Terahertz wave interferometry apparatus and terahertz wave interferometry method

Abstract

To provide a terahertz wave interferometry device that can shorten measurement time. [Solution] The terahertz wave interference measuring device 1A comprises an optical pulse train output unit 10A, a first conversion unit 21, a second conversion unit 22, a multiplexing optical system 23, and a detection unit 40A. The optical pulse train output unit 10A outputs a periodic first optical pulse train P1 having a first repetition frequency, and a periodic second optical pulse train P2 having a second repetition frequency smaller than the first repetition frequency. The first conversion unit 21 converts the first optical pulse train P1 into a first terahertz wave T1. The second conversion unit 22 converts the second optical pulse train P2 into a second terahertz wave T2. The multiplexing optical system 23 combines the first terahertz wave T1 with the second terahertz wave T2 to generate a third terahertz wave T3. The detection unit 40A detects the third terahertz wave T3.

Description

[Technical Field]

[0001] This disclosure relates to a terahertz wave interferometry apparatus and a terahertz wave interferometry method. [Background technology]

[0002] Terahertz waves are light in a frequency band intermediate between light waves and radio waves. Because they exhibit unique absorption spectra for analytes such as drugs that are not found in other wavelength bands, they are expected to be useful for identifying analytes. Various analytical techniques using terahertz waves are known.

[0003] Terahertz time-domain spectroscopy (THz-TDS) measures the time waveform of terahertz waves transmitted, reflected, or totally reflected by the analyte, and performs a Fourier transform on the time waveform of the electric field amplitude of the terahertz wave obtained from this measurement to analyze the analyte (Non-patent documents 1, 2). In this technique, a lock-in amplifier is used when measuring the time waveform of the terahertz wave.

[0004] Furthermore, analysis of the analyte can also be performed by Fourier spectroscopy using interference measurements with terahertz waves, based on a measurement principle similar to that of Fourier transform infrared spectroscopy (FTIR) (Non-patent Literature 3). In this technique, a thermal detector is used to detect the interference of terahertz waves. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2022-538534 [Non-patent literature]

[0006] [Non-Patent Document 1] Jens Neu and Charles A. Schmuttenmaer, "An Introduction to Terahertz Time Domain Spectroscopy (THz-TDS)", Journal of Applied Physics,124, 231101 (2018).

Outdoor Tool2

Outdoor Tools3

Outdoor Tools 4

Direct Environment 5

Outdoor Configuration6

[0007] When a lock-in amplifier is used to measure the time waveform of terahertz waves, a long integration time by the lock-in amplifier is required. Also, when a thermal detector is used to detect the interference of terahertz waves, the response of the thermal detector is slow, so the measurement time is long. As such, conventional analysis techniques using terahertz waves require a long time for measurement.[[ID=​​This disclosure aims to provide a terahertz wave interference measurement device and a terahertz wave interference measurement method that can shorten the measurement time. [Means for solving the problem]

[0009] [1] A terahertz wave interference measuring device according to one aspect of the present disclosure comprises an optical pulse train output unit, a first conversion unit, a second conversion unit, a multiplexing optical system, a trigger generation unit, and a detection unit. The optical pulse train output unit outputs a periodic first optical pulse train having a first repetition frequency, and also outputs a periodic second optical pulse train having a second repetition frequency smaller than the first repetition frequency. The first conversion unit converts the first optical pulse train into a first terahertz wave. The second conversion unit converts the second optical pulse train into a second terahertz wave. The multiplexing optical system combines the first terahertz wave with the second terahertz wave to generate a third terahertz wave. The trigger generation unit generates a trigger signal indicating the timing for detecting the third terahertz wave. The detection unit includes an electron emission unit that emits electrons upon receiving the third terahertz wave, and an electron multiplication unit that emits secondary electrons upon receiving electrons. The detection unit detects the third terahertz wave at the timing indicated by the trigger signal.

[0010] The terahertz wave interferometry apparatus described in [1] above allows for suitable dual-comb spectroscopic measurements using terahertz waves. Specifically, the first terahertz wave and the second terahertz wave are generated from a first optical pulse train and a second optical pulse train, respectively, which have different repetition frequencies. Therefore, the first terahertz wave and the second terahertz wave also differ from each other in terms of pulse repetition frequencies. These first and second terahertz waves are combined by a multiplexing optical system to obtain a third terahertz wave, which includes a pulse waveform obtained by temporally extending the pulse waveform of either the first or second terahertz wave. The third terahertz wave is then detected using a detection unit that can detect the third terahertz wave, in other words, a detection unit that is sensitive to a wavelength band including terahertz waves. This makes it possible to know the time waveform of the pulses contained in the third terahertz wave, and to perform spectroscopic measurements of the object under test based on that time waveform. The object to be measured is placed on the optical path of the first terahertz wave between the first conversion unit and the multiplexing optical system, or on the optical path of the second terahertz wave between the second conversion unit and the multiplexing optical system.

[0011] In addition, the terahertz wave interferometry device described in [1] above has a detection unit which emits electrons upon receiving a third terahertz wave and an electron multiplication unit which emits secondary electrons upon receiving electrons. This makes it possible to detect the time waveform of pulses contained in the third terahertz wave in a short time without using conventional methods that require a long time, such as a lock-in amplifier or a thermal detector. Therefore, the terahertz wave interferometry device described in [1] above can shorten the measurement time compared to conventional methods.

[0012] [2] In the terahertz wave interference measuring device described in [1] above, the detection unit may have a photomultiplier tube including multiple stages of dynodes as an electron multiplication unit. In this case, by converting the secondary electrons output from the final stage dynode into a voltage signal, a signal showing the time waveform of the pulses contained in the third terahertz wave can be obtained. Therefore, the time waveform of the pulses contained in the third terahertz wave can be detected in a shorter time.

[0013] [3] In the terahertz wave interferometry apparatus described in [2] above, both the first repetition frequency and the second repetition frequency may be within the range of 1 MHz or more and 1 GHz or less. By having both the first repetition frequency and the second repetition frequency of 1 MHz or more, the measurement time can be sufficiently shortened. By having both the first repetition frequency and the second repetition frequency of 1 GHz or less, for example, a fiber laser can be used to generate the first optical pulse train and the second optical pulse train.

[0014] [4] In the terahertz wave interferometry apparatus described in [1] above, the detection unit may include an image intensifier and an imaging unit. The image intensifier includes a microchannel plate as an electron multiplication unit and a phosphor that converts secondary electrons emitted from the microchannel plate into a fluorescence image. The imaging unit captures the fluorescence image output from the image intensifier. In this case, a signal showing the time waveform of pulses contained in the third terahertz wave is obtained two-dimensionally from the light intensity distribution shown by the image data obtained in the imaging unit. Therefore, a spectroscopic measurement of a region of a certain extent in the object to be measured can be performed in a single measurement.

[0015] [5] In the terahertz wave interferometry apparatus described in [4] above, both the first repetition frequency and the second repetition frequency may be within the range of 1 MHz or more and 500 MHz or less. By having both the first repetition frequency and the second repetition frequency of 1 MHz or more, the measurement time can be sufficiently shortened. By having both the first repetition frequency and the second repetition frequency of 500 MHz or less, the measurement time can be shortened while taking into account the relaxation time of the phosphor.

[0016] [6] In the terahertz wave interferometry apparatus described in [1] to [5] above, the trigger generation unit may include a nonlinear crystal and a photodetector. The nonlinear crystal generates difference frequency light or sum frequency light from a first optical pulse train and a second optical pulse train. The photodetector detects the difference frequency light or sum frequency light from the nonlinear crystal and generates a detection signal. The trigger generation unit may use the detection signal or a signal based on the detection signal as the trigger signal.

[0017] [7] In the terahertz wave interference measuring apparatus described in [1] to [6] above, the difference between the first repetition frequency and the second repetition frequency may be 100 Hz or more. In this case, the measurement time can be significantly reduced.

[0018] [8] In the terahertz wave interference measuring apparatus described in [1] to [7] above, when Δfrep is defined as the difference between the first repetition frequency frep1 and the second repetition frequency frep2, and the measurement bandwidth Δν is defined as Δν = (frep1 × frep2) / 2Δfrep, the measurement bandwidth Δν may be greater than the spectral sensitivity spectral width of the detection unit. In this case, the time waveform of pulses contained in the third terahertz wave can be suitably detected.

[0019] [9] A terahertz wave interference measurement method according to one aspect of the present disclosure comprises the steps of: outputting a periodic first optical pulse train having a first repetition frequency and outputting a periodic second optical pulse train having a second repetition frequency smaller than the first repetition frequency; converting the first optical pulse train into a first terahertz wave and converting the second optical pulse train into a second terahertz wave; generating a third terahertz wave by combining the first terahertz wave with the second terahertz wave; generating a trigger signal indicating the timing for detecting the third terahertz wave; and detecting the third terahertz wave at the timing indicated by the trigger signal using a device having an electron emission unit that emits electrons upon receiving the third terahertz wave and an electron multiplier unit that emits secondary electrons upon receiving electrons.

[0020] According to the terahertz wave interferometry measurement method described in [9] above, dual-comb spectroscopic measurements using terahertz waves can be suitably performed, similar to the terahertz wave interferometry device described in [1] above. In addition, the terahertz wave interferometry measurement method described in [9] above uses a device having an electron emission unit that emits electrons upon receiving a third terahertz wave and an electron multiplier unit that emits secondary electrons upon receiving electrons to detect the third terahertz wave. This makes it possible to detect the time waveform of pulses contained in the third terahertz wave in a short time without using conventional methods that require a long time, such as lock-in amplifiers or thermal detectors. Therefore, the terahertz wave interferometry measurement method described in [9] above can shorten the measurement time compared to conventional methods. [Effects of the Invention]

[0021] This disclosure provides a terahertz wave interference measurement device and a terahertz wave interference measurement method that can shorten the measurement time. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic diagram showing the configuration of a terahertz wave interference measurement device according to the first embodiment. [Figure 2] Figure 2 is a cross-sectional view showing an example of the configuration of a photomultiplier tube. [Figure 3] Figure 3 is a partially enlarged view showing an example of the configuration of a photomultiplier tube. [Figure 4] Figure 4 is a partially enlarged view showing an example of a metasurface configuration. [Figure 5] Figure 5 is a graph showing the relationship between electric field amplitude and average current. [Figure 6] Figure 6 is a graph showing the relationship between electric field amplitude and current value. [Figure 7] Figure 7 is a flowchart showing the terahertz wave interference measurement method according to the first embodiment. [Figure 8] Figure 8 is a schematic diagram showing the configuration of a terahertz wave interference measurement device according to the second embodiment. [Figure 9]Figure 9 is a cross-sectional view showing an example of the configuration of an image intensifier. [Figure 10] Figure 10 shows the configuration of the optical pulse train output unit according to a modified example. [Modes for carrying out the invention]

[0023] Specific examples of the present disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as defined by the claims. In the following description, identical elements in the drawings are denoted by the same reference numerals, and redundant descriptions are omitted.

[0024] [First Embodiment] Figure 1 is a schematic diagram showing the configuration of a terahertz wave interferometry device 1A according to the first embodiment of this disclosure. Using the terahertz wave interferometry device 1A of this embodiment, for example, dual-comb spectroscopic measurements using terahertz waves can be performed. As shown in Figure 1, the terahertz wave interferometry device 1A of this embodiment includes an optical pulse train output unit 10A, a first conversion unit 21, a second conversion unit 22, a multiplexing optical system 23, beam splitters 24 and 25, a mirror 26, a trigger generation unit 30, and a detection unit 40A.

[0025] The optical pulse train output unit 10A outputs a periodic first optical pulse train P1 (first frequency comb) having a first repetition frequency frep1. Simultaneously, the optical pulse train output unit 10A outputs a periodic second optical pulse train P2 (second frequency comb) having a second repetition frequency frep2 that is slightly smaller than the first repetition frequency frep1. The wavelength bands of the first optical pulse train P1 and the second optical pulse train P2 are within the range of 500 nm to 3000 nm when using a fiber laser having an excitation medium such as erbium, yttrivium, thulium, or niodium and a wavelength conversion method such as second harmonic generation.

[0026] The optical pulse train output unit 10A of this embodiment includes two repetition frequency controllers 111 and 112, and two femtosecond lasers 121 and 122 that are synchronized with each other. The femtosecond laser 121 generates and outputs a first optical pulse train P1. The femtosecond laser 122 generates and outputs a second optical pulse train P2. The femtosecond lasers 121 and 122 are, for example, fiber lasers that amplify light using optical fibers. The time width of the optical pulses included in the first optical pulse train P1 and the second optical pulse train P2 is, for example, within the range of 10 fs to 10 ps. The first repetition frequency frep1 and the second repetition frequency frep2 are, for example, within the range of 1 MHz to 1 GHz, or within the range of 1 MHz to 250 MHz, or 1 MHz or less. The difference between the first repetition frequency frep1 and the second repetition frequency frep2 is, for example, 100 Hz or more.

[0027] Let Δfrep be the difference between the first repetition frequency frep1 and the second repetition frequency frep2. Furthermore, the measurement bandwidth Δν (Nyquist spectral bandwidth) is defined as Δν = (frep1 × frep2) / 2Δfrep. The first repetition frequency frep1 and the second repetition frequency frep2 are set such that the measurement bandwidth Δν is greater than the spectral sensitivity spectral width of the photomultiplier tube 41 (described later). The spectral sensitivity spectral width of the photomultiplier tube 41 is, for example, 2 THz.

[0028] The repetition frequency controller 111 is electrically connected to the femtosecond laser 121 and controls the first repetition frequency frep1. The repetition frequency controller 112 is electrically connected to the femtosecond laser 122 and controls the second repetition frequency frep2. The first repetition frequency frep1 and the second repetition frequency frep2 are variable by the repetition frequency controllers 111 and 112, respectively.

[0029] Beam splitter 24 is optically coupled to femtosecond laser 121. Beam splitter 24 receives a first optical pulse train P1 from femtosecond laser 121 and outputs the first optical pulse train P1 by splitting it into two optical paths. Beam splitter 25 is optically coupled to femtosecond laser 122. Beam splitter 25 receives a second optical pulse train P2 from femtosecond laser 122 and outputs the second optical pulse train P2 by splitting it into two optical paths.

[0030] The first conversion unit 21 is optically coupled to the femtosecond laser 121 via a beam splitter 24. The first conversion unit 21 receives a first optical pulse train P11, which is one of two first optical pulse trains split by the beam splitter 24, and converts the first optical pulse train P11 into a first terahertz wave T1. The first conversion unit 21 includes a terahertz wave generating element 211. The second conversion unit 22 receives a second optical pulse train P21, which is one of two second optical pulse trains split by the beam splitter 25, and converts the second optical pulse train P21 into a second terahertz wave T2. The second conversion unit 22 includes a terahertz wave generating element 221. The terahertz wave generating elements 211 and 221 are, for example, photoconductive antennas. Alternatively, the terahertz wave generating elements 211 and 221 may be nonlinear crystals such as ZnTe. The terahertz wave generator 211 converts the incident first optical pulse train P11 into a first terahertz wave T1. The first terahertz wave T1 includes a periodic pulse train having a first repetition frequency frep1. The terahertz wave generator 221 converts the incident second optical pulse train P21 into a second terahertz wave T2. The second terahertz wave T2 includes a periodic pulse train having a second repetition frequency frep2. The multiplexing optical system 23 is optically coupled to both the first converter 21 and the second converter 22. The multiplexing optical system 23 combines the first terahertz wave T1 with the second terahertz wave T2 to generate the third terahertz wave T3. In Figure 1, for ease of understanding, the combined first terahertz wave T1 is shown separated from the second terahertz wave T2, but in reality, the combined first terahertz wave T1 travels along the same optical axis as the second terahertz wave T2. The multiplexing optical system 23 includes, for example, a half-mirror 231.

[0031] The object B to be measured is placed on the optical path of the first terahertz wave T1 between the first conversion unit 21 and the multiplexing optical system 23, or on the optical path of the second terahertz wave T2 between the second conversion unit 22 and the multiplexing optical system 23.

[0032] The trigger generation unit 30 generates a trigger signal TR that indicates the timing for detecting the third terahertz wave T3. The trigger generation unit 30 includes, for example, a lens 31, a nonlinear crystal 32, an aperture 33, and a photodetector 34.

[0033] Lens 31 is optically coupled to the femtosecond laser 121 via a beam splitter 24. In addition, lens 31 is optically coupled to the femtosecond laser 122 via a beam splitter 25 and a mirror 26. Lens 31 receives the first optical pulse train P12, which is the other of two first optical pulse trains split by the beam splitter 24. At the same time, lens 31 receives the second optical pulse train P22, which is the other of two second optical pulse trains split by the beam splitter 25. The incident points of the first optical pulse train P12 and the incident points of the second optical pulse train P22 to lens 31 are separated from each other with the optical axis of lens 31 in between. As a result, the first optical pulse train P12 and the second optical pulse train P22 emitted from lens 31 intersect each other at a certain point.

[0034] The nonlinear crystal 32 is positioned at a location that coincides with the point where the first optical pulse train P12 and the second optical pulse train P22 intersect. The nonlinear crystal 32 generates difference frequency light P3 from the first optical pulse train P12 and the second optical pulse train P22. The nonlinear crystal 32 may also generate sum frequency light from the first optical pulse train P12 and the second optical pulse train P22. The aperture 33 is positioned between the lens 31 and the nonlinear crystal 32. The difference frequency light P3 or sum frequency light generated by the nonlinear crystal 32 passes through the aperture 33. The first optical pulse train P12 and the second optical pulse train P22 that have passed through the nonlinear crystal 32 are shielded by the aperture 33.

[0035] The photodetector 34 detects difference frequency light P3 or sum frequency light from the nonlinear crystal 32 and generates a detection signal indicating the incidence timing of the difference frequency light P3 or sum frequency light. The trigger generation unit 30 outputs the detection signal, or a signal generated based on that detection signal, as a trigger signal TR.

[0036] The configuration of the trigger generation unit 30 is not limited to the above. For example, the photodetector 34 may detect a first optical pulse train P12 or a second optical pulse train P22 instead of a difference frequency light P3 or a sum frequency light. The trigger generation unit 30 may then output the detected signal, or a signal generated based on that detected signal, as a trigger signal TR.

[0037] The detection unit 40A detects the third terahertz wave T3 at the timing indicated by the trigger signal TR. The detection unit 40A in this embodiment includes a photomultiplier tube 41 and a time waveform measuring instrument (oscilloscope) 401. The photomultiplier tube 41 generates an electrical signal DS corresponding to the intensity of the third terahertz wave T3. The time waveform measuring instrument 401 is electrically connected to the photomultiplier tube 41 and receives the electrical signal DS from the photomultiplier tube 41. The time waveform measuring instrument 401 is also electrically connected to the photodetector 34 of the trigger generation unit 30 and receives the trigger signal TR from the photodetector 34. The time waveform measuring instrument 401 repeatedly measures the magnitude of the electrical signal DS output from the photomultiplier tube 41, in other words, the magnitude of the pulses contained in the third terahertz wave T3, in accordance with the timing indicated by the trigger signal TR. As a result, the time waveform measuring instrument 401 acquires the time waveform of the pulse contained in the first terahertz wave T1 that has passed through the object B under test, or the time waveform of the pulse contained in the second terahertz wave T2 that has passed through the object B under test.

[0038] The time waveform data acquired by the detection unit 40A is provided to a computer (not shown). The computer performs Fourier spectroscopy analysis of the object B based on this time waveform. However, since the relationship between the output current from the photomultiplier tube 41 and the electric field amplitude of the third terahertz wave T3 is nonlinear, the calculation is performed taking this nonlinearity into account.

[0039] Figure 2 is a cross-sectional view showing an example configuration of a photomultiplier tube 41. The photomultiplier tube 41 has an electron emitter 42, an electron multiplier 43, an electron collector 413, a housing 414, and a plurality of wires 417. The housing 414 has a valve 415 and a stem 416. The valve 415 has a window 411 that transmits terahertz waves. The electron emitter 42 emits electrons in response to the incidence of a third terahertz wave T3.

[0040] Figure 3 is a partially enlarged view showing an example configuration of a photomultiplier tube 41. The electron emission section 42 has a substrate 421 and a metasurface 422. The substrate 421 includes a main surface 421a facing the electron multiplier section 43 and a main surface 421b facing the window 411. The third terahertz wave T3 transmitted through the window 411 is incident on the main surface 421b of the substrate 421. The metasurface 422 is provided on the main surface 421a. The third terahertz wave T3 is incident on the metasurface 422 after passing through the substrate 421. The metasurface 422 is contained in a patterned oxide layer or a patterned metal layer on the main surface 421a of the substrate 421. The oxide layer is, for example, titanium oxide (TiO2). The metal layer is, for example, gold (Au). Figure 4 is a partially enlarged view showing an example configuration of the metasurface 422. In this example, the metal layer constituting the passive type metasurface 422 forms multiple antennas 423 on the main surface 421a. The smaller the size of the antennas 423, the more sensitive it is to terahertz waves with shorter wavelengths, i.e., terahertz waves with higher frequencies. By changing the structure of the antennas 423, the metasurface 422 can support frequency bands of, for example, 0.01 THz to 10 THz or 10 THz to 50 THz. The metasurface 422 emits an amount of electrons corresponding to the intensity of the third terahertz wave T3.

[0041] Refer to Figure 2 again. The electron multiplication unit 43 receives electrons emitted from the electron emission unit 42 and emits multiplied secondary electrons. The electron multiplication unit 43 includes a focusing electrode 412 that focuses electrons emitted from the metasurface 422, and a so-called line-focus type multi-stage dynode 43a to 43j. The dynodes 43a to 43j multiply the electrons that pass through the opening of the focusing electrode 412 according to the potential applied through the wiring 417. The electrons are sequentially sent from the first-stage dynode 43a to the final-stage dynode 43j, while being multiplied. The electron collection unit 413 collects the electrons multiplied in the electron multiplication unit 43. The collected electrons are output as a current signal from the electron collection unit 413 through the wiring 417. The current signal is converted into a voltage signal by a circuit (not shown), and this voltage signal is output to the time waveform measuring instrument 401 as an electrical signal DS (see Figure 1).

[0042] The input / output characteristics of the photomultiplier tube 41 are not linear. The output value from the photomultiplier tube 41 may be described by a polynomial with the electric field amplitude E of the electromagnetic wave incident on the photomultiplier tube 41 as a variable, or it may be described using the following equation (1) which represents the efficiency of electron emission in the metasurface 422 (see Non-Patent Literature 11 for details). This equation represents the relationship between the current J emitted from the metasurface 422 and the electric field amplitude E of the incident terahertz wave, and is called the Fowler-Nordheim relations (hereinafter referred to as the "FN equation").

number

[0043] a in the FN formula FN and b FN It is necessary to obtain their respective values. To do so, the electric field amplitude E of the incident terahertz wave is set to each value, the output current of the photomultiplier tube 41 is measured, and fitting processing using these measured currents is performed to obtain FN a and b FN Their respective values can be obtained.

[0044] When the third terahertz wave T3 is incident on the photomultiplier tube 41, the average value J of the output current from the photomultiplier tube 41 avg is obtained by the following formula. J avg (E) = J(E) × f rep × CE × G However, f rep is the repetition frequency of the pulses included in the third terahertz wave T3, CE is the collection efficiency, and G is the gain of the electron multiplication section 43. In order to accurately detect the third terahertz wave T3 in the photomultiplier tube 41, the average current J avg should satisfy J min < J avg < J f e sat Here, J min is a value defined by the magnitude of the dark current. Also, J sat is a value defined by the DC linearity (the limit of maintaining linearity between input and output) of the electron multiplication section 43. <00002 FIG. 5 is a graph showing the relationship between the electric field amplitude E and the average current J avg In FIG. 5, the horizontal axis represents the electric field amplitude E (kV / cm), and the vertical axis represents the average current J avg(A) is shown. The triangular plot, quadrilateral plot, and circular plot represent the repetition frequency f, respectively. rep This shows the cases where the frequency is 100MHz, 10MHz, and 1MHz. The dashed line in the figure represents the limit of DC linearity, i.e., the saturation level (J). sat The dashed line shows the lower limit J based on the magnitude of the dark current. min This is shown. Referring to Figure 5, the repetition frequency f rep The measurement range is widest when the repetition frequency f is 1 MHz. rep It can be seen that the larger the value, the narrower the measurement range becomes. In this example, the measurement range of the electron multiplier unit 43 is approximately 38 nA to 20 μA. From the graph shown in Figure 5, the repetition frequency f rep The repetition frequency f may be 100 MHz or less, 10 MHz or less, or 1 MHz or less. rep The first repetition frequency frep1 and the second repetition frequency frep2, which are very close to or equivalent to the first repetition frequency, may also be 100 MHz or less, 10 MHz or less, or 1 MHz or less.

[0046] Figure 6(a) is a graph showing the relationship between the electric field amplitude and the average current value when the terahertz wave incident on the photomultiplier tube 41 is a continuous wave. In Figure 6(a), the horizontal axis represents the electric field amplitude (kV / cm) and the vertical axis represents the average current (A). Figure 6(b) is a graph showing the relationship between the electric field amplitude and the peak current value when the terahertz wave incident on the photomultiplier tube 41 includes a repeating pulse with a repetition frequency of 1 MHz. In Figure 6(b), the horizontal axis represents the electric field amplitude (kV / cm) and the vertical axis represents the peak current (A). The saturation level of the photomultiplier tube 41 also differs depending on whether the incident terahertz wave is a continuous wave or a pulse. For example, in the examples shown in Figures 6(a) and 6(b), when the incident terahertz wave is a continuous wave, the electric field amplitude required to reach the saturation level is smaller than when the incident terahertz wave is a pulse. Therefore, the saturation level (J) calculated from the current J when the terahertz wave is a continuous wave is... sat ) due to the average current J avgThis is limited. Conversely, when the terahertz wave is pulsed, if the electric field amplitude that reaches the saturation level is smaller than when the terahertz wave is a continuous wave, the average current J when the terahertz wave is pulsed is limited. avg Saturation level (J) calculated from sat ) due to the average current J avg This is restricted.

[0047] Figure 7 is a flowchart showing a terahertz wave interference measurement method according to the first embodiment of this disclosure. As shown in Figure 7, the terahertz wave interference measurement method of this embodiment comprises steps ST1 to ST5.

[0048] In step ST1, the optical pulse train output unit 10A outputs a periodic first optical pulse train P1 having a first repetition frequency frep1, and a periodic second optical pulse train P2 having a second repetition frequency frep2 that is smaller than the first repetition frequency frep1. In step ST2, the first conversion unit 21 converts the first optical pulse train P1 into a first terahertz wave T1, and the second conversion unit 22 converts the second optical pulse train P2 into a second terahertz wave T2. In step ST3, the multiplexing optical system 23 combines the first terahertz wave T1 with the second terahertz wave T2 to generate a third terahertz wave T3. In step ST4, the trigger generation unit 30 generates a trigger signal TR indicating the timing for detecting the third terahertz wave T3. In step ST5, the third terahertz wave T3 is detected at the timing indicated by the trigger signal TR using a photomultiplier tube 41 having an electron emission unit 42 that emits electrons in response to the third terahertz wave T3 and an electron multiplication unit 43 that emits secondary electrons in response to the electrons.

[0049] The effects obtained by the terahertz wave interferometry apparatus 1A and terahertz wave interferometry method of this embodiment, as described above, will now be explained. According to the terahertz wave interferometry apparatus 1A and terahertz wave interferometry method of this embodiment, dual-comb spectroscopic measurements using terahertz waves can be suitably performed. That is, the first terahertz wave T1 and the second terahertz wave T2 are each generated from the first optical pulse train P1 and the second optical pulse train P2, respectively, which have different repetition frequencies. Therefore, the first terahertz wave T1 and the second terahertz wave T2 also differ from each other in terms of pulse repetition frequencies. By combining these first terahertz wave T1 and the second terahertz wave T2 by the combined optical system 23, a third terahertz wave T3 is obtained, which includes a pulse waveform obtained by temporally extending the pulse waveform of either the first terahertz wave T1 or the second terahertz wave T2. Then, the third terahertz wave T3 is detected using a detection unit 40A that is capable of detecting the third terahertz wave T3, in other words, has sensitivity to the wavelength band including terahertz waves. This makes it possible to know the time waveform of the pulse contained in the third terahertz wave T3, and based on that time waveform, Fourier spectroscopy analysis of the object B under test can be performed.

[0050] In addition, in this embodiment, the detection unit 40A includes an electron emission unit 42 that emits electrons upon receiving a third terahertz wave T3, and an electron multiplication unit 43 that emits secondary electrons upon receiving electrons. This makes it possible to detect the time waveform of pulses contained in the third terahertz wave T3 in a short time (e.g., less than 1 ms) without using conventional methods that require a long time, such as a lock-in amplifier or a thermal detector. Therefore, according to this embodiment, the measurement time can be significantly reduced compared to conventional methods.

[0051] Conventional measuring devices have several drawbacks, including difficulty in confirming interference between terahertz waves, a small detection area of ​​terahertz wave detection elements (less than 1 mm), and a sharp decrease in sensitivity at frequencies above 1 THz for terahertz wave detectors (such as semiconductor devices like Schottky barrier diodes). While thermal detectors exist, their response speed is slow, making them unsuitable for femtosecond lasers with repetition rates of, for example, 50 MHz. Therefore, conventional measuring devices have not employed methods that involve interference between terahertz waves.

[0052] On the other hand, the photomultiplier tube 41 used in this embodiment for terahertz wave detection has a response speed of 100 MHz or more, sensitivity to bandwidths of 1 THz or more, and a detection area of, for example, 6 mm or more. Therefore, the photomultiplier tube 41 can be said to be suitable for interference measurement of terahertz waves.

[0053] As in this embodiment, the detection unit 40A may have a photomultiplier tube 41 including multiple stages of dynodes 43a to 43j as an electron multiplication unit 43. In this case, by converting the secondary electrons output from the final stage dynode 43j into a voltage signal, a signal showing the time waveform of the pulse contained in the third terahertz wave T3 can be obtained. Therefore, the time waveform of the pulse contained in the third terahertz wave T3 can be detected in a shorter time. Furthermore, simple interference measurement using a single element is possible, which is useful for spectroscopic measurement or tomography, so-called THz coherence tomography (THz-OCT).

[0054] As in this embodiment, both the first repetition frequency frep1 and the second repetition frequency frep2 may be within the range of 1 MHz to 1 GHz, or 1 MHz to 250 MHz. By having both the first repetition frequency frep1 and the second repetition frequency frep2 be 1 MHz or higher, the measurement time can be sufficiently shortened. By having both the first repetition frequency frep1 and the second repetition frequency frep2 be 1 GHz or lower, for example, a fiber laser can be used to generate the first optical pulse train P1 and the second optical pulse train P2.

[0055] The upper limits (1 GHz or 250 MHz) of the first repetition frequency frep1 and the second repetition frequency frep2 are due to the response speed of the photomultiplier tube 41 of the detection unit 40A. The response speed of the photomultiplier tube 41 is largely limited by the response speed of the electron multiplier unit 43. The photomultiplier tube 41 outputs a pulse current with a time width (full width at half maximum) of, for example, 2 ns in response to a terahertz wave pulse input with a time width (full width at half maximum) of about 1 ps. In this case, the maximum repetition frequency to which the photomultiplier tube 41 can respond is 1 / (2 ns × 2) = 250 MHz. Furthermore, if a photomultiplier tube 41 with an electron multiplier unit 43 capable of faster response is used, the photomultiplier tube 41 can output a pulse current with a time width (full width at half maximum) of, for example, 0.5 ns in response to a terahertz wave pulse input with a time width (full width at half maximum) of about 1 ps. In that case, the maximum repetition frequency to which the photomultiplier tube 41 can respond is 1 GHz.

[0056] As in this embodiment, the trigger generation unit 30 may include a nonlinear crystal 32 and a photodetector 34. The nonlinear crystal 32 generates difference frequency light P3 or sum frequency light from a first optical pulse train P1 and a second optical pulse train P2. The photodetector 34 detects the difference frequency light P3 or sum frequency light from the nonlinear crystal 32 and generates a detection signal. The trigger generation unit 30 may use the detection signal or a signal based on the detection signal as the trigger signal TR. In this case, a trigger generation unit 30 capable of generating a trigger signal TR with high timing accuracy can be easily configured.

[0057] As in this embodiment, the difference Δfrep between the first repetition frequency frep1 and the second repetition frequency frep2 may be 100 Hz or more. In this case, the measurement time can be significantly reduced.

[0058] As in this embodiment, the measurement bandwidth Δν may be larger than the spectral sensitivity spectral width of the detection unit 40A. The larger the difference in repetition frequencies Δfrep, the shorter the measurement time, but the larger the difference in repetition frequencies Δfrep, the narrower the measurement bandwidth Δν (Nyquist spectral bandwidth). Therefore, there is a trade-off relationship between measurement time and measurement bandwidth Δν, and the shorter the measurement time, the narrower the measurement bandwidth Δν becomes. By reducing Δfrep to such an extent that the measurement bandwidth Δν is larger than the spectral sensitivity spectral width of the detection unit 40A, the time waveform of pulses contained in the third terahertz wave T3 can be suitably detected. [Second Embodiment]

[0059] Figure 8 is a schematic diagram showing the configuration of a terahertz wave interferometry device 1B according to the second embodiment of this disclosure. The terahertz wave interferometry device 1B of this embodiment differs from the terahertz wave interferometry device 1A of the first embodiment in that it includes a detection unit 40B instead of a detection unit 40A, and is otherwise identical to the terahertz wave interferometry device 1A of the first embodiment. The detection unit 40B detects the third terahertz wave T3 at the timing indicated by the trigger signal TR. The detection unit 40B of this embodiment includes an image intensifier 44 and an imaging unit (camera) 45.

[0060] The image intensifier 44 generates a fluorescence image FL corresponding to the planar intensity distribution of the third terahertz wave T3. The imaging unit 45 is optically coupled to the image intensifier 44 and receives the fluorescence image FL from the image intensifier 44. The imaging unit 45 is also electrically connected to the photodetector 34 of the trigger generation unit 30 and receives a trigger signal TR from the photodetector 34. The imaging unit 45 repeatedly captures the fluorescence image FL output from the image intensifier 44 in accordance with the timing indicated by the trigger signal TR. In this way, the imaging unit 45 acquires two-dimensionally the time waveform of the pulse contained in the first terahertz wave T1 that has passed through the object B, or the time waveform of the pulse contained in the second terahertz wave T2 that has passed through the object B.

[0061] Figure 9 is a cross-sectional view showing an example configuration of an image intensifier 44. The image intensifier 44 includes an electron emission section 42, an entrance window 46, an electron multiplication section 47, a phosphor 48, a fiber optic plate (FOP) 49, and a housing 440. The entrance window 4 blocks one end of the cylindrical housing 440, and the FOP 49 blocks the other end of the cylindrical housing 440. The entrance window 4 transmits at least a portion of the third terahertz wave T3. The electron emission section 42 is fixed to the back surface of the entrance window 4, that is, the surface opposite to the surface into which the third terahertz wave T3 is incident. The configuration of the electron emission section 42 is the same as in the first embodiment described above. The electron multiplication section 47 in this embodiment is a microchannel plate (MCP). The MCP has a plurality of holes (capillaries) arranged in two dimensions, and multiplies the electrons passing through each hole.

[0062] The phosphor 48 is positioned between the electron emission unit 42 and the electron multiplication unit 47. The phosphor 48 converts secondary electrons emitted from the MCP into a fluorescence image. The phosphor 48 is formed, for example, by coating a fluorescent material onto the end face of the FOP 49. The FOP 49 guides the fluorescence image output from the phosphor 48 to the outside of the image intensifier 44 while maintaining its image shape. This fluorescence image is output from the image intensifier 44 as the fluorescence image FL shown in Figure 8.

[0063] In this embodiment, the first repetition frequency frep1 and the second repetition frequency frep2 are, for example, within the range of 1 MHz to 500 MHz, within the range of 1 MHz to 250 MHz, or 1 MHz or less. The difference between the first repetition frequency frep1 and the second repetition frequency frep2 is, as in the first embodiment, for example, 100 Hz or more. The first repetition frequency frep1 and the second repetition frequency frep2 are set such that the measurement bandwidth Δν is greater than the spectral sensitivity spectral width of the image intensifier 44. The spectral sensitivity spectral width of the image intensifier 44 is, for example, 2 THz.

[0064] In the terahertz wave interference measurement method according to this embodiment, in step ST5 of Figure 7, the third terahertz wave T3 is detected at the timing indicated by the trigger signal TR using an image intensifier 44 having an electron emission unit 42 that emits electrons upon receiving a third terahertz wave T3 and an electron multiplication unit 47 that emits secondary electrons upon receiving electrons. Steps ST1 to ST4 are the same as in the first embodiment.

[0065] According to this embodiment, dual-comb spectroscopic measurement using terahertz waves can be suitably performed, similar to the first embodiment. In addition, in this embodiment, the detection unit 40B has an electron emission unit 42 that emits electrons upon receiving a third terahertz wave T3, and an electron multiplication unit 47 that emits secondary electrons upon receiving electrons. This makes it possible to detect the time waveform of pulses contained in the third terahertz wave T3 in a short time without using conventional methods that require a long time, such as a lock-in amplifier or a thermal detector. Therefore, in this embodiment as well, the measurement time can be significantly reduced compared to conventional methods. Furthermore, according to this embodiment, a signal indicating the time waveform of pulses contained in the third terahertz wave T3 can be obtained two-dimensionally from the light intensity distribution shown in the image data obtained by the imaging unit 45. Therefore, spectroscopic measurement of a region with a certain extent in the object B to be measured can be performed in a single measurement.

[0066] As in this embodiment, both the first repetition frequency frep1 and the second repetition frequency frep2 may be within the range of 1 MHz to 500 MHz. By having both the first repetition frequency frep1 and the second repetition frequency frep2 be 1 MHz or higher, the measurement time can be sufficiently shortened. By having both the first repetition frequency frep1 and the second repetition frequency frep2 be 500 MHz or lower, the measurement time can be shortened while taking into account the relaxation time of the phosphor 48.

[0067] The upper limits (500 MHz) of the first repetition frequency frep1 and the second repetition frequency frep2 are due to the relaxation time of the phosphor 48 in the image intensifier 44 of the detection unit 40B. The relaxation time is, for example, 1 ms or 0.2 μs to 0.4 μs. Some high-speed phosphors developed in recent years have relaxation times of as long as 1 ns. If the relaxation time is 1 ns, the maximum repetition frequency that the image intensifier 44 can respond to is 1 / (1 ns × 2) = 500 MHz.

[0068] [Differentiation] Figure 10 shows the configuration of the optical pulse train output unit 10B according to a modified example. The terahertz wave interference measurement device 1A of the first embodiment and the terahertz wave interference measurement device 1B of the second embodiment described above may be equipped with the optical pulse train output unit 10B of this modified example instead of the optical pulse train output unit 10A.

[0069] The optical pulse train output unit 10B is a bidirectional dual-comb laser light source that outputs a first optical pulse train P1 and a second optical pulse train P2. The optical pulse train output unit 10B outputs a first optical pulse train P1 that oscillates clockwise (CW) and a second optical pulse train P2 that oscillates counterclockwise (CCW). In the optical pulse train output unit 10B, light from a light source 13, such as a laser diode, is sent to a doped fiber 14, such as an erbium-doped fiber, and amplified. The amplified light circulates in two different directions, clockwise and counterclockwise, within the loop optical path 15. The loop optical path 15 is provided with a nonlinear polarization rotation unit 16 that controls the intensity and phase of the light by changing the polarization state of the light, and a semiconductor saturation absorption mirror 17, which is a device for generating light pulses. A portion of the light circulating clockwise within the loop optical path 15 is taken out by a coupler 18 and output as the first optical pulse train P1. A portion of the light circulating counterclockwise within the loop optical path 15 is extracted by the coupler 19 and output as the second optical pulse train P2.

[0070] As shown in this modified example, the optical pulse train output section may include a dual-comb laser light source. Even in this case, the same effects as those of the above embodiments can be obtained. For a detailed description of the bidirectional oscillation type dual-comb laser light source, please refer to the description in Non-Patent Document 4.

[0071] In the above example, the optical pulse train output unit 10B uses a nonlinear polarization rotation unit 16 as the mode-locking method, but is not limited to this. For example, a non-reciprocal phase shifter, a nonlinear loop mirror, and a saturable absorber that absorbs only continuous light and has high transmittance of pulsed light may be used as the mode-locking method. Preferably, a non-reciprocal phase shifter or a saturable absorber using a polarization-maintaining fiber that is robust to disturbances may be used as the mode-locking method. The gain medium for the laser light in the optical pulse train output unit 10B is not particularly limited and may be any of the following: erbium, yttrivium, thulium, and niodium.

[0072] In this modified example, a bidirectional dual-comb laser light source is used as the optical pulse train output unit 10B. However, the configuration and type of the optical pulse train output unit 10B are not particularly limited, as long as it can output a first optical pulse train P1 and a second optical pulse train P2. For example, the optical pulse train output unit 10B may be a dual-comb laser light source with two synchronized units. In this case, noise can be suppressed.

[0073] Alternatively, the optical pulse train output unit 10B may be a dual-unit synchronous type, a mechanically shared type, a multi-polarization type, or a microcomb type. For the configuration of a dual-unit synchronous type dual-comb laser light source, refer to the description in Non-Patent Document 12. For the configuration of a mechanically shared type dual-comb laser light source, refer to the description in Non-Patent Document 13. For the configuration of a multi-polarization type dual-comb laser light source, refer to the description in Non-Patent Document 9. For the configuration of a microcomb type dual-comb laser light source, refer to the description in Non-Patent Document 14.

[0074] The terahertz wave interferometry apparatus and terahertz wave interferometry method according to this disclosure are not limited to the embodiments described above, and various other modifications are possible. For example, in each of the above embodiments, a fiber laser was exemplified as the light source of the optical pulse train output unit, but the light source is not limited to this, and may be a microcomb light source combining, for example, a continuous wave (CW) laser and SiN. [Explanation of symbols]

[0075] 1A,1B…Terahertz wave interferometry device, 4…Ingress window, 10A,10B…Optical pulse train output unit, 13…Light source, 14…Doped fiber, 15…Loop optical path, 16…Nonlinear polarization rotation unit, 17…Semiconductor saturation absorption mirror, 18,19…Coupler, 21…First conversion unit, 22…Second conversion unit, 23…Multiplier optical system, 24,25…Beam splitter, 26…Mirror, 30…Trigger generation unit, 31…Lens, 32…Nonlinear crystal, 33…Aperture, 34…Photodetector, 40A,40B…Detection unit, 41…Photomultiplier tube, 42…Electron emission unit, 43…Electron multiplication unit, 43a~43j…Dynode, 44…Image intensifier, 45…Imaging unit, 46…Ingress window, 47…Electron multiplication unit, 48…Phosphor, 49…Fiber optic plate (FOP), 111,112... Repetition frequency controller, 121,122... Femtosecond laser, 211,221... Terahertz wave generator, 231... Half mirror, 401... Time waveform measuring instrument (oscilloscope), 411... Window, 412... Focusing electrode, 413... Electron collection unit, 414... Housing, 415... Valve, 416... Stem, 417... Wiring, 421... Circuit board 421a, 421b…Main surface, 422…Metasurface, 423…Antenna, 440…Housing, B…Object under test, DS…Electrical signal, FL…Fluorescence image, P1, P11, P12…First optical pulse train, P2, P21, P22…Second optical pulse train, P3…Difference frequency light, T1…First terahertz wave, T2…Second terahertz wave, T3…Third terahertz wave, TR…Trigger signal.

Claims

1. An optical pulse train output unit that outputs a periodic first optical pulse train having a first repetition frequency and a periodic second optical pulse train having a second repetition frequency smaller than the first repetition frequency, A first conversion unit that converts the first optical pulse train into a first terahertz wave, A second conversion unit that converts the second light pulse train into a second terahertz wave, A multiplexing optical system that combines the first terahertz wave with the second terahertz wave to generate a third terahertz wave, A trigger generation unit that generates a trigger signal indicating the timing for detecting the third terahertz wave, The device includes an electron emission unit that emits electrons upon receiving the third terahertz wave, and an electron multiplication unit that emits secondary electrons upon receiving the electrons, and a detection unit that detects the third terahertz wave at the timing indicated by the trigger signal, A terahertz wave interferometry device equipped with the following features.

2. The terahertz wave interference measuring apparatus according to claim 1, wherein the detection unit has a photomultiplier tube including a plurality of dynodes as the electron multiplication unit.

3. The terahertz wave interference measuring device according to claim 2, wherein both the first repetition frequency and the second repetition frequency are within the range of 1 MHz to 1 GHz.

4. The detection unit is An image intensifier comprising a microchannel plate as the electron multiplication unit, and a phosphor that converts the secondary electrons emitted from the microchannel plate into a fluorescent image, An imaging unit that captures the fluorescence image output from the image intensifier, A terahertz wave interference measuring device according to claim 1, having the following features.

5. The terahertz wave interference measuring device according to claim 4, wherein both the first repetition frequency and the second repetition frequency are within the range of 1 MHz to 500 MHz.

6. The trigger generation unit, A nonlinear crystal that generates difference frequency light or sum frequency light from the first and second optical pulse trains, A photodetector that detects the difference frequency light or the sum frequency light from the nonlinear crystal and generates a detection signal, It has, The terahertz wave interference measuring apparatus according to any one of claims 1 to 5, wherein the trigger generation unit uses the detection signal or a signal based on the detection signal as the trigger signal.

7. A terahertz wave interference measuring device according to any one of claims 1 to 5, wherein the difference between the first repetition frequency and the second repetition frequency is 100 Hz or more.

8. The terahertz wave interference measuring device according to any one of claims 1 to 5, wherein Δfrep is defined as the difference between a first repetition frequency frep1 and a second repetition frequency frep2, and the measurement bandwidth Δν is defined as Δν = (frep1 × frep2) / 2Δfrep, and the measurement bandwidth Δν is greater than the spectral sensitivity spectral width of the detection unit.

9. The steps include outputting a periodic first light pulse train having a first repetition frequency and outputting a periodic second light pulse train having a second repetition frequency smaller than the first repetition frequency, The steps include converting the first light pulse train into a first terahertz wave and converting the second light pulse train into a second terahertz wave, The steps include generating a third terahertz wave by combining the first terahertz wave with the second terahertz wave, The steps include generating a trigger signal indicating the timing for detecting the third terahertz wave, A device having an electron emission unit that emits electrons upon receiving the third terahertz wave and an electron multiplication unit that emits secondary electrons upon receiving the electrons, to detect the third terahertz wave at the timing indicated by the trigger signal, A terahertz wave interference measurement method comprising the following features.

Images