Interference measuring device and interference measuring method

Abstract

An interference measuring device 1A comprises a light source 10, an interference optical system 20A, a photomultiplier tube 30, an interference intensity measuring unit 40, a unit 50 for calculating the electric field amplitude, and an analysis unit 60. The light source 10 emits light in a band (a light band containing terahertz waves) to which the photomultiplier tube 30 is sensitive. The photomultiplier tube 30 is sensitive in a light band containing terahertz waves and outputs an electrical signal with a value corresponding to the intensity of the incident light.The unit 50 for calculating the electric field amplitude converts a value of intensity V of interference light, measured by the interference intensity measuring unit 40, into a value of electric field amplitude E, based on a relationship between a value of the electric field amplitude of light incident on the photomultiplier tube 30 and a value of the electrical signal output by the photomultiplier tube 30. It obtains the value of the electric field amplitude E of the interference light for each value of a time difference Δt, which corresponds to an optical path length difference Δd. Thus, an interference measuring device and an interference measurement method are realized that can perform Fourier spectroscopy by interference measurement using a terahertz wave quickly and accurately.

Description

Technical field

[0001] The present invention relates to an interference measuring device and an interference measuring method. background

[0002] A terahertz wave is light in a band intermediate between light waves and radio waves (a band with a frequency around 1 THz) and has a characteristic absorption spectrum for an analytical target, such as a drug, which is not observed in other wavelength bands. Therefore, its application is expected to be for identifying the analytical target and the like. Various techniques are known as analytical techniques using terahertz waves.

[0003] Terahertz time-domain spectroscopy (THz-TDS) is a method in which a temporal waveform of the terahertz wave passing through, being reflected, or totally reflected by the target object is measured, and the analysis of the target object can be performed by performing a Fourier transform on the temporal waveform of an electric field amplitude of the terahertz wave obtained from the above measurement (non-patent document 1). Hereinafter, the above-mentioned method is referred to as the "prior art." In the prior art, a lock-in amplifier is used to measure the temporal waveform of the terahertz wave.

[0004] By using a terahertz wave light source with a variable output wavelength and by scattering and detecting the terahertz wave passing through, reflecting, or totally reflecting the target object, the analysis of the target object can be performed (non-patent 2). Hereinafter, the above-mentioned technique is referred to as the "second prior art." In the second prior art, a thermal detector is used to detect the terahertz wave.

[0005] By performing Fourier spectroscopy via interference measurement using the terahertz wave, based on a measurement principle similar to that of Fourier-transform infrared spectroscopy (FTIR), the analysis of the target object can also be carried out (non-patent 3). Hereinafter, the above-mentioned technique is referred to as the "third prior art." In the third prior art, a thermal detector is used to detect the interference of the terahertz wave. List of citations Patent literature

[0006] Patent document 1: Published Japanese patent application no. 2022-538534 Non-patented literature Non-patent document 1: Jens Neu et al., “Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS),” J. Appl. Phys., 124, 231101 (2018), pp. 231101-1-231101-14 Non-patent document 2: K. Murate et al., “Perspective: Terahertz wave parametric generator and its applications,” J. Appl. Phys., 124, 160901 (2018), pp. 160901-1-160901-10 Non-Patent Document 3: Masashi Yamaguchi et al., “Terahertz wave generation in nitrogen gas using shaped optical pulses,” J. Opt. Soc. On. B, Vol.26, No.9 (2009), pp.A90-A94 Non-patent document 4: Simon Lehnskov Lange et al., “Ultrafast THz-driven electron emission from metal metasurfaces”, J. Appl. Phys., 128, 070901 (2020), p. 070901-1-070901-21 Summary of the invention; Technical task

[0007] In the first state of the art described above, measuring the temporal waveform of the terahertz wave requires a long integration time due to the lock-in amplifier. Furthermore, the second and third state of the art described above employ a thermal detector with a slow response time, resulting in a long measurement time. Conventional analysis techniques using terahertz waves, including the first to third state of the art, therefore require a long measurement time.

[0008] As part of their research into high-speed analysis techniques using terahertz waves, the inventors considered using a photomultiplier tube as described in Patent Document 1. The photomultiplier tube described in Patent Document 1 can be sensitive within a band of light encompassing terahertz waves and can output an electrical signal with a value corresponding to the intensity of the incident light.

[0009] The photomultiplier tube described above comprises an electron emission unit for emitting electrons upon exposure to light, an electron multiplication unit for multiplying the emitted electrons, and a signal output unit for collecting the multiplied electrons and outputting them as an electrical signal or as an optical image converted by a phosphor. The electron emission unit, the electron multiplication unit, and the signal output unit are arranged within a housing, the interior of which is kept under vacuum. A window is provided in the housing, and light passing through this window falls onto the electron emission unit.

[0010] The electron emission unit has a configuration in which a metamaterial structure (a metasurface) corresponding to the band of light from a detection target is formed on a main surface of a substrate and can emit photoelectrons upon incident light on the metasurface. The electron multiplication unit comprises several stages of dynodes or a microchannel plate. In the case where the electron multiplication unit comprises the microchannel plate, imaging of an incident light intensity distribution is possible.

[0011] By using the above photomultiplier tube instead of the thermal detector in the third prior art configuration, the inventors have found a way to accelerate Fourier spectroscopy by interference measurement using the terahertz wave, and have furthermore found that simply performing the same processing as in conventional FTIR on the output signal of the above-described photomultiplier tube does not allow for an accurate analysis of the target object.

[0012] The present invention is based on the findings of the inventors described above and aims to provide an interference measurement device and an interference measurement method that can perform Fourier spectroscopy quickly and accurately by interference measurement using a terahertz wave. Solution to the problem

[0013] One embodiment of the present invention is an interference measuring device. The interference measuring device comprises (1) a photomultiplier tube sensitive in a band of light including a terahertz wave and outputting an electrical signal with a value corresponding to an incident light intensity; (2) a light source for outputting light in the band in which the photomultiplier tube is sensitive; (3) an interference-optical system for splitting the light output by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through an analysis target, and for shining the combined light into the photomultiplier tube, wherein the optical path length difference between the first split light and the second split light is variable;(4) an interference intensity measuring unit for measuring the intensity of interference light produced by the first split light and the second split light incident on the photomultiplier tube, based on the electrical signal output by the photomultiplier tube; (5) a unit for calculating the electric field amplitude, which, based on a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, converts a value of the intensity of the interference light measured by the interference intensity measuring unit into a value of the electric field amplitude and determines the value of the electric field amplitude of the interference light for each value of a time difference corresponding to the optical path length difference;and (6) an analysis unit for analyzing the target object by performing a Fourier transform based on a dependence of the value of the electric field amplitude of the interference light determined by the electric field amplitude calculation unit on the value of the time difference.

[0014] One embodiment of the present invention is an interference measurement method. The interference measurement method is a method for analyzing a target object using (1) a photomultiplier tube with sensitivity in a light band containing a terahertz wave and for outputting an electrical signal with a value corresponding to an incident light intensity; (2) a light source for outputting light in the band in which the photomultiplier tube has the sensitivity;and (3) an interference-optical system for splitting the light emitted by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through the target object for analysis, and for shining the combined light onto the photomultiplier tube, wherein an optical path length difference between the first split light and the second split light is variable, and the method comprises (4) an interference intensity measurement step for measuring an intensity of interference light produced by the first split light and the second split light incident on the photomultiplier tube, based on the electrical signal emitted by the photomultiplier tube;(5) a step for calculating the electric field amplitude, wherein, based on a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, a value of the intensity of the interference light measured in the interference intensity measurement step is converted into the value of the electric field amplitude, and the value of the electric field amplitude of the interference light is determined for each value of a time difference corresponding to the optical path length difference; and (6) an analysis step for analyzing the target object by performing a Fourier transform based on a dependence of the value of the electric field amplitude of the interference light obtained in the electric field amplitude calculation step on the value of the time difference. Advantageous effects of the invention

[0015] According to the embodiments of the present invention, it is possible to perform Fourier spectroscopy quickly and accurately by interference measurement using a terahertz wave. Brief description of the drawings Fig. Figure 1 is a diagram illustrating a configuration example of an interference measurement device. Fig. Figure 2 is a diagram illustrating another configuration example of the interference measurement device. Fig. Figure 3 is a block diagram illustrating a configuration of a photomultiplier tube 30. Fig. Figure 4 is a diagram showing a relationship between an intensity of interference light, measured by an interference intensity measuring unit 40 based on an electrical signal output from the photomultiplier tube 30, and a time difference Δt. Fig. Figure 5 is a diagram showing a relationship between the intensity of the interference light and the time difference Δt, as shown in non-patent document 3. Fig. 6 contains (a) a diagram showing the input-output characteristics of the photomultiplier tube 30, and (b) a diagram showing the time dependence of a voltage signal V output by the photomultiplier tube 30. Fig. Figure 7 contains (a) a diagram showing the dependence of Vp-p on an optical path length difference Δd, and (b) a diagram showing the dependence of Vp-p on the time difference Δt. Fig. Figure 8 contains (a) a diagram showing the dependence of an electric field amplitude E of the interference light on the time difference Δt, and (b) a diagram showing an amplitude spectrum and a phase spectrum of the electric field amplitude E of the interference light. Fig. Figure 9 is a diagram showing a relationship (the FN equation) between an output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave obtained by matching processing. Fig. Table 10 describes a correspondence example between the electric field amplitude E of the incident terahertz wave and the output value V of the photomultiplier tube 30, which is calculated using the FN equation. Fig. 11 contains (a) a diagram showing the dependence of the electric field amplitude E of the interference light on the time difference Δt, and (b) a diagram showing the phase spectrum of the electric field amplitude E of the interference light. Fig. 12 contains (a) a diagram showing the amplitude spectrum of the electric field amplitude E of the interference light, (b) a diagram showing a spectrum of an absorption coefficient α(ω), and (c) a diagram showing a spectrum of a refractive index n(ω). Fig. Figure 13 is a diagram showing a relationship between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave. Description of embodiments

[0016] The following are detailed descriptions of embodiments of an interference measuring device and an interference measuring method with reference to the accompanying drawings. In the description of the drawings, the same elements are designated with the same reference numerals, and redundant descriptions are omitted. The present invention is not limited to these examples, and the claims, their equivalents, and all modifications within the scope are to be understood as falling within the scope of the present invention.

[0017] An interference measuring device of the present embodiment can be configured such that it is a configuration of an interference measuring device 1A ( Fig. 1) or a configuration of an interference measurement device 1B ( Fig. 2) exhibits, as described below.

[0018] Fig. Figure 1 is a diagram illustrating a configuration example of an interference measurement device. The interference measurement device 1A shown in this diagram comprises a light source 10, an interference optical system 20A, a photomultiplier tube 30, an interference intensity measurement unit 40, a unit 50 for calculating the electric field amplitude, and an analysis unit 60.

[0019] The light source 10 emits light in a band (a band of light containing a terahertz wave) to which the photomultiplier tube 30 is sensitive. The light emitted by the light source 10 can be pulsed or continuous.

[0020] Examples of light sources capable of emitting the terahertz pulsed light wave include configurations combining a femtosecond laser light source (e.g., a Ti-sapphire laser light source) and a nonlinear optical crystal (e.g., ZnTe), a parametric THz generator with injection seeding (is-TPG), and similar devices. Examples of light sources capable of emitting the terahertz continuous-wave light wave include a resonant tunnel diode (RTD), a pulsed avalanche and transit-time diode (IM-PATT diode), a quantum cascade laser light source, a THz gas laser light source, and similar devices.

[0021] The interference-optical system 20A comprises a beam splitter 21, a mirror 23, and a mirror 24 and has a Michelson interferometer configuration. The beam splitter 21 splits the light emitted by the light source 10 into two beams: a first split light and a second split light. It outputs the first split light of one beam to mirror 23 and the second split light of the other beam to mirror 24. Furthermore, the beam splitter 21 receives the first split light reflected by mirror 23, receives the second split light reflected by mirror 24, combines the first and second split light as described above, and outputs the combined light to the photomultiplier tube 30. The beam splitter 21 can be made, for example, of silicon or an ITO mirror.

[0022] An analysis target S is positioned on the beam path of the first split light between the beam splitter 21 and the mirror 23. The analysis target S can also be positioned on the beam path of the second split light. The mirror 23 and the mirror 24 can both, or one of them, be moved in a direction perpendicular to a reflective surface, thereby variably adjusting the optical path length difference between the first split light and the second split light.

[0023] The photomultiplier tube 30 exhibits sensitivity in the light band including the terahertz wave and outputs an electrical signal with a value corresponding to the incident light intensity. Details of the photomultiplier tube 30 will be given later with reference to Fig. 3 described.

[0024] The interference intensity measuring unit 40 measures an intensity of interference light produced by the first split light and the second split light incident on the photomultiplier tube 30, based on the electrical signal output by the photomultiplier tube 30 (an interference intensity measurement step).

[0025] Unit 50, used to calculate the electric field amplitude, converts an intensity value V of the interference light measured by the interference intensity measuring unit 40 into the value of the electric field amplitude E, based on a relationship between a value of the electric field amplitude of the light incident on the photomultiplier tube 30 and a value of the electrical signal output by the photomultiplier tube 30. It then obtains the value of the electric field amplitude E of the interference light for each value of a time difference Δt, which corresponds to the optical path length difference Δd (electric field amplitude calculation step). The optical path length difference Δd corresponds to twice the distance between the beam splitter and the two mirrors. The relationship between the optical path length difference Δd and the time difference Δt is given by Δt = Δd / c, where c is the speed of light in a vacuum.

[0026] The analysis unit 60 analyzes the analysis target object S by performing a Fourier transformation based on the dependence of the value of the electric field amplitude E of the interference light obtained by the unit 50 for calculating the electric field amplitude on the value of the time difference Δt (one analysis step).

[0027] The details of the processing carried out by each of the interference intensity measurement unit 40, the electric field amplitude calculation unit 50 and the analysis unit 60 are also described later.

[0028] An interference measurement method of the present embodiment is a method for analyzing the target object S using the light source 10, the interference optical system 20A and the photomultiplier tube 30 described above and comprises the interference intensity measurement step, the step for calculating the electric field amplitude and the analysis step.

[0029] Fig. Figure 2 is a diagram illustrating another configuration example of the interference measuring device. The interference measuring device 1B shown in this diagram comprises the light source 10, an interference optical system 20B, the photomultiplier tube 30, the interference intensity measuring unit 40, the electric field amplitude calculation unit 50, and the analysis unit 60. In comparison to the configuration of the interference measuring device 1A ( Fig. 1) the interference measuring device 1B differs ( Fig. 2) by including the interference optical system 20B instead of the interference optical system 20A.

[0030] The interference-optical system 20B comprises beam splitters 21 and 22 and mirrors 23 to 26 and has a Mach-Zehnder interferometer configuration. Beam splitter 21 splits the light emitted by light source 10 into two beams, one a first split light and the other a second split light. It outputs the first split light of one beam to mirror 23 and the second split light of the other beam to mirror 24. The second split light output to mirror 24 is reflected by mirror 24, mirror 25, and mirror 26, respectively. Beam splitter 22 receives the first split light reflected by mirror 23, receives the second split light reflected by mirror 26, combines the first and second split light as described above, and outputs the combined light to photomultiplier tube 30.

[0031] The target object S is positioned on the beam path of the first split light from beam splitter 21 through mirror 23 to beam splitter 22. The target object S can also be positioned on the beam path of the second split light. Mirror 24 and mirror 25 are movable, allowing the optical path length difference between the first split light and the second split light to be adjusted.

[0032] The Michelson interferometer is preferred over the Mach-Zehnder interferometer because the number of optical components (beam splitter and mirrors) is small and the mechanism for changing the optical path length difference is simple. Therefore, the configuration of the interference measuring device 1A ( Fig. 1) compared to the configuration of the interference measuring device 1B ( Fig. 2) preferred.

[0033] Fig. Figure 3 is a block diagram illustrating a configuration of the photomultiplier tube 30. The photomultiplier tube 30 has a configuration in which an electron emission unit 31, an electron multiplication unit 32, and a signal output unit 33 are arranged within a housing 34, the interior of which is kept in a vacuum. The housing 34 is provided with a window section 35.

[0034] The electron emission unit 31 emits electrons e upon incident light when light v transmitted through the window section 35 strikes it. The electron emission unit 31 is a photoelectric conversion unit designed to be sensitive in the band of light including the terahertz wave of the detection target, and, for example, has a configuration in which a metamaterial structure (a metasurface) is formed on a major surface of a substrate and the electrons e can be emitted upon incident light on the metasurface.

[0035] The electron multiplication unit 32 multiplies the electrons e emitted by the electron emission unit 31. The electron multiplication unit 32 comprises several stages of dynodes or a microchannel plate. The electron multiplication factor in the electron multiplication unit 32 depends on the voltage applied to the several stages of dynodes or the microchannel plate.

[0036] The signal output unit 33 collects the electrons e multiplied by the electron multiplication unit 32 and outputs them as a current signal J. The interference intensity measurement unit 40 can input the current signal J output by the signal output unit 33 or a voltage signal obtained by converting the current signal J into the voltage signal using an I / O conversion circuit.

[0037] Fig. Figure 4 is a diagram showing the relationship between the intensity of the interference light, measured by the interference intensity measuring unit 40 based on the electrical signal output from the photomultiplier tube 30, and the time difference Δt. According to the diagram, there is a peak in the intensity of the interference light at a time difference Δt = 0, and there are also peaks in the intensity of the interference light in a time difference range Δt = -16 to -11 ps and in a time difference range Δt = +11 to +16 ps. Conversely, the intensity of the interference light is very low in the time difference ranges Δt outside the aforementioned peak ranges.

[0038] The peak value of the interference light intensity at the position of the time difference Δt = 0 arises from the interference of the first and second split light without multiple reflections, and the half-width is approximately 0.4 ps. It is confirmed that the interference light intensity can be obtained using the 30 photomultiplier tube.

[0039] The peaks of interference light intensity in the time difference range Δt = -16 to -11 ps and in the time difference range Δt = +11 to +16 ps are considered to be the result of an etalon effect of the terahertz wave in the window region 35 of the photomultiplier tube 30 or in the substrate of the electron emission unit 31. The aforementioned peaks of interference light intensity, generated due to the etalon effect, are sufficiently separated from the peak of interference light intensity at the time difference Δt = 0 and do not become noise.

[0040] For example, if the window section 35 of the photomultiplier tube 30 is made of synthetic quartz (with a refractive index of 1.5) and has a thickness of 1 mm, reflected pulses are generated in a time interval of 13.3 ps due to the etalon effect. In the case where the substrate of the electron emission unit 31 of the photomultiplier tube 30 is made of silicon (with a refractive index of 3.3) and has a thickness of 0.525 mm, reflected pulses are generated in a time interval of 12.0 ps due to the etalon effect.

[0041] The aforementioned time intervals (13.3 ps, 12.0 ps) of the reflected pulses generated by the etalon effect are sufficiently larger than the half-width (approximately 0.4 ps) of the peak of the interference light intensity at the position of the time difference Δt = 0. Therefore, there is no influence of the etalon effect of the terahertz wave in window section 35 of the photomultiplier tube 30 or the substrate of the electron emission unit 31.

[0042] Fig. Figure 5 is a diagram showing a relationship between the intensity of the interference light and the time difference Δt, as shown in Non-Patent Document 3. Non-Patent Document 3 uses a thermal detector. When comparing Fig. 4 and Fig. Figure 5 shows that the dependence of the intensity of the interference light on the time difference Δt between the photomultiplier tube 30 and the thermal detector is very different.

[0043] The dependence of the intensity of the interference light on the time difference Δt, obtained when using the photomultiplier tube 30, shows a weak correlation with the waveform of the terahertz wave incident on the photomultiplier tube 30. Therefore, accurate analysis of the target object is not possible if the same processing as in conventional FTIR is performed on the output signal of the photomultiplier tube 30.

[0044] This is attributed to the fact that the relationship between the value of the electric field amplitude of the terahertz wave incident on the photomultiplier tube 30 and the value of the electrical signal output by the photomultiplier tube 30 is different between the photomultiplier tube 30 and the thermal detector. The present invention was made based on the aforementioned findings of the inventors.

[0045] Fig. 6 to Fig. Figure 8 shows diagrams describing the processing details performed by the interference intensity measurement unit 40, the electric field amplitude calculation unit 50, and the analysis unit 60, respectively. The interference intensity measurement unit 40 performs the operations described in (b) in Fig. 6 and (a) in Fig. The processing described in 7 is carried out. The unit 50 for calculating the electric field amplitude performs the process described in (b) in Fig. 7 and (a) in Fig. The processing described in (b) is carried out. The analysis unit 60 performs the processing described in (b) in Fig. 8 processing described above. In addition, the horizontal axis and the vertical axis in (a) are Fig. 6 and the vertical axis in (b) in Fig. 7 are logarithmic scales, while the other scales on the horizontal and vertical axes are linear scales.

[0046] (a) in Fig. Figure 6 is a diagram showing the input-output characteristics of the photomultiplier tube 30. The horizontal axis represents the electric field amplitude E of the light incident on the photomultiplier tube 30. The vertical axis shows the electrical signal (the voltage signal V) output by the photomultiplier tube 30. As shown in this diagram, the input-output characteristics of the photomultiplier tube 30 are non-linear. The input-output characteristics of the photomultiplier tube 30 mentioned above are obtained in advance.

[0047] (am Fig. Figure 6 is a diagram showing the time dependence of the voltage signal V output by the photomultiplier tube 30. The optical path length difference Δd is set to a specific value, and a change in the voltage signal V output by the photomultiplier tube 30 is obtained over time. The magnitude of the amplitude Vp-p of the voltage signal V is read from this diagram.

[0048] (a) in Fig. Figure 7 is a diagram showing the dependence of Vp-p on the optical path length difference Δd. This diagram is obtained by determining Vp-p for each value of the optical path length difference Δd.

[0049] (am Fig. Figure 7 is a diagram showing the dependence of Vp-p on the time difference Δt. The relationship between the optical path length difference Δd and the time difference Δt is given by Δt = Δd / c, so the time difference Δt can be obtained from the optical path length difference Δd.

[0050] (a) in Fig. Figure 8 is a diagram showing the dependence of the electric field amplitude E of the interference light on the time difference Δt. Using the input-output characteristics of the photomultiplier tube 30 ((a) in Fig. 6) Is it possible to go from (b) to Fig. 7 to (a) in Fig. to convert 8.

[0051] (am Fig. Figure 8 is a diagram showing the amplitude spectrum and phase spectrum of the electric field amplitude E of the interference light. This diagram is obtained by Fourier transforming the dependence of the electric field amplitude E of the interference light on the time difference Δt.

[0052] According to the above description, in the present embodiment the input-output characteristics of the photomultiplier tube 30 (the relationship between the electric field amplitude E of the light incident on the photomultiplier tube 30 and the electrical signal (the voltage signal V) output by the photomultiplier tube 30) are obtained in advance, and using this, the magnitude of the amplitude Vp-p of the voltage signal V output by the photomultiplier tube 30 is converted into the electric field amplitude E of the interference light. By using the photomultiplier tube 30 and performing the above conversion, it is possible to carry out Fourier spectroscopy quickly and accurately by interference measurement using the terahertz wave.

[0053] According to the description above, the input-output properties ((a) in Fig. 6) of the photomultiplier tube 30 is non-linear. The output value of the photomultiplier tube 30 can be described by a polynomial formula in which the electric field amplitude E of the light incident on the photomultiplier tube 30 is a variable and can also be described using the following formula (1), which represents the efficiency of electron emission in the metasurface (non-patent document 4). This formula represents the relationship between the current JFN emitted by the metasurface and the electric field amplitude E of the incident terahertz wave and is known as the Fowler-Nordheim relation (hereinafter referred to as the “FN equation”). [Formula 1] JFN(E)=aFNtF2(βE)2Φexp(−νFbFNΦ3 / 2βE)

[0054] In the above FN equation, a FN and b FNThese are referred to as FN constants and they exhibit certain constant values. β is the field amplification factor, which is approximately 400 in non-patent document 4. Φ is the work function of the metasurface material of electron emission unit 31, which is 3.5 eV for gold. F and v F are constants. In the case where the electric field amplitude of the incident terahertz wave is not large, the values ​​of t can be F and v F set to 1. In this case, the FN equation is represented by the following formula (2). [Formula 2] JFN(E)=aFN(βE)2Φexp(−bFNΦ3 / 2βE)

[0055] The FN equation represents the relationship between the current JFN emitted by the electron emission unit 31 of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave. Furthermore, the relationship between the output value of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave can also be represented in the same way.

[0056] It is necessary to determine the respective values ​​of a FN and b FN to determine in the FN equation. For this purpose, the electric field amplitude E of the incident terahertz wave is set to each value, the output value V of the photomultiplier tube 30 is measured, and by performing a matching process using the values ​​measured above, the values ​​of a FN and b FN be determined. Fig. Figure 9 is a diagram showing the relationship (the FN equation) between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave obtained through matching processing. In this diagram, five measured values ​​are represented by circles.

[0057] To obtain the electric field amplitude E of the incident terahertz wave from the output value V of the photomultiplier tube 30 using the FN equation, the following procedure can be used. By calculating using the FN equation, the output value V of the photomultiplier tube 30 is obtained for each value of the electric field amplitude E of the incident terahertz wave. Fig. Table 10 describes a correspondence example between the electric field amplitude E of the incident terahertz wave and the output value V of the photomultiplier tube 30, obtained by calculation using the FN equation. Unit 50 for calculating the electric field amplitude obtains the electric field amplitude E of the incident terahertz wave that is closest to the matching value from the actual output value V of the photomultiplier tube 30. Additionally, the electric field amplitude E of the incident terahertz wave can be obtained by interpolation.

[0058] The analysis unit 60 performs the analysis of the target object S by carrying out the Fourier transform based on the dependence of the value of the electric field amplitude E of the interference light, obtained by the electric field amplitude calculation unit 50, on the value of the time difference Δt. Specifically, the process is as follows.

[0059] In the interference measuring device 1A ( Fig. 1), provided by the interference-optical system 20A with the Michelson interferometer configuration, the terahertz wave passes through the target object S twice. A phase refractive index of the target object S is set to n(ω), an extinction coefficient of the target object S is set to k(ω), and a complex refractive index of the target object S is set to n'(ω) = n(ω) + ik(ω). ω is an angular velocity of the terahertz wave. When the frequency of the terahertz wave is set to f, ω = 2πf. π is the mathematical constant pi. i is the imaginary unit.

[0060] The electric field amplitude of the interference light obtained in the case where the target object S is positioned is given by E sample (ω) is set and the electric field amplitude of the interference light obtained in the case where the analysis target object S is not positioned is set to E ref(ω) is set, and the ratio T(ω) of the above values ​​is represented by the following formula (3). t as The interfacial amplitude transmittance of air to the target object S is represented by the following formula (4). sa The interfacial amplitude transmittance of the target object S to the air is represented by the following formula (5). d is the thickness of the target object S. c is the speed of light in a vacuum. [Formula 3] T(ω)=Esample(ω)Eref(ω)=(tastsa)2exp[2i{n'(ω)−1}ωdc]=(tastsa)2exp[−2kωdc]exp[2i{n(ω)−1}ωdc] [Formula 4] tas=2n(ω)+1 [Formula 5] tsa=2n(ω)n(ω)+1

[0061] By decomposing the above formula (3) into a real part and an imaginary part, the following formulas (6) to (8) are obtained. f(ω) is the phase spectrum. α(ω) is the absorption coefficient. In the interference measuring device 1A ( Fig. 1) The analysis unit 60 can analyze the analysis target object S based on these formulas. [Formula 6] n(ω)=1+c2ωdϕ(ω) [Formula 7] k(ω)=c2ωd[In{4n(ω)n(ω)+1}2−In{T(ω)}] [Formula 8] α(ω)=2ωk(ω)c

[0062] In the interference measuring device 1B ( Fig. 2), which is provided with the interference optical system 20B with the Mach-Zehnder interferometer configuration, the terahertz wave passes through the target object S once. Therefore, instead of the above formulas (3), (6) and (7), the following formulas (9) to (11) are used. In the interference measuring device 1B ( Fig. 2) The analysis unit 60 can analyze the analysis target object S based on these formulas. [Formula 9] T(ω)=Esample(ω)Eref(ω)=tastsaexp[i{n'(ω)−1}ωdc]=tastsaexp[−kωdc]exp[i{n(ω)−1}ωdc] [Formula 10] n(ω)=1+cωdϕ(ω) [Formula 11] k(ω)=cωd[In{4n(ω)n(ω)+1}−In{T(ω)}]

[0063] Fig. 11 and Fig. Figure 12 contains diagrams that show an example of the results of the measurement or analysis. The one in (a) in Fig. Diagram 11 shown is obtained using the unit 50 for calculating the electric field amplitude. The respective diagrams shown in (b) in Fig. 11 and (a) to (c) in Fig. The 12 shown are obtained by the analysis unit 60.

[0064] (a) in Fig. Figure 11 is a diagram showing the dependence of the electric field amplitude E of the interference light on the time difference Δt. (b) in Fig. Figure 11 is a diagram showing the phase spectrum of the electric field amplitude E of the interference light. (a) in Fig. Figure 12 is a diagram showing the amplitude spectrum of the electric field amplitude E of the interference light. Each of the diagrams above shows the case where the target object S is positioned and the case where the target object S is not. Lactose is used as the target object S.

[0065] (am Fig. Figure 12 is a diagram showing the spectrum of the absorption coefficient α(ω). (c) in Fig. Figure 12 is a diagram showing the spectrum of the refractive index n(ω). The diagrams above show the analysis results according to the present embodiment and the analysis results using THz-TDS from the prior art.

[0066] As can be seen by comparing the analysis results using the present embodiment and the analysis results using THz-TDS of the prior art, the positions of the absorption peaks appearing in the spectrum of the absorption coefficient α(ω) are identical. From the above, it can be said that the target object S can be analyzed using the present embodiment.

[0067] In the interference intensity measurement step, it is preferred that the interference intensity measurement unit 40 sets a measurement range at the time of measurement of the intensity of the interference light based on the electrical signal output from the photomultiplier tube 30 such that a saturation of the measured value of the intensity of the interference light is suppressed and a decrease of the measured value of the intensity of the interference light to a value of a noise level or less is suppressed.

[0068] This means that if the optical path length difference Δd is changed, the magnitude of the amplitude Vp-p of the voltage signal V output by the photomultiplier tube 30 changes by approximately three orders of magnitude. In this case, if the measurement range is fixed, the interference light intensity measurement may saturate or fall below the noise level. By adjusting the measurement range when measuring the interference light intensity, the interference intensity measurement unit 40 can suppress saturation of the interference light intensity measurement and prevent the interference light intensity measurement from falling to or below the noise level.

[0069] In the interference intensity measurement step, it is preferred that the interference intensity measurement unit 40 measures a quantity of a specific frequency component, which is obtained by performing the Fourier transform on the temporal waveform of the electrical signal that is output by the photomultiplier tube 30, as the intensity of the interference light.

[0070] This means that it is preferred that the interference intensity measurement unit 40 uses an FFT function provided in an oscilloscope. Using the FFT function, the value of the electrical signal can be logarithmically transformed, and thus it is possible to suppress the saturation of the interference light intensity measurement even if the measurement range is not matched. Furthermore, it is possible to suppress the decrease in the interference light intensity measurement to the level of the noise or less. It is sufficient to measure the magnitude of the specific frequency component obtained by the Fourier transform as the interference light intensity.

[0071] In general, the lower the frequency, the higher the signal-to-noise ratio. A high-frequency component depends on the type of photomultiplier tube. Therefore, from this perspective as well, it is preferable to analyze a low-frequency component. Furthermore, at a frequency of 0 Hz (i.e., the DC component), there is no significant difference due to the signal. It is also preferred to avoid the component that corresponds to the repetition frequency of the pulsed light emitted by the light source 10. Therefore, it is preferred that the interference intensity measuring unit 40 measures the intensity of the interference light as a component with the lowest possible frequency, excluding the DC component and the repetition frequency of the pulsed light.

[0072] In the interference intensity measurement step, it is preferred that the interference intensity measurement unit 40 adjusts the dynamic range or the sensitivity at the time of measurement of the intensity of the interference light based on the electrical signal by adjusting the voltage applied to the photomultiplier tube 30.

[0073] This means that, as in Fig. As shown in Figure 9, the output value V of the photomultiplier tube 30 increases non-linearly with the increase in the electric field amplitude E of the incident terahertz wave. With increasing electric field amplitude E of the incident terahertz wave, the increase in the output value V of the photomultiplier tube 30 tends to be less pronounced. By changing the voltage applied to the electron multiplication unit 32 of the photomultiplier tube 30, a sensitivity curve, which shows the relationship between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave, can be shifted horizontally.

[0074] Fig.Figure 13 is a diagram showing the relationship between the output voltage V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave. This diagram depicts three sensitivity curves A to C with different voltage values ​​applied to the electron multiplication unit 32 of the photomultiplier tube 30. The specifications of the light source 10 and the range of change ΔE of the electric field amplitude E of the incident terahertz wave are fixed.

[0075] In the case where the applied voltage is increased with respect to sensitivity curve A, the sensitivity curve shifts to the left and becomes sensitivity curve B. Compared to the change range ΔV A The change range ΔV of the initial value V of the photomultiplier tube 30 in the sensitivity curve A is BThe output value V of the photomultiplier tube 30 in the sensitivity curve B is smaller, which increases the dynamic range of the measurement, which is advantageous, for example, for spectroscopic measurement using the photomultiplier tube 30.

[0076] On the other hand, if the applied voltage is reduced relative to sensitivity curve A, the sensitivity curve shifts to the right and becomes sensitivity curve C. Compared to the change range ΔV A The change range ΔV of the initial value V of the photomultiplier tube 30 in the sensitivity curve A is C The output value V of the photomultiplier tube 30 in the sensitivity curve C is larger, which increases the measurement sensitivity, which is advantageous, for example, for detection using the photomultiplier tube 30.

[0077] According to the above description, the interference intensity measuring unit 40 can adjust the dynamic range or sensitivity when measuring the intensity of the interference light based on the electrical signal by adjusting the voltage applied to the photomultiplier tube 30.

[0078] The photomultiplier tube 30 can be a photomultiplier tube for measuring the incident light intensity or a photomultiplier tube that can image the incident light intensity distribution. In the case where the electron multiplication unit 32 includes the microchannel plate (for example, an image intensifier), imaging of the incident light intensity distribution is possible. By using the photomultiplier tube 30 described above, it becomes possible to perform an analytical image of the target object S.

[0079] The interference measuring device and the interference measuring method are not limited to the embodiments and configuration examples described above, and various modifications are possible.

[0080] The interference measuring device of a first aspect according to the above embodiment comprises (1) a photomultiplier tube having sensitivity in a band of light including a terahertz wave and outputting an electrical signal with a value corresponding to an incident light intensity; (2) a light source for emitting light in the band in which the photomultiplier tube has sensitivity; (3) an interference-optical system for splitting the light emitted by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through an analysis target object, and for shining the combined light onto the photomultiplier tube, wherein an optical path length difference between the first split light and the second split light is variable;(4) an interference intensity measuring unit for measuring the intensity of interference light produced by the first split light and the second split light incident on the photomultiplier tube, based on the electrical signal output by the photomultiplier tube; (5) a unit for calculating the electric field amplitude, which, based on a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, converts a value of the intensity of the interference light measured by the interference intensity measuring unit into a value of the electric field amplitude and obtains the value of the electric field amplitude of the interference light for each value of a time difference corresponding to the optical path length difference;and (6) an analysis unit for analyzing the target object by performing a Fourier transform based on a dependence of the value of the electric field amplitude of the interference light obtained by the electric field amplitude calculation unit on the value of the time difference.

[0081] In the interference measurement device of a second aspect, in the configuration of the first aspect, the interference intensity measurement unit can set a measurement range when measuring the intensity of the interference light based on the electrical signal output from the photomultiplier tube in such a way that a saturation of a measured value of the intensity of the interference light is suppressed and a decrease of the measured value of the intensity of the interference light to a value of a noise level or less is suppressed.

[0082] In the interference measuring device of a third aspect, in the configuration of the first or second aspect, the interference intensity measuring unit can measure a quantity of a specific frequency component, obtained by performing a Fourier transform on a temporal waveform of the electrical signal output by the photomultiplier tube, as the intensity of the interference light.

[0083] In the interference measuring device of a fourth aspect, in the configuration of any of the first to third aspects, the interference intensity measuring unit can set a dynamic range or sensitivity when measuring the intensity of the interference light based on the electrical signal by adjusting a voltage applied to the photomultiplier tube.

[0084] In the interference measurement device of a fifth aspect, the photomultiplier tube can image an incident light intensity distribution in any of the first to fourth aspect configurations. The analysis unit can then perform an analytical image of the target object.

[0085] The interference measurement method of a first aspect according to the above embodiment is a method for analyzing an analysis target object using (1) a photomultiplier tube having sensitivity in a band of light including a terahertz wave and for outputting an electrical signal with a value corresponding to an incident light intensity; (2) a light source for outputting light in the band in which the photomultiplier tube has sensitivity;and (3) an interference-optical system for splitting the light emitted by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through the target object and causing the combined light to strike the photomultiplier tube, wherein an optical path length difference between the first split light and the second split light is variable. The method comprises (4) an interference intensity measurement step for measuring an intensity of interference light produced by the first split light and the second split light that strikes the photomultiplier tube, based on the electrical signal emitted by the photomultiplier tube;(5) a step for calculating the electric field amplitude, wherein, based on a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, a value of the intensity of the interference light measured in the interference intensity measurement step is converted into the value of the electric field amplitude, and the value of the electric field amplitude of the interference light is obtained for each value of a time difference corresponding to the optical path length difference; and (6) an analysis step for analyzing the target object by performing a Fourier transform based on a dependence of the value of the electric field amplitude of the interference light obtained in the electric field amplitude calculation step on the value of the time difference.

[0086] In the interference measurement procedure of a second aspect, in the configuration of the first aspect in the interference intensity measurement step, a measurement range can be set when measuring the intensity of the interference light based on the electrical signal output by the photomultiplier tube in such a way that a saturation of a measured value of the intensity of the interference light is suppressed and a decrease of the measured value of the intensity of the interference light to a value of a noise level or less is suppressed.

[0087] In the interference measurement method of a third aspect, in the configuration of the first or second aspect in the interference intensity measurement step, a quantity of a specific frequency component, which is obtained by performing a Fourier transformation on a temporal waveform of the electrical signal output by the photomultiplier tube, can be measured as the intensity of the interference light.

[0088] In the interference measurement method of a fourth aspect, in the configuration of any of the first to third aspects in the interference intensity measurement step, a dynamic range or sensitivity can be set when measuring the intensity of the interference light based on the electrical signal by adjusting a voltage applied to the photomultiplier tube.

[0089] In the interference measurement method of a fifth aspect, the photomultiplier tube, which can map an incident light intensity distribution, can be used in the configuration of any of the first to fourth aspects, and in the analysis step an analysis image of the target object can be performed. Industrial applicability

[0090] The present invention can be used as an interference measuring device and interference measuring method that is capable of performing Fourier spectroscopy quickly and accurately by interference measurement using a terahertz wave. List of reference symbols

[0091] 1A, 1B - Interference measuring device, 10 - Light source, 20A, 20B - Interference optical system, 21, 22 - Beam splitter, 23 - 26 - Mirrors, 30 - Photomultiplier tube, 31 - Electron emission unit, 32 - Electron multiplication unit, 33 - Signal output unit, 34 - Housing, 35 - Window section, 40 - Interference intensity measuring unit, 50 - Unit for calculating the electric field amplitude, 60 - Analysis unit, S - Target object for analysis. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2022-538534

[0006] Cited non-patent literature

[0000] Jens Neu et al., “Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS),” J. Appl. Phys., 124, 231101 (2018), pp. 231101-1-231101-14

[0006] K. Murate et al., “Perspective: Terahertz wave parametric generator and its applications,” J. Appl. Phys., 124, 160901 (2018), pp. 160901-1-160901-10

[0006] Masashi Yamaguchi et al., “Terahertz wave generation in nitrogen gas using shaped optical pulses,” J. Opt. Soc. On. B, Vol.26, No.9 (2009), pp.A90-A94

[0006] Simon Lehnskov Lange et al., „Ultrafast THz-driven electron emission from metal metasurfaces“, J. Appl. Phys., 128, 070901 (2020), S. 070901-1-070901-21

[0006]

Claims

[1] Interference measuring device comprising: a photomultiplier tube with sensitivity in a light band encompassing terahertz waves, and for outputting an electrical signal with a value corresponding to an incident light intensity; a light source for emitting light in the band in which the photomultiplier tube has the sensitivity; an optical interference system for splitting the light emitted by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through an analysis target object, and for shining the combined light onto the photomultiplier tube, wherein an optical path length difference between the first split light and the second split light is variable; an interference intensity measuring unit for measuring the intensity of interference light produced by the first split light and the second split light incident on the photomultiplier tube, based on the electrical signal output by the photomultiplier tube; a unit for calculating the electric field amplitude, in order to convert a value of the intensity of the interference light measured by the interference intensity measuring unit into the value of the electric field amplitude on the basis of a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, and to obtain the value of the electric field amplitude of the interference light for each value of a time difference corresponding to the optical path length difference; and an analysis unit for analyzing the target object by performing a Fourier transformation based on a dependence of the value of the electric field amplitude of the interference light obtained by the electric field amplitude calculation unit on the value of the time difference. [2] Interference measuring device according to claim 1, wherein the interference intensity measuring unit sets a measuring range when measuring the intensity of the interference light based on the electrical signal output from the photomultiplier tube, such that a saturation of a measured value of the intensity of the interference light is suppressed and a decrease of the measured value of the intensity of the interference light to a value of a noise level or less is suppressed. [3] Interference measuring device according to claim 1 or 2, wherein the interference intensity measuring unit measures a quantity of a specific frequency component obtained by Fourier transformation of a temporal waveform of the electrical signal output by the photomultiplier tube as the intensity of the interference light. [4] Interference measuring device according to one of claims 1 to 3, wherein the interference intensity measuring unit sets a dynamic range or sensitivity when measuring the intensity of the interference light based on the electrical signal by adjusting a voltage applied to the photomultiplier tube. [5] Interference measuring device according to one of claims 1 to 4, wherein the photomultiplier tube is designed to image an incident light intensity distribution and the analysis unit performs an analysis image of the target object. [6] Interference measurement methods for analyzing an analysis target object using: a photomultiplier tube which has a sensitivity in a light band including a terahertz wave and outputs an electrical signal with a value corresponding to an incident light intensity; a light source for emitting light in the band in which the photomultiplier tube has sensitivity; and an interference optics system for splitting the light emitted by the light source into a first split light and a second split light, for combining the first split light and the second split light that have passed through the target object and for shining the combined light onto the photomultiplier tube, wherein an optical path length difference between the first split light and the second split light is variable, the procedure includes: an interference intensity measurement step for measuring the intensity of interference light produced by the first split light and the second split light falling on the photomultiplier tube, based on the electrical signal output by the photomultiplier tube; a step for calculating the electric field amplitude, in which, based on a relationship between a value of an electric field amplitude of the light incident on the photomultiplier tube and a value of the electrical signal output by the photomultiplier tube, a value of the intensity of the interference light measured in the interference intensity measurement step is converted into the value of the electric field amplitude, and the value of the electric field amplitude of the interference light is obtained for each value of a time difference corresponding to the optical path length difference; and an analysis step to analyze the target object by performing a Fourier transformation based on a dependence of the value of the electric field amplitude of the interference light determined in the step to calculate the electric field amplitude on the value of the time difference. [7] Interference measurement method according to claim 6, wherein in the interference intensity measurement step a measurement range for measuring the intensity of the interference light is set based on the electrical signal output by the photomultiplier tube such that a saturation of a measured value of the intensity of the interference light is suppressed and a decrease of the measured value of the intensity of the interference light to a value of a noise level or less is suppressed. [8] Interference measurement method according to claim 6 or 7, wherein in the interference intensity measurement step a quantity of a certain frequency component, which is obtained by Fourier transformation of a temporal waveform of the electrical signal output by the photomultiplier tube, is measured as the intensity of the interference light. [9] Interference measurement method according to one of claims 6 to 8, wherein in the interference intensity measurement step a dynamic range or sensitivity is set when measuring the intensity of the interference light based on the electrical signal by adjusting a voltage applied to the photomultiplier tube. [10] Interference measurement method according to one of claims 6 to 9, wherein the photomultiplier tube which can image an incident light intensity distribution is used and in the analysis step an analysis image of the target object is performed.

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