Terahertz wave generating device, terahertz wave imaging device, and terahertz wave generating method

Abstract

To provide a terahertz wave generation device that can efficiently generate a terahertz wave while minimally suppressing light density of a plurality of optical pulse trains mutually different in a frequency.SOLUTION: A terahertz wave generation device 1 comprises: an optical pulse train generation unit 10 that spatially lines and simultaneously outputs a first optical pulse train PT1 and second optical pulse train PT2 mutually different in a pulse interval; a terahertz wave generation unit 20 that is optically coupled to the optical pulse train generation unit 10, receives the first optical pulse train PT1 to cause a first terahertz wave TW1 to generate, and receives the second optical pulse train PT2 to cause a second terahertz wave TW2 different in a frequency from the first terahertz wave TW1 to generate; and a converging optical system 30 that is arranged on an optical path between the optical pulse train generation unit 10 and the terahertz wave generation unit 20, and causes the first optical pulse train PT1 and second optical pulse train PT2 outputted from the optical pulse train generation unit 10 to be converged to the same one spot 21 in the terahertz wave generation unit 20.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Non-Patent Document 1 discloses a technique for generating terahertz waves by irradiating a nonlinear optical crystal with an optical pulse train. Non-Patent Document 2 discloses a technique for generating frequency-tunable terahertz waves with energy on the order of μJ using intensity-modulatable pseudo-sine wave light. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] J. Ahn, AV Efimov, RD Averitt, and AJ Taylor, "Terahertzwaveform synthesis via optical rectification of shaped ultrafast laser pulses", Optics Express, Vol. 11, No. 20, 2486 (2003) [Non-patent document 2] Zhao Chen, Xibin Zhou, Christopher A. Werley, and Keith A. Nelson, "Generation of high power tunable multicycle teraherz pulses", Applied Physics Letters,Vol.99, 071102 (2011) Summary of the Invention [Problem to be solved by the invention]

[0004] A conventional technique is known in which a nonlinear optical crystal is irradiated with a train of optical pulses having a pulse width on the order of femtoseconds as excitation light, and terahertz waves are generated by difference frequency generation in the nonlinear optical crystal (see Non-Patent Document 1). To obtain multiple terahertz waves with different frequencies, it is conceivable to irradiate multiple optical pulse trains with different pulse intervals onto corresponding individual nonlinear optical crystals (or onto different positions on the nonlinear optical crystal). However, in order to efficiently generate terahertz waves, it is desirable to increase the optical density of the excitation light. When irradiating multiple optical pulse trains onto corresponding individual nonlinear optical crystals (or onto different positions on the nonlinear optical crystal), the optical density of the multiple optical pulse trains must be increased for each optical pulse train, which requires a large overall amount of energy.

[0005] One embodiment of the present disclosure has been made in consideration of these problems, and aims to provide a terahertz wave generator, a terahertz wave imaging device, and a terahertz wave generating method that can efficiently generate terahertz waves while keeping the optical density of each of a plurality of optical pulse trains having different pulse intervals low. [Means for solving the problem]

[0006] In order to solve the above-described problems, a terahertz wave generating device according to an embodiment of the present disclosure includes: an optical pulse train generating unit that spatially arranges and simultaneously outputs a first optical pulse train and a second optical pulse train, the first optical pulse train and the second optical pulse train having different pulse intervals; a terahertz wave generating unit that is optically coupled to the optical pulse train generating unit and receives the first optical pulse train to generate a first terahertz wave, and receives the second optical pulse train to generate a second terahertz wave having a frequency different from that of the first terahertz wave; and a focusing optical system that is arranged on an optical path between the optical pulse train generating unit and the terahertz wave generating unit and focuses the first optical pulse train and the second optical pulse train output from the optical pulse train generating unit onto the same spot within the terahertz wave generating unit.

[0007] A terahertz wave generating method according to an embodiment of the present disclosure includes the steps of spatially arranging and simultaneously outputting a first optical pulse train and a second optical pulse train, the pulse intervals of which are different from each other, and generating a first terahertz wave upon receiving the first optical pulse train, and generating a second terahertz wave having a frequency different from that of the first terahertz wave upon receiving the second optical pulse train, wherein in the generating step, the first optical pulse train and the second optical pulse train are focused onto a single spot, and the first terahertz wave and the second terahertz wave are generated at the single spot.

[0008] In these terahertz wave generating devices and terahertz wave generating methods, a first optical pulse train and a second optical pulse train, which have different pulse intervals and are spatially aligned and simultaneously output, are focused on the same spot, and the first terahertz wave and the second terahertz wave are generated at the spot. This allows the optical density at the single spot to be increased while keeping the optical density of each of the first optical pulse train and the second optical pulse train low, thereby enabling efficient generation of terahertz waves.

[0009] In the above-described terahertz wave generator, the terahertz wave generating unit may include one or more selected from the group consisting of ZnTe crystal, PPLN crystal, PPLT crystal, GaSe crystal, GaP crystal, DAST crystal, DASC crystal, DSTMS crystal, a photoconductive antenna, and HMQ-TMS. In this case, the first terahertz wave and the second terahertz wave can be suitably generated in response to the first optical pulse train and the second optical pulse train.

[0010] In the above-described terahertz wave generator, the optical pulse train generation unit may include a pulse light source that outputs an initial optical pulse, and a waveform shaper optically coupled to the pulse light source that generates a first optical pulse train and a second optical pulse train in parallel from the initial optical pulse. The waveform shaper may include a spatial light modulator that modulates the spectral phase of the initial optical pulse to generate the first optical pulse train and the second optical pulse train. In this case, the first optical pulse train and the second optical pulse train can be suitably generated by controlling the modulation pattern presented to the spatial light modulator.

[0011] The above-described terahertz wave generator may further include a multiplexing unit that multiplexes the first terahertz wave and the second terahertz wave output from the terahertz wave generating unit. Since the first terahertz wave and the second terahertz wave have different frequencies, a terahertz wave pulse can be generated by combining them. In this case, the terahertz wave generator may further include a time delay control unit that controls the time delay between the first optical pulse train and the second optical pulse train. In this case, the waveform of the terahertz wave pulse can be easily changed.

[0012] A terahertz wave imaging device according to an embodiment of the present disclosure includes any one of the terahertz wave generators described above and an imaging unit that captures first and second terahertz waves irradiated from the terahertz wave generator onto an object and output from the object. This terahertz wave imaging device can efficiently generate terahertz waves by increasing the light density at the spot of the terahertz wave generating unit while keeping the light density of each of the first and second optical pulse trains low. Furthermore, spectroscopic imaging using terahertz waves can be suitably performed. [Effects of the Invention]

[0013] According to a terahertz wave generating device, a terahertz wave imaging device, and a terahertz wave generating method according to an embodiment of the present disclosure, terahertz waves can be efficiently generated while keeping the optical density of each of a plurality of optical pulse trains having different pulse intervals low. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram schematically illustrating the configuration of a terahertz wave generating device according to a first embodiment of the present disclosure. [Figure 2] Parts (a) and (b) of FIG. 2 show examples of the time intensity waveforms of the first optical pulse train and the second optical pulse train, respectively. [Figure 3]FIG. 3 is a diagram illustrating an example of the configuration of a waveform shaper. [Figure 4] FIG. 4 is a diagram showing the modulation surface of the spatial light modulator. [Figure 5] Part (a) of Fig. 5 shows the spectral waveform of the initial optical pulse, and part (b) of Fig. 5 shows the temporal intensity waveform of the initial optical pulse. [Figure 6] Part (a) of Figure 6 shows the spectral waveform of an optical pulse train when a rectangular-wave spectral phase modulation is applied by a spatial light modulator, and part (b) of Figure 6 shows the temporal intensity waveform of the optical pulse train. [Figure 7] Fig. 7 is a diagram showing an example of an optical pulse train. Part (a) of Fig. 7 is a spectrogram, with the horizontal axis representing time and the vertical axis representing wavelength, and the optical intensity is represented by the shade of color. Part (b) of Fig. 7 shows the time waveform of the optical pulse train. Part (c) of Fig. 7 shows the spectrum obtained by combining three optical pulses. [Figure 8] Parts (a) and (b) of FIG. 8 show examples of the time intensity waveforms of the first terahertz wave and the second terahertz wave, respectively. [Figure 9] FIG. 9 is a flowchart showing the terahertz wave generating method according to the first embodiment. [Figure 10] FIG. 10 is a diagram illustrating a schematic configuration of a data creation device. [Figure 11] FIG. 11 is a diagram illustrating an example of the hardware configuration of a data creation device. [Figure 12] FIG. 12 is a block diagram showing the internal configuration of the spectrum design unit. [Figure 13] FIG. 13 is a block diagram showing the procedure for calculating the phase spectral function in the spectrum design unit. [Figure 14] FIG. 14 is a diagram showing, by means of equations, the procedure for calculating the phase spectral function in the spectrum design unit. [Figure 15]Part (a) of Fig. 15 is a graph schematically showing an intensity spectrum function and a phase spectrum function as examples of initial spectrum functions, and part (b) of Fig. 15 is a graph schematically showing a time intensity waveform function and a time phase waveform function of a second waveform function in the first cycle, which are Fourier transformed from the first waveform function shown in part (a) of Fig. 15. [Figure 16] Parts (a) and (b) of FIG. 16 are diagrams showing an intensity spectrogram and a phase spectrogram, respectively, converted from the second waveform function shown in part (b) of FIG. [Figure 17] Part (a) of Fig. 17 is a graph schematically showing the time intensity waveform function and time phase waveform function of the third waveform function in the first cycle, which have been subjected to inverse STFT from the intensity spectrogram and phase spectrogram shown in parts (c) and (d) of Fig. 16. Part (b) of Fig. 17 is a graph schematically showing the intensity spectrum function and phase spectrum function of the fourth waveform function in the first cycle, which have been subjected to inverse Fourier transform from the third waveform function shown in part (a) of Fig. 17. [Figure 18] Part (a) of Fig. 18 is a graph schematically showing the intensity spectrum function and phase spectrum function of the first waveform function at the third cycle (n=3) after part (b) of Fig. 17. Part (b) of Fig. 18 is a graph schematically showing the time intensity waveform function and time phase waveform function of the second waveform function at the third cycle, which are Fourier transformed from the first waveform function shown in part (a) of Fig. 18. [Figure 19] Parts (a) and (b) of Fig. 19 respectively show an intensity spectrogram and a phase spectrogram at the third turn, which are converted from the second waveform function shown in part (b) of Fig. 18. Part (c) of Fig. 19 shows an intensity spectrogram at the third turn. Part (d) of Fig. 19 shows a constrained phase spectrogram at the third turn. [Figure 20]Part (a) of Fig. 20 is a graph schematically showing the time intensity waveform function and time phase waveform function of the third waveform function at the third revolution, which have been subjected to the inverse STFT from the intensity spectrogram and phase spectrogram shown in parts (c) and (d) of Fig. 19. Part (b) of Fig. 20 is a graph schematically showing the intensity spectrum function and phase spectrum function of the fourth waveform function at the third revolution, which have been subjected to the inverse Fourier transform from the third waveform function shown in part (a) of Fig. 20. [Figure 21] FIG. 21 is a block diagram showing the functional configuration of an apparatus for creating an intensity spectrogram. [Figure 22] Part (a) of Figure 22 is a graph showing an intensity spectrum function and a phase spectrum function as examples of target waveform functions, and part (b) of Figure 22 is a graph showing a time intensity waveform function and a time phase waveform function generated from the target waveform function shown in part (a) of Figure 22 as examples of time domain waveform functions. [Figure 23] Part (a) of Fig. 23 is a diagram showing, as an example of an intensity spectrogram, an intensity spectrogram generated from the waveform function shown in part (b) of Fig. 22. Part (b) of Fig. 23 is a diagram showing an intensity spectrogram obtained by superimposing the intensity spectrograms for three optical pulses PL on each other. [Figure 24] FIG. 24 is a diagram showing a configuration of a comparative example. [Figure 25] FIG. 25 is a diagram showing a configuration of a comparative example. [Figure 26] FIG. 26 shows the entire spectrum of the terahertz wave. [Figure 27] FIG. 27 shows the spectrum of a terahertz wave in a part of space. [Figure 28] FIG. 28 shows the spectrum of terahertz waves in another part of space. [Figure 29] FIG. 29 is a diagram schematically illustrating the configuration of a terahertz wave generating device according to the second embodiment of the present disclosure. [Figure 30] Parts (a), (b), and (c) of FIG. 30 show examples of the time intensity waveform of the terahertz wave generated at the spot. [Figure 31] FIG. 31 is a diagram schematically illustrating the configuration of a terahertz wave imaging device according to the third embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of a terahertz wave generating device, a terahertz wave imaging device, and a terahertz wave generating method according to the present disclosure will be described in detail with reference to the accompanying drawings. Note that in the description of the drawings, identical elements are designated by the same reference numerals, and duplicated explanations will be omitted. [First embodiment]

[0016] 1 is a diagram schematically illustrating the configuration of a terahertz wave generator 1 according to a first embodiment of the present disclosure. The terahertz wave generator 1 of this embodiment is a device that outputs a plurality of terahertz waves TW, including a first terahertz wave TW1 and a second terahertz wave TW2, in a spatially aligned manner. As shown in FIG. 1, the terahertz wave generator 1 includes an optical pulse train generating unit 10, a terahertz wave generating unit 20, a focusing optical system 30, and a collimating optical system 40.

[0017] The optical pulse train generation unit 10 outputs a plurality of optical pulse trains PT including a first optical pulse train PT1 and a second optical pulse train PT2. Parts (a) and (b) of FIG. 2 show examples of the time intensity waveforms of the first optical pulse train PT1 and the second optical pulse train PT2, respectively. In these figures, the horizontal axis represents time, and the vertical axis represents optical intensity. Each of the plurality of optical pulse trains PT includes a plurality of optical pulses PL generated at a constant time interval ΔT. The pulse intervals of the plurality of optical pulse trains PT, i.e., the time interval ΔT between the intensity peaks of temporally adjacent optical pulses PL, are set for each optical pulse train PT and differ from one another among the optical pulse trains PT. In other words, the periods of the plurality of optical pulse trains PT are set for each optical pulse train PT and differ from one another among the optical pulse trains PT. The optical pulse train generation unit 10 simultaneously outputs a plurality of optical pulses PL spatially arranged in a plane intersecting the optical axis.

[0018] The optical pulse train generating unit 10 includes a pulse light source 11 and a waveform shaper 12. The pulse light source 11 outputs an initial optical pulse La. The pulse light source 11 is, for example, a laser light source such as a solid-state laser light source, and the initial optical pulse La is, for example, a coherent single optical pulse. The pulse light source 11 is, for example, a femtosecond laser, and in one embodiment, a LD-directly pumped Yb:YAG pulse laser. The time intensity waveform of the initial optical pulse La is, for example, a Gaussian function. The full width at half maximum (FWHM) of the initial optical pulse La is, for example, within a range of 10 fs to 10,000 fs, and in one example, is 100 fs. The initial optical pulse La has a certain bandwidth and includes multiple continuous wavelength components. In one embodiment, the bandwidth of the initial optical pulse La is 10 nm, and the center wavelength of the initial optical pulse La is 800 nm.

[0019] The waveform shaper 12 is optically coupled to the pulse light source 11. The waveform shaper 12 generates a plurality of optical pulse trains PT in parallel from an initial optical pulse La. The waveform shaper 12 simultaneously outputs a plurality of optical pulses PL, spatially arranging them in a plane intersecting the optical axis. The waveform shaper 12 has a spatial light modulator (SLM) 13, which receives a control signal SC from a control unit 18. The SLM 13 modulates the spectral phase of the initial optical pulse La to generate a plurality of optical pulse trains PT. The control signal SC is generated based on data for controlling the SLM 13, i.e., data including the intensity of a complex amplitude distribution or the intensity of a phase distribution. The data is, for example, a computer-generated hologram (CGH).

[0020] 3 is a diagram showing an example configuration of the waveform shaper 12. The waveform shaper 12 has a diffraction grating 121, a lens 122, an SLM 13, a lens 123, and a diffraction grating 124. The diffraction grating 121 is a spectroscopic element, and is optically coupled to the pulse light source 11. The SLM 13 is optically coupled to the diffraction grating 121 via the lens 122. The diffraction grating 121 spatially separates the multiple wavelength components contained in the initial optical pulse La into individual wavelengths. Note that instead of the diffraction grating 121, other optical components such as a prism may be used as the spectroscopic element.

[0021] The initial light pulse La is incident obliquely on the diffraction grating 121 and is split into multiple wavelength components. The light Lb containing these multiple wavelength components is condensed into each wavelength component by the lens 122 and is imaged on the modulation surface of the SLM 13. The lens 122 may be a convex lens made of a light-transmitting member, or may be a concave mirror having a concave light-reflecting surface.

[0022] The SLM 13 shifts the phases of the multiple wavelength components output from the diffraction grating 121 to convert the initial light pulse La into a train of multiple light pulses PT. To do this, the SLM 13 receives a control signal SC from the control unit 18 and simultaneously performs phase modulation and intensity modulation of the light Lb. Note that the SLM 13 may perform only phase modulation or only intensity modulation. The SLM 13 is, for example, a phase modulation type. In one embodiment, the SLM 13 is an LCOS (Liquid Crystal on Silicon) type. Note that while the figure shows a transmissive SLM 13, the SLM 13 may also be a reflective type. Furthermore, the SLM 13 is not limited to a phase modulation type spatial light modulator, but may also be an intensity modulation type spatial light modulator such as a DMD (Digital Micro Mirror Device) or a phase-intensity modulation type spatial light modulator.

[0023] FIG. 4 is a diagram showing the modulation surface 17 of the SLM 13. As shown in FIG. 4, on the modulation surface 17, multiple modulation regions 17a are arranged along a direction AA, and each modulation region 17a extends in a direction AB that intersects with the direction AA. The direction AA is the direction of light dispersion by the diffraction grating 121. The modulation surface 17 functions as a Fourier transform plane, and each of the multiple modulation regions 17a is incident on a corresponding wavelength component after dispersion. The SLM 13 modulates the phase and intensity of each incident wavelength component in each modulation region 17a independently from the other wavelength components. Note that the SLM 13 of this embodiment is a phase modulation type, and therefore intensity modulation is achieved by a phase pattern (phase image) presented on the modulation surface 17.

[0024] Each wavelength component of modulated light Lc modulated by SLM 13 is focused by lens 123 to a single point on diffraction grating 124. Lens 123 and diffraction grating 124 function as an optical system that focuses modulated light Lc. Lens 123 may be a convex lens made of a light-transmitting member, or a concave mirror with a concave light-reflecting surface. Diffraction grating 124 also functions as a combining optical system that combines each modulated wavelength component. That is, lens 123 and diffraction grating 124 focus and combine the multiple wavelength components of modulated light Lc to form a multiple optical pulse train PT. When SLM 13 is a reflective type, lenses 122 and 15 may be formed by a common lens, and diffraction gratings 121 and 16 may be formed by a common diffraction grating.

[0025] The region in front of the lens 123 (spectral region) and the region behind the diffraction grating 124 (time domain) have a Fourier transform relationship with each other, and the distribution of phase modulation in the spectral domain affects the spatial distribution of the time-intensity waveform in the time domain. Therefore, the multiple optical pulse trains PT can have desired time-intensity waveforms (pulse widths and pulse intervals) that are different from the initial optical pulses La and that differ from each other according to the modulation pattern of the SLM 13. Part (a) of FIG. 5 shows, as an example, the spectral waveform (spectral phase G11 and spectral intensity G12) of a single-pulse initial optical pulse La, and part (b) of FIG. 5 shows the time-intensity waveform of the initial optical pulse La. Part (a) of FIG. 6 shows, as an example, the spectral waveform (spectral phase G21 and spectral intensity G22) of the optical pulse train PT when a rectangular-wave spectral phase modulation is applied by the SLM 13, and part (b) of FIG. 6 shows the time-intensity waveform of the optical pulse train PT. In part (a) of Figure 5 and part (a) of Figure 6, the horizontal axis represents wavelength (nm), the left vertical axis represents intensity values ​​(arbitrary units) of the intensity spectrum, and the right vertical axis represents phase values ​​(rad) of the spectral phase. Also, in part (b) of Figure 5 and part (b) of Figure 6, the horizontal axis represents time (femtoseconds), and the vertical axis represents optical intensity (arbitrary units). In this example, by providing an initial optical pulse La with a rectangular phase spectral waveform, the single pulse of the initial optical pulse La is converted into a double pulse accompanied by higher-order light as an optical pulse train PT. Note that the spectrum and waveform shown in Figure 6 are just one example, and the pulse width and pulse interval of the optical pulse train PT can be shaped to various lengths by combining various spectral phases and spectral intensities.

[0026] FIG. 7 is a diagram showing an example of an optical pulse train PT. In this example, an optical pulse train PT consisting of three optical pulses PL is shown. Part (a) of FIG. 7 is a spectrogram, with the horizontal axis representing time and the vertical axis representing wavelength, and the optical intensity being represented by the shade of color. Part (b) of FIG. 7 shows the time waveform of the optical pulse train PT. The time waveform of each optical pulse PL is, for example, in the form of a Gaussian function.

[0027] As shown in parts (a) and (b) of Figure 7, the peaks of the three light pulses PL are spaced apart in time, and the propagation timings of the three light pulses PL are shifted from one another. In other words, one light pulse PL is time-delayed relative to another light pulse PL, and yet another light pulse PL is time-delayed relative to the other light pulse PL. However, the tail portions of adjacent light pulses PL may overlap. The time interval (peak interval) between adjacent light pulses PL is, for example, within a range of 10 fs to 10,000 fs, and is, for example, 2,000 fs. Furthermore, the FWHM of each light pulse PL is, for example, within a range of 10 fs to 5,000 fs, and is, for example, 300 fs.

[0028] Part (c) of Fig. 7 shows the spectrum obtained by combining three light pulses PL. As shown in part (c) of Fig. 7, the spectrum obtained by combining three light pulses PL has a single peak. The spectrum having a single peak shown in part (c) of Fig. 7 is almost the same as the spectrum of the initial light pulse La. Note that, although the central wavelengths of the three light pulses PL match each other in the example shown in part (a) of Fig. 7, the central wavelengths of the three light pulses PL may be shifted from each other.

[0029] Referring again to FIG. 1, the terahertz wave generation unit 20 is optically coupled to the optical pulse train generation unit 10. The terahertz wave generation unit 20 receives a plurality of optical pulse trains PT and generates a plurality of terahertz waves TW having different frequencies. That is, the terahertz wave generation unit 20 receives a first optical pulse train PT1 and generates a first terahertz wave TW1, which is one of the plurality of terahertz waves TW. The terahertz wave generation unit 20 also receives a second optical pulse train PT2 and generates a second terahertz wave TW2, which is one of the plurality of terahertz waves TW. Parts (a) and (b) of FIG. 8 show examples of the time-intensity waveforms of the first terahertz wave TW1 and the second terahertz wave TW2, respectively. In these figures, the horizontal axis represents time, and the vertical axis represents intensity. The frequency of the second terahertz wave TW2 is different from the frequency of the first terahertz wave TW1. The frequency of each terahertz wave TW is determined according to the pulse interval of the corresponding optical pulse train PT. The intensity of each terahertz wave TW is determined according to the optical intensity of the corresponding optical pulse train PT. The terahertz wave generation unit 20 simultaneously outputs a plurality of terahertz waves TW spatially arranged in a plane intersecting the optical axis. At this time, the arrangement order of the plurality of terahertz waves TW is reversed to the arrangement order of the corresponding plurality of optical pulse trains PT when viewed from the optical axis direction.

[0030] The terahertz wave generating unit 20 may be a nonlinear optical crystal capable of generating terahertz waves, a device capable of generating terahertz waves, or a spatial phenomenon capable of generating terahertz waves. The nonlinear optical crystal may be an inorganic nonlinear optical crystal or an organic nonlinear optical crystal.

[0031] The inorganic nonlinear optical crystal includes one or more crystals selected from the group consisting of zinc telluride (ZnTe) crystal, periodically poled lithium niobate (PPLN) crystal, periodically poled lithium tantalate (PPLT) crystal, gallium selenide (GaSe) crystal, and gallium phosphide (GaP) crystal. The organic nonlinear optical crystal includes one or more crystals selected from the group consisting of DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) crystal, DASC (4-dimethylamino-N-methyl-4-stilbazolium-chlorobenzenesulfonate) crystal, DSTMS (4-N,N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate) crystal, and HMQ-TMS (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 2,4,6-trimethylbenzenesulfonate). A nonlinear optical crystal is excited by an optical pulse train PT and generates terahertz waves by difference frequency generation. Devices capable of generating terahertz waves include photoconductive antennas. A photoconductive antenna has a photoconductive film and a pair of electrodes. The electrode structure can be, for example, a dipole, stripline, or spiral type. Free electrons in the photoconductive film are excited by the optical pulse train PT. The free electrons are accelerated by a voltage applied between the pair of electrodes, thereby generating terahertz waves TW. Spatial phenomena that can generate terahertz waves include air plasma. When high-intensity ultrashort optical pulses, which serve as the optical pulse train PT, are focused in a gas such as neon (Ne) or in air to generate plasma, terahertz waves TW are generated from the plasma.

[0032] The focusing optical system 30 is disposed on an optical path along which the plurality of optical pulse trains PT propagate between the optical pulse train generation unit 10 and the terahertz wave generation unit 20. That is, the focusing optical system 30 is optically coupled to the optical pulse train generation unit 10 and the terahertz wave generation unit 20. The focusing optical system 30 receives the plurality of optical pulse trains PT, including the first optical pulse train PT1 and the second optical pulse train PT2, output from the optical pulse train generation unit 10, and focuses the plurality of optical pulse trains PT at the same single spot 21 in the terahertz wave generation unit 20. In other words, the plurality of optical pulse trains PT are focused by the focusing optical system 30 at the single spot 21 in the terahertz wave generation unit 20. The focusing optical system 30 includes a lens 31. The lens 31 may be a light-transmitting convex lens or a light-reflecting lens (concave mirror). The condensed diameter (minimum diameter) of the spot 21 in a plane perpendicular to the optical axis of the lens 31 is, for example, 1 μm or more and 10 mm or less.

[0033] The collimating optical system 40 is optically coupled to the terahertz wave generating unit 20, and is provided at a position sandwiching the terahertz wave generating unit 20 between it and the focusing optical system 30. The collimating optical system 40 receives the plurality of terahertz waves TW output from the terahertz wave generating unit 20, collimates the plurality of terahertz waves TW, and outputs the parallelized (collimated) waves. The collimating optical system 40 includes a lens 41. The lens 41 may be a light-transmitting convex lens or a light-reflecting lens (concave mirror). The plurality of terahertz waves TW are output in a state where they are spatially aligned in a plane intersecting the optical axis of the lens 41.

[0034] 9 is a flowchart showing a terahertz wave generating method according to this embodiment. This terahertz wave generating method is preferably carried out using the terahertz wave generating device 1 described above.

[0035] First, in step ST1, a plurality of optical pulse trains PT, each having a different pulse interval, are spatially aligned and simultaneously output from the optical pulse train generation unit 10. The plurality of optical pulse trains PT include a first optical pulse train PT1 and a second optical pulse train PT2, each having a different pulse interval. Next, in step ST2, the terahertz wave generation unit 20 receives the plurality of optical pulse trains PT and generates a plurality of terahertz waves TW. For example, the terahertz wave generation unit 20 receives the first optical pulse train PT1 to generate a first terahertz wave TW1, and receives the second optical pulse train PT2 to generate a second terahertz wave TW2. At this time, the plurality of optical pulse trains PT are focused by the focusing optical system 30 onto the same single spot 21, and a plurality of terahertz waves TW are generated at the single spot 21. Then, in step ST3, the plurality of terahertz waves TW output from the terahertz wave generation unit 20 are collimated by the collimating optical system 40.

[0036] Here, a method and apparatus for generating data related to the phase modulation pattern presented to the SLM 13 will be described in detail. FIG. 10 is a diagram illustrating the configuration of the data generating device 6. The data generating device 6 is electrically connected to the control unit 18 and generates data related to a phase modulation pattern for adjusting the time intensity waveform of a plurality of optical pulse trains PT, particularly the pulse width and pulse interval, to desired values, and provides the phase modulation pattern to the control unit 18. The control unit 18 then provides a control signal SC containing the data to the SLM 13. The data generating device 6 includes a spectrogram setting unit 6a, a spectrum design unit 6b, and a data generating unit 6c. The data generating device 6 may be, for example, a personal computer; a smart device such as a smartphone or tablet; or a computer with a processor, such as a cloud server. The data generating device 6 may also function as the control unit 18.

[0037] Fig. 11 is a diagram schematically illustrating an example of the hardware configuration of the data creation device 6. As shown in Fig. 11, the data creation device 6 can be physically configured as a normal computer including a processor (CPU) 61, main storage devices such as a ROM 62 and a RAM 63, input devices 64 such as a keyboard, mouse, and touch screen, output devices 65 such as a display (including a touch screen), a communication module 66 such as a network card for transmitting and receiving data to and from other devices, and an auxiliary storage device 67 such as a hard disk.

[0038] The computer processor 61 can realize each function of the data creation device 6 by using a data creation program. Therefore, the data creation program causes the computer processor 61 to operate as the spectrogram setting unit 6a, the spectrum design unit 6b, and the data generating unit 6c in the data creation device 6. The modulation pattern calculation program is stored in a storage device (storage medium) inside or outside the computer. The storage device may be a non-transitory recording medium. Examples of the recording medium include recording media such as a flexible disk, CD, DVD, recording media such as ROM, semiconductor memory, and cloud server.

[0039] The spectrogram setting unit 6a accepts input of data related to a desired intensity spectrogram (target intensity spectrogram) from an operator. The operator inputs data related to a target intensity spectrogram prepared in advance to the spectrogram setting unit 6a. Alternatively, the spectrogram setting unit 6a may store a target intensity spectrogram in advance. The data related to the target intensity spectrogram is provided to the spectrum design unit 6b. The spectrum design unit 6b calculates a phase spectrum function suitable for realizing the provided target intensity spectrogram.

[0040] Based on the phase spectral function determined by the spectrum design unit 6b, the data generation unit 6c calculates a phase modulation pattern (e.g., a computer-generated hologram) to be applied to the initial optical pulse La in the SLM 13, and generates data related to the phase modulation pattern. This data is then provided to the control unit 18, and a control signal SC including the phase modulation pattern is provided from the control unit 18 to the SLM 13. The SLM 13 is controlled based on the control signal SC.

[0041] Fig. 12 is a block diagram showing the internal configuration of the spectrum design unit 6b. As shown in Fig. 12, the spectrum design unit 6b has a Fourier transform unit 6b1, a spectrogram conversion unit 6b2, a spectrogram substitution unit 6b3, a spectrogram inverse conversion unit 6b4, and an inverse Fourier transform unit 6b5. The spectrum design unit 6b calculates a phase spectral function that forms the basis of the modulation pattern using the calculation method described below. Fig. 13 is a block diagram showing the calculation procedure of the phase spectral function in the spectrum design unit 6b. Fig. 14 is a diagram showing the calculation procedure of the phase spectral function in the spectrum design unit 6b using mathematical expressions.

[0042] First, an initial spectral function A1, i.e., an initial intensity spectral function A0(ω) and a phase spectral function Φ0(ω), which are functions of frequency ω, are prepared. In one example, these intensity spectral function A0(ω) and phase spectral function Φ0(ω) represent the intensity spectrum and spectral phase of the initial optical pulse La, respectively, but are not limited to this. Part (a) of FIG. 15 is a graph schematically showing the intensity spectral function A0(ω) and the phase spectral function Φ0(ω) as an example of the initial spectral function A1. In part (a) of FIG. 15, graph G31 represents the intensity spectral function A0(ω), and graph G32 represents the phase spectral function Φ0(ω). The horizontal axis represents wavelength, and the vertical axis represents the intensity value of the intensity spectral function or the phase value of the phase spectral function.

[0043] The first waveform function A2 is expressed by the following formula (1), where n is the repetition number (n=1, 2, . . . , N) and i is an imaginary number. In the first iteration (n=1), the initial spectral function A1 is set to the first waveform function A2. That is, the first waveform function A2 in the first iteration is set to Φ1(ω)=Φ0(ω).

number

number

[0044] Next, spectrogram conversion unit 6b2 of spectrum design unit 6b performs a short-time Fourier transform (STFT) on second waveform function A3 (arrow B2 in the figure), thereby obtaining intensity spectrogram A41 and phase spectrogram A42 expressed by the following equation (3).

number

[0045] The processing performed by the spectrogram conversion unit 6b2 to convert the second waveform function A3 into the intensity spectrogram A41 and the phase spectrogram A42 is not limited to STFT and may be other processing. The process of converting a time-intensity waveform into a spectrogram, including STFT, is called time-frequency transformation. In time-frequency transformation, a composite signal such as a time-intensity waveform is subjected to frequency filtering or numerical calculation processing (multiplying by a window function while shifting it to derive a spectrum for each time) to generate three-dimensional information consisting of time, frequency, and the strength of the signal component (intensity spectrum). In this embodiment, the transformation result (time, frequency, intensity spectrum) is defined as a "spectrogram." In addition to STFT, time-frequency transformations include wavelet transforms (Haar wavelet transform, Gabor wavelet transform, Mexican Hat wavelet transform, and Morlet wavelet transform).

[0046] Next, the spectrogram replacement unit 6b3 of the spectrum design unit 6b replaces the intensity spectrogram A41 with a previously created intensity spectrogram A43 (target intensity spectrogram) and constrains the phase spectrogram A42 (arrow B3 in the figure). The intensity spectrogram A43 is provided from the spectrogram setting unit 6a (see FIG. 1). Constraining the phase spectrogram A42 means that the phase spectrogram A42 is not changed (left as is). As a result, the above equation (3) is replaced with the following equation (4). TSG(ω,t) is the target intensity spectrogram function.

number

[0047] Next, the spectrogram inverse transform unit 6b4 of the spectrum design unit 6b performs an inverse STFT on the intensity spectrogram A43 and the phase spectrogram A42 (arrow B4 in the figure). Note that, as with the spectrogram transform unit 6b2, a time-frequency transform other than the STFT may be used here as well. As a result, the time-intensity waveform function a' shown by the following formula (5) is obtained. n (t) and the time-phase waveform function φ' n A third waveform function A5 in the time domain including (t) is obtained, where n=1 at the first cycle.

number

[0048] Next, the inverse Fourier transform unit 6b5 of the spectrum design unit 6b performs an inverse Fourier transform from the time domain to the frequency domain on the third waveform function A5 (arrow B5 in the figure). As a result, the intensity spectrum function A5 is obtained as shown in the following formula (6): n (ω) and the phase spectrum function Φ' n A fourth waveform function A6 in the frequency domain containing (ω) is obtained, where n=1 in the first round.

number

[0049] Thereafter, spectrum design unit 6b replaces the phase spectral function Φ1(ω) of first waveform function A2 with the phase spectral function Φ'1(ω) of the fourth waveform function while constraining the intensity spectral function A0(ω) of first waveform function A2 (i.e., Φ2(ω)=Φ'1(ω) of first waveform function A2; see arrow B6 in the figure). Then, the operations of the Fourier transform unit 6b1, spectrogram conversion unit 6b2, spectrogram substitution unit 6b3, spectrogram inverse conversion unit 6b4, and inverse Fourier transform unit 6b5 described above are repeated N times until the phase spectral function converges. In this way, spectrum design unit 6b replaces the phase spectral function Φ1(ω) of first waveform function A2 with the phase spectral function Φ'1(ω) of the fourth waveform function while constraining the intensity spectral function A0(ω) of first waveform function A2. n+1 (ω) is the phase spectrum function Φ' of the fourth waveform function at the nth cycle n While replacing with (ω), Fourier transform unit 6b1, spectrogram transform unit 6b2, spectrogram substitution unit 6b3, spectrogram inverse transform unit 6b4, and inverse Fourier transform unit 6b5 operate repeatedly in this order.

[0050] Part (a) of FIG. 18 is a graph schematically illustrating the intensity spectral function A0(ω) (graph G71) and the phase spectral function Φ3(ω) (graph G72) of the first waveform function A2 at the third cycle (n=3) after Part (b) of FIG. 17. It can be seen that the waveform of the phase spectral function Φ3(ω) has changed from the first cycle. Part (b) of FIG. 18 is a graph schematically illustrating the time intensity waveform function a3(t) (graph G81) and the time phase waveform function φ3(t) (graph G82) of the second waveform function A3 at the third cycle, which have been Fourier transformed from the first waveform function A2 shown in Part (a) of FIG. 18. Parts (a) and (b) of FIG. 19 are graphs respectively illustrating the intensity spectrogram A41 and the phase spectrogram A42 at the third cycle, which have been converted from the second waveform function A3 shown in Part (b) of FIG. 18. It can be seen that three optical pulses PL having different center wavelengths and different time differences from one another are beginning to be generated. Part (c) of Figure 19 shows an intensity spectrogram A43 in the third cycle, which is the same as the intensity spectrogram A43 in the first cycle shown in part (c) of Figure 16. Part (d) of Figure 19 shows a constrained phase spectrogram A42 in the third cycle, which is the same as the phase spectrogram A42 in part (b) of Figure 19. Part (a) of Figure 20 is a graph schematically showing the time intensity waveform function a'3(t) (graph G91) and the time phase waveform function φ'3(t) (graph G92) of the third waveform function A5 in the third cycle, which have been subjected to inverse STFT from the intensity spectrogram A43 and the phase spectrogram A42 shown in parts (c) and (d) of Figure 19. Part (b) of Figure 20 is a graph schematically showing the intensity spectrum function A3(ω) (graph G101) and the phase spectrum function Φ'3(ω) (graph G102) of the fourth waveform function A6 at the third rotation, which is obtained by inverse Fourier transform from the third waveform function A5 shown in part (a) of Figure 20.

[0051] By repeating the above operation, the phase spectral function Φ' is calculated so that the intensity spectrogram A41 gradually approaches the intensity spectrogram A43. n(ω) is corrected. Finally, the phase spectrum function Φ' included in the fourth waveform function A6 is N (ω) is the desired spectral phase solution Φ result (ω). This spectral phase solution Φ result (ω) is provided to the data generator 6c.

[0052] The data generating unit 6c generates the spectral phase solution Φ calculated by the spectrum design unit 6b. result Data relating to a phase modulation pattern (for example, a computer-generated hologram) for imparting a spectral phase and / or a spectral intensity based on (ω) to the initial light pulse La is calculated.

[0053] Next, an apparatus for creating a target intensity spectrogram A43 used to create data for controlling the SLM 13 will be described. FIG. 21 is a block diagram showing the functional configuration of an apparatus 100 for creating an intensity spectrogram A43. The apparatus 100 creates a target intensity spectrogram A43 for an optical pulse train PT including a plurality of optical pulses (e.g., the above-mentioned optical pulse PL) having a time difference from one another. As shown in FIG. 21, the apparatus 100 includes a waveform function setting unit 101, a Fourier transform unit 102, a spectrogram conversion unit 103, and a target generation unit 104. The apparatus 100 may have the same hardware configuration as the above-mentioned data creation apparatus 6 (see FIG. 11). In this case, the computer processor 61 can realize each function of the above-mentioned apparatus 100 by a target intensity spectrogram creation program. Therefore, the target intensity spectrogram creation program causes the computer processor 61 to operate as a waveform function setting unit 101 , a Fourier transform unit 102 , a spectrogram conversion unit 103 , and a target generation unit 104 .

[0054] The waveform function setting unit 101 sets a target waveform function in the frequency domain, including an intensity spectral function and a phase spectral function, for each optical pulse. For example, the waveform function setting unit 101 sets a target waveform function for an optical pulse PL, sets another target waveform function for another optical pulse PL, and sets yet another target waveform function for yet another optical pulse PL. Parameters of the target waveform function include, for example, the following: a) The shape of the intensity spectrum function of each light pulse (e.g., Gaussian) b) The spectral energy content of the intensity spectrum function of each light pulse c) The bandwidth (full width at half maximum) of the intensity spectral function of each light pulse d) The central wavelength of the intensity spectrum function of each light pulse e) Phase spectral function of each optical pulse Note that, when the spectral phase of each optical pulse is linear with respect to frequency, the slope of the linear function corresponds to the amount of time shift of each optical pulse. Part (a) of Fig. 22 is a graph showing an intensity spectral function G131 and a phase spectral function G132 as examples of target waveform functions. In part (a) of Fig. 22, the horizontal axis represents wavelength (nm), and the vertical axis represents the intensity value (arbitrary unit) of the intensity spectral function G131 and the phase value (rad) of the phase spectral function G132. Since this example is a target waveform function for the central optical pulse PL of three optical pulses PL that are equally spaced in time, the slope of the phase spectral function G132 (i.e., the amount of time shift) is set to zero.

[0055] The Fourier transform unit 102 transforms the target waveform function of each of the multiple optical pulses into a time-domain waveform function including a time-intensity waveform function and a time-phase waveform function. Part (b) of Fig. 22 is a graph showing, as an example of a time-domain waveform function, a time-intensity waveform function G141 and a time-phase waveform function G142 generated from the target waveform function shown in part (a) of Fig. 22. In part (b) of Fig. 22, the horizontal axis represents time (fs), and the vertical axis represents the intensity value (arbitrary unit) of the time-intensity waveform function G141 and the phase value (rad) of the time-phase waveform function G142.

[0056] The spectrogram conversion unit 103 generates an intensity spectrogram from the waveform function in the time domain of each of the multiple optical pulses. Part (a) of Fig. 23 shows, as an example of an intensity spectrogram, an intensity spectrogram generated from the waveform function shown in part (b) of Fig. 22. The method for creating the intensity spectrogram and the definition of the spectrogram are the same as those described for the spectrogram conversion unit 6b2 of the spectrum design unit 6b. Also, the spectrogram conversion unit 103 is not limited to STFT, and other time-frequency transformations (e.g., wavelet transforms) may be used.

[0057] The target generation unit 104 generates an intensity spectrogram A43 by superimposing the intensity spectrograms of the respective multiple optical pulses on one another. Part (b) of FIG. 23 shows, as an example, an intensity spectrogram A43 obtained by superimposing the intensity spectrograms of three optical pulses PL on one another. Note that the target generation unit 104 may multiply the superimposed intensity spectrograms of the respective multiple optical pulses by a correction coefficient. The correction coefficient is, for example, a coefficient for bringing the spectral intensity distribution of the generated target intensity spectrogram A43 closer to the spectral intensity distribution of the optical pulse of the initial optical pulse La.

[0058] The effects obtained by the terahertz wave generator 1 and terahertz wave generation method of the present embodiment described above will be described. To generate multiple terahertz waves TW with different frequencies, it is possible to prepare multiple terahertz wave generation units 20 and focus an optical pulse train PT on each of them, as shown in FIG. 24 . Alternatively, it is possible to focus an optical pulse train PT at different positions within a single terahertz wave generation unit 20, as shown in FIG. 25 . However, in these methods, in order to efficiently generate terahertz waves, the optical density of the multiple optical pulse trains PT must be increased for each optical pulse train PT, which requires a large amount of energy overall. In contrast, in the terahertz wave generator 1 and terahertz wave generation method of the present embodiment, multiple optical pulse trains PT with different pulse intervals ΔT, spatially aligned, and simultaneously output are focused on the same spot 21 within the terahertz wave generation unit 20. As a result, the optical density at the spot 21 is the sum of the optical densities of the multiple optical pulse trains PT. Therefore, the light density in the spot 21 can be increased while keeping the light density of each optical pulse train PT low, thereby enabling efficient generation of terahertz waves.

[0059] Furthermore, the method shown in FIG. 24 requires a number of terahertz wave generating units 20 corresponding to the number of terahertz waves TW, resulting in a problem of a large number of parts. Furthermore, the method shown in FIG. 25 requires the preparation of large crystals, for example, with a diameter of about 10 mm, but the production of large crystals with high purity is extremely difficult. Furthermore, while large crystals are commercially available, they are also very expensive. According to the terahertz wave generating device 1 and terahertz wave generating method of this embodiment, the number of terahertz wave generating units 20 can be reduced, and production is easy because only small crystals are required.

[0060] It is a previously unknown finding that even when a plurality of optical pulse trains PT, each having a different pulse interval ΔT, are focused at a single point (spot 21) in the terahertz wave generating unit 20, a plurality of terahertz waves TW having frequencies corresponding to the plurality of optical pulse trains PT are generated. FIGS. 26 to 28 are graphs showing the spectra of terahertz waves actually obtained by focusing an optical pulse train having a time interval corresponding to a terahertz wave with a frequency of 1 THz and an optical pulse train having a time interval corresponding to a terahertz wave with a frequency of 2 THz at a single point in a nonlinear optical crystal. FIG. 26 shows the overall spectrum of the terahertz wave. FIG. 27 shows the spectrum of the terahertz wave in a portion of space. FIG. 28 shows the spectrum of the terahertz wave in another portion of space. As shown in FIG. 26, the overall spectrum of the terahertz wave mainly includes frequency components of 1 THz and 2 THz. Furthermore, as shown in Fig. 27, the spectrum of the terahertz waves in a part of the space contains a frequency component of 2 THz, and as shown in Fig. 28, the spectrum of the terahertz waves in another part of the space contains a frequency component of 1 THz. This shows that even when a plurality of optical pulse trains with different time intervals are focused on one point on the terahertz wave generating unit, each terahertz wave has a frequency corresponding to each optical pulse train.

[0061] As in the present embodiment, the terahertz wave generating unit 20 may include one or more selected from the group consisting of ZnTe crystal, PPLN crystal, PPLT crystal, GaSe crystal, GaP crystal, DAST crystal, DASC crystal, DSTMS crystal, photoconductive antenna, and HMQ-TMS. In this case, a plurality of terahertz waves TW can be suitably generated upon receiving a plurality of optical pulse trains PT.

[0062] As in the present embodiment, the optical pulse train generation unit 10 may include a pulse light source 11 that outputs an initial optical pulse La, and a waveform shaper 12 that is optically coupled to the pulse light source 11 and generates a plurality of optical pulse trains PT in parallel from the initial optical pulse La. The waveform shaper 12 may include an SLM 13 that modulates the spectral phase of the initial optical pulse La to generate a plurality of optical pulse trains PT. In this case, the plurality of optical pulse trains PT can be suitably generated by controlling the modulation pattern presented to the SLM 13. [Second embodiment]

[0063] FIG. 29 is a diagram schematically illustrating a configuration of a terahertz wave generator 2 according to a second embodiment of the present disclosure. The terahertz wave generator 2 includes a multiplexing unit 70 in addition to the configuration of the terahertz wave generator 1 according to the first embodiment. The multiplexing unit 70 is disposed after the collimating optical system 40, i.e., after the terahertz wave generation unit 20, and is optically coupled to the terahertz wave generation unit 20 with the collimating optical system 40 sandwiched therebetween. The multiplexing unit 70 multiplexes multiple terahertz waves TW, including a first terahertz wave TW1 and a second terahertz wave TW2, output from the terahertz wave generation unit 20. In one example, the multiplexing unit 70 includes a condensing lens 71. The condensing lens 71 condenses the multiple terahertz waves TW into one spot 72. At the spot 72, the multiple terahertz waves TW are multiplexed to generate a single terahertz wave.

[0064] Furthermore, in this embodiment, the control unit 18 controls the time delay between the multiple optical pulse trains PT (for example, the time delay between the first optical pulse train PT1 and the second optical pulse train PT2) by selecting a phase pattern to be provided to the SLM 13. That is, the control unit 18 constitutes a time delay control unit in this embodiment. In this way, the time delay between the multiple optical pulse trains PT, in other words, the phase of each optical pulse train PT, is variable by the control unit 18. Parts (a), (b), and (c) of FIG. 30 show examples of the time intensity waveform of the terahertz wave generated at the spot 72. By varying the time delay between the multiple optical pulse trains PT to various magnitudes, various time intensity waveforms of the terahertz wave can be realized, as shown in FIG. 30.

[0065] As in this embodiment, the terahertz wave generator may further include a multiplexing section 70 that multiplexes multiple terahertz waves TW output from the terahertz wave generating section 20. Since the multiple terahertz waves TW each have a different frequency, a terahertz wave pulse can be generated by multiplexing these. In this case, the terahertz wave generator may include a time delay control section (control section 18) that controls the time delay between the multiple optical pulse trains PT, thereby making it easy to change the waveform of the terahertz wave pulse.

[0066] If the frequency of the terahertz wave pulse generated at an arbitrary crystal position is f and the time delay of the optical pulse train PT is Δt, the phase shift Δφ of the terahertz wave pulse can be expressed as Δφ=2πfΔt. Therefore, by controlling the time delay of the optical pulse train PT, the phase of the terahertz wave pulse can be controlled. Furthermore, by controlling the intensity of the optical pulse train PT, the amplitude of the terahertz wave pulse can also be controlled. [Third embodiment]

[0067] FIG. 31 is a diagram schematically illustrating a configuration of a terahertz wave imaging device 3 according to a third embodiment of the present disclosure. The terahertz wave imaging device 3 includes an imaging unit 50 in addition to the configuration of the terahertz wave generator 1 according to the first embodiment. The imaging unit 50 captures a plurality of terahertz waves TW irradiated from the terahertz wave generator 1 to an object B and output from the object B. In one example, the imaging unit 50 includes an image intensifier 51 and a two-dimensional imaging element 52. The image intensifier 51 includes a photocathode that converts the plurality of terahertz waves TW into photoelectrons, a microchannel plate (MCP) that multiplies the photoelectrons output from the photocathode and outputs secondary electrons, and a phosphor screen that converts the multiplied secondary electrons into light. With this configuration, the image intensifier 51 converts the plurality of incident terahertz waves TW into light and outputs the light. The two-dimensional imaging element 52 converts the light image output from the image intensifier 51 into electrical image data.

[0068] According to the terahertz wave imaging device 3 of this embodiment, it is possible to efficiently generate terahertz waves by increasing the light density at the spot 21 of the terahertz wave generating unit 20 while keeping the light density of each of the plurality of optical pulse trains PT small. Also, it is possible to suitably perform spectral imaging using terahertz waves.

[0069] The terahertz wave generator, terahertz wave imaging device, and terahertz wave generating method according to the present disclosure are not limited to the above-described embodiments and may be modified in various ways. For example, while several specific examples of the terahertz wave generator have been described in the above embodiments, the terahertz wave generator is not limited to these, and various devices capable of receiving an optical pulse train and generating terahertz waves can be used. Furthermore, while the above embodiments have been described as examples of the optical pulse train generator including a pulse light source and a waveform shaper, the configuration of the optical pulse train generator is not limited to this. For example, the optical pulse train generator may include multiple pulse light sources that each output multiple optical pulse trains with different pulse intervals. [Explanation of symbols]

[0070] 1, 2... Terahertz wave generator, 3... Terahertz wave imaging device, 6... Data creation device, 6a... Spectrogram setting unit, 6b... Spectrum design unit, 6b1... Fourier transform unit, 6b2... Spectrogram conversion unit, 6b3... Spectrogram substitution unit, 6b4... Spectrogram inverse conversion unit, 6b5... Inverse Fourier transform unit, 6c... Data generation unit, 10... Optical pulse train generation unit, 11... Pulse light source, 12... Waveform shaper, 13... spatial light modulator (SLM), 17... modulation surface, 17a... modulation region, 18... control unit, 20... terahertz wave generating unit, 21... spot, 30... focusing optical system, 31, 41... lens, 40... collimating optical system, 50... imaging unit, 51... image intensifier, 52... two-dimensional imaging element, 61... processor (CPU), 62... ROM, 64... input device, 65... output device, 66... ​​communication Module, 67... auxiliary storage device, 70... multiplexing unit, 71... condensing lens, 72... spot, 100... device, 101... waveform function setting unit, 102... Fourier transform unit, 103... spectrogram conversion unit, 104... target generation unit, 121... diffraction grating, 122, 123... lens, 124... diffraction grating, A1... initial spectral function, A2... first waveform function, A3... second waveform function, A5... third waveform function, A6... third 4 waveform functions, A41...intensity spectrogram, A42...phase spectrogram, A43...target intensity spectrogram, AA, AB...direction, B...object, La...initial light pulse, Lb...light, Lc...modulated light, PL...light pulse, PT...light pulse train, PT1...first light pulse train, PT2...second light pulse train, SC...control signal, TW...terahertz wave, TW1...first terahertz wave, TW2...second terahertz wave.

Claims

1. an optical pulse train generator that spatially arranges and simultaneously outputs a first optical pulse train and a second optical pulse train having different pulse intervals; a terahertz wave generation unit optically coupled to the optical pulse train generation unit, receiving the first optical pulse train to generate a first terahertz wave, and receiving the second optical pulse train to generate a second terahertz wave having a frequency different from that of the first terahertz wave; a focusing optical system that is arranged on an optical path between the optical pulse train generation unit and the terahertz wave generation unit and focuses the first optical pulse train and the second optical pulse train output from the optical pulse train generation unit onto a single spot within the terahertz wave generation unit; A terahertz wave generating device comprising:

2. 2. The terahertz wave generating device of claim 1, wherein the terahertz wave generating unit includes one or more selected from the group consisting of a ZnTe crystal, a PPLN crystal, a PPLT crystal, a GaSe crystal, a GaP crystal, a DAST crystal, a DASC crystal, a DSTMS crystal, a photoconductive antenna, and HMQ-TMS.

3. The optical pulse train generation unit a pulsed light source that outputs an initial optical pulse; a waveform shaper optically coupled to the pulse light source, which generates the first optical pulse train and the second optical pulse train in parallel from the initial optical pulse; and 3. The terahertz wave generating device according to claim 1, wherein the waveform shaper includes a spatial light modulator that modulates a spectral phase of the initial optical pulse to generate the first optical pulse train and the second optical pulse train.

4. 4. The terahertz wave generating device according to claim 1, further comprising a multiplexing section that multiplexes the first terahertz wave and the second terahertz wave output from the terahertz wave generating section.

5. The terahertz wave generating device according to claim 4 , further comprising a time delay control unit that controls a time delay between the first optical pulse train and the second optical pulse train.

6. The terahertz wave generating device according to any one of claims 1 to 5, an imaging unit that captures an image of the first terahertz wave and the second terahertz wave that are irradiated from the terahertz wave generating device onto an object and output from the object; A terahertz wave imaging device comprising:

7. a step of spatially arranging and simultaneously outputting a first optical pulse train and a second optical pulse train having different pulse intervals; receiving the first optical pulse train to generate a first terahertz wave, and receiving the second optical pulse train to generate a second terahertz wave having a frequency different from that of the first terahertz wave; Including, A terahertz wave generating method, wherein in the generating step, the first optical pulse train and the second optical pulse train are focused onto the same single spot, and the first terahertz wave and the second terahertz wave are generated at the single spot.

Images