JP4452045B2 - Terahertz wave generator - Google Patents

Description

  The present invention relates to a terahertz wave generating apparatus that generates a terahertz wave that is an electromagnetic wave around a frequency of 1 THz (terahertz).

  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. Yes, application to various fields is being studied.

  In the generation of terahertz waves using short pulse light as excitation light, a method that utilizes 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.

This optical rectification effect corresponds to a case where the frequency difference of incident light is made as close as possible by difference frequency mixing which is a secondary passive nonlinear optical effect. Utilizing such an effect, a terahertz wave generating device can be realized by causing the excitation light to enter the crystal and generating a terahertz wave as described above. Moreover, ZnTe, GaP, DAST, etc. are known as non-linear optical crystals that cause such an optical rectifying effect. In recent years, N-benzyl-MNA crystals (BNA crystals) have attracted attention as organic nonlinear optical crystals capable of generating high-intensity terahertz waves (see Non-Patent Document 1).
Hideki Hashimoto et al., “Development of organic nonlinear optical crystals using p-nitroaniline-type molecules”, Solid Physics Vol.35 No.6, p.417 (2000)

  The electric field intensity of the terahertz wave generated by the optical rectification effect is proportional to the light intensity of the excitation light incident on the crystal. Therefore, if the incident light intensity is increased, a terahertz wave can be obtained with high efficiency. However, if the incident light intensity is increased by a method such as condensing excitation light, the crystal used for generating the terahertz wave is damaged. There is.

  In order to obtain a high-intensity terahertz wave without damaging the crystal, it is conceivable to use a crystal having a large area of the incident surface and make the excitation light incident without condensing it. On the other hand, since there is a phase matching length with respect to the thickness of the crystal, the intensity of the terahertz wave obtained cannot be increased efficiently even if the thickness of the crystal is a certain level or more. Therefore, in order to obtain a high-intensity terahertz wave by utilizing the optical rectification effect in the crystal, it is necessary to use a thin nonlinear optical crystal having a large area.

  Here, when a large crystal is manufactured in order to make the incident surface of the excitation light large in area, the thickness of the crystal with respect to the traveling direction of the light increases, and it is difficult to form a thin crystal. For this reason, in order to obtain a thin crystal with a large area, it is indispensable to process the crystal by a method such as polishing and grinding. However, since organic crystals are generally very soft, it is difficult to process them by methods such as polishing and grinding, and there is a problem that thin crystals cannot be formed. Further, when performing such processing, it is necessary to obtain the relationship between the crystal growth surface and the crystal axis and to cut out at an appropriate angle.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a terahertz wave generator capable of efficiently obtaining a high-intensity terahertz wave using a thin crystal with a large area. And

In order to achieve such an object, a terahertz wave generator according to the present invention includes (1) excitation light supply means for supplying excitation light used for generating terahertz waves, and (2) N-benzyl-MNA crystal ( Terahertz wave generating means for generating terahertz waves by making excitation light incident on the cleavage plane of the N-benzyl-MNA crystal as an incident surface , and a BNA crystal , N-benzyl-2-methyl-4-nitroaniline crystal ) It is characterized by providing.

  As a BNA crystal that is an organic nonlinear optical crystal, an as-grown crystal (the state immediately after growth) has been used so far because of the above-described difficulty in processing. On the other hand, as a result of examining the characteristics of the BNA crystal, the present inventor has found that the BNA crystal, which is a hexagonal columnar crystal, can be cleaved with respect to a specific plane orientation of the crystal cross section, and has reached the present invention. .

  That is, in the above-described terahertz wave generator, terahertz waves are generated with the cleavage plane of the BNA crystal as the incident surface of the excitation light. Thus, by cleaving and using the BNA crystal, it is possible to obtain a crystal having a large incident light incident surface and a thin thickness in the light traveling direction, and efficiently obtaining a high-intensity terahertz wave. It becomes possible. Further, such a BNA crystal has excellent characteristics in terms of conversion efficiency from excitation light to terahertz waves.

  Here, the excitation light supply means is preferably a pulse laser light source that supplies femtosecond pulse laser light as excitation light. As described above, by using the excitation light as pulse light, particularly femtosecond pulse light, it is possible to generate a pulsed terahertz wave having sufficient peak intensity.

  Further, the excitation light supply means may supply laser light having two or more wavelengths as excitation light. By supplying such excitation light, a terahertz wave can be suitably generated using the optical rectification effect.

  The terahertz wave generation device may include a polarization control unit that makes excitation light incident between the excitation light supply unit and the terahertz wave generation unit and controls the polarization to be a predetermined polarization. good. Thereby, the intensity and spectrum of the obtained terahertz wave can be easily controlled via the polarization state of the excitation light. In particular, by controlling the polarization of the excitation light with the polarization control means installed in the previous stage of the crystal, adjustment of the optical system accompanying the control of the terahertz wave becomes unnecessary.

  In this case, the polarization control means preferably includes at least one of a half-wave plate, a Soleil-Babinet phase compensation plate, and a Pockels cell. By using such a polarization control element, it is possible to suitably control the polarization state of the excitation light.

  Further, the generator includes a terahertz wave detecting unit that detects a part of the terahertz wave generated by the terahertz wave generating unit, and sets or changes the generation condition of the terahertz wave based on the detection result by the terahertz wave detecting unit. It is good also as a structure. By adopting such a feedback control configuration, the terahertz wave generation conditions can be reliably controlled as necessary.

  Alternatively, the generator sets the generation condition of the terahertz wave based on the measurement result of the reaction in the sample by the reaction measurement unit installed on the sample irradiated with the terahertz wave generated by the terahertz wave generation unit or It is also possible to adopt a configuration that changes. Even with such a configuration, the generation condition of the terahertz wave can be reliably controlled as necessary.

  According to the terahertz wave generator according to the present invention, a cleaved surface of an N-benzyl-MNA crystal is used as an incident surface for excitation light, whereby a high-intensity terahertz wave can be efficiently obtained using a thin crystal with a large area. It becomes possible.

  Hereinafter, preferred embodiments of a terahertz wave generator 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.

  FIG. 1 is a block diagram schematically showing a configuration of a first embodiment of a terahertz wave generator according to the present invention. This terahertz wave generating device generates a terahertz wave by utilizing the optical rectification effect in a nonlinear optical crystal, and includes a laser light source 1, a light guide unit 2, and a terahertz wave generation unit 3. Has been.

  The laser light source 1 is excitation light supply means for supplying excitation light used for generating terahertz waves. The laser light source 1 is preferably a femtosecond pulse laser light source that outputs femtosecond (fs) pulse laser light. Further, the pulse excitation light has a spectrum spread to some extent.

  The terahertz wave generating unit 3 is configured to have an N-benzyl-MNA crystal (hereinafter referred to as a BNA crystal) 30 as a nonlinear optical crystal with respect to the pulsed excitation light supplied from the laser light source 1. When pulse excitation light from the laser light source 1 enters the BNA crystal 30, a terahertz wave is generated in the crystal 30 due to the optical rectification effect. A predetermined surface of the BNA crystal 30 is a cleavage plane, and the cleavage plane is disposed so as to be an incident surface for excitation light. Thereby, a high-intensity terahertz wave is efficiently generated in the crystal 30. The crystal characteristics such as the cleavage plane of the BNA crystal 30 will be described later in detail. The terahertz wave emitted from the crystal 30 is, for example, an electromagnetic wave in the middle to far infrared region having a wavelength of 30 mm to 3 μm (frequency 0.01 THz to 100 THz).

  In the embodiment shown in FIG. 1, the polarization controller 20 is installed in the light guide 2 that guides the excitation light from the laser light source 1 to the BNA crystal 30 of the terahertz wave generator 3. The polarization controller 20 receives the excitation light supplied from the laser light source 1, controls the polarized light to be a predetermined polarization, and emits it to the terahertz wave generator 3. On the other hand, the intensity and spectrum of the terahertz wave generated in the BNA crystal 30 depend on the polarization state of the incident excitation light. Therefore, by controlling the polarization state of the excitation light by the polarization controller 20, the terahertz wave generated by the terahertz wave generator 3 is controlled to a desired intensity and spectrum.

  The effect of the terahertz wave generator according to the present embodiment will be described.

  In the terahertz wave generator shown in FIG. 1, a BNA crystal 30 is used as a nonlinear optical crystal for generating a terahertz wave, and a cleavage plane of the BNA crystal 30 is used as an incident surface for pulse excitation light from the laser light source 1. . A BNA crystal is an organic nonlinear optical crystal capable of generating a high-intensity terahertz wave. Furthermore, by cleaving and using the BNA crystal, it is possible to obtain a crystal having a large incident light incident surface and a small thickness with respect to the light traveling direction. Therefore, it is possible to efficiently obtain a high-intensity terahertz wave. Further, such a BNA crystal has excellent characteristics in terms of conversion efficiency from excitation light to terahertz waves.

  2A is a perspective view of a bulk single crystal and FIG. 2B is a cross-sectional view showing the crystal structure of the BNA crystal 30 used in the terahertz wave generator shown in FIG. The BNA crystal (N-benzyl-MNA crystal) has a hexagonal columnar crystal shape of ABCDEF-A'B'C'D'E'F 'as shown in FIG. 2A in the as-grown state. FIG. 2B is a cross-sectional view of the hexagonal columnar BNA crystal, and the angles are (∠C = ∠F)> (∠A = ∠D)> (∠B = ∠E). ing.

  The inventor of the present application has found for the first time that a BNA crystal having such a shape can be cleaved with respect to a specific plane orientation. By processing the BNA bulk crystal using cleavage, a thin BNA crystal having a large area can be produced. Then, by applying such a BNA crystal to a nonlinear optical crystal that generates a terahertz wave with a cleavage plane as an incident surface of excitation light, a terahertz wave generator capable of generating a high-intensity terahertz wave is realized. Specifically, the cleavage plane in the BNA crystal is expressed by ABBB′-A ′ (or EDDD′-E) in the hexagonal columnar crystal structure shown in FIGS. It is parallel to '). The plane determined by AB-B'-A 'of the crystal cut out by cleavage is a rectangle.

  Here, as the laser light source 1 for supplying the pulse excitation light to the BNA crystal 30 of the terahertz wave generation unit 3, by applying a pulse laser light source, preferably a femtosecond pulse laser light source as described above, sufficient peak intensity can be obtained. It is possible to generate a pulsed terahertz wave having Thereby, when the terahertz wave emitted from the terahertz wave generation device is applied to living body measurement or the like, it is possible to perform measurement with a favorable S / N ratio.

  Moreover, it is preferable that the excitation light supplied from the excitation light supply means such as the laser light source 1 has a spectrum that spreads over a certain frequency range as described above. Alternatively, laser light having two or more wavelengths may be supplied as excitation light. According to such excitation light, a terahertz wave can be suitably generated using the optical rectification effect.

  Further, in the terahertz wave generation device of the present embodiment, the light guide unit 2 is provided with the polarization control unit 20, and excitation light whose polarization state is controlled by the polarization control unit 20 is incident on the BNA crystal 30. This makes it possible to control the intensity and spectrum of the terahertz wave obtained from the BNA crystal 30 using the relationship between the polarization state of the excitation light and the crystal axis of the crystal 30.

  In particular, in the above configuration, the polarization of the excitation light is controlled by the polarization controller 20 installed in the previous stage of the crystal 30, so that adjustment of the optical system associated with the control of the terahertz wave becomes unnecessary. That is, with such a configuration, the incident position of the excitation light on the crystal 30 and the emission position of the terahertz wave do not change. Further, since the direction of the crystal axis of the crystal 30 does not change when changing the intensity or spectrum of the terahertz wave, a change in the polarization direction of the obtained terahertz wave is prevented.

  Specifically, the polarization controller 20 preferably includes at least one of a half-wave plate, a Soleil-Babinet phase compensator, and a Pockels cell. By using such a polarization control element, it is possible to suitably control the polarization state of the excitation light. Further, other optical elements may be used, or a plurality of optical elements may be used in combination. The polarization control unit 20 is configured not to install the polarization control unit 20 when it is not necessary to control the spectrum of the terahertz wave or when spectrum control is performed by a method other than the control of the polarization state of the excitation light. good.

  A specific configuration example of the terahertz wave generation device illustrated in FIG. 1 will be described. FIG. 3 is a diagram illustrating a configuration of an example of the terahertz wave generation device. In this configuration example, a pulsed laser light source that outputs pulsed light having a center wavelength of 815 nm, a full width at half maximum of the wavelength spectrum of 35 nm, and a pulse time width of 40 fs is used as the laser light source 1 that supplies the excitation light. For this pulsed excitation light, the polarization controller 20 is configured using a half-wave plate 21. Further, as described above, as the nonlinear optical crystal of the terahertz wave generation unit 3, the BNA crystal 30 having the cleavage plane AB-B'-A 'as the excitation light incident surface 30a is used. The BNA crystal 30 used has a crystal size of 5 mm × 4 mm and a thickness of 780 μm.

  In such a configuration, the pulse excitation light from the laser light source 1 is incident on the half-wave plate 21 to rotate its polarization, and the intensity and spectrum of the terahertz wave generated in the BNA crystal 30 are controlled. Specifically, pulsed excitation light from the laser light source 1 is incident perpendicularly to the cleavage plane 30 a of the BNA crystal 30. Then, the half-wave plate 21 in the front stage of the crystal 30 rotates the polarization of the excitation light in the ABBB′-A ′ plane, and the intensity spectrum of the terahertz wave obtained by the BNA crystal 30 and the excitation The change of the spectrum with the polarization state of light was measured.

  FIG. 4 is a graph showing changes in the frequency spectrum of the terahertz wave generated in the BNA crystal 30. This graph shows the electric field intensity spectrum of a terahertz wave obtained by performing fast Fourier transform (FFT) on a time waveform. The horizontal axis represents frequency (THz), and the vertical axis represents the electric field of terahertz wave. Intensity (arb. Units) is shown.

  In this graph, the polarization angle θ (see FIG. 3) of the pulse excitation light incident on the BNA crystal 30 is changed in the range of −100 degrees to 100 degrees at intervals of 10 degrees, and the frequency spectrum of the terahertz wave obtained for each is obtained. Is shown. However, in order to make it easy to see the difference in spectrum, the zero points of the FFT intensity are sequentially shifted in the graph corresponding to each polarization angle. As for the polarization angle θ, θ = 0 degrees indicates the polarization in the AB direction, and θ = 90 degrees indicates the polarization in the A-A ′ direction (see FIGS. 2A and 3).

  Referring to the graph of FIG. 4, it can be seen that the terahertz wave is efficiently generated by using the BNA crystal 30 having the cleavage plane as the incident surface 30a of the excitation light. Further, the generation condition of the terahertz wave changes corresponding to the change in the polarization angle θ of the excitation light, and the intensity and spectrum of the obtained terahertz wave are controlled. Specifically, the radiation intensity of the generated terahertz wave is large when excitation light having polarization in the AB direction is incident. Moreover, it turns out that a high intensity | strength terahertz wave can be radiated | emitted in the range whose polarization | polarized-light is about +/- 30 degree from AB direction by the dependence with respect to the polarization of excitation light.

  FIG. 5 is a block diagram showing a configuration of the second embodiment of the terahertz wave generation device. This terahertz wave generator includes a laser light source 1, a light guide 2 provided with a polarization controller 20, a terahertz wave generator 3 having a BNA crystal 30, and a terahertz wave detector 4. Yes. Among these, the configurations of the laser light source 1 and the terahertz wave generator 3 are the same as those in the embodiment shown in FIG.

  In the present embodiment, a terahertz wave detection unit 4 is further provided for the terahertz wave generation unit 3. The terahertz wave detection unit 4 detects a part of the terahertz wave generated by the BNA crystal 30 of the terahertz wave generation unit 3. The terahertz wave is detected by, for example, installing an optical branching unit for the terahertz wave in the middle to far-infrared region emitted from the crystal 30 in the terahertz wave generation unit 3, and the branched terahertz wave is detected by the terahertz wave detection unit. This can be done by the configuration detected at 4. As such a terahertz wave detection unit 4, for example, an electro-optic crystal or an optical switch can be used.

  The detection result of the terahertz wave by the terahertz wave detection unit 4 is fed back to the laser light source 1. Thereby, in the laser light source 1, the intensity of the pulse excitation light supplied to the BNA crystal 30 is set or changed based on the input detection result of the terahertz wave detection unit 4.

  In the present embodiment, the detection result by the terahertz wave detection unit 4 is also fed back to the polarization control unit 20 installed in the light guide unit 2. On the other hand, the polarization control unit 20 includes a polarization control element 22 made of a half-wave plate or the like, and an element drive unit 23 that drives and controls the polarization control element 22. The element driving unit 23 drives and controls the polarization control element 22 based on the input detection result in the terahertz wave detection unit 4, and thereby the polarization control condition of the excitation light in the polarization control unit 20 is set or changed. The

  In the configuration of the present embodiment, the terahertz wave detection unit 4 is installed, and the laser light source 1 and the polarization control unit 20 are feedback-controlled based on the detection result of the detection unit 4, thereby setting or changing the generation condition of the terahertz wave is doing. Thereby, the generation conditions of the terahertz wave and the intensity and spectrum of the terahertz wave generated in the BNA crystal 30 can be reliably controlled as necessary.

  FIG. 6 is a block diagram illustrating a configuration of the third embodiment of the terahertz wave generation device. The terahertz wave generator includes a laser light source 1, a light guide 2 provided with a polarization controller 20, and a terahertz wave generator 3 having a BNA crystal 30. Among these, the configurations of the laser light source 1 and the terahertz wave generator 3 are the same as those in the embodiment shown in FIG.

  In the present embodiment, the reaction measurement unit 5 is installed on the sample S irradiated with the terahertz wave generated by the terahertz wave generation unit 3 and emitted from the generation device. The reaction measuring unit 5 is a measuring unit that measures a reaction in the sample S caused by irradiating the terahertz wave emitted from the terahertz wave generating device. As a measuring method of the reaction in the sample S by the reaction measuring unit 5, for example, there is a method of measuring the temperature of the sample S, the transmittance of the terahertz wave in the sample S, the reflectance, the amount of the product in the sample S, and the like. .

  The reaction measurement result by the reaction measurement unit 5 is fed back to the laser light source 1. Thereby, in the laser light source 1, the intensity | strength etc. of the pulse excitation light supplied with respect to the BNA crystal | crystallization 30 are set or changed based on the measurement result in the input reaction measurement part 5. FIG.

  In the present embodiment, the measurement result obtained by the reaction measurement unit 5 is also fed back to the polarization control unit 20 installed in the light guide unit 2. On the other hand, the polarization control unit 20 includes a polarization control element 22 and an element driving unit 23. The element drive unit 23 drives and controls the polarization control element 22 based on the input measurement result of the reaction measurement unit 5, and thereby, the control condition of the polarization of the excitation light in the polarization control unit 20 is set or changed. .

  In the configuration of the present embodiment, the terahertz wave generation condition is set or changed by performing feedback control of the laser light source 1 and the polarization control unit 20 based on the measurement result in the reaction measurement unit 5. Thereby, the generation conditions of the terahertz wave and the intensity and spectrum of the terahertz wave generated in the BNA crystal 30 can be reliably controlled as necessary.

  The terahertz wave generator according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications can be made. For example, in the configuration example described above, the excitation light from the laser light source 1 is incident perpendicular to the cleavage plane of the BNA crystal 30. The incident angle of this excitation light is also approximately the same as the rotation angle of the polarization. Tolerable. For example, referring to the graph of FIG. 4, it can be seen that a high-intensity terahertz wave can be emitted by entering the excitation light at an incident angle in a range of about ± 30 degrees from the vertical.

  The apparatus according to the present invention can be used as a terahertz wave generator capable of efficiently obtaining a high-intensity terahertz wave using a thin crystal with a large area.

It is a block diagram which shows the structure of 1st Embodiment of a terahertz wave generator. It is (a) perspective view and (b) sectional drawing which show about the crystal structure of a BNA crystal. It is a block diagram which shows one Example of a terahertz wave generator. It is a graph which shows about the change of the frequency spectrum of the terahertz wave which generate | occur | produces in a BNA crystal. It is a block diagram which shows the structure of 2nd Embodiment of a terahertz wave generator. It is a block diagram which shows the structure of 3rd Embodiment of a terahertz wave generator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2 ... Light guide part, 20 ... Polarization control part, 21 ... 1/2 wavelength plate, 22 ... Polarization control element, 23 ... Element drive part, 3 ... Terahertz wave generation part, 30 ... BNA crystal (N -Benzyl-MNA crystal), 30a ... incidence plane, 4 ... terahertz wave detection unit, 5 ... reaction measurement unit.

Claims (7)

Excitation light supply means for supplying excitation light used for generation of terahertz waves;
It has an N-benzyl-MNA crystal (N-benzyl-2-methyl-4-nitroaniline crystal) and generates terahertz waves by making the excitation light incident on the cleavage plane of the N-benzyl-MNA crystal as an incident surface. And a terahertz wave generating means.   2. The terahertz wave generator according to claim 1, wherein the excitation light supply means is a pulse laser light source that supplies femtosecond pulse laser light as the excitation light.   The terahertz wave generation device according to claim 1, wherein the excitation light supply unit supplies laser light having two or more wavelengths as the excitation light.   2. A polarization control unit for controlling the polarization of the excitation light to be a predetermined polarization by entering the excitation light between the excitation light supply unit and the terahertz wave generation unit. The terahertz wave generator as described in any one of -3.   5. The terahertz wave generating apparatus according to claim 4, wherein the polarization control means includes at least one of a half-wave plate, a Soleil-Babinet phase compensator, and a Pockels cell.   Terahertz wave detection means for detecting a part of the terahertz wave generated by the terahertz wave generation means, and setting or changing the generation condition of the terahertz wave based on a detection result by the terahertz wave detection means. The terahertz wave generation device according to claim 1, wherein the terahertz wave generation device is characterized.   Setting or changing the generation condition of the terahertz wave based on the measurement result of the reaction in the sample by the reaction measurement unit installed on the sample irradiated with the terahertz wave generated by the terahertz wave generation unit The terahertz wave generator according to claim 1, wherein the terahertz wave generator is provided.

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