JP2010231018A - Wavelength conversion light generator and generation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion light generator capable of variably controlling wavelength of wavelength conversion light to be generated by incidence of pulse exciting light, and to provide a generation system. <P>SOLUTION: The wavelength conversion light generator 10 is constituted by using: a plurality of layers of crystal layers 21 which are constituted by nonlinear optical crystal, generate the wavelength conversion light T by the incidence of the pulse exciting light L, and are arranged in order so as to be adjacent to each other at distance; and generation elements 20 having layered structure with at least one layer of adjustment layer 22, which are constituted by a prescribed material, arranged alternately with the crystal layers 21 so as to be interposed between the two layers of crystal layers 21 adjacent to each other, and are used in adjustment of phase matching conditions. In addition, an optical distance adjustment part 30, which variably adjusts optical distance in a state that physical distance between the adjacent crystal layers via the adjustment layer 22 is fixed, is provided for the crystal layers 21 of the generation elements 20. <P>COPYRIGHT: (C)2011,JPO&INPIT

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 terahertzradiation via optical rectification of femtosecond pulses in periodically poledlithium 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 (Tokushima University, Autumn 2005) 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, by forming a periodic inverted structure of the nonlinear optical constant in the nonlinear optical crystal used for generation of terahertz waves (such as LiNbO _3), the terahertz wave to achieve quasi-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, the wavelength-converted light generator according to the present invention is composed of a nonlinear optical crystal, generates wavelength-converted light by incidence of pulsed excitation light, and is adjacent to each other at a distance. A plurality of crystal layers arranged in order and a predetermined material, and alternately arranged with a plurality of crystal layers so as to be interposed between two adjacent crystal layers, For at least one adjustment layer used for adjustment and multiple crystal layers, the optical distance is variably adjusted while the physical distance between two adjacent crystal layers is fixed via the adjustment layer. And an optical distance adjusting means.

  A wavelength-converted light generation system according to the present invention includes a wavelength-converted light generator configured as described above, and excitation light supply means for supplying pulsed pump light used for generating wavelength-converted light to the wavelength-converted light generator. It is characterized by providing.

  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 optical distance adjustment means adjusts the phase matching condition in the laminated structure by variably adjusting the optical distance while the physical distance between the two crystal layers is fixed. Yes. According to such a configuration, by adjusting the optical distance between the crystal layers corresponding to the optical path length in the adjustment layer, the effective period of the laminated structure used for generating the wavelength-converted light, and the phase caused thereby By setting or changing the matching condition, it is possible to variably control the wavelength of the wavelength converted light such as the terahertz wave generated.

  Here, regarding the material of the adjustment layer interposed between the crystal layers, the adjustment layer is preferably constituted by a liquid crystal layer disposed 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, a laminated structure in which the optical 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 optical distance, the optical distance adjusting means applies a voltage to the adjusting layer to change its optical characteristics, thereby changing the optical distance variably. A structure having an application unit can be used. In this case, it is preferable that the optical distance adjusting means has an electrode provided between the layers and used for applying a voltage by the voltage applying means in a laminated structure including the crystal layer and the adjusting layer.

  In addition, regarding the stacked structure of the crystal layer and the adjustment layer in the wavelength conversion light generator, n crystal layers (n is an integer of 3 or more) are provided as a plurality of crystal layers. It is preferable that the optical distance is variably adjusted in a state where n−1 optical 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 optical distance adjusting unit in addition to the wavelength-converted light generator and the excitation light supply unit. 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. And adjusting the phase matching condition in the laminated structure by adjusting the optical distance variably while the physical distance between the two crystal layers is fixed by the optical distance adjusting means. It becomes possible to variably control the wavelength of the wavelength-converted light.

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 figure shown about the structure of the conventional wavelength conversion light generation element. It is a figure which shows typically the structural example of a terahertz wave generator.

  Hereinafter, preferred embodiments of a wavelength-converted light generator 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. 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 an optical distance adjustment 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. The plurality of crystal layers 21 are preferably fixed to each other and have the same thickness in the plurality of layers. When a plurality of adjustment layers 22 are provided, the thicknesses thereof are preferably fixed to each other and set to the same thickness in the plurality of layers.

  In the generating element 20 of the present embodiment, a liquid crystal material is used as the material of the adjustment layer 22, and the adjustment layer 22 is configured by a liquid crystal layer formed by surrounding the liquid crystal material with a fixing material such as an adhesive. Yes. Further, in the stacked structure of the element 20 composed of the crystal layer 21 and the adjustment layer 22, one surface thereof is the excitation light incident surface 201 on which the excitation light L is incident, and the surface opposite to the incident surface 201 is in the generation element 20. The terahertz wave output surface 202 for outputting the terahertz wave T generated in step 1 to the outside.

  An optical distance adjusting unit 30 is provided for the terahertz wave generating element 20 having such a configuration. The optical distance adjusting unit 30 can change the optical distance of a plurality of crystal layers 21 of the generating element 20 in a state where the physical distance between two adjacent crystal layers 21 via the adjustment layer 22 is fixed. It is the adjustment means to adjust.

  That is, in the terahertz wave generation device 10 of FIG. 3, the optical element between the crystal layers 21 corresponding to the optical path length (optical thickness) in the adjustment layer 22 along the traveling direction of the excitation light L in the generation element 20. The optical distance can be variably adjusted by the adjustment unit 30 to variably adjust the period of the laminated structure in the element 20 and the phase matching condition thereby. Is done. Specifically, in this configuration example, the optical distance adjustment unit 30 variably adjusts the optical distance between the crystal layers 21 by applying a voltage to the adjustment layer 22 to change its optical characteristics. A voltage application unit 31 is included.

  Corresponding to the voltage applying unit 31 for adjusting the optical distance, in the laminated structure of the generating element 20 composed of the crystal layer 21 and the adjusting layer 22, electrodes 36 and 37 for applying voltage are provided between the layers. . That is, the first electrode 36 is provided between the crystal layer 21 and the adjustment layer 22 on the downstream side in the traveling direction of the excitation light L with respect to the stacked structure of the crystal layer 21 and the adjustment layer 22. . A second electrode 37 is provided between the crystal layer 21 and the adjustment layer 22 on the upstream side thereof. In the present embodiment, the optical distance adjusting unit 30 is configured using the voltage applying unit 31 and the electrodes 36 and 37.

  In such a configuration, the voltage application unit 31 applies the voltage V to the upstream first electrode 36 and the downstream second electrode 37 to the ground potential G () with respect to the adjustment layer 22 formed of the liquid crystal layer. The voltage is 0). At this time, the voltage V is variably applied to the liquid crystal layer, so that the optical path length of light propagating in the liquid crystal material in the adjustment layer 22 changes, thereby changing the effective period of the stacked structure. When the effective period of the laminated structure is variably set, a terahertz wave centered on a frequency at which phase matching can be obtained in the laminated structure is generated and output. 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 optical 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 FIG. 3, a terahertz wave generating structure is formed by a laminated structure of a plurality of crystal layers 21 made of nonlinear optical crystals and at least one adjustment layer 22 interposed therebetween. The generating element 20 serving as a wavelength conversion medium is configured. In the laminated structure, the optical distance adjusting unit 30 adjusts the optical distance variably while the physical distance D between the two crystal layers 21 is fixed, thereby satisfying the phase matching condition in the laminated structure. It is adjusting.

  According to such a configuration, by adjusting the optical distance OD between the crystal layers 21 corresponding to the optical path length in the adjustment layer 22, the effective period of the laminated structure used for generating the terahertz wave T, In addition, the wavelength (frequency) of the terahertz wave T can be variably controlled in the same device without changing the device configuration itself by setting or changing the phase matching condition.

  Here, FIG. 4 is a diagram showing a configuration of a conventional wavelength-converted light generating element. In the configuration shown in FIG. 4A, 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. In addition, the configuration shown in FIG. 4B 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 FIG. 3, the crystal layer 21 along the traveling direction of the excitation light L can be obtained without changing the incident position of the excitation light on the element or rotating the crystal. By adjusting the optical distance variably while the physical distance between them is fixed, it is possible to suitably and easily execute the 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 optical distance adjustment unit 30 has n−1 optical distances (optical path lengths in the n−1 adjustment layers 22) between the two adjacent crystal layers 21 via the adjustment layer 22. It is preferable to variably adjust the optical distance 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 constituted by a liquid crystal layer (liquid crystal material) disposed between two adjacent crystal layers 21 as described above with reference to FIG. preferable. 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, the laminated structure in which the optical distance between the crystal layers 21 is variably adjusted can be suitably realized. In particular, in the configuration in which a liquid crystal material is used for the adjustment layer 22, the optical path length in the adjustment layer 22 can be easily and reliably changed by applying a voltage or the like.

  In addition, regarding the specific configuration of the optical distance adjusting unit 30 for adjusting the optical distance between the crystal layers 21, the optical distance is changed by applying a voltage to the adjusting layer 22 to change its optical characteristics. A configuration in which a voltage application unit 31 that is variably adjusted can be used. In addition, when voltage is applied in this way, the optical distance adjusting unit 30 is an electrode that is provided between the layers and used for applying a voltage by the voltage applying unit 31 in the stacked structure including the crystal layer 21 and the adjusting layer 22. A configuration having 36 and 37 is preferable. 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, a generation control unit that controls the phase matching condition in the generation device 10 by controlling the operation of the optical 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 FIG. 3 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. In addition, as the material of the adjustment layer 22, for example, as described above, it is preferable to use a liquid crystal material capable of changing optical characteristics by applying a voltage. An example of such a liquid crystal material is 5CB. 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 generation element 20 having such a configuration, when the optical distance is set to OD1 with respect to the fixed physical distance D between the crystal layers 21, the frequency f1 at which phase matching is effectively achieved in the stacked structure is the center. The terahertz wave is generated and output with high efficiency. Further, when the optical distance between the crystal layers 21 is changed to OD2, a terahertz wave centered at a frequency f2 different from f1 that can effectively achieve phase matching in the stacked structure is generated and output with high efficiency.

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 crystal 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. If the liquid crystal material of the adjustment layer 22 is 5 CB, the refractive index for pulsed excitation light is 1.72 and the refractive index for terahertz waves is 1.83. In this case, the thickness of the adjustment layer 22 (physical distance between the crystal layers 21) is D = 0.405 mm.

  Next, an AC electric field of 590 V / cm is applied as the voltage V to the liquid crystal material 5CB of the adjustment layer 22 by the voltage application unit 31 and the electrodes 36 and 37. In this case, the refractive index of the adjustment layer 22 is increased by about 0.2, the optical path length for the pulse excitation light when the voltage V is applied is 0.774 mm, and the optical path length for the terahertz wave is 0.824 mm. Thus, by applying a voltage to the adjustment layer 22 to adjust the optical distance, the center frequency of the terahertz wave generated by phase matching is changed from f1 = 1.4 THz to f2 = 1.23 THz. It can be variably controlled.

  In addition, when the thickness of the liquid crystal of the adjustment layer 22 is D = 0.405 mm as described above, the AC electric field of 590 V / cm corresponds to the applied voltage V = 24V. As the material of the adjustment layer 22, for example, A / O crystal, E / O crystal, or the like can be used in addition to the liquid crystal material. Further, a ZnTe (100) crystal can be used as the material of the adjustment layer 22. In a ZnTe (100) crystal, when pulse excitation light is incident perpendicularly to the crystal, no terahertz wave is generated because the nonlinear optical constant is zero. In addition, when a voltage is applied in parallel with the traveling direction of the excitation light, the refractive index changes, whereby the optical path length for the excitation light and the terahertz wave can be variably adjusted.

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

  FIG. 5 is a diagram schematically illustrating a configuration example of the terahertz wave generation device. A terahertz wave generator 10A according to this configuration example generates a terahertz wave including a crystal layer 21 (211 to 214) made of a nonlinear optical crystal and an adjustment layer 22 (221 to 223) made of a liquid crystal material interposed therebetween. The element 20 is included. Further, the end face of the crystal layer 211 which is one surface of the element 20 is an excitation light incident surface 201, and the end face of the crystal layer 214 which is the opposite surface is a terahertz wave output surface 202.

  With respect to the laminated structure having the crystal layers 211 to 214 and the adjustment layers 221 to 223, the crystal layer 211 and the adjustment layer 221, the crystal layer 212 and the adjustment layer 222, and the crystal layer 213 and the adjustment layer. The first electrodes 361, 362, and 363 are provided between the first and second electrodes 223, respectively. In addition, second electrodes 371, 372, and 373 are provided between the adjustment layer 221 and the crystal layer 212, between the adjustment layer 222 and the crystal layer 213, and between the adjustment layer 223 and the crystal layer 214, respectively. ing.

  The first electrodes 361 to 363 and the second electrodes 371 to 373 have openings for passing the excitation light L and the terahertz wave T between the respective layers, as shown in FIG. 5 for the electrodes 361 and 371. Are formed in a ring-shaped electrode pattern. Such an electrode can be formed by vapor deposition on the surface of a nonlinear optical crystal such as a ZnTe crystal constituting the crystal layers 211 to 214, for example.

  In such a configuration, when the voltage application unit 31 applies the voltage V to the first electrodes 361 to 363 and the second electrodes 371 to 373 are set to the ground potential G, the value of the applied voltage V is By being variably adjusted, the optical characteristics of the liquid crystal material of the adjustment layers 221 to 223 are changed, and the optical distance between the crystal layers is variably adjusted. Further, in the configuration shown in FIG. 5, the voltage values applied to the respective adjustment layers are equal, so that the optical distances (optical path lengths) of the adjustment layers 221 to 223 are held in substantially the same state. .

  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 stacked structure of the crystal layer and the adjustment layer, the number of layers, the constituent material of each layer, the configuration of the optical distance adjustment unit, and the like, 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 ... Terahertz wave generator, 20 ... Terahertz wave generation element, 21 ... Nonlinear optical crystal layer, 22 ... Adjustment layer, 201 ... Excitation light incident surface, 202 ... Terahertz wave output surface,
DESCRIPTION OF SYMBOLS 30 ... Optical distance adjustment part, 31 ... Voltage application part, 36 ... 1st electrode, 37 ... 2nd electrode, 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 wavelength-converted light by incidence of pulsed excitation light, and are arranged in order 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,
An optical distance adjusting means for variably adjusting the optical distance of the plurality of crystal layers in a state where a physical distance between two adjacent crystal layers via the adjustment layer is fixed. A wavelength-converted light generator.   2. The wavelength-converted light generator according to claim 1, wherein the adjustment layer includes a liquid crystal layer disposed between two adjacent crystal layers.   3. The wavelength-converted light generator according to claim 1, wherein the adjustment layer is made of a material that does not substantially have a nonlinear optical effect.   The said optical distance adjustment means has a voltage application means which adjusts the said optical distance variably by applying a voltage with respect to the said adjustment layer, and changing the optical characteristic. The wavelength-converted light generator according to any one of the above.   5. The optical distance adjusting means includes an electrode that is provided between the layers and used for applying a voltage by the voltage applying means in a laminated structure including the crystal layer and the adjusting layer. Wavelength conversion light generator. N layers (n is an integer of 3 or more) are provided as the plurality of crystal layers,
The optical distance adjusting means variably adjusts the optical distance in a state where n-1 optical distances between two crystal layers adjacent to each other through the adjustment layer are substantially equal to each other. The wavelength conversion light generator as described in any one of Claims 1-5. The wavelength-converted light generator according to any one of claims 1 to 6,
A wavelength-converted light generation system comprising: excitation light supply means for supplying pulsed excitation light used for generating wavelength-converted light to the wavelength-converted light generator.   8. The wavelength-converted light generation system according to claim 7, further comprising phase matching control means for controlling the phase matching condition in the wavelength-converted light generator by controlling the operation of the optical distance adjusting means.

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