Terahertz wave detection apparatus and terahertz wave detection method

Abstract

To enable highly sensitive and high-speed detection of terahertz waves.SOLUTION: A terahertz wave detection apparatus 10 includes: a QCL element 2 which has a light incident surface 2a and a light emitting surface 2b; an optical member 3 which is disposed so that laser light L emitted from the light emitting surface 2b passes therethrough; and a light detector 4 which detects the laser light L that has passed through the optical member 3. The QCL element 2 includes: a substrate 21; and a semiconductor layer 22 that includes an active layer 224 in which unit laminates 50 composed of an emission layer 51 and an injection layer 52 are stacked in multiple stages. The unit laminate 50 is configured to be capable of oscillating first laser light being mid-infrared light in a wavelength λ1 and second laser light being mid-infrared light in a wavelength λ2. The QCL element 2 is configured to change the oscillation intensity of the first laser light and the second laser light in response to the incidence of a terahertz wave T on the light incident surface 2a. The optical member 3 is configured to allow either the first or second laser light to be incident on the light detector 4.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a terahertz wave detection device and a terahertz wave detection method. [Background technology]

[0002] Conventionally, as detectors of terahertz waves, Golay cells (eg, Patent Document 1), pyro detectors, Schottky barrier diodes (eg, Patent Document 2), Fermi level controlled barrier diodes, and the like are known. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-278366 A [Patent Document 2] JP 2013-178212 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, Golay cells and pyro detectors have the problem of slow response speed despite their high sensitivity, while Schottky barrier diodes and Fermi-level controlled barrier diodes can detect terahertz waves relatively quickly but have the problem of low sensitivity in the terahertz band above 1 THz.

[0005] Therefore, an object of one aspect of the present disclosure is to provide a terahertz wave detection device and a terahertz wave detection method that are capable of detecting terahertz waves with high sensitivity and at high speed. [Means for solving the problem]

[0006] The present disclosure includes the following terahertz wave detection devices [1] to [6] and terahertz wave detection methods [7] to

[10] .

[0007] [1] A quantum cascade laser element having a light incident surface on which a terahertz wave to be detected is incident and a light exit surface from which a laser beam different from the terahertz wave is emitted; an optical member arranged so that the laser light emitted from the light emitting surface passes through; a photodetector that detects the laser light that has passed through the optical member, The quantum cascade laser device includes: A substrate; a semiconductor layer including an active layer provided on the substrate, the active layer having a cascade structure in which unit laminate bodies each including a light emitting layer and an injection layer are laminated in multiple stages, the light emitting layer and the injection layer being alternately laminated; The unit laminate body has a first emission upper level, a second emission upper level that is an energy level higher than the first emission upper level, and an emission lower level in a subband level structure, and is configured to be able to oscillate a first laser light which is mid-infrared light of a first wavelength generated by a transition from the first emission upper level to the emission lower level, and a second laser light which is mid-infrared light of a second wavelength generated by a transition from the second emission upper level to the emission lower level, the quantum cascade laser element is configured to change an oscillation degree of the first laser light and the second laser light in response to the terahertz wave being incident on the light incident surface; The optical member is configured to cause either the first laser light or the second laser light to be incident on the photodetector.

[0008] According to the above terahertz wave detection device, when the terahertz wave to be detected is incident on the light incident surface of a quantum cascade laser element (hereinafter referred to as "QCL element"), the oscillation degree of the first laser light and the second laser light can be changed. This can cause a difference in the detection amount of the first laser light or the second laser light in the photodetector between a first state in which the terahertz wave is incident on the light incident surface and a second state in which the terahertz wave is not incident on the light incident surface. By detecting such a difference in the detection amount, the terahertz wave to be detected can be substantially detected. In addition, since it is possible to cause the first laser light or the second laser light, which is mid-infrared light, to be incident on the photodetector instead of directly causing the terahertz wave to be detected on the photodetector, the terahertz wave can be detected with high sensitivity and high speed using a photodetector capable of detecting mid-infrared light.

[0009] [2] a switch that is disposed upstream of the light incident surface on an optical path of the terahertz wave and that periodically switches between an on state in which the terahertz wave passes and an off state in which the terahertz wave does not pass; The terahertz wave detection device of [1] further includes a measuring instrument that acquires a first detection signal detected by the photodetector in the on state and a second detection signal detected by the photodetector in the off state based on a signal indicating a repetition frequency of the switch.

[0010] According to the configuration [2] above, even if the signal value detected by the photodetector is weak, the terahertz wave can be detected with high accuracy by analyzing the difference between the first detection signal and the second detection signal.

[0011] [3] The terahertz wave detection device according to [1] or [2], wherein the photodetector is any one of an MCT detector, an InAs detector, an InAsSb detector, and a quantum well infrared detector.

[0012] According to the configuration [3] above, by using a photodetector that operates at high sensitivity and high speed as a detector for the mid-infrared light range, it is possible to detect mid-infrared light emitted from the quantum cascade laser element with high sensitivity and high speed, and ultimately to detect terahertz waves with high sensitivity and high speed.

[0013] [4] The terahertz wave detection device of any of [1] to [3], wherein the quantum cascade laser element is configured such that, when a predetermined drive current corresponding to the terahertz wave is injected, in a first state in which the terahertz wave is not incident on the light incident surface, the first laser light oscillates while the second laser light does not oscillate, and in a second state in which the terahertz wave is incident on the light incident surface, the second laser light oscillates.

[0014] According to the configuration of [4] above, the second laser light is generated only when the terahertz wave is incident on the light incident surface, and the first laser light can be reduced accordingly. This makes it possible to generate a more significant difference in the amount of the first laser light or the second laser light detected by the photodetector between the first state and the second state, thereby enabling the terahertz wave to be detected with high accuracy.

[0015] [5] The substrate has a first end face which is an end face of the substrate on one side in a first direction which is a resonance direction of the quantum cascade laser element and constitutes the light incident surface, and a second end face located on the opposite side to the first end face in the first direction, The terahertz wave detecting device according to any one of [1] to [4], wherein the first end face is inclined with respect to the first direction so as to move away from the active layer toward the second end face along the first direction.

[0016] According to the configuration [5] above, by making the terahertz wave to be detected incident on the first end face in a direction inclined with respect to the first direction (a direction inclined so as to approach the active layer as it moves from the first end face side to the second end face side along the first direction), it is possible to suppress optical loss of the terahertz wave due to reflection at the first end face and to suitably guide the terahertz wave to the active layer.

[0017] [6] The terahertz wave detection device of [5], wherein an inclination angle of the first end face with respect to a plane perpendicular to the first direction is approximately equal to a Cherenkov radiation angle corresponding to the active layer.

[0018] According to the configuration [6], the terahertz wave to be detected is incident on the first end face substantially perpendicularly, so that the degree of oscillation of the first laser light or the second laser light can be changed more efficiently while suppressing the optical loss of the terahertz wave due to reflection at the first end face. As a result, the terahertz wave can be detected with even higher accuracy.

[0019] [7] A terahertz wave detection method for detecting a terahertz wave to be detected using the terahertz wave detection device according to any one of [1] to [6], continuously acquiring a signal value indicative of a detection result of the photodetector during a predetermined time period; detecting the terahertz wave in response to detection of a change in the signal value.

[0020] According to the terahertz wave detection method [7] above, the terahertz wave detection device is used to detect a change in the output value (signal value) of the photodetector that occurs when terahertz waves are incident on the light incident surface, thereby making it possible to detect terahertz waves with high sensitivity and high speed.

[0021] [8] The terahertz wave detection method of [7], further comprising a step of setting the polarization plane of the terahertz wave incident on the light incident surface and the polarization plane of the laser light emitted from the light exit surface so as to be parallel to each other.

[0022] According to the configuration of [8] above, the polarization plane of the terahertz wave is made to coincide with the polarization plane of the laser light emitted from the light emission surface (i.e., a plane parallel to the resonance direction of the QCL element and the lamination direction of the active layer), thereby making it possible to increase the effect of the terahertz wave on the active layer. As a result, it is possible to increase the change in the oscillation degree of the first laser light and the second laser light according to the incidence of the terahertz wave on the light incidence surface, and it is possible to detect the terahertz wave with higher accuracy.

[0023] [9] The terahertz wave detection method of [6] or [7], further comprising a step of controlling the operation of the quantum cascade laser element so that the difference between the frequency of the first laser light and the frequency of the second laser light approximately matches the frequency of the terahertz wave.

[0024] According to the configuration of [9], it is possible to suitably increase the change in the oscillation degree of the first laser light and the second laser light in response to the incidence of the terahertz wave on the light incident surface, and as a result, it is possible to detect the terahertz wave with higher accuracy.

[0025]

[10] The terahertz wave detection method according to any of [7] to [9], further comprising a step of controlling operation of the quantum cascade laser element so that in a first state in which the terahertz wave is not incident on the light incident surface, the first laser light oscillates while the second laser light does not oscillate, and in a second state in which the terahertz wave is incident on the light incident surface, the second laser light oscillates.

[0026] According to the configuration of

[10] above, the second laser light is generated only when the terahertz wave is incident on the light incident surface, and the first laser light can be reduced accordingly. This makes it possible to generate a more significant difference in the amount of the first laser light or the second laser light detected by the photodetector between the first state and the second state, thereby enabling the terahertz wave to be detected with higher accuracy. Effect of the Invention

[0027] According to one aspect of the present disclosure, it is possible to provide a terahertz wave detection device and a terahertz wave detection method capable of detecting terahertz waves with high sensitivity and at high speed. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of an experimental system including a terahertz wave detection device according to an embodiment. [Diagram 2] FIG. 2 is a schematic diagram of a QCL element. [Diagram 3] FIG. 3 is a diagram showing a schematic stack structure of a QCL element. [Figure 4] FIG. 4 is a diagram showing an example of a subband level structure in the active layer of a QCL device. [Diagram 5] FIG. 5 is a diagram showing an example of the configuration of a unit laminate body constituting an active layer. [Figure 6] FIG. 6 is a diagram showing an example of the structure of a unit laminate structure for one period in the active layer. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the polarization of the terahertz wave and the MCT signal value. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are designated by the same reference numerals, and duplicated description will be omitted.

[0030] With reference to Fig. 1, an optical system 1 as an experimental system including a terahertz wave detecting device 10 of this embodiment will be described. As shown in Fig. 1, the terahertz wave detecting device 10 is incorporated into the optical system 1 as a part that detects the terahertz wave T to be detected. In this embodiment, the terahertz wave detecting device 10 includes a quantum cascade laser element (hereinafter referred to as "QCL element") 2, an optical member 3, a photodetector 4, an optical chopper 5 (switching device), and a lock-in amplifier 6 (measuring device). In addition to the terahertz wave detecting device 10, the optical system 1 includes a light source 7, a polarizing element 8, a condensing lens 9, and a control unit 11.

[0031] [Structure of QCL element] First, the configuration of the QCL element 2 will be described with reference to FIGS. 2 to 6. As shown in FIGS. 1 and 2, the QCL element 2 has a light incident surface 2a on which the terahertz wave T to be detected is incident, and a light emitting surface 2b from which laser light L different from the terahertz wave T is emitted. The light incident surface 2a is a surface on one side (left side in FIG. 2) in a direction D1 (first direction) which is a resonance direction of the QCL element 2 (a direction in which end faces 22a and 22b of a semiconductor layer 22 described later that constitute a resonator face each other). The light emitting surface 2b is a surface on the other side (right side in FIG. 2) in the direction D1. The laser light L is mid-infrared light (light with a wavelength of about 3 μm to 20 μm) generated by laser resonance between end faces 22a and 22b of the semiconductor layer 22 of the QCL element 2 (specifically, between end faces of a lower guide layer 223, an active layer 224, and an upper guide layer 225 described later).

[0032] The QCL element 2 is configured to be capable of oscillating a first laser light, which is mid-infrared light with a wavelength λ1 (first wavelength), and is also configured to be capable of oscillating a second laser light, which is mid-infrared light with a wavelength λ2 (second wavelength) different from the wavelength λ1. The QCL element 2 is also configured to change the degree of oscillation of the first laser light (λ1) and the second laser light (λ2) in response to the incidence of the terahertz wave T on the light incident surface 2a. The wavelength λ1 and the wavelength λ2 depend on the magnitude of the drive current of the QCL element 2, but in this embodiment, as an example, under a predetermined current condition (in this embodiment, when a drive current of 490 mA is injected into the QCL element 2), the wavelength λ1 is 6.9 μm and the wavelength λ2 is 6.6 μm. Furthermore, the QCL element 2 is configured such that, when a predetermined drive current I (490 mA as described above in this embodiment) corresponding to the terahertz waves T is injected, in a first state in which the terahertz waves T are not incident on the light incident surface 2a, the first laser light oscillates but the second laser light does not oscillate, and in a second state in which the terahertz waves T are incident on the light incident surface 2a, the second laser light oscillates. An example of the configuration of the QCL element 2 having the above-mentioned characteristics will be described below.

[0033] The QCL element 2 has a substrate 21 and a semiconductor layer 22. The substrate 21 is, for example, a semiconductor substrate such as an InP single crystal substrate. The substrate 21 is formed in a rectangular plate shape and has an end face 21a (first end face), an end face 21b (second end face), a main face 21c, and a back face 21d. The end face 21a is a face on one side (left side in FIG. 2) in the direction D1, and constitutes a part of the light incident face 2a of the QCL element 2. The end face 21b is a face on the other side (right side in FIG. 2) in the direction D1, and constitutes a part of the light emitting face 2b of the QCL element 2. The main face 21c is a face on which the semiconductor layer 22 is provided, and the back face 21d is a face opposite to the main face 21c.

[0034] The principal surface 21c and the back surface 21d are parallel to each other. The end surface 21b is a surface perpendicular to the direction D1, that is, a surface parallel to the direction D2 (second direction) perpendicular to the principal surface 21c. On the other hand, as shown in FIG. 2, the end surface 21a is inclined by an inclination angle θ with respect to a surface perpendicular to the direction D1 when viewed from a direction perpendicular to the directions D1 and D2. The end surface 21a is inclined with respect to the direction D1 so as to move away from the active layer 224 (semiconductor layer 22) along the direction D1 toward the end surface 21b side. In other words, the end surface 21a is inclined with respect to the direction D2 so as to move closer to the end surface 21b as it moves away from the active layer 224 along the direction D2 (i.e., from the principal surface 21c side toward the back surface 21d side).

[0035] The length of substrate 21 (length in direction D1) is, for example, several hundred μm to several mm. The width of substrate 21 (length in a direction perpendicular to directions D1 and D2) is, for example, several hundred μm to several mm. The thickness of substrate 21 (length in direction D2) is, for example, several hundred μm. As an example, the length of substrate 21 is about 3 mm, the width of substrate 21 is about 1 mm, and the thickness of substrate 21 is about 300 μm.

[0036] The semiconductor layer 22 is provided on the main surface 21c of the substrate 21. The thickness (length in the direction D2) of the semiconductor layer 22 is, for example, about 10 μm to 20 μm. The semiconductor layer 22 has an end face 22a that is continuous with the end face 21a of the substrate 21 and forms part of the light incident surface 2a, and an end face 22b that is continuous with the end face 21b of the substrate 21 and forms part of the light emitting surface 2b. The end faces 22a and 22b are surfaces perpendicular to the direction D1. The end faces 22a and 22b are, for example, cleavage surfaces formed by cleavage.

[0037] As shown in FIG. 3, the semiconductor layer 22 is formed by stacking a lower contact layer 221, a lower cladding layer 222, a lower guide layer 223, an active layer 224, an upper guide layer 225, an upper cladding layer 226, and an upper contact layer 227 in this order on the primary surface 21c of the substrate 21.

[0038] The lower contact layer 221 is, for example, a high-concentration Si-doped InGaAs layer (Si: 1.0×10 18 cm -3 ) and is provided on the main surface 21c of the substrate 21.

[0039] The lower cladding layer 222 is, for example, a Si-doped InP layer (Si: 1.5×10 16 cm -3 ) and is provided on the lower contact layer 221.

[0040] The lower guide layer 223 is, for example, a Si-doped In layer having a thickness of about 100 nm. 0.53 Ga 0.47 As layer (Si: 1.5 × 10 16 cm -3 ) and is disposed on the lower cladding layer 222 .

[0041] The active layer 224 is a layer in which a quantum cascade structure is formed, and is provided on the lower guide layer 223. The active layer 224 will be described in detail later.

[0042] The upper guide layer 225 is, for example, a Si-doped In layer having a thickness of about 300 nm. 0.53 Ga 0.47 As layer (Si: 1.5 × 10 16 cm -3 ) and is provided on the active layer 224.

[0043] The upper cladding layer 226 is, for example, a Si-doped InP layer (Si: 1.5×10 16 cm -3 ) and is disposed on the upper guiding layer 225.

[0044] The upper contact layer 227 is, for example, a high-concentration Si-doped InP layer (Si: 1.5×10 18 cm -3 ) and is disposed on the upper cladding layer 226 .

[0045] The structure of the active layer 224 of this embodiment will be described with reference to Fig. 4 to Fig. 6. The active layer 224 has a cascade structure in which quantum well light emitting layers (light emitting layers 51) used to generate light and electron injection layers (injection layers 52) used to inject electrons (carriers) into the light emitting layers 51 are alternately stacked in multiple stages. Specifically, a semiconductor stack structure consisting of the light emitting layers 51 and the injection layers 52 is defined as one period of unit stack bodies 50, and the unit stack bodies 50 are stacked in multiple stages to form the active layer 224 having the cascade structure.

[0046] 4, each of the multiple unit laminate structures 50 included in the active layer 224 is composed of a light emitting layer 51 and an injection layer 52. As described below, the light emitting layer 51 and the injection layer 52 are formed to have a predetermined quantum well structure including quantum well layers (well layers W1 to W11) and quantum barrier layers (barrier layers B1 to B11). As a result, a subband level structure, which is an energy level structure due to the quantum well structure, is formed in the unit laminate structure 50.

[0047] The active layer 224 has a dual-upper-state (DAU) structure as shown in, for example, Japanese Patent No. 5523759, Japanese Patent No. 6276758, etc. That is, the unit laminate structure 50 included in the active layer 224 has two emission upper levels. As an example, in the subband level structure of the unit laminate structure 50, a first emission upper level L up1 and a second emission upper level L which is a higher energy level than the first emission upper level. up2 and a single emitting lower level L low However, the level structure of the unit laminate body 50 is not limited to the above. For example, the unit laminate body 50 may have a plurality of light emission lower levels.

[0048] In addition, in the subband level structure shown in FIG. 4, the emission lower level L low The relaxation level L is a lower energy level than r This relaxation level L r is the electron that has undergone the radiative transition to the lower radiative level L lowis preferably the light emitting lower level L low and the relaxation level L r The energy difference between the LO phonon and LO In this embodiment, the relaxation level L r is the first emission upper level L in the light emitting layer 51b of the subsequent unit laminate structure 50 under the operating electric field condition. up1 It is designed to bind tightly to

[0049] In the unit laminate body 50 shown in FIG. 4, electrons e injected from the injection layer 52a to the light emitting layer 51 are present between the light emitting layer 51 and the injection layer 52a of the previous unit laminate body 50. - An injection barrier layer for electrons entering the injection layer 52 is provided between the light emitting layer 51 and the injection layer 52. In addition, an exit barrier layer for electrons entering the injection layer 52 from the light emitting layer 51 is provided between the light emitting layer 51 and the injection layer 52. These barrier layers are provided as necessary depending on the specific stack structure and subband level structure of the active layer 224 including the light emitting layer 51 and the injection layer 52.

[0050] In this embodiment, in such a subband level structure, the relaxation level L r Electron e from - is injected into the light emitting layer 51 through the injection barrier, and thereby reaches the relaxation level L r The first emission upper level L bound to up1 is strongly excited.

[0051] First emission upper level L up1 The electrons injected into the lower light level L low The first emission upper level L up1 and the lower emission level L low Light with a wavelength λ1 corresponding to the energy difference between the subband levels of L and L is generated and emitted. low The electrons that transition to the relaxation level L r Thus, the relaxation level L r Using the lower emission level Llow By removing an electron from the first emission upper level L up1 and the lower emission level L low A population inversion is formed between the lower emission level L low From relaxation level L r The electrons relaxed to the relaxation level L r The first emission upper level L up1 is injected in a cascade manner.

[0052] By repeating such electron injection, luminescence transition, and relaxation in the multiple unit laminate bodies 50, cascaded generation of light occurs in the active layer 224. That is, by alternately stacking a large number of light emitting layers 51 and injection layers 52, electrons move in a cascaded manner in the multiple unit laminate bodies 50 one after another, and light with wavelength λ1 is generated during intersubband transition in each unit laminate body 50. Then, such light is resonated in the optical resonator of the QCL element 2 (i.e., the resonator formed by the end faces 22a, 22b of the lower guide layer 223, the active layer 224, and the upper guide layer 225), and laser light with wavelength λ1 (first laser light) is generated.

[0053] As shown in FIG. 5, in this embodiment, one period of unit laminate structure 50 is configured as a quantum well structure in which eleven well layers W1 to W11 and eleven barrier layers B1 to B11 are alternately laminated.

[0054] The well layers W1 to W11 are In 0.53 Ga 0.47 The barrier layers B1 to B11 are made of In. 0.48 Al 0.52 As a result, the active layer 224 is configured with an InGaAs / InAlAs quantum well structure that is lattice-matched to the substrate 21, which is an InP substrate.

[0055] In the unit laminate body 50, a laminate portion consisting of the well layers W1 to W4 and the barrier layers B1 to B4 mainly functions as a light emitting layer 51. In addition, a laminate portion consisting of the well layers W5 to W11 and the barrier layers B5 to B11 mainly functions as an injection layer 52.

[0056] Among the semiconductor layers of the light-emitting layer 51, the first barrier layer B1 is located between the preceding injection layer 52a (see FIG. 4) and the light-emitting layer 51, and serves as an injection barrier layer for electrons from the preceding injection layer 52a to the light-emitting layer 51. Similarly, among the semiconductor layers of the injection layer 52, the first barrier layer B5 is located between the light-emitting layer 51 and the injection layer 52, and serves as an extraction barrier layer for electrons from the light-emitting layer 51 to the injection layer 52. FIG. 6 shows an example of a specific structure of one period of the unit laminate body 50 in the active layer 224. In this embodiment, the designed oscillation wavelength of the QCL element 2 is set to about 6.5 μm by such a layer structure.

[0057] In this embodiment, due to the structure of the active layer 224 (unit laminate body 50) described above, when a predetermined driving current I is injected into the QCL element 2, the first emission upper level L up1 Therefore, when the QCL device 2 starts to operate, the first emission upper level L up1 to the lower emission level L low As described above, the relaxation level L r However, the first emission upper level L up1 In this way, by adjusting the driving current I of the QCL element 2, in the above-mentioned first state (i.e., a state in which the terahertz wave T is not incident on the light incident surface 2a), the first emission upper level L up1 The second emission upper level L up2It is possible to prevent oscillation from occurring. Furthermore, in this embodiment, the upper guide layer 225 is provided with a diffraction grating (in other words, a diffraction grating pattern configured to reduce optical loss at wavelength λ1) for promoting oscillation at wavelength λ1 (6.9 μm in this embodiment). More specifically, by providing the upper guide layer 225 with a diffraction grating having a period of 1080 nm, the QCL element 2 is configured to oscillate mainly at wavelength λ1.

[0058] On the other hand, in this embodiment, by adjusting the driving current I according to the frequency of the terahertz wave T to be detected, etc., in the above-mentioned second state (i.e., the state in which the terahertz wave T is incident on the light incident surface 2a and is incident on the active layer 224), the second emission upper level L up2 That is, when the terahertz wave T enters the active layer 224 (unit laminate body 50), the first emission upper level L up1 and the second emission upper level L up2 The distribution of carriers in the first laser light and the second laser light changes, and as a result, the oscillation degree of the first laser light and the second laser light changes. More specifically, the first emission upper level L up1 Some of the electrons concentrated in the second emission upper level L up2 By moving to the second emission upper level L up2 As shown in FIG. 5, in this embodiment, a sufficient number of carriers are excited at the second emission upper level L up2 and the lower emission level L low The overlap of the wave functions with the second emission upper level L up2 to the lower emission level L low The transition intensity to the second emission upper level L up2 As a result, the second emission upper level L up2 The electrons injected into the lower light level L low The second emission upper level L up2 and the lower emission level L lowLight of wavelength λ2 corresponding to the energy difference between the subband levels of the first and second laser beams is generated and emitted, and the degree of oscillation of the second laser beam increases. up1 From the second emission upper level L up2 As a result of the movement of carriers to the first laser beam, the oscillation intensity of the first laser beam decreases accordingly.

[0059] With the configuration described above, the QCL element 2 is configured, under specified current conditions (a state in which the driving current I adjusted as described above is injected and the QCL element 2 operates), to output laser light L containing only the first laser light of wavelength λ1 in a first state in which the terahertz wave T is not incident on the QCL element 2, and to output laser light L containing not only the first laser light of wavelength λ1 but also the second laser light of wavelength λ2 in a second state in which the terahertz wave T is incident on the QCL element 2.

[0060] As described above, the subband level structure in the unit laminate structure 50 depends on the magnitude of the driving current I. That is, the first emission upper level L up1 The wavelength λ1 of the first laser light and the second emission upper level L up2 The wavelength λ2 of the second laser light generated based on this can be changed by the magnitude of the driving current I. From the viewpoint of efficiently oscillating the second laser light by incidenting the terahertz wave T into the QCL element 2, it is preferable to adjust the driving current I so that the difference frequency Δf (=c(λ1-λ2) / (λ1·λ2)) between the frequency (c / λ1) of the first laser light and the frequency (c / λ2) of the second laser light approximately matches the frequency of the terahertz wave T. By incidenting such a terahertz wave T into the active layer 224, the second emission upper level L as described above is generated. up2 Therefore, the supply of carriers to the second laser beam can be promoted, and the oscillation degree of the second laser beam can be effectively increased. In the above, c represents the speed of light.

[0061] [Explanation of each component of the optical system] Next, an example of the configuration of the optical system 1 for detecting the terahertz wave T using the QCL element 2 having the characteristics as described above will be described.

[0062] The light source 7 is a light source that emits terahertz waves T to be detected. The frequency range of the terahertz waves T emitted from the light source 7 is, for example, 0.1 THz to 10.0 THz. In this embodiment, the light source 7 is a THz gas laser light source configured to emit terahertz waves T with a frequency of 2.52 THz.

[0063] The polarizing element 8 is an optical element for changing the polarization direction (electric field vibration direction) of the terahertz wave T emitted from the light source 7. The polarizing element 8 is composed of, for example, a wire grid polarizer, a wavelength plate, etc. The polarizing element 8 may also be composed of a combination of a plurality of optical elements (for example, the above-mentioned polarizers, wavelength plates, etc.). The polarizing element 8 can be used to make the polarization plane of the terahertz wave T parallel to the polarization plane of the laser light L emitted from the light emission surface 2b of the QCL element 2 (i.e., a plane parallel to both the direction D1, which is the resonance direction, and the direction D2, which is the stacking direction of the active layer 224). Note that, when it is possible to make the polarization plane of the terahertz wave T parallel to the polarization plane of the QCL element 2 by adjusting the relative positional relationship (direction) between the light source 7 and the QCL element 2, the polarizing element 8 may be omitted.

[0064] The optical chopper 5 is disposed upstream of the light incident surface 2a of the QCL element 2 on the optical path of the terahertz wave T, and periodically switches between an on state in which the terahertz wave T passes and an off state in which the terahertz wave T does not pass. A signal (reference signal) indicating the timing (repetition frequency) of switching between the on state and the off state by the optical chopper 5 is sent to the lock-in amplifier 6.

[0065] The focusing lens 9 is a lens member disposed between the optical chopper 5 and the light incident surface 2a of the QCL element 2, and focuses the terahertz wave T that has passed through the optical chopper 5 onto the light incident surface 2a (in this embodiment, the end surface 21a of the substrate 21). The focusing lens 9 is, for example, a lens for the terahertz wave band such as Tsurupica (registered trademark) having a focal length of 40 mm.

[0066] 2, in this embodiment, the QCL element 2 is disposed so that the terahertz waves T condensed by the condenser lens 9 are incident on the end face 21a of the substrate 21 substantially perpendicularly. The terahertz waves T incident on the end face 21a in this manner pass through the inside of the substrate 21 and reach the active layer 224 of the semiconductor layer 22. As a result, in the active layer 224, the first emission upper level L up1 and the second emission upper level L up2 The carrier distribution of the first laser light (λ1) and the second laser light (λ2) changes, and the oscillation degree of the first laser light (λ1) and the second laser light (λ2) changes. Specifically, the second emission upper level L up2 While the number of carriers (electrons) supplied to the first emission upper level L up1 The number of carriers (electrons) supplied to the laser beam L decreases. As a result, the amount of the first laser light contained in the laser beam L decreases, while the second laser light comes to be contained in the laser beam L. The laser beam L is emitted radially from the end face 22b of the semiconductor layer 22 (mainly the end face of the active layer 224) along the direction D1.

[0067] The control unit 11 injects the driving current I into the QCL device 2, and also controls the start or end of the injection, the amount of injection, etc. The control unit 11 includes, for example, a computer device equipped with a processor, a memory, and the like.

[0068] The optical member 3 is disposed so that the laser light L emitted from the light emission surface 2b of the QCL element 2 passes through it. As an example, the optical member 3 has a collimator lens 31, an optical filter 32, and a condenser lens 33. By including the optical filter 32, the optical member 3 is configured to cause either the first laser light (λ1) or the second laser light (λ2) (in this embodiment, only the second laser light) to be incident on the photodetector 4.

[0069] The collimating lens 31 is a lens that collimates the laser light L radially emitted from the light emitting surface 2b. The collimating lens 31 is, for example, a ZnSe lens with a focal length of 1 mm.

[0070] The optical filter 32 is configured to guide only one of the first laser light (λ1) and the second laser light (λ2) to the downstream collecting lens 33. As the optical filter 32, for example, a bandpass filter that transmits only a specific wavelength band, a diffraction grating that reflects only a specific wavelength, etc. can be used. In this embodiment, the optical filter 32 is a bandpass filter configured to transmit the wavelength λ2 while not transmit (absorb or reflect) the wavelength λ1.

[0071] The condenser lens 33 is a lens that condenses the laser light L (the second laser light in this embodiment) that has passed through the optical filter 32 toward the light receiving surface of the photodetector 4. The condenser lens 33 is, for example, a ZnSe lens with a focal length of 50 mm.

[0072] The photodetector 4 detects the laser light L that has passed through the optical member 3. In this embodiment, the photodetector 4 is an MCT detector that detects the second laser light that has passed through the optical filter 32 and is incident on the light receiving surface of the photodetector 4. Here, in the first state in which the terahertz wave T is not incident on the QCL element 2, the second laser light is not generated, and only the first laser light is output from the QCL element 2 as the laser light L. Therefore, in the first state, all of the laser light L including only the first laser light of wavelength λ1 is cut by the optical filter 32, and the signal value of the photodetector 4 is close to 0. On the other hand, in the second state, the second laser light of wavelength λ2 passes through the optical filter 32, and the second laser light (laser light L) is detected by the photodetector 4, and a significant signal value is obtained.

[0073] The photodetector 4 has good sensitivity to mid-infrared light and preferably has a faster response speed than detectors that can be used in the terahertz wave band, such as pyro detectors and Golay cells. By detecting the laser light L (in this embodiment, the second laser light with a wavelength λ2 derived from the terahertz wave T) using such a photodetector 4, the terahertz wave T can be detected at high speed without impairing the detection sensitivity, compared to the case where the terahertz wave T is directly detected by a pyro detector and Golay cell, etc. From the above viewpoint, it is preferable to use, for example, any of an MCT detector, an InAs detector, an InAsSb detector, and a quantum well infrared detector (QWIP: Quantum Well Infrared Photodetector) as the photodetector 4. A specific example of the QWIP is, for example, a quantum cascade photodetector (QCD: Quantum Cascade Photodetector).

[0074] The lock-in amplifier 6 acquires a first detection signal detected by the photodetector 4 in the on state and a second detection signal detected by the photodetector 4 in the off state based on a signal (reference signal) indicating the repetition frequency of the optical chopper 5.

[0075] [Effects of the terahertz wave detection device] According to the terahertz wave detection device 10 described above, when the terahertz wave T to be detected is incident on the light incident surface 2a of the QCL element 2 (in other words, when the terahertz wave T to be detected is generated), the degree of oscillation of the first laser light and the second laser light in the QCL element 2 can be changed. This can cause a difference in the amount of detection of the first laser light or the second laser light (in this embodiment, the second laser light) in the photodetector 4 between a first state in which the terahertz wave T is incident on the light incident surface 2a (in this embodiment, the end surface 21a of the substrate 21) and a second state in which the terahertz wave T is not incident on the light incident surface 2a. In this embodiment, in the first state, the signal value indicating the amount of detection of the second laser light in the photodetector 4 is 0, and in the second state, the signal value indicating the amount of detection of the second laser light in the photodetector 4 is a value greater than 0 according to the light intensity. By detecting such a difference in the amount of detection, the terahertz wave T to be detected can be substantially detected. Furthermore, instead of directly incident the terahertz waves T to be detected on the photodetector 4, it is possible to incident the first laser light or the second laser light (in this embodiment, the second laser light), which is mid-infrared light, on the photodetector 4. Therefore, the terahertz waves T can be detected with high sensitivity and high speed using the photodetector 4 capable of detecting mid-infrared light.

[0076] The terahertz wave detection device 10 also includes the optical chopper 5 and the lock-in amplifier 6 described above. As a result, even if the signal value detected by the photodetector 4 (the signal value corresponding to the second laser light detected in the second state) is weak, the terahertz wave T can be detected with high accuracy by analyzing the difference between the first detection signal and the second detection signal. For example, if the terahertz wave T is emitted from the light source 7, in the off state in which the terahertz wave T is blocked by the optical chopper 5, the terahertz wave T does not reach the QCL element 2. For this reason, during the period in which the terahertz wave T is emitted from the light source 7, the first detection signal detected in the on state in which the terahertz wave T reaches the QCL element 2 indicates a signal value corresponding to the detection result of the second laser light generated in response to the terahertz wave T. On the other hand, the second detection signal indicates a signal value in a state in which the second laser light is not detected. Here, the second detection signal is ideally considered to be 0, but it may also indicate a value greater than 0 due to the influence of noise. Furthermore, when the second laser light generated by the incidence of the terahertz waves T is weak, the signal value of the first detection signal is very small, and when the first detection signal is viewed alone, it may be impossible to determine whether the value is due to the second laser light or noise. In such a case, by analyzing the difference between the first detection signal and the second detection signal, it becomes possible to detect with high accuracy that the second laser light has been detected in the on state (i.e., that the terahertz waves T have been incident on the QCL element 2).

[0077] The photodetector 4 may be any one of an MCT detector, an InAs detector, an InAsSb detector, and a QWIP. In this case, by using the photodetector 4 that operates with high sensitivity and high speed as a detector for the mid-infrared light region, it is possible to detect the mid-infrared light (laser light L including the second laser light having a wavelength λ2 in this embodiment) emitted from the QCL element 2 with high sensitivity and high speed, and therefore it is possible to detect the terahertz wave T with high sensitivity and high speed.

[0078] Furthermore, the QCL element 2 is configured such that, when a predetermined drive current I corresponding to the terahertz wave T is injected, in a first state in which the terahertz wave T is not incident on the light incident surface 2a, the first laser light oscillates while the second laser light does not oscillate, and in a second state in which the terahertz wave T is incident on the light incident surface 2a, the second laser light oscillates. According to the above configuration, the second laser light is generated only when the terahertz wave T is incident on the light incident surface 2a, and the first laser light can be reduced accordingly. This makes it possible to generate a more significant difference between the first state and the second state in the amount of detection of the first laser light or the second laser light (the second laser light in this embodiment) in the photodetector 4. As a result, the terahertz wave T can be detected with high accuracy.

[0079] 2, the end surface 21a of the substrate 21 constituting the light incident surface 2a is inclined with respect to the direction D1 so as to move away from the active layer 224 as it moves toward the end surface 21b side (i.e., the right side in FIG. 2) along the direction D1. According to the above configuration, the terahertz wave T to be detected is made incident on the end surface 21a in a direction inclined with respect to the direction D1 (as shown in FIG. 2, the direction inclined so as to approach the active layer 224 as it moves from the end surface 21a side to the end surface 21b side along the direction D1), thereby reducing the incident angle of the terahertz wave T on the end surface 21a, thereby suppressing the optical loss of the terahertz wave T due to reflection at the end surface 21a, and the terahertz wave T can be suitably guided to the active layer 224. More specifically, since the thickness (length in the direction D2) of the active layer 224 is very small, it is not easy to make a sufficient amount of terahertz wave T incident from the end surface 22a of the active layer 224. In contrast, as shown in FIG. 2, by irradiating terahertz waves T via substrate 21 in a direction inclined with respect to direction D1, terahertz waves T can be made to be incident on active layer 224 efficiently.

[0080] Moreover, it is preferable that the inclination angle θ of the end face 21a with respect to the plane perpendicular to the direction D1 is set so as to substantially coincide with the Cherenkov radiation angle corresponding to the active layer 224. Here, the "Cherenkov radiation angle corresponding to the active layer 224" refers to the angle that the radiation direction of the terahertz wave forms with the direction D1 when both the above-mentioned first laser light (λ1) and second laser light (λ2) oscillate in the active layer 224 and a terahertz wave corresponding to the difference frequency Δf (=c(λ1-λ2) / (λ1·λ2)) between the frequency of the first laser light (c / λ1) and the frequency of the second laser light (c / λ2) is generated in the active layer 224. The group refractive index of the substrate 21 with respect to mid-infrared light is defined as n MIR The refractive index of the substrate 21 with respect to the terahertz wave is expressed as n THz When expressed as above, the Cherenkov radiation angle θ C is expressed by the following formula (1). Note that the Cherenkov radiation angle θ C Although it depends on the material of the substrate 21 (that is, the refractive index corresponding to the material) and the frequency of the terahertz wave, it is, for example, about 5 degrees to 30 degrees.

[0081] θ C =cos -1 (n MIR / n THz ) …(1)

[0082] According to the above configuration, by making the terahertz waves T to be detected incident on the end face 21a approximately perpendicularly, it is possible to effectively increase the influence of the terahertz waves T on the active layer 224 while suppressing the optical loss of the terahertz waves T due to reflection at the end face 21a, and it is possible to change the oscillation degree of the first laser light or the second laser light more efficiently. As a result, it is possible to detect the terahertz waves T with even higher accuracy. More specifically, the incidence angle θ of the terahertz waves T with respect to the active layer 224 is THz and Cherenkov radiation angle θ CIf they do not match, the mid-infrared light (MIR light) generated by the incidence of the terahertz wave T on the active layer 224 may be output in a direction different from the resonance direction (direction D1) of the QCL element 2, and may not be efficiently detected by the photodetector 4. C and the terahertz wave T is incident on the end face 21a substantially perpendicularly, so that the incidence angle θ of the terahertz wave T is THz and Cherenkov radiation angle θ C As a result, the emission direction of the mid-infrared light generated by the incidence of the terahertz wave T on the active layer 224 can be made to coincide with the resonance direction (direction D1), so that the mid-infrared light detected by the photodetector 4 (i.e., the mid-infrared light emitted from the light emitting surface 2b along the direction D1) can be efficiently generated.

[0083] [Terahertz wave detection method] Next, an example of a terahertz wave detection method for detecting the terahertz wave T as a detection target using the terahertz wave detection device 10 will be described.

[0084] 1, each component is arranged so that the terahertz wave T to be detected is incident on the light incident surface 2a of the QCL element 2. At this time, it is preferable to set so that the polarization plane of the terahertz wave T incident on the light incident surface 2a and the polarization plane of the laser light L emitted from the light emitting surface 2b (plane parallel to both directions D1 and D2) are parallel to each other. For example, the above setting may be performed by placing a polarizing element 8 between the light source 7 and the QCL element 2, or may be performed by arranging (adjusting the orientation) the light source 7 or the QCL element 2 without using the polarizing element 8.

[0085] Furthermore, the operation of the QCL element 2 may be controlled so that in a first state in which the terahertz waves T are not incident on the light incident surface 2a, the first laser light oscillates but the second laser light does not oscillate, and in a second state in which the terahertz waves T are incident on the light incident surface 2a, the second laser light oscillates. The above control (hereinafter referred to as "first control") is performed, for example, by adjusting the drive current I injected from the control unit 11 to the QCL element 2.

[0086] Furthermore, the operation of the QCL element 2 may be controlled so that the difference frequency Δf (=c(λ1-λ2) / (λ1·λ2)) between the frequency of the first laser light (c / λ1) and the frequency of the second laser light (c / λ2) approximately matches the frequency of the terahertz wave T. The above control (hereinafter referred to as "second control") is performed, for example, by adjusting the drive current I injected from the control unit 11 to the QCL element 2.

[0087] With the above, preparations for detecting terahertz waves T are completed in the optical system 1. In this state, a signal value indicating the detection result of the photodetector 4 is continuously acquired during a predetermined target period. Here, the "predetermined target period" is a period during which the terahertz waves T to be detected may be generated, and is a period during which it is desired to detect the terahertz waves T when they are generated (in this embodiment, when the terahertz waves T are emitted from the light source 7).

[0088] Next, the terahertz wave T is detected in response to detection of a change (temporal change) in the signal value continuously acquired as described above. In this embodiment, the photodetector 4 detects the second laser light (laser light L) only when the terahertz wave T is incident on the light incident surface 2a, and the photodetector 4 does not detect the second laser light in other cases. Therefore, when the state in which the terahertz wave T is not incident on the light incident surface 2a changes to the state in which the terahertz wave T is incident on the light incident surface 2a, the output value (signal value) of the photodetector 4 changes. Therefore, according to the above-mentioned terahertz wave detection method using the terahertz wave detection device 10, the terahertz wave T can be detected with high sensitivity and high speed by detecting a change in the output value (signal value) of the photodetector 4 that occurs when the terahertz wave T is incident on the light incident surface 2a.

[0089] Moreover, in the above-mentioned terahertz wave detection method, the polarization plane of the terahertz wave T incident on the light incident surface 2a and the polarization plane of the laser light L are set to be parallel to each other. According to the above-mentioned configuration, by matching the polarization plane of the terahertz wave T with the polarization plane of the laser light L (i.e., a plane parallel to the direction D1 which is the resonance direction of the QCL element 2 and the direction D2 which is the lamination direction of the active layer 224), the influence of the terahertz wave T on the active layer 224 (in this embodiment, the second emission upper level L up2 As a result, it is possible to increase the change in the oscillation degree of the first laser light and the second laser light in response to the incidence of the terahertz wave T on the light incident surface 2a, and it is possible to detect the terahertz wave T with higher accuracy.

[0090] The above effect will be supplemented with reference to FIG. 7. FIG. 7 shows the results of measuring the output value (mV) of the photodetector 4 (MCT detector) (i.e., the signal value corresponding to the intensity of the second laser light detected by the photodetector 4) while changing the output intensity (mW) of the terahertz wave T from the light source 7 for each of the cases where the polarization plane of the terahertz wave T and the polarization plane of the laser light L are parallel to each other (circle), are shifted by 45 degrees from each other (triangle), and are shifted by 90 degrees from each other (square) by using the optical system 1. It was confirmed that in the region A1 where the output intensity of the terahertz wave T is relatively small, the photodetector 4 tends to obtain a higher output value when the polarization deviation is small (circle or triangle) than when the polarization deviation is large (square). It was also confirmed that in the region A2 where the output intensity of the terahertz wave T is medium, the photodetector 4 tends to obtain a higher output value when there is no polarization deviation (circle) than when there is a polarization deviation (triangle).

[0091] Moreover, according to the first control, the second laser light is generated only when the terahertz wave T is incident on the light incident surface 2a, and the first laser light can be reduced accordingly. This makes it possible to generate a more significant difference between the first state and the second state in the amount of the first laser light or the second laser light detected by the photodetector 4 (the amount of the second laser light detected in this embodiment). This allows the terahertz wave T to be detected with higher accuracy.

[0092] In addition, according to the second control, it is possible to suitably increase the change in the oscillation degree of the first laser light and the second laser light in response to the incidence of the terahertz wave T on the light incident surface 2a. In this embodiment, the second emission upper level L up2 As a result, it becomes possible to preferably oscillate the second laser light, and as a result, it becomes possible to detect the terahertz wave T with higher accuracy.

[0093] [Variations] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the configurations shown in the above embodiments. The materials and shapes of each component are not limited to the specific materials and shapes described above, and various materials and shapes other than those described above can be used. In addition, some of the components included in the above embodiments may be omitted or modified as appropriate.

[0094] In the above embodiment, the optical filter 32 is configured to pass only the second laser light (λ2) generated by the terahertz wave T, and the photodetector 4 is configured to detect the second laser light. However, the optical filter 32 may be configured to pass only the first laser light (λ1) oscillating in a stationary state in which the terahertz wave T is not incident on the QCL element 2, and the photodetector 4 may be configured to detect the first laser light. As described above, when the terahertz wave T is incident on the light incident surface 2a, it is considered that the oscillation degree of the second laser light increases while the oscillation degree of the first laser light decreases. Therefore, even when the photodetector 4 is configured to detect the first laser light, it is possible to detect the difference in the detection amount of the first laser light between the first state and the second state, and the terahertz wave T can be detected based on the difference in the detection amount. In other words, when the detection amount of the first laser light decreases, it is possible to determine that the terahertz wave T is incident on the QCL element 2.

[0095] Furthermore, in the terahertz wave detecting device 10, if a difference occurs in the detected amount of the first laser light or the second laser light between the first state and the second state, the difference can be detected to detect the terahertz wave T. Therefore, the QCL element 2 only needs to be configured so that at least a difference occurs in the detected amount of the first laser light or the second laser light between the first state and the second state, and may be configured so that both the first laser light and the second laser light are oscillated in both the first state and the second state.

[0096] Furthermore, in order to detect the terahertz wave to be detected using the terahertz wave detection device 10, the QCL element 2 can be arranged so that the light incident surface 2a of the QCL element 2 is located on the optical path of the terahertz wave, and the device can be applied to various optical systems other than the optical system 1 of the above embodiment.

[0097] In addition, in the above embodiment, the light incident surface 2a through which the terahertz waves T are incident on the QCL element 2 and the light emitting surface 2b through which the mid-infrared light (laser light L) is emitted are different from each other, but the light incident surface and the light emitting surface may be the same surface. For example, in the above embodiment, the terahertz waves T may be incident on the light emitting surface 2b. In this case, the light emitting surface 2b functions as a surface that emits the mid-infrared light and also functions as a light incident surface on which the terahertz waves T are incident. [Explanation of symbols]

[0098] 1...optical system, 2...QCL element, 2a...light incident surface, 2b...light exit surface, 3...optical component, 4...photodetector, 5...optical chopper (switch), 6...lock-in amplifier (measurement device), 7...light source, 8...polarizing element, 9...collecting lens, 10...terahertz wave detection device, 11...control unit, 21...substrate, 21a...end surface (first end surface), 21b...end surface (second end surface), 50...unit laminate, 51...light emitting layer, 52...injection layer, 224...active layer, D1...direction (first direction), D2...direction (second direction), L...laser light, T...terahertz wave.

Claims

1. A quantum cascade laser element having a light incident surface on which the terahertz wave to be detected is incident and a light emitting surface from which a laser light different from the terahertz wave is emitted; an optical member arranged so that the laser light emitted from the light emitting surface passes through; a photodetector that detects the laser light that has passed through the optical member, The quantum cascade laser device includes: A substrate; a semiconductor layer including an active layer provided on the substrate, the active layer having a cascade structure in which unit laminate bodies each including a light emitting layer and an injection layer are laminated in multiple stages, the light emitting layer and the injection layer being alternately laminated; The unit laminate body has a first emission upper level, a second emission upper level that is an energy level higher than the first emission upper level, and an emission lower level in a subband level structure, and is configured to be able to oscillate a first laser light which is mid-infrared light of a first wavelength generated by a transition from the first emission upper level to the emission lower level, and a second laser light which is mid-infrared light of a second wavelength generated by a transition from the second emission upper level to the emission lower level, the quantum cascade laser element is configured to change an oscillation degree of the first laser light and the second laser light in response to the terahertz wave being incident on the light incident surface, The optical member is configured to cause either the first laser light or the second laser light to be incident on the photodetector.

2. a switch that is disposed upstream of the light incident surface on an optical path of the terahertz wave and that periodically switches between an on state in which the terahertz wave passes and an off state in which the terahertz wave does not pass; 2. The terahertz wave detection device according to claim 1, further comprising a measuring instrument that acquires a first detection signal detected by the photodetector in the on state and a second detection signal detected by the photodetector in the off state based on a signal indicating a repetition frequency of the switch.

3. The terahertz wave detection device according to claim 1 , wherein the photodetector is any one of an MCT detector, an InAs detector, an InAsSb detector, and a quantum well type infrared detector.

4. 2. The terahertz wave detection device of claim 1, wherein the quantum cascade laser element is configured such that, when a predetermined drive current corresponding to the terahertz wave is injected, in a first state in which the terahertz wave is not incident on the light incident surface, the first laser light oscillates while the second laser light does not oscillate, and in a second state in which the terahertz wave is incident on the light incident surface, the second laser light oscillates.

5. the substrate has a first end face which is an end face of the substrate on one side in a first direction which is a resonance direction of the quantum cascade laser element and constitutes the light incident surface, and a second end face which is located on the opposite side to the first end face in the first direction, The terahertz wave detection device according to claim 1 , wherein the first end face is inclined with respect to the first direction so as to move away from the active layer toward the second end face along the first direction.

6. The terahertz wave detection device according to claim 5 , wherein an inclination angle of the first end face with respect to a plane perpendicular to the first direction is substantially equal to a Cherenkov radiation angle corresponding to the active layer.

7. A terahertz wave detection method for detecting a terahertz wave of a detection target using the terahertz wave detection device according to claim 1, comprising the steps of: continuously acquiring a signal value indicative of a detection result of the photodetector during a predetermined time period; detecting the terahertz wave in response to detection of a change in the signal value.

8. The terahertz wave detection method according to claim 7 , further comprising a step of setting the polarization plane of the terahertz wave incident on the light incident surface and the polarization plane of the laser light emitted from the light exit surface so as to be parallel to each other.

9. The terahertz wave detection method according to claim 7 , further comprising a step of controlling the operation of the quantum cascade laser element so that a difference between a frequency of the first laser light and a frequency of the second laser light approximately matches a frequency of the terahertz wave.

10. 8. The terahertz wave detection method of claim 7, further comprising a step of controlling operation of the quantum cascade laser element so that in a first state in which the terahertz wave is not incident on the light incident surface, the first laser light oscillates while the second laser light does not oscillate, and in a second state in which the terahertz wave is incident on the light incident surface, the second laser light oscillates.

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