz wave generating element 80 shown in FIG. 1, the excitation light L is converted into a terahertz wave T in a nonlinear optical crystal (frequency conversion). ) First, in the first crystal region A1 closest to the excitation light incident surface 81, a terahertz wave T1 having a positive electric field is generated. Next, in the second crystal region B1, since the axis of the nonlinear optical constant is inverted from that of the first crystal region A1, a terahertz wave T2 having a negative electric field is generated by reversing positive and negative.

  Here, in the nonlinear optical crystal constituting the generating element 80, the refractive index (phase refraction) for the terahertz wave T generated in the crystal is higher than the refractive index (group refractive index) for the pulse excitation light L such as pulse laser light. Rate) is greater. For this reason, as schematically shown in FIG. 1, the pulse excitation light L travels first in the crystal and the terahertz wave T is delayed.

  In the generating element 80 shown in FIG. 1, in consideration of such light propagation conditions in the crystal, the terahertz wave T1 generated by the excitation light L in the first crystal region A1 and propagated to the second crystal region B1. And the terahertz wave T2 generated by the excitation light L in the second crystal region B1, the thickness of the crystal regions A1 and B1 (period of the inversion structure) is set so that the phase is shifted by 180 °. Thereby, the terahertz waves T1 and T2 become one continuous wave with phase matching.

  When the phase difference between the terahertz waves T1 and T2 is different from 180 °, the respective electric fields cancel each other and the output of the terahertz wave T decreases. On the other hand, by setting the period of the inversion structure so as to satisfy the phase matching condition, it is possible to generate a terahertz wave with high efficiency and a narrow band. Further, similarly, a terahertz wave T3 having a positive electric field generated in the crystal region A2, a terahertz wave T4 having a negative electric field generated in the crystal region B2, and a terahertz wave T5 having a positive electric field generated in the crystal region A3, The terahertz wave T6 having a negative electric field generated in the crystal region B3 becomes one continuous wave in phase matching with the terahertz waves T1 and T2, and is finally output from the output surface 82 as the terahertz wave T.

  FIG. 2 is a diagram illustrating another example of generation of terahertz waves by quasi phase matching. The terahertz wave generating element 85 shown in FIG. 2 has first crystal regions (crystal layers) A1, A2, and A3 having a thickness Da made of a nonlinear optical crystal having a predetermined nonlinear optical constant, and a nonlinear optical constant of 0. A laminated structure in which second material regions (material layers) C1, C2, and C3 having a thickness Dc and made of a normal material having no nonlinear optical effect are alternately arranged along the traveling direction of the pulse excitation light L have. In the generating element 85, the end surface on the first crystal region A1 side is an excitation light incident surface 86, and the end surface on the second material region C3 side is a terahertz wave output surface 87.

  In such a configuration, similarly to the configuration shown in FIG. 1, in order to avoid light reflection loss at the interface between the first crystal region and the second material region, the first crystal region A and the second material region It is preferable to use a material having the same refractive index as C.

  When the pulsed excitation light L is incident on the terahertz wave generating element 85 shown in FIG. 2, the excitation light L is wavelength-converted into a terahertz wave T in the nonlinear optical crystal. First, a terahertz wave T1 having a positive electric field is generated in the first crystal region A1 closest to the excitation light incident surface 86 side. Next, since the second material region C1 does not have a nonlinear optical effect, a terahertz wave is not generated and the wave T2 has an intensity of 0. Further, in the subsequent first crystal region A2, a terahertz wave T3 having a positive electric field is generated as in the crystal region A1.

  In the generating element 85 shown in FIG. 2, in consideration of the light propagation conditions in the crystal, the light is generated by the excitation light L in the first crystal region A1 and propagates to the first crystal region A2 through the second material region C1. The thickness of the crystal regions A1, A2 and the material region C1 (stacked structure) so that the phase difference between the terahertz wave T1 and the terahertz wave T3 generated by the excitation light L in the first crystal region A2 is 0 °. Cycle). As a result, the terahertz waves T1 and T3 become one continuous wave with phase matching.

  When the phase difference between the terahertz waves T1 and T3 is different from 0 °, the respective electric fields cancel each other and the output of the terahertz wave T decreases. On the other hand, by setting the period of the laminated structure so as to satisfy the phase matching condition, terahertz waves can be generated with high efficiency and in a narrow band. Furthermore, similarly, the terahertz wave T5 having a positive electric field generated in the crystal region A3 becomes one continuous wave in phase matching with the terahertz waves T1 and T3, and is finally output from the output surface 87 as the terahertz wave T. Is done.

  In the terahertz wave generating element using the quasi phase matching using the inversion structure of the nonlinear optical constant in the nonlinear optical crystal shown in FIG. 1 or the laminated structure of the nonlinear optical crystal and the normal crystal shown in FIG. Thus, once the period of the structure is determined, it is difficult to adjust and change the wavelength of the output terahertz wave. On the other hand, the terahertz wave generation device and the generation system according to the present invention enable variable control of the wavelength of the terahertz wave in the generation of the terahertz wave using the quasi phase matching type element.

  FIG. 3 is a block diagram schematically showing a configuration of an embodiment of a terahertz wave generation system including a terahertz wave generation device according to the present invention. A terahertz wave generation system 1A according to the present embodiment generates a terahertz wave that is wavelength-converted light by using an optical rectification effect in a nonlinear optical crystal, and includes a terahertz wave generation device 10, an excitation light source 50, And a terahertz wave generation control unit 55.

  The pulse excitation light source 50 is excitation light supply means for supplying the pulse excitation light L used for generating the terahertz wave to the terahertz wave generator 10. As the excitation light source 50, a pulse laser light source such as a femtosecond pulse laser light source that outputs femtosecond (fs) pulse laser light can be preferably used.

  For this excitation light source 50, the terahertz wave generation device 10 is configured to include a nonlinear optical crystal serving as a medium for generating terahertz waves (for wavelength conversion). Specifically, the generator 10 according to the present embodiment includes a terahertz wave generating element 20 including a nonlinear optical crystal and a physical distance adjusting unit 30.

  The terahertz wave generating element 20 is composed of a non-linear optical crystal having a predetermined non-linear optical constant, and is a multi-layer crystal (three layers in FIG. 3) that generates a terahertz wave T that is wavelength converted light upon incidence of pulsed excitation light L. The layer 21 is composed of a material different from that of the crystal layer 21, and includes a plurality of crystal layers 21 and at least one adjustment layer 22 (two layers in FIG. 3) arranged alternately. .

  The crystal layer 21 corresponds to the first crystal region A in the stacked structure shown in FIG. 2, and is sequentially arranged so as to be adjacent to each other with a distance along the traveling direction of the excitation light L. The adjustment layer 22 corresponds to the second material region C in the stacked structure shown in FIG. 2 and is disposed so as to be interposed between two adjacent crystal layers 21. The adjustment layer 22 is a layer used for adjusting the phase matching condition in the generating element 20.

  In the generating element 20 of this embodiment, the liquid material 25 accommodated in the outer container 26 is used as the material of the adjustment layer 22. The crystal layer 21 is accommodated in the container 26 together with the liquid material 25, and a portion of the liquid material 25 that fills the space between the crystal layers 21 functions as the adjustment layer 22. In addition, in the container 26, one surface thereof is an excitation light incident surface 261 on which the excitation light L is incident, and a surface opposite to the incident surface 261 is a terahertz wave that outputs the terahertz wave T generated in the generating element 20 to the outside. An output surface 262 is formed.

  A physical distance adjusting unit 30 is provided for the terahertz wave generating element 20 having such a configuration. The physical distance adjustment unit 30 is an adjustment unit that variably adjusts the physical distance between two adjacent crystal layers 21 via the adjustment layer 22 with respect to the plurality of crystal layers 21 of the generation element 20.

  That is, the terahertz wave generation device 10 of FIG. 3 has a configuration in which the physical distance D between the crystal layers 21 corresponding to the thickness of the adjustment layer 22 can be changed in the generation element 20. By adjusting the physical distance D variably by the adjusting unit 30, the period of the laminated structure in the element 20 and the phase matching condition thereby are variably adjusted. Specifically, in the present configuration example, the physical distance adjustment unit 30 includes a plurality of crystal layers 21 in the material 25 constituting the adjustment layer 22 along the stacking direction (advancing direction of the excitation light L). The crystal layer driving unit 31 is configured to variably adjust the physical distance D by being moved.

  Here, FIG. 4 is a diagram schematically showing a method of adjusting the phase matching condition in the terahertz wave generation apparatus 10 shown in FIG. In the configuration shown in FIG. 4A, the generating element 20 includes a non-linear optical crystal crystal layer 211, 212, 213 having the same thickness in a plurality of layers, and a liquid material 25 interposed between the crystal layers. The adjustment layers 221 and 222 are included. The physical distance between two adjacent crystal layers is set to D = D1, and this physical distance D1 is the thickness of each of the adjustment layers 221 and 222.

  In such a configuration, the position of the crystal layer 211 is fixed, the adjustment distance is set to ΔD = D2−D1, and the crystal layer driving unit 31 (see FIG. 3) moves the crystal layer 212 by ΔD in the arrangement direction. The crystal layer 213 is moved in the arrangement direction by 2 × ΔD. At this time, as shown in FIG. 4B, the physical distance D between the crystal layers is changed from D1 to D2. Thereby, the phase matching condition in the laminated structure of the generating element 20 is variably adjusted, and the wavelength λ and the frequency f of the terahertz wave T generated in the element 20 are variably controlled.

  Further, in the generation system 1 </ b> A illustrated in FIG. 3, a terahertz wave generation control unit 55 is provided for the excitation light source 50 and the generation device 10. The generation control unit 55 has a function as a phase matching control unit that controls the phase matching condition in the generation device 10 by controlling the operation of the physical distance adjusting unit 30. Such a control part 55 can be comprised by the computer with which the predetermined | prescribed control program is run, for example. In addition, the generation control unit 55 may be configured to control the operation of the excitation light source 50 together with the operation of the generation device 10.

  The effects of the terahertz wave generation device 10 and the terahertz wave generation system 1A according to the above embodiment will be described.

  In the terahertz wave generation device 10 and the generation system 1A shown in FIGS. 3 and 4, the terahertz structure has a stacked structure of a plurality of crystal layers 21 made of nonlinear optical crystals and at least one adjustment layer 22 interposed therebetween. A generation element 20 is formed as a wavelength conversion medium for generating waves. In the stacked structure, the physical distance adjusting unit 30 variably adjusts the physical distance D between the two crystal layers 21 to adjust the phase matching condition in the stacked structure.

  According to such a configuration, by adjusting the physical distance D between the crystal layers 21 corresponding to the thickness of the adjustment layer 22, the period of the laminated structure used for generating the terahertz wave T and the phase caused thereby It is possible to variably control the wavelength (frequency) of the terahertz wave T in the same device without setting or changing the matching condition and changing the device configuration itself.

  Here, FIG. 5 is a diagram showing a configuration of a conventional wavelength-converted light generating element. In the configuration shown in FIG. 5A, the periodic structure of polarization inversion in the nonlinear optical crystal changes in a fan shape in the direction orthogonal to the light propagation direction, and the incident position of the excitation light is changed. Thus, the polarization inversion period can be selected. The configuration shown in FIG. 5B is a circular element structure having a polarization inversion structure, and the effective polarization inversion period is changed by rotating the element with respect to the traveling direction of the excitation light. It has become. On the other hand, in the terahertz wave generation device 10 shown in FIGS. 3 and 4, the crystal along the traveling direction of the excitation light L without changing the incident position of the excitation light on the element, rotating the crystal, or the like. By adjusting the physical distance D between the layers 21 variably, it is possible to suitably and easily execute adjustment of the period of the laminated structure, the phase matching condition, and the wavelength of the obtained terahertz wave.

  In the terahertz wave generator 10 shown in FIG. 3, the stacked structure including the crystal layer 21 and the adjustment layer 22 uses a configuration in which n crystal layers 21 (n is an integer of 3 or more) are provided as crystal layers. Can do. In this case, the physical distance adjustment unit 30 has n−1 physical distances D (thicknesses of the n−1 adjustment layers 22) between two adjacent crystal layers 21 via the adjustment layer 22. It is preferable to variably adjust the physical distance D while maintaining a substantially equal state. According to such a configuration, the phase matching condition can be suitably controlled in the entire stacked structure including three or more crystal layers 21. Note that the number of crystal layers 21 and adjustment layers 22 may be appropriately set according to a specific laminated structure.

  As for the material of the adjustment layer 22, the adjustment layer 22 is preferably composed of a liquid material 25 that fills between two adjacent crystal layers 21 as described above with reference to FIG. 3. The adjustment layer 22 is preferably made of a material that does not substantially have a nonlinear optical effect (a material whose nonlinear optical constant is substantially 0). Thereby, it is possible to preferably realize a stacked structure in which the physical distance D between the crystal layers 21 is variably adjusted. In particular, in the configuration using the liquid material 25 for the adjustment layer 22, the thickness of the adjustment layer 22 can be easily and reliably changed as the physical distance between the crystal layers 21 is adjusted.

  In addition, regarding a specific configuration of the physical distance adjustment unit 30 for adjusting the physical distance between the crystal layers 21, each of the plurality of crystal layers 21 is moved in the arrangement direction in the material constituting the adjustment layer 22. Thus, a configuration in which the crystal layer driving unit 31 that variably adjusts the physical distance D can be used. Further, the physical distance adjusting unit 30 may use a configuration in which a plurality of crystal layers 21 include an elastic member that connects two adjacent crystal layers 21. The configuration of the adjustment unit 30 will be specifically described later.

  In addition, in the generation system 1A shown in FIG. 3, in addition to the generation device 10 and the excitation light source 50, the generation control unit that controls the phase matching condition in the generation device 10 by controlling the operation of the physical distance adjustment unit 30. 55 is provided. Thereby, the phase matching condition in the laminated structure of the generating element 20 and the wavelength of the generated terahertz wave T can be suitably controlled to the desired condition and wavelength. Such a control unit 55 may be configured not to be provided if unnecessary, for example, when an operator manually controls the operation of each unit of the apparatus.

  A specific example of the configuration of the terahertz wave generation device 10 illustrated in FIGS. 3 and 4 will be described. As the nonlinear optical crystal that forms the crystal layer 21 with the terahertz wave generating element 20, for example, a ZnTe (110) crystal having a predetermined thickness can be used. As the liquid material 25 of the adjustment layer 22, it is preferable to use a material that is transparent to both the excitation light L and the terahertz wave T and transmits them. As such a liquid material 25, fats and oils, such as salad oil, can be used, for example. In order to avoid light reflection loss at the interface between the crystal layer 21 and the adjustment layer 22, it is preferable to use materials having the same refractive index in the crystal layer 21 and the adjustment layer 22.

  In the generating element 20 having such a configuration, when the distance between the crystal layers 21 is set to D = D1, as shown in FIG. 4A, the terahertz centered on the frequency f1 at which phase matching can be achieved in the stacked structure. Waves are generated and output with high efficiency. Also, as shown in FIG. 4B, when the distance between the crystal layers 21 is changed to D = D2, terahertz waves centered at a frequency f2 different from f1 that can achieve phase matching in the stacked structure are highly efficient. Is generated and output.

Here, consider a case where the wavelength of the pulse excitation light L is 620 nm, the pulse width is τ = 150 fs, and the frequency of the terahertz wave T generated in the stacked structure of the element 20 is 1.4 THz. If the nonlinear optical crystal constituting the crystal layer 21 is ZnTe, the refractive index for the excitation light L having a wavelength of 620 nm is n _L = 3.0. On the other hand, the refractive index for the terahertz wave T having a frequency of 1.4 THz is n _T = 3.19.

Further, in the element 20 having such a configuration, the period Λ of the laminated structure capable of achieving phase matching is given by Λ = 2cτ / (n _T −n _L ) (see Non-Patent Document 1). Therefore, the period in which phase matching can be obtained around the frequency f1 = 1.4 THz using the ZnTe crystal is Λ = 0.464 mm. When the refractive index of the liquid material of the adjustment layer 22 is the same as that of the ZnTe crystal, the thickness of each of the crystal layer 21 and the adjustment layer 22 is 0.232 mm. Further, when the liquid material of the adjustment layer 22 is oil or fat, its refractive index is about 1.5, which is half that of the ZnTe crystal. In this case, the thickness of the adjustment layer 22 that is the physical distance between the crystal layers 21 is D1 = 0.464 mm.

  Next, while keeping the thickness of the ZnTe crystal of the crystal layer 21 as it is, only the physical distance between the crystal layers 21 corresponding to the thickness of the adjustment layer 22 is changed, and the phase is centered on the frequency f2 = 1.2 THz. Consider matching. In this case, the thickness of the adjustment layer 22 necessary for achieving phase matching is D2 = 0.263 mm. Therefore, in the configuration of FIG. 3, when a ZnTe crystal having a thickness of 0.232 mm is used as the crystal layer 21 and oil is used as the liquid material 25 of the adjustment layer 22, the physical distance between the crystal layers 21 is D1 = 0.464 mm. By changing from D2 to 0.526 mm, the center frequency of the terahertz wave generated in the element 20 can be variably controlled from f1 = 1.4 THz to f2 = 1.2 THz (see FIG. 4). .

  The configuration of the terahertz wave generation device 10 according to the above embodiment and the adjustment of the physical distance D between crystal layers in the generation device 10 will be further described together with a specific configuration example.

  FIG. 6 is a diagram schematically illustrating a first configuration example of the terahertz wave generation device. FIG. 7 is a diagram illustrating a configuration of a physical distance adjustment unit in the terahertz wave generation device illustrated in FIG. 6. The terahertz wave generating apparatus 10A according to the present configuration example includes a crystal layer 21 (211 to 214) having a substantially triangular planar shape and a liquid material 25 constituting the adjustment layer 22 (221 to 223) interposed therebetween. The terahertz wave generating element 20 is accommodated in the container 26. Further, in the container 26, one surface thereof is an incident surface 261 provided with an excitation light incident window 266, and the opposite surface is an output surface 262 provided with a terahertz wave output window 267. As the terahertz wave output window 267, for example, high resistance silicon can be suitably used.

  For such a terahertz wave generating element 20, the crystal layer driving unit of the adjusting unit that variably adjusts the physical distance D between the crystal layers includes a support member 311, a driving member 312, a rod 313, and a spring 314. Configured. Of the crystal layers 211 to 214, the crystal layer 211 closest to the incident surface 261 is fixedly supported by a bar-shaped support member 311 extending upward. Further, three rods 313 extending along the traveling direction of the excitation light L are fixed to the crystal layer 211. The crystal layers 212, 213, and 214 are each provided with three through holes corresponding to the rods 313, and the rods 313 are inserted into the through holes so that the crystal layers 212 to 214 are crystal layers. 211 is held and arranged in a fixed positional relationship.

  In addition, between the crystal layers 211 and 212, between the crystal layers 212 and 213, and between the crystal layers 213 and 214, an elastic member that connects two adjacent crystal layers together with the three rods 313, respectively. These three springs 314 are movably provided. All of these springs 314 have the same spring characteristics, so that the physical distance between two adjacent crystal layers via the adjustment layers 221, 222, and 223 is substantially equal to each other. Yes.

  The crystal layer 214 closest to the output surface 262 is supported by a bar-shaped member 312 extending upward. The member 312 is a drive member 312 that moves along the direction in which the crystal layers are arranged. The support member 311 attached to the crystal layer 211 and the drive member 312 attached to the crystal layer 214 are installed in a state where the upper portions thereof are exposed on the liquid surface of the liquid material 25. In such a configuration, when the crystal layer 214 is moved in the arrangement direction by the driving member 312, the crystal layers 212 and 213 maintain a state where the physical distances between the crystal layers are substantially equal to each other by the action of the rod 313 and the spring 314. While moving with the crystal layer 214, the physical distance between the crystal layers is variably adjusted.

  FIG. 8 is a diagram schematically illustrating a second configuration example of the terahertz wave generation device. The terahertz wave generating device 10B according to this configuration example includes a terahertz wave generating element 20 in which a plurality of crystal layers 21 and a liquid material 25 constituting an adjustment layer 22 interposed therebetween are accommodated in a container 26. doing. Further, the physical distance adjusting unit for the generating element 20 is configured by using an elastic member 32 that can be expanded and contracted by external control using an elastic body such as a polymer.

  In this configuration example, in the plurality of crystal layers 21, two adjacent crystal layers are connected via an elastic member 32. In such a configuration, when the elastic member 32 is expanded or contracted by external control, the liquid material 25 constituting the adjustment layer 22 flows in or out between the crystal layers 21, thereby causing physical properties between the crystal layers 21. The distance and the thickness of the adjustment layer 22 are variably adjusted. In addition, as such an elastic member 32, for example, a thermally expandable polymer that expands due to a temperature rise accompanying light irradiation can be used.

  FIG. 9 is a diagram schematically illustrating a third configuration example of the terahertz wave generation device. The configuration of the terahertz wave generation device 10C according to the present configuration example is basically the same as the configuration of the generation device 10B illustrated in FIG. 8, but a container 26 is provided for the liquid material 25 forming the adjustment layer 22. The difference is that the liquid material 25 is sealed inside the crystal layer 21 and the elastic member 32. In such a configuration, when the elastic member 32 is expanded or contracted, the height of the liquid surface of the liquid material 25 constituting the adjustment layer 22 varies between the crystal layers 21 and the physical distance between the crystal layers 21. , And the thickness of the adjustment layer 22 is variably adjusted.

  The wavelength-converted light generation apparatus and the generation system according to the present invention are not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, regarding the laminated structure of the crystal layer and the adjustment layer, the number of layers, the constituent material of each layer, the configuration of the physical distance adjustment unit, and the like, specifically, various configurations other than the above configuration may be used. In the above description, a terahertz wave is assumed as wavelength-converted light. However, the wavelength-converted light generator and the generation system according to the present invention are similarly applied to generation of wavelength-converted light in a wavelength region other than the terahertz wave. Is possible.

  The present invention can be used as a wavelength-converted light generator and a wavelength-converted light generation system that can variably control the wavelength of wavelength-converted light such as terahertz waves generated by incidence of pulsed excitation light.

DESCRIPTION OF SYMBOLS 1A ... Terahertz wave generation system, 10, 10A-10C ... Terahertz wave generator, 20 ... Terahertz wave generation element, 21 ... Nonlinear optical crystal layer, 22 ... Adjustment layer, 25 ... Liquid material, 26 ... Outer container, 261 ... Excitation light incident surface, 262... Terahertz wave output surface, 266... Excitation light incident window, 267.
DESCRIPTION OF SYMBOLS 30 ... Physical distance adjustment part, 31 ... Crystal layer drive part, 311 ... Support member, 312 ... Drive member, 313 ... Rod, 314 ... Spring, 32 ... Elastic member, 50 ... Pulse excitation light source, 55 ... Terahertz wave generation control part .

Claims (8)

A plurality of crystal layers that are configured by a nonlinear optical crystal, generate a terahertz wave that is wavelength-converted light by incidence of pulsed excitation light, and are sequentially arranged so as to be adjacent to each other at a distance;
At least one adjustment layer that is made of a predetermined material and is alternately arranged with the plurality of crystal layers so as to be interposed between two adjacent crystal layers, and is used for adjusting the phase matching condition When,
Physical distance adjusting means for variably adjusting the physical distance between two adjacent crystal layers via the adjustment layer for the plurality of crystal layers ;
The adjustment layer is composed of a liquid material that substantially does not have a nonlinear optical effect and fills between two adjacent crystal layers.
The physical distance adjusting means, in the crystal layer of the plural layers, a terahertz wave generating device according to claim Rukoto to have a resilient member connecting the crystal layer of two layers adjacent to each other. The first crystal layer on the most incident side of the pulse excitation light among the plurality of crystal layers is fixedly supported by a support member,
In the plurality of crystal layers, a spring which is the elastic member is movably provided between two adjacent crystal layers,
Most said terahertz wave output side of the crystal layer among the crystal layers of the plurality of layers, the terahertz wave according to claim 1, characterized in that it is supported by the drive member to move along the array direction of the crystal layer Generator. A rod extending along the traveling direction of the pulse excitation light is fixed to the first crystal layer,
Of the plurality of crystal layers, the crystal layers excluding the first crystal layer are inserted into through holes provided corresponding to the rods, so that the crystal layers are converted into the first crystals. The terahertz wave generation device according to claim 2 , wherein the terahertz wave generation device is held and arranged in a fixed positional relationship with respect to the layer . The elastic member, a terahertz wave generating apparatus according to claim 1 Symbol mounting, characterized in that it is configured with the expansion and contraction is resilient body according to external control. Wherein the elastic body constituting the elastic member, a terahertz wave generating apparatus according to claim 4 Symbol mounting, characterized in that a thermally expandable polymer. N layers (n is an integer of 3 or more) are provided as the plurality of crystal layers,
The physical distance adjusting means variably adjusts the physical distance in a state where n-1 physical distances between two crystal layers adjacent to each other through the adjustment layer are substantially equal to each other. The terahertz wave generator according to any one of claims 1 to 5. The terahertz wave generator according to any one of claims 1 to 6,
Terahertz wave generation system characterized by comprising relative to the terahertz wave generating device, and a pumping light supply means for supplying pulsed excitation light used for generation of terahertz waves. 8. The terahertz wave generation system according to claim 7, further comprising phase matching control means for controlling the phase matching condition in the terahertz wave generation device by controlling the operation of the physical distance adjusting means.

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