Terahertz wave observation device

Abstract

This technology provides a way to directly observe terahertz waves generated inside a nonlinear crystal in a terahertz wave parametric generator. [Solution] The terahertz wave observation device 101 comprises a terahertz wave generating nonlinear crystal 1 that generates terahertz waves 3 from excitation light 2 by optical parametric generation, an excitation light source 21 that incidents excitation light 2 onto the terahertz wave generating nonlinear crystal 1, a probe light source 22 that incidents probe light 5 onto the terahertz wave generating nonlinear crystal 1, a first prism coupler 31 for causing the probe light 5 to be incident inside the terahertz wave generating nonlinear crystal 1, and an imaging device 41. The imaging device 41 is characterized by imaging the detected Stokes light 6 generated by the interaction between the probe light 5 and the terahertz waves 3 generated by the terahertz wave generating nonlinear crystal 1.

Description

[Technical Field]

[0001] This disclosure relates to a terahertz wave observation device. [Background technology]

[0002] Terahertz waves are electromagnetic waves with frequencies ranging from approximately 0.3 THz to 10 THz and wavelengths ranging from approximately 1 mm to 30 μm, i.e., they exist in the millimeter wave to far-infrared region. These terahertz waves possess excellent properties such as directivity, material penetration, non-invasiveness to living organisms, safety for the human body, and unique absorption spectra for reagents, making them promising for applications in a wide range of fields.

[0003] As a high-power terahertz wave light source, a light-injection type terahertz wave parametric generation light source has been proposed (see, for example, Patent Document 1).

[0004] Furthermore, methods for numerically simulating parametric wavelength conversion have been proposed (see, for example, Non-Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2002-72269 [Non-patent literature]

[0006] [Non-Patent Document 1] K. Weipeng et al., JOSA B , 8 ,2479.84 (2020) [Non-Patent Document 2] Shinichiro Hayashi, Koji Nawata, Akimichi Kawase, and Yasuaki Minamide, "High-Power, Wavelength-Tunable Terahertz Parametric Light Source," Laser Research 40 (7), 486-, 2012. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In terahertz wave parametric generation, terahertz waves are generated by stimulated polariton scattering utilizing second- and third-order nonlinear optical effects. Therefore, a complex wavelength conversion process occurs within the crystal, making direct observation of its generation difficult. While the technology disclosed in Non-Patent Document 1 numerically simulates terahertz parametric wavelength conversion, direct observation has not yet been achieved.

[0008] This disclosure is made in view of these circumstances, and its purpose is to provide a technique for directly observing terahertz waves generated inside a nonlinear crystal of a terahertz wave parametric generator. [Means for solving the problem]

[0009] To solve the above problems, a terahertz wave observation apparatus according to one aspect of the present invention comprises a nonlinear crystal for generating terahertz waves from excitation light by optical parametric generation, an excitation light source for injecting excitation light into the nonlinear crystal for generating terahertz waves, a probe light source for injecting probe light into the nonlinear crystal for generating terahertz waves, a prism coupler for injecting probe light into the interior of the nonlinear crystal for generating terahertz waves, and an imaging apparatus. The imaging apparatus captures detected Stokes light generated by the interaction between the probe light and the terahertz waves generated in the nonlinear crystal for generating terahertz waves. [Effects of the Invention]

[0010] According to this disclosure, terahertz waves generated inside a nonlinear crystal of a terahertz wave parametric generator can be directly observed. [Brief explanation of the drawing]

[0011] [Figure 1] This is the dispersion curve of polaritons when LiNbO3 is used as a nonlinear crystal. [Figure 2] This is a schematic diagram illustrating the principle of parametric generation of terahertz waves. [Figure 3]It is a schematic diagram showing the principle of optical injection type terahertz wave parametric generation. [Figure 4] It is a functional block diagram of a terahertz wave observation apparatus according to the first embodiment. [Figure 5] It is a schematic diagram of a terahertz wave observation apparatus in which the polarization direction of the excitation light and the polarization direction of the probe light are parallel. [Figure 6] It is a schematic diagram of a terahertz wave observation apparatus in which the polarization direction of the excitation light and the polarization direction of the probe light are orthogonal. [Figure 7] It is a functional block diagram of a terahertz wave observation apparatus according to the second embodiment. [Figure 8] It is a photograph of the detected Stokes light when the probe light is swept along the excitation light, taken with the terahertz wave observation apparatus of the second embodiment.

Embodiments for Carrying Out the Invention

[0012] Before explaining specific embodiments, the underlying knowledge will be explained. First, terahertz wave parametric generation will be explained (see, for example, Non-Patent Document 2).

[0013] When an electromagnetic wave having a strong electromagnetic field such as a pulsed laser passes through a non-linear optical crystal having a transverse optical phonon mode, photons and phonons combine to form an elementary excitation state called a polariton. Fig. 1 shows the dispersion curve of polaritons when LiNbO3 is used as the non-linear crystal. Polaritons behave phonon-like near the resonance frequency ω TO but behave photon-like in the low-frequency region sufficiently far from the resonance frequency. Therefore, broadband terahertz wave generation can be performed by induced Raman scattering (induced polariton scattering) via polaritons. Induced polariton scattering is observed in polar crystals such as LiNbO3, LiTaO3, and GaP. In particular, LiNbO3 has no strong absorption in the broadband (0.4 to 5.5 μm) in the visible to infrared region and has high resistance to optical damage (damage threshold > several 100 MW / cm 2) It is useful for high-power and broadband terahertz wave generation because it has excellent characteristics such as high gain in stimulated scattering and low loss of terahertz waves in the crystal compared to other materials.

[0014] Figure 2 is a schematic diagram showing the principle of terahertz wave parametric generation. 1 is a nonlinear optical crystal (e.g., LiNbO3), 2 is the pump light (e.g., YAG laser light), 3 is the terahertz wave, and 4 is the Stokes light. The pump light, terahertz wave, and Stokes light may also be called pump light, signal light, and idler light, respectively.

[0015] As described above, the nonlinear optical crystal 1 is a crystal in which the passing electromagnetic wave has a transverse optical phonon mode. When the pump light 2 (usually near-infrared light or visible light) is incident in a certain direction, the terahertz wave 3 and the Stokes light 4 are generated through the elementary excitation wave (polariton) of the substance by the induced Raman effect (or parametric interaction). Let the frequency of the pump light 2 be ω P , and the wave vector be k P . Let the frequency of the terahertz wave 3 be ω T , and the wave vector be k T . Let the frequency of the Stokes light 4 be ω S , and the wave vector be k S . At this time, between the pump light 2, the terahertz wave 3, and the Stokes light 4, the energy conservation law shown in Equation (1) and the momentum conservation law (non-collinear phase matching condition) shown in Equation (2) hold. The non-collinear phase matching condition is represented by the vector diagram in the upper right of Figure 2.

[0016] ω P = ω T + ω S ···(1) k P = k T + k S ···(2)

[0017] The terahertz waves 3 and Stokes light 4 generated at this time have a spatial extent. Their wavelengths change continuously depending on the emission angle. Figure 2 illustrates terahertz waves 3 at 1.0 THz, 1.5 THz, and 2.0 THz, and their corresponding Stokes light 4. The generation mechanism of terahertz waves and Stokes light in this single-pass configuration is called terahertz-wave paramatric generation (THz-wave Paramatric Generation) or optical paramatric generation.

[0018] A basic optical parametric process is defined by the annihilation of one excited photon and the simultaneous creation of one terahertz photon and one Stokes photon. Parametric oscillation occurs when the excitation light intensity exceeds a certain threshold. Stimulated Raman scattering is the annihilation of one excited photon and the simultaneous creation of one Stokes photon and one polariton. These are included in parametric interactions in a broad sense. Thus, terahertz wave parametric generation converts near-infrared or visible light to terahertz waves.

[0019] However, the broadband terahertz waves generated by the single-pass terahertz wave generator shown in Figure 2 are weak, and most of them are absorbed while traveling several hundred micrometers through the nonlinear optical crystal. These weak, broadband terahertz waves generated by this parametric terahertz wave generation mechanism are also called parametric fluorescence.

[0020] Figure 3 is a schematic diagram of a light-injection type terahertz wave parametric generator that solves this problem. In this device, in addition to excitation light 2, laser light (injection light 7) of the same frequency as the Stokes light 4, which is paired with the target terahertz wave 3 (1.5 THz in the example of Figure 3), is incident onto the nonlinear optical crystal 1 in the same direction as the generation direction of the Stokes light 4.

[0021] As shown in Figure 3, the inventors have for the first time in the world confirmed that in parametric oscillation under non-collinear phase matching conditions in a nonlinear optical crystal, by exciting with a single-frequency laser beam (excitation beam) and injecting with a laser beam (injection beam) of the same direction and frequency as the Stokes beam, the spectral width of the generated terahertz wave can be narrowed to approximately the sum of the spectral widths of the excitation beam and the injection beam, and the terahertz wave output is significantly increased.

[0022] The generation mechanism of terahertz waves and Stokes light in the system shown in Figure 3 is called injection-seeded THz-wave parametric generator (is-TPG). Because it possesses high peak power (~100kW), wide wavelength tunability (0.4~5THz), and Fourier-limited linewidth, the injection-seeded THz-wave parametric generator is extremely useful as a powerful light source for monochromatic terahertz waves.

[0023] In such terahertz wave parametric generation (or light-injection type terahertz wave parametric generation), being able to observe the terahertz wave intensity inside the nonlinear crystal of the light source using visible light is expected to be a useful means for optimizing parameters such as excitation light and crystal, and for performing system checks toward practical application.

[0024] [First Embodiment] Figure 4 is a functional block diagram of the terahertz wave observation device 101 according to the first embodiment. The terahertz wave observation device 101 comprises a nonlinear crystal 1 for terahertz wave generation, an excitation light source 21, a probe light source 22, a first prism coupler 31, an imaging device 41, a nonlinear crystal 11 for detecting Stokes light amplification, and a second prism coupler 32.

[0025] The nonlinear crystal 1 for terahertz wave generation generates terahertz waves 3 and Stokes rays 4 from excitation light 2 through optical parametric generation. The nonlinear crystal 1 for terahertz wave generation is, for example, a polar crystal such as LiNbO3, LiTaO3, or GaP. Particularly preferred is LiNbO3 for terahertz wave generation (LiNbO3 may also be doped with MgO). The mechanism of optical parametric generation by the nonlinear crystal 1 for terahertz wave generation is as described above.

[0026] The excitation light source 21 incidents excitation light 2 onto the nonlinear crystal 1 for terahertz wave generation. The excitation light 2 has, for example, a light intensity of 100 MW / cm². 2 While a nanosecond pulse laser of a certain magnitude or greater (e.g., a YAG laser) is preferred, it is not limited to this; any suitable light source capable of achieving optical parametric generation will suffice.

[0027] The probe light source 22 incidents probe light 5 onto the nonlinear crystal 1 for terahertz wave generation. The probe light 5 has an intensity and frequency such that it interacts with the terahertz waves 3 generated in the nonlinear crystal 1 for terahertz wave generation to generate detected Stokes light 6. The probe light source 22 incidents the probe light 5 onto the nonlinear crystal 1 for terahertz wave generation at a timing and direction that allows the probe light 5 to interact with the terahertz waves 3.

[0028] A first prism coupler 31 is provided on the surface of the nonlinear crystal 1 for terahertz wave generation to which the probe light 5 is incident. The first prism coupler 31 causes the probe light 5 to enter the interior of the nonlinear crystal 1 for terahertz wave generation without total internal reflection at the interface between free space and the nonlinear crystal 1 for terahertz wave generation. For example, the first prism coupler 31 is a glass prism. However, it is not limited to this, and the material, shape, and size of the first prism coupler 31 are arbitrary as long as the probe light can enter the interior of the nonlinear crystal 1 for terahertz wave generation. Note that if total internal reflection does not occur at the interface, the first prism coupler 31 is unnecessary.

[0029] The imaging device 41 captures the detected Stokes light 6 generated by the interaction between the probe light 5 and the terahertz wave 3 generated by the nonlinear crystal 1 for terahertz wave generation. The imaging device 41 is, for example, a CMOS camera, but is not limited to this and may be any suitable imaging device such as a CCD camera.

[0030] The detected Stokes light 6 emitted from the terahertz wave generating nonlinear crystal 1 may, for example, propagate through a free space or cavity optical waveguide to reach the imaging device 41. However, in that case, the light reaching the imaging device 41 is weak. This is because the interaction length between the probe light 5 and the terahertz wave 3 is approximately the same as the beam diameter of the excitation light 2. In such a case, the imaging device 41 needs to be an extremely sensitive camera.

[0031] To solve this problem, the terahertz wave observation device 101 in Figure 4 includes a nonlinear crystal 11 for amplifying detected Stokes light between the nonlinear crystal 1 for terahertz wave generation and the imaging device 41. The nonlinear crystal 11 for amplifying detected Stokes light is a nonlinear crystal similar to the nonlinear crystal 1 for terahertz wave generation. Preferably, the nonlinear crystal 11 for amplifying detected Stokes light is LiNbO3.

[0032] The nonlinear crystal 11 for amplifying the detected Stokes light parametrically amplifies the detected Stokes light 6. This ensures that the detected Stokes light 6 has sufficient intensity when it reaches the imaging device 41. In this case, the imaging device 41 can be an easily available device such as a CMOS camera.

[0033] The terahertz wave observation device 101 in Figure 4 is equipped with a second prism coupler 32 between the terahertz wave generation nonlinear crystal 1 and the detection Stokes light amplification nonlinear crystal 11. The second prism coupler 32 causes the detection Stokes light 6 to enter the detection Stokes light amplification nonlinear crystal 11 without total internal reflection at the interface between the terahertz wave generation nonlinear crystal 1 and the detection Stokes light amplification nonlinear crystal 11. If the relationship between the refractive index of the terahertz wave generation nonlinear crystal 1 and the refractive index of the detection Stokes light amplification nonlinear crystal 11, and the relationship between the critical angle and total internal reflection are appropriate, instead of using the second prism coupler 32, the detection Stokes light amplification nonlinear crystal 11 may be formed in a trapezoidal shape to fill the space corresponding to the second prism coupler 32. If total internal reflection does not occur at the interface, the second prism coupler 32 is unnecessary.

[0034] The desirable incidence angle of the probe light 5 to the nonlinear crystal 1 for terahertz wave generation is described below. As shown in Figure 4, let θ1 be the angle between the incidence direction of the excitation light 2 and the generation direction of the terahertz wave 3. Let θ2 be the angle between the generation direction of the terahertz wave 3 and the probe light 5. In this case, the generation intensity of the detected Stokes light 6 is maximum when θ1 = θ2 is satisfied, so this is the most desirable time. When the nonlinear crystal 11 for amplification of the detected Stokes light is LiNbO3, θ1 is approximately 65 degrees. In reality, even if there is some error between θ1 and θ2, the generation intensity of the detected Stokes light 6 does not decrease significantly. Specifically, it is desirable that the angular error between θ1 and θ2 be around ±10%. That is, it is desirable that 0.9θ1 ≤ θ2 ≤ 1.1θ1. However, even if the angular error exceeds ±10%, although the generation intensity of the detected Stokes light 6 decreases, it does not hinder the purpose of the invention of this technology.

[0035] As described above, according to this embodiment, terahertz waves generated inside the nonlinear crystal of the terahertz wave parametric generator can be directly observed using an imaging device.

[0036] Figure 5 is a schematic diagram of a terahertz wave observation device 103 in which the polarization direction of the excitation light 2 and the polarization direction of the probe light 5 are parallel. In this example, the excitation light 2 is incident on the nonlinear crystal 1 for terahertz wave generation as light polarized in the z-axis direction from the x-plane of the crystal. On the other hand, the probe light 5 is incident on the nonlinear crystal 1 for terahertz wave generation as light polarized in the z-axis direction from the y-plane of the crystal. Thus, both the excitation light 2 and the probe light 5 are polarized in the z-axis direction, and their polarization directions are parallel.

[0037] According to this embodiment, the generated terahertz waves can be observed with a polarization plane perpendicular to the waves themselves.

[0038] Note that the polarization direction of excitation light 2 and the polarization direction of probe light 5 do not need to be strictly parallel; they only need to be close to parallel. In this specification, this is referred to as "approximately parallel," where "approximately" includes a state where they are tilted within ±10% of the strictly parallel state. Depending on the crystal used, circular polarization may also be applied. In this case, particular attention does not need to be paid to polarization.

[0039] Figure 6 is a schematic diagram of a terahertz wave observation device 104 in which the polarization direction of the excitation light 2 and the polarization direction of the probe light 5 are orthogonal. In this example, the excitation light 2 is incident on the nonlinear crystal 1 for terahertz wave generation as light polarized in the z-axis direction from the x-plane of the crystal. On the other hand, the probe light 5 is incident on the nonlinear crystal 1 for terahertz wave generation as light polarized in the x-axis direction from the z-plane of the crystal. Thus, since the excitation light 2 is polarized in the z-axis direction and the probe light 5 is polarized in the x-axis direction, the polarization directions of the two are orthogonal.

[0040] According to this embodiment, the generated terahertz waves can be observed with a polarization plane parallel to the waves themselves.

[0041] Note that the polarization direction of excitation light 2 and the polarization direction of probe light 5 do not need to be strictly orthogonal; they only need to be close to orthogonal. In this specification, this is referred to as "approximately orthogonal," where "approximately" means that it also includes a state where they are tilted within ±10% of the strictly orthogonal state. Similar to the case of parallel polarization, depending on the crystal used, circular polarization or other polarizations can also be applied. In this case as well, there is no need to pay particular attention to polarization.

[0042] [Second Embodiment] Figure 7 is a functional block diagram of the terahertz wave observation device 102 according to the second embodiment. The terahertz wave observation device 102 comprises a nonlinear crystal 1 for terahertz wave generation, an excitation light source 21, a probe light source 22, a first prism coupler 31, an imaging device 41, a nonlinear crystal 11 for detecting Stokes light amplification, a second prism coupler 32, and an injection light source 23. In other words, the terahertz wave observation device 102 is equipped with an injection light source 23 in addition to the configuration of the terahertz wave observation device 101 shown in Figure 5. The other components of the terahertz wave observation device 102 are the same as those of the terahertz wave observation device 101.

[0043] The injection light source 23 injects laser light 7 (injection light) with the same frequency as the Stokes light 4 into the nonlinear crystal 1 for terahertz wave generation, in the same direction as the generation direction of the Stokes light 4 which is paired with the target terahertz wave 3. The laser light 7 causes optical injection type parametric generation of terahertz waves, generating narrow-linewidth monochromatic terahertz waves with high peak power.

[0044] According to this embodiment, terahertz waves generated inside the nonlinear crystal of a light-injection type terahertz wave parametric generator can be directly observed using an imaging device.

[0045] [Verification experiment] The inventors conducted verification experiments to confirm the usefulness of the technology described herein.

[0046] Figure 8 is a photograph of the detected Stokes light taken with the terahertz wave observation device of the second embodiment when the probe light is swept along the excitation light.

[0047] The vertical axis represents the sweep distance of the probe light from the excitation light incident surface. The horizontal axis represents the frequency of the terahertz wave corresponding to the phase matching angle. The vertical line observed around 1.75 THz is the detected Stokes light. It was confirmed that the targeted terahertz wave was generated from approximately 1 cm to 4 cm from the crystal incident surface. In addition, noise due to parametric fluorescence was generated around 3.5 cm from the crystal incident surface, visualizing the coexistence of signal and noise. As described above, this experiment confirmed that the process of parametric generation of terahertz waves by light injection can be visualized.

[0048] [Each aspect of this disclosure] The following summarizes various aspects of this disclosure. A terahertz wave observation device in one aspect of this disclosure comprises a nonlinear crystal for generating terahertz waves from excitation light by optical parametric generation, an excitation light source for injecting excitation light into the nonlinear crystal for generating terahertz waves, a probe light source for injecting probe light into the nonlinear crystal for generating terahertz waves, a prism coupler for injecting probe light into the interior of the nonlinear crystal for generating terahertz waves, and an imaging device. The imaging device images the detected Stokes light generated by the interaction between the probe light and the terahertz waves generated in the nonlinear crystal for generating terahertz waves.

[0049] According to this embodiment, terahertz waves generated inside the nonlinear crystal of a terahertz wave parametric generator can be directly observed.

[0050] In one embodiment, the terahertz wave observation device further includes an injection light source for injecting laser light of the same frequency as the Stokes light, which is paired with the target terahertz wave, into the terahertz wave generation nonlinear crystal in the same direction as the generation of the Stokes light.

[0051] According to this embodiment, terahertz waves generated inside the nonlinear crystal of a light-injection type terahertz wave parametric generator can be directly observed.

[0052] In one embodiment, the system further comprises a nonlinear crystal for amplifying the detected Stokes light.

[0053] According to this embodiment, since the generated weak detected Stokes light is parametrically amplified, a readily available near-infrared detector such as a CMOS camera can be used as the imaging device.

[0054] In one embodiment, the nonlinear crystal for terahertz wave generation and the nonlinear crystal for detecting Stokes light amplification are LiNbO3.

[0055] According to this embodiment, the amplification of terahertz waves and the amplification of detected Stokes light can be achieved with higher efficiency.

[0056] According to this embodiment, detected Stokes light can be generated with high efficiency.

[0057] In one embodiment, when the angle θ is the angle between the incident direction of the excitation light and the direction of generation of the terahertz wave, the angle between the direction of generation of the terahertz wave and the incident direction of the probe light is 0.9θ ≤ θ ≤ 1.1θ.

[0058] According to this embodiment, detected Stokes light can be generated with even greater efficiency.

[0059] According to this embodiment, the generated terahertz waves can be observed with a polarization plane perpendicular to the waves themselves.

[0060] In one embodiment, the polarization direction of the excitation light and the polarization direction of the probe light are approximately orthogonal.

[0061] According to this embodiment, the generated terahertz waves can be observed with a polarization plane parallel to the waves themselves.

[0062] The present disclosure has been described above based on embodiments. These embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications are also within the scope of the present disclosure. [Explanation of symbols]

[0063] 1. Nonlinear crystal for terahertz wave generation. 2. Excitation light, 3. Terahertz waves, 4. Stokes Light, 5. Probe light, 6. Detect Stokes light, 7... Injected light, 11. Nonlinear crystal for detection and amplification of Stokes light. 21. Excitation light source, 22. Probe light source, 23... Injection light source, 31. First prism coupler, 32. Second prism coupler, 41. Imaging device, 101. Terahertz wave observation device, 102. Terahertz wave observation device, 103. Terahertz wave observation device, 104...Terahertz wave observation device.

Claims

1. A nonlinear crystal for generating terahertz waves that generates terahertz waves from excitation light by optical parametric generation, An excitation light source is used to inject excitation light into the aforementioned nonlinear crystal for generating terahertz waves, A probe light source that incidents probe light onto the terahertz wave generating nonlinear crystal, A prism coupler for injecting the probe light into the interior of the terahertz wave generating nonlinear crystal, Imaging device and Equipped with, The imaging device is a terahertz wave observation device characterized by imaging detected Stokes light generated by the interaction between the probe light and the terahertz waves generated by the nonlinear crystal for terahertz wave generation.

2. The terahertz wave observation apparatus according to claim 1, further comprising an injection light source for injecting laser light of the same frequency as the Stokes light, which is paired with the target terahertz wave, into the nonlinear crystal for terahertz wave generation in the same direction as the generation direction of the Stokes light.

3. The terahertz wave observation apparatus according to claim 1 or 2, further comprising a nonlinear crystal for amplifying detected Stokes light for amplifying the detected Stokes light.

4. The nonlinear crystal for terahertz wave generation and the nonlinear crystal for detecting Stokes light amplification are made of LiNbO ３ The terahertz wave observation device according to claim 3, characterized in that it is the same as described in claim 3.

5. When the angle θ is the angle between the incident direction of the excitation light and the generation direction of the terahertz wave, The terahertz wave observation apparatus according to claim 1 or 2, characterized in that the angle between the direction of generation of the terahertz wave and the incident direction of the probe light is 0.9θ ≤ θ ≤ 1.1θ.

6. The terahertz wave observation apparatus according to claim 1 or 2, characterized in that the polarization direction of the excitation light and the polarization direction of the probe light are substantially parallel.

7. The terahertz wave observation apparatus according to claim 1 or 2, characterized in that the polarization direction of the excitation light and the polarization direction of the probe light are substantially orthogonal.

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