JP6238058B2 - Terahertz spectroscopy system - Google Patents

Description

  The present invention relates to a terahertz spectroscopy system that includes a terahertz wave (electromagnetic wave of 100 GHz to 10 THz) generation unit and a detection unit for the electromagnetic wave and can perform terahertz time domain spectroscopy.

  The present invention can be used for terahertz time-domain spectroscopy, generates a terahertz wave using an ultrashort light pulse, irradiates the measurement sample, and detects the terahertz wave from the measurement sample. It is for spectroscopic measurement, and the terahertz wave is detected in combination with the terahertz wave from the measurement sample so that the terahertz wave propagates directly from the generation part to the detection part. .

  Conventional terahertz time domain spectroscopy is disclosed in, for example, Patent Document 1. FIG. 1 is a block diagram showing an example of the configuration. Here, the code | symbol in FIG. 1 is a code | symbol in patent document 1. FIG. The conventional time-resolved spectroscopic apparatus is roughly composed of a mode-locked laser that is an ultrashort optical pulse laser as the laser light source 3, a beam splitter 5 that branches the laser light, a time delay unit 53 that uses a mechanically driven movable mirror, A terahertz wave generator 21 that converts an ultrashort light pulse into a terahertz wave pulse and a terahertz wave detector 22 are included. The light from the laser light source 3 is branched into two by the beam splitter 5 and the first light pulse is incident on the terahertz electromagnetic wave generator 21 to generate a terahertz wave. The generated terahertz wave is guided to the terahertz electromagnetic wave receiver 22 using the concave reflecting mirrors 31 to 34 which are terahertz optical systems. The terahertz electromagnetic wave receiver 22 detects the terahertz wave by causing the other second branched optical pulse to be incident simultaneously with the terahertz wave from the terahertz optical system. Here, the arrival time of one of the two branches is changed with respect to the other optical pulse by using the time delay unit 53.

  In the time-resolved spectroscopic device described in Patent Document 1, a confidential container is used, and a terahertz wave generated by a terahertz wave generator is guided to a terahertz detector via a terahertz optical system. The confidential container is evacuated as necessary, and the problem of changes in waveform and spectrum shape is suppressed due to absorption by a medium (for example, air) on the path through which the terahertz wave propagates. However, this does not suppress problems such as terahertz wave loss and waveform distortion caused by the terahertz optical system.

  On the other hand, Patent Document 2 discloses a terahertz wave generation / detection probe having a fiber pigtail, and Patent Document 3 discloses a terahertz spectroscopy system using the probe as shown in FIG. .

  In the conventional terahertz time domain spectroscopy including those disclosures, it is difficult to determine the time origin or initial phase and obtain the reference spectrum in the terahertz wave time waveform measurement performed by one time sweep. Therefore, in order to acquire a transmission spectrum of the sample to be measured, it is necessary to perform time sweep measurement of the terahertz wave waveform at least twice in an independent process. That is, first, a first time sweep measurement is performed to obtain a terahertz wave waveform (reference waveform) when there is no sample, and then a terahertz wave waveform (sample waveform) transmitted or reflected by the sample is obtained. 2. Perform time sweep measurement of 2. Thereafter, a transmission spectrum of the sample is derived by analyzing both waveforms. Therefore, a long time measurement is required to acquire one transmission spectrum. Further, since the two waveform measurements are performed in independent processes, it is very difficult to eliminate errors due to reproducibility in measurement system repeated measurement and fluctuations due to external factors. For this reason, it has been difficult to determine the frequency, intensity, and phase (or phase change) of the obtained spectrum with high accuracy.

JP 2004-101257 A Special table 2003-515917 gazette Special table 2003-518617 gazette

  The present invention makes it possible to acquire a reference waveform and a sample waveform by a single time sweep in a time waveform measurement of a terahertz wave time domain spectroscopy by a small system in which a terahertz wave generator and a detector are integrated. Another object of the present invention is to provide a terahertz spectroscopy system capable of acquiring a transmission spectrum of a sample to be measured and the like, and a spectrum with high accuracy with respect to frequency, intensity, and phase.

The terahertz spectroscopy system of the present invention roughly includes a terahertz wave source that generates a terahertz wave by irradiation with a first light pulse, a configuration that irradiates the sample to be measured with the generated terahertz wave, and the sample to be measured or the sample to be measured. Detecting means for inputting and detecting a terahertz transmitted wave or reflected wave reflected by the provided reflecting means, and the detecting means outputs a terahertz transmitted wave or reflected wave intensity at the time of irradiation by irradiation of the second light pulse. The time waveform of the reflected terahertz wave from the sample to be measured is acquired from the output obtained by changing the time of the second light pulse irradiation time with respect to the first light pulse irradiation time, and the spectrum is acquired by the signal processing. This is a terahertz time-resolved spectroscopy system that has the following features. However, as a matter of course, a laser pulse light source used in a normal terahertz time-resolved spectroscopy system, a signal processing means for obtaining the reflection spectrum, and the like are necessary.
That is, the terahertz wave source has an optical pulse-terahertz wave conversion unit for converting an optical pulse to a terahertz wave provided on the first optical crystal region, and the detection means is at least partially on the second optical crystal region. A terahertz wave detector provided; and a first path through which a direct terahertz wave can be propagated directly from the optical pulse-terahertz wave converter to the terahertz wave detector without reflection. Here, at least partially means at least a part of the detection means. For example, when an antenna-type detector is used, the case where at least a part of the antenna is provided on the second optical crystal region is included. In addition, for example, in the case where a light pulse is modulated with a terahertz wave and detected using a detection optical system and a photoelectric converter, this includes the case where a means for modulating is provided on the second optical crystal region.
The arrival time of the second optical pulse at the terahertz wave detection unit is the arrival time of the terahertz transmitted wave or reflected wave from the direct terahertz wave at the terahertz wave detection unit It is a plurality of time points at which the time zone including the time point is discretely swept. Here, discretely sweeping means that a plurality of second optical pulses are discretely distributed in the time zone.

  Further, the second optical pulse reflecting means for the second optical pulse is provided on the back surface of the second optical crystal region on the incident side of the second optical pulse. In this case, the reflecting means includes, in addition to the reflecting film, for example, a reflecting means using reflection due to a change in the refractive index on the optical path.

  The first optical crystal region and the second optical crystal region are regions of optical crystals having the same composition. It is assumed that the first optical crystal region and the second optical crystal region are provided in one crystal, or the case where they are the same crystal but have different crystal plane orientations. The first optical crystal region and the second optical crystal region may be connected via a spacer.

  The wavelength of the first light pulse can be shorter than the wavelength of the second light pulse. This improves the efficiency by using optical pulses of different wavelengths for generation and detection of terahertz waves. It is assumed that different optical pulse sources are used, or that only the first optical pulse is wavelength-converted using a nonlinear optical crystal. As for the wavelength conversion, as well known, there are a harmonic and a sum frequency mixing method.

  A terahertz wave condensing system is provided on the terahertz wave propagation path reflected from the terahertz wave source to the terahertz wave detection unit and reaching the terahertz wave detection unit, thereby collecting the terahertz wave on the measurement sample and the terahertz wave detection unit. The signal to noise ratio can be improved. As a condensing system, it is well known, and in addition to a lens by refraction or diffraction, a lens whose refractive index is spatially changed may be used. Further, the lens is not limited to a complete lens, and may be an eccentric portion of the lens.

  The terahertz spectroscopy system includes a signal extraction unit that extracts and outputs a terahertz wave and a terahertz reflected wave or transmitted wave directly from the acquired terahertz wave time waveform, and further transmits a terahertz transmitted wave or reflected wave from the extracted terahertz wave waveform. A spectrum calculation unit for calculating an amplitude spectrum or a phase spectrum of the wave is provided. For example, the spectrum calculation unit performs a Fourier transform on the signal output from the signal extraction unit and outputs an amplitude spectrum and a phase spectrum of a terahertz reflected wave or transmitted wave.

  The spectrum calculation unit calculates the amplitude and phase spectrum of each reflected wave with respect to the direct terahertz wave of the reflected terahertz wave at the stack interface when the sample to be measured is a laminate of a plurality of layers. By calculating the amplitude and phase spectrum of each reflected wave, the absorption spectrum and refractive index of each layer can be calculated. Furthermore, by measuring the position of each reflected wave in the time domain with respect to a reference waveform that is a direct terahertz wave, it is possible to separately measure changes in the film thickness and refractive index of each layer.

  A plurality of wave sources can be used as the terahertz wave source. That is, it is assumed that the terahertz wave source includes a plurality of optical pulse-terahertz wave conversion units arranged in a one-dimensional or two-dimensional distribution. Here, the first optical pulse is a plurality of optical pulses incident on each of the plurality of optical pulse-terahertz wave conversion units in a predetermined order. In addition, in the detection, since the analysis may be difficult if the terahertz transmitted wave or the reflected wave overlaps, the direct terahertz wave by each light pulse of the first light pulse is incident on the terahertz wave detection unit. The time period from the time to the time when the terahertz transmitted wave or reflected wave enters the terahertz wave detection unit is set so as not to overlap.

  Further, by using a plurality of terahertz detection means for terahertz detection, it is possible to detect terahertz transmitted waves or reflected waves at different positions simultaneously. For this purpose, each of the terahertz detection means includes a plurality of terahertz wave detection units provided at least partially on the second optical crystal region, and the second optical pulse is transmitted to each of the plurality of terahertz wave detection units. It is assumed that the light pulse is incident. At this time, each time zone from the time when the direct terahertz wave by the first light pulse is incident on the terahertz wave detecting unit to the time when the terahertz transmitted wave or reflected wave is incident on the terahertz wave detecting unit is respectively It is assumed that the terahertz wave detection unit is overlapped. In this configuration, it is possible to select some of the many terahertz detection means by selecting the irradiation of the second light pulse. The outputs from the terahertz detection means are preferably processed in parallel, but the output can be processed by switching the terahertz wave detection units one by one.

  The terahertz wave propagation path that is reflected from the terahertz wave source and reflected by the sample to be measured to reach the terahertz wave detection unit is provided with a protection means for protecting from a failure caused by the outside. As the protection means, a housing that covers from the terahertz wave source to the sample to be measured and a terahertz waveguide extending from the terahertz wave source to the sample to be measured are assumed. It is possible to easily realize gripping by this protective portion.

  According to the present invention, the terahertz time-resolved spectroscopic system can be miniaturized, simplified, highly stabilized, measured at high speed, and highly accurate. In addition, for example, measurement in a gripped state or in a liquid can be performed under adverse conditions that are impossible with the conventional method. Furthermore, an absolute measurement of the phase spectrum becomes possible.

It is a block diagram which shows the structural example disclosed by patent document 1 by the conventional terahertz time-domain spectroscopy. It is a figure which shows the terahertz spectroscopy system using the terahertz wave generation and detection probe which comprises a fiber pigtail regarding the indication of patent document 3. FIG. (A) is a schematic diagram of the terahertz wave generation / detection unit of the present invention, and (b) is observed as a temporally separated signal when the waveform of the terahertz wave is measured using a time-resolved measurement method. FIG. It is a figure which shows the structure of the terahertz wave generation and detection part 100 in a present Example. It is a figure for demonstrating the various optical systems provided in the optical crystal. It is a figure for demonstrating the structure in the case of using a some optical fiber for the input / output of pulsed light and terahertz light in the optical crystal. (A) It is a figure for demonstrating the housing of the terahertz wave generation and detection probe 150, (b) It is a figure which shows the case where the propagation path of a terahertz wave is provided in the terahertz waveguide. 1 is a block diagram of an example of a terahertz spectroscopy system in Example 1. FIG. FIG. 9 is a block diagram of an example of a terahertz spectroscopy system in Example 1 that is different from the configuration of FIG. 8. 1 is a block diagram of an example of a terahertz spectroscopy system when an optical fiber array 16 arrayed in one or two dimensions is used for an optical fiber 6. FIG. FIG. 6 is a diagram for explaining Example 2 according to the present invention, and FIG. 5A is a diagram showing an example of an optical crystal 1 provided with stripline antennas 29 and 30 for generating and detecting terahertz waves. (B) is a schematic diagram of a terahertz wave generation / detection unit. FIG. 6 is a block diagram of an example of a terahertz spectroscopy system of Example 2. It is a figure for demonstrating the 3rd Example which concerns on this invention. It is a figure which shows the example of an observation in case the to-be-measured sample 4 has a multilayer structure.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  The present invention is provided in a direct wave of a terahertz wave generated by a terahertz wave generator and a sample to be measured or the sample to be measured by adopting a configuration in which the terahertz wave generator and the detector are integrated. It is possible to detect a reflected wave from the reflecting means, and thereby acquire a reference waveform and a sample waveform in time-resolved measurement within one time sweep measurement.

FIG. 3A is a schematic diagram of the terahertz wave generation / detection unit of the present invention. The present terahertz wave generation / detection unit includes an optical crystal 1 that generates and detects a terahertz wave using an optical pulse. Examples of the optical crystal 1 include gallium arsenide (GaAs), indium gallium arsenide (InGaAs), zinc telluride (ZnTe), lithium niobate (LiNbO ₃ ), gallium phosphide (GaP), 4-dimethylamino-N-methyl- 4-stilbazolium tosylate (DAST) or the like is preferably used. On the optical crystal 1, a terahertz wave generation region and a terahertz wave detection region are provided at spatially separated positions. However, the separated position is a distance at which the terahertz wave generated in the terahertz wave generation region can be detected in the terahertz detection region, for example, from 0.1 times the wavelength corresponding to the highest frequency of the generated terahertz wave. It is desirable to be between 1000 times.

  When the optical pulse 2 is incident on the terahertz wave generation region of the optical crystal 1, a pulsed terahertz wave is generated. In addition, when the optical pulse 3 is incident on the terahertz wave detection region of the optical crystal 1, the terahertz wave guided to the terahertz wave detection region can be detected. By giving a variable time delay to the optical pulse 2 and the optical pulse 3, the time waveform of the terahertz wave generated by the optical pulse 2 can be acquired by the optical pulse 3.

  Methods for generating and detecting terahertz waves using optical crystals are already well known. For example, an antenna structure may be formed in the terahertz wave generation region and detection region, thereby enhancing the electric field of terahertz waves. Expected to be effective. A voltage may be applied to the antenna structure formed in the terahertz wave generation region, and an increase in the intensity of the terahertz wave generated thereby is expected. Further, a method of measuring a minute current flowing in an antenna structure formed in the terahertz wave detection region may be used for terahertz wave detection.

The terahertz wave generated when the optical pulse is incident on the terahertz wave generation region propagates along the following paths A, B, and C.
Path A: A path that is radiated in a spherical wave shape from the terahertz wave generation region of the optical crystal 1 and propagates as a direct wave to the terahertz wave detection region.
Path B: A path that is reflected by the back surface of the optical crystal 1 and propagates to the terahertz wave detection region.
Path C: A path that is radiated to the outside of the optical crystal 1 and reflected by the sample to be measured or the reflecting means provided on the sample to be measured, and then propagates to the terahertz wave detection region.

  Since the above three paths have different optical path lengths, when the waveform of the terahertz wave is measured using a time-resolved measurement method, the time is swept in one time sweep, for example, as shown in FIG. Observed as a separated signal. The terahertz wave A propagated along the path A is a direct detection of the terahertz wave generated by the optical crystal 1 without reflection. The terahertz wave B propagated through the path B is reflected at least once by the optical crystal 1 and is slightly affected by absorption and dispersion by the optical crystal 1 and its reflecting surface. The terahertz wave C propagated through the path C reflects its absorption characteristic or reflection characteristic with the return light from the sample to be measured. Terahertz waves A, B, and C are separated and extracted from the acquired time waveform. The time width to be extracted is such that it does not include adjacent waveforms, and the resolution of the desired Fourier transform spectrum can be obtained. Thereafter, the spectrum (sample spectrum) obtained by Fourier transforming the time waveform (sample waveform) of the terahertz wave C is divided by the Fourier transform spectrum (reference spectrum) of the time waveform (reference waveform) of the terahertz wave A or terahertz wave B. By doing so, an amplitude spectrum in the terahertz wave band of the sample to be measured can be obtained. Further, the phase spectrum of the sample to be measured can be obtained by taking the difference between the phase spectrum of the sample waveform and the phase spectrum of the reference waveform. Here, the distance of the path A is determined by the distance L between the terahertz wave generation region and the detection region, and the distance of the path B is determined by the L and the thickness d of the optical crystal 1. Therefore, the phase of the reference waveform is constant with respect to other measured samples regardless of other optical systems, that is, optical systems arranged between the optical crystal 1 and the measured sample 4. Thereby, as will be described below, the frequency and intensity spectrum can be measured with high accuracy, and the phase and its change can also be measured.

FIG. 4 shows a configuration of the terahertz wave generation / detection unit 100 in the present embodiment. The terahertz wave generation / detection unit 100 includes an optical fiber 5, an optical fiber 6, and an optical crystal 1.
Examples of the optical crystal 1 include gallium arsenide (GaAs), zinc telluride (ZnTe), lithium niobate (LiNbO ₃ ), gallium phosphide (GaP), 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). Etc. are preferably used. One end face of the optical crystal 1 is an optical pulse input end, and the other face is a terahertz wave input / output end. A highly reflective film 7 for light pulses is applied to the entire surface or a part of the terahertz wave input / output end. It is desirable that the highly reflective film 7 exhibits high transparency with respect to terahertz waves. An anti-reflection coating for incident light may be applied to the light pulse input end.

  Examples of the optical fibers 5 and 6 are a single mode fiber, a polarization maintaining fiber, a large mode area fiber, and the like. The output ends of the optical fibers 5 and 6 are cleaved and emit light pulses out of the fiber. In order to reduce the reflection loss, a tip end processing or a non-reflection coating may be applied to the fiber end face as necessary. The end faces of the optical fibers 5 and 6 are connected to the optical pulse input end of the optical crystal 1, and light pulses are incident on the optical crystal 1, respectively.

  The region of the optical crystal 1 to which the optical fiber 5 is connected is a terahertz wave generation region, and the region to which the optical fiber 6 is connected is a terahertz wave detection region. The optical fiber 6 is disposed so as to face the high reflection film applied to the terahertz wave input / output end of the optical crystal 1, and the optical pulse emitted from the optical fiber 6 is reflected by the high reflection film and coupled to the optical fiber 6 again. To do. In the terahertz wave generation region, a terahertz wave is generated by a nonlinear optical effect.

  The terahertz wave generated in the terahertz wave generation region is incident on the terahertz wave detection region via the paths A and B in the optical crystal 1 and the path C that is reflection from the sample 4 to be measured. The distance between the optical crystal 1 and the sample 4 to be measured is within a distance corresponding to the maximum delay amount of the optical delay device used for time sweep. In the terahertz wave detection region, the polarization of the light pulse changes due to the electrooptic effect induced by the electric field of the terahertz wave. The electric field of the terahertz wave can be measured by measuring the amount of change in polarization with a detection optical system and a photodetector.

  In order to adjust the polarization of the input light, a half-wave plate or a quarter-wave plate may be inserted into a part or the entire surface of the optical pulse input end of the optical crystal 1, and they adhere to the optical crystal 1 May be.

FIG. 5 is a diagram for explaining an optical system provided in the optical crystal 1.
As shown in FIG. 5A, the optical crystal 1 may be one obtained by bonding a plurality of crystals. For example, different optical crystals 8 and 9 are used in the terahertz wave generation region and the terahertz wave detection region, and are joined so that the terahertz waves propagate in the crystal. Thereby, the efficiency of generation and detection of terahertz waves can be optimized independently. Of course, optical crystals of the same substance with different plane orientations may be used. Generation and detection of terahertz waves can be optimized by using optical crystals with plane orientations suitable for each.

  Further, as shown in FIG. 5B, a spacer 10 may be inserted between the optical crystals 8 and 9 and bonded. The spacer 10 serves to reduce absorption when the optical crystals 8 and 9 have absorption in the terahertz wave band. It can also be used to adjust the optical distance of terahertz waves (paths A and B) propagating in the crystal.

  As shown in FIG. 5C, a nonlinear optical crystal 11 may be disposed between the optical crystal 1 and the optical fiber 6. If the nonlinear optical crystal 11 that generates the second harmonic of the optical pulse is used, the generation efficiency of the terahertz wave in the optical crystal 1 can be increased. For the nonlinear optical crystal 11, for example, barium borate (BBO), lithium triborate (LBO) or the like is preferably used.

  As shown in FIG. 5D, a lens 12 serving as a condensing system may be disposed on the terahertz wave emission end face of the terahertz wave generation region of the optical crystal 1. The lens 12 efficiently radiates out of the crystal by converting the terahertz wave generated in a spherical wave shape by the optical crystal 1 into a parallel beam or a condensed beam and matching the refractive index. The lens 12 can also be used to adjust the propagation direction of the terahertz wave. As the lens 12, a whole or a part of a refractive lens, a diffractive lens, a cylindrical lens, or the like can be used. Here, the number of parts can be reduced by processing the optical crystal 1 into a lens shape.

  As shown in FIG. 5 (e), a lens 13 serving as a condensing system may be disposed on the terahertz wave incident end face of the terahertz wave detection region of the optical crystal 1. The lens 13 efficiently causes the terahertz wave reflected from the sample to be measured to be incident on the optical crystal 1 and is condensed on the crystal. The lens 13 can also be used to adjust the incident angle of the terahertz wave. As the lens 13, a whole or a part of a refractive lens, a diffractive lens, a cylindrical lens, or the like can be used. Similarly to the above case, the number of parts can be reduced by processing the optical crystal 1 into a lens shape.

  As shown in FIG. 5 (f), both lenses 12 and 13 may be arranged. These lenses may be different or the same, or may be a part of the lens.

  As shown in FIG. 5G, a spacer 14 may be disposed on the rear surface of the optical crystal 1. By providing the spacer 14 with a wedge structure, the propagation path B of the terahertz wave can be adjusted optimally. As the spacer 14, a silicon crystal or a germanium crystal is preferably used.

FIG. 6 is a diagram for explaining a configuration in the case where a plurality of optical fibers are used for input and output for pulsed light and terahertz light for the optical crystal 1.
As the optical fiber 5, as shown in the terahertz wave generation / detection unit 111 in FIG. 6A, an optical fiber array 15 arrayed in one or two dimensions may be used. As a result, for example, the irradiation angle of the terahertz wave can be changed, and the time waveform and the emission direction of the terahertz wave can be controlled by injecting optical pulses with time delays into the respective optical fibers. it can. The array spacing is, for example, between 0.01 and 10 times the wavelength corresponding to the highest frequency of the generated terahertz wave.

  Similarly, as the optical fiber 6, as shown in the terahertz wave generation / detection unit 112 in FIG. 6B, an optical fiber array 16 arrayed in one or two dimensions may be used. Thereby, for example, measurement is possible even when the reflection direction of the terahertz wave is changed due to the unevenness of the sample to be measured. Further, the spatial distribution of the terahertz wave reflected from the sample to be measured can be measured, and the surface shape and the like seen with the terahertz wave can be measured. The array spacing is, for example, between 0.01 and 10 times the wavelength corresponding to the highest frequency of the generated terahertz wave.

  As shown in the terahertz wave generation / detection unit 113 of FIG. 6C, optical fiber arrays 15 and 16 arrayed one-dimensionally or two-dimensionally on the optical fiber 5 and the optical fiber 6 may be used, respectively. Thereby, the three-dimensional structure seen by the terahertz wave of the sample to be measured can be observed.

FIG. 7 is a diagram for explaining protection means for the terahertz wave propagation path of the terahertz wave generation / detection probe 150.
As shown in FIG. 7A, the optical crystal 1 is fixed in the housing 17, and the end surface of the housing 17 from the optical crystal 1 is set to an optimum distance for terahertz wave measurement. By providing a hole or window on the end face of the housing 17, the sample to be measured can be irradiated with the terahertz wave. As the window material, for example, Teflon (registered trademark), silicon, germanium, or the like is preferably used. A method of keeping the distance from the optical crystal 1 to the sample 4 to be measured constant by bringing the sample to be measured into close contact with or close to the end face of the housing may be used. Further, the absorption of the terahertz wave may be reduced by sealing the inside of the housing 17 with a gas that absorbs less terahertz wave such as dry nitrogen, or by reducing the pressure.

  As shown in FIG. 7B, a terahertz waveguide 18 for propagating a terahertz wave may be inserted between the optical crystal 1 and the sample 4 to be measured. The terahertz waveguide 18 is preferably made of a material that does not absorb terahertz waves. For example, Teflon (registered trademark), silicon, germanium, or the like is preferably used. Depending on conditions, a liquid may be used. Further, by providing a tapered shape or the like, the terahertz waveguide 18 can be used as a terahertz wave guide, and the irradiation position of the terahertz wave can be easily specified. If necessary, the end surface in contact with the sample 4 to be measured may be a spherical surface. Further, by selecting the refractive index of the terahertz waveguide 18 so that the refractive indexes of the terahertz waveguide 18 and the sample 4 to be measured are approximately the same, reflection on the surface of the sample 4 to be measured can be reduced. it can.

8, 9, and 10 show block diagrams of the terahertz spectroscopy system in this embodiment.
In FIG. 8, the optical pulse emitted from the pulse light source 19 is bifurcated by the optical splitter 20. As the pulse light source, for example, a mode-locked semiconductor laser, a mode-locked fiber laser, or the like is preferably used. One of the branched beams is incident on the optical fiber 5 and guided to the terahertz wave generation region of the optical crystal 1 in the terahertz wave generation / detection unit 100. The other passes through the optical delay device 21 and the optical circulator 22, enters the optical fiber 6, and is guided to the terahertz wave detection region of the optical crystal 1 in the terahertz wave generation / detection unit 100. The light reflected by the highly reflective film 7 of the optical crystal 1 passes through the optical circulator 22 and enters the polarizing beam splitter 23 that is a detection optical system. The polarization beam splitter 23 separates the input light into components orthogonal to each other and enters each input port of the balanced photodetector 24 to detect a change in polarization caused by the terahertz wave in the optical crystal 1. . A time waveform of the terahertz wave is obtained by changing the delay time of the optical pulse by the optical delay device 21 and recording the delay time and the output value of the balanced photodetector 24 at each time delay together.

Further, the terahertz spectroscopy system may be configured as shown in FIG.
After passing through the optical delay device 21, a part of the optical pulse is input to one port of the balanced photodetector 24 by the polarization beam splitter 25. The light pulse that has passed through the polarization beam splitter 25 passes through the Faraday rotator 26 and the polarization beam splitter 27 and is incident on the optical fiber 6. The light reflected by the highly reflective film 7 of the optical crystal 1 is guided to the balanced photodetector 24 by the polarization beam splitter 27.

  When the optical fiber array 16 is used for light input to the terahertz wave detection region, as shown in FIG. 10, for example, in addition to the configuration of FIG. 8, the optical path switch 28 is provided between the optical circulator 22 and the terahertz wave unit 100. It is good also as a structure which inserted.

  The output of the balanced photodetector 24 in FIGS. 8, 9 and 10 is input to the signal processing unit 50. In particular, the signal processing unit 50 includes a signal extraction unit 52 that separates and extracts the direct terahertz wave and the transmitted terahertz wave or reflected wave from the terahertz wave waveform, and a terahertz transmitted wave or reflected wave from the extracted terahertz wave waveform. The spectrum calculation unit 51 for calculating the amplitude spectrum or the phase spectrum is provided. For spectrum calculation, Fourier transform is performed.

FIGS. 11A and 11B show a second embodiment according to the present invention. The terahertz wave generation / detection unit 120 includes an optical crystal 1, an optical fiber 5, and an optical fiber 6, as shown in FIG.
For example, gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium phosphide (GaP), or the like is preferably used for the optical crystal 1 in FIG. One end face of the optical crystal 1 is an optical pulse input end, and the other face is a terahertz wave input / output end. An anti-reflection coating for incident light may be applied to the light pulse input end. Stripline antennas 29 and 30 as shown in FIG. 11A are fabricated at the optical pulse input end of the optical crystal 1 in FIG. As other antenna structures, for example, a dipole antenna, a bowtie antenna, a spiral antenna, a slot antenna, or the like is preferably used, and an area for irradiating an optical pulse is provided.

  A voltage is applied to the antenna 29 using a voltage source 31. This voltage is, for example, between 1V and 10kV. By irradiating the antenna 29 with a light pulse, an instantaneous current is generated in the antenna, and a terahertz wave is radiated due to a time change of the current.

  The terahertz wave radiated from the antenna 29 is incident on the terahertz wave detection region via the paths A and B in the optical crystal 1 and the path C which is the reflection from the sample 4 to be measured in FIG. The distance between the optical crystal 1 and the sample 4 to be measured is within a distance corresponding to the maximum delay amount of the optical delay device used for time sweep. In the terahertz wave detection region, a minute current is generated in the antenna by the action of the optical pulse applied to the antenna 30 and the electric field of the terahertz wave. By measuring this minute current, the electric field of the terahertz wave can be measured.

  Examples of the optical fibers 5 and 6 in FIG. 11B are a single mode fiber, a polarization maintaining fiber, a large mode area fiber, and the like. The output ends of the optical fibers 5 and 6 are cleaved and emit light pulses out of the fiber. In order to reduce the reflection loss, a tip end processing or a non-reflection coating may be applied to the fiber end face as necessary. The output end of the optical fiber 5 is connected to the light pulse irradiation region of the antenna 29. Similarly, the optical fiber 6 is connected to the light pulse irradiation region of the antenna 30.

  FIG. 12 shows a block diagram of the terahertz spectroscopy system in the present embodiment. A voltage is applied to the antenna 29 using the voltage source 31. The optical pulse emitted from the pulse light source 19 is bifurcated by the optical splitter 20. As the pulse light source, a mode-locked semiconductor laser, a mode-locked fiber laser, or the like is preferably used. One of the two branches enters the optical fiber 5 and irradiates the antenna 29. The other passes through the optical delay device 21 and then enters the optical fiber 6 and irradiates the antenna 30. A minute current generated between the antenna elements by the terahertz wave is measured using an ammeter 32. A time waveform of a terahertz wave is acquired by changing the delay time of the optical pulse by the optical delay device 21 and recording the output value of the ammeter 32 at each time delay. The output of the antenna 30 may be connected to a current amplifier, and the output of the current amplifier may be measured using a voltmeter. The current amplifier may be integrated on the optical crystal 1.

  Similarly to the first embodiment, the optical crystal 1 may be formed by bonding a plurality of crystals, and a spacer may be inserted between them.

  Similarly to the first embodiment, a lens may be disposed at the terahertz wave output end of the optical crystal 1.

FIG. 13 shows a third embodiment according to the present invention. The terahertz waves A to C acquire their time waveforms by one time sweep by time-resolved measurement.
First, the terahertz wave A in the detection unit is used as a reference waveform, the terahertz wave C is used as a sample waveform, and the time difference Δt is calculated. The terahertz wave B may be a reference waveform, and the time difference between the terahertz wave B and the terahertz wave C may be Δt. Further, the distance between the terahertz wave generation region and the terahertz wave detection region is L, the thickness and refractive index of the optical crystal 1 are d and n ₁ , the distance between the optical crystal 1 and the sample 4 to be measured is x, and the optical crystal When the refractive index of the medium between 1 and the sample 4 to be measured is n ₂ , x is determined using geometric optics and triangulation from the relationship of Δt, L, n ₁ , d, x, and n ₂ , for example. And can be calculated by using the following equation.

  However, the reference plane for measurement is the terahertz wave output end of the optical crystal 1. L is preferably 1/5 times or less than x.

When the sample 4 to be measured has a multilayer structure, the terahertz wave C is observed by separating multiple terahertz waves on the time axis by multiple reflection. For example, when the sample 4 to be measured has the layer 33 and the layer 34, the reflected wave C ₁ at the surface of the sample 4 to be measured and the reflected wave C ₂ at the interface between the layer 33 and the layer 34 have a time difference Δt _2. Observed. By calculating the amplitude and phase spectrum of the reflected wave C ₁ and the reflected wave C ₂ with respect to the reference spectrum of the obtained waveform, and further calculating the amplitude and phase spectrum of the reflected wave C ₂ with respect to the reflected wave C ₁ , the layer 33 The absorption spectrum and refractive index of can be calculated. Furthermore, by measuring the positions of the reference waveform, the reflected wave C ₁ and the reflected wave C _{2 in} the time domain, it is possible to separately measure changes in the film thickness and refractive index of the layer 33. Even when there is a third layer, a fourth layer, etc., the absorption spectrum and refractive index of each layer can be measured by the same method.

  Examples of the sample 4 to be measured include a semiconductor gain medium. In the gain medium, it is known that the absorption characteristics change and the refractive index also changes depending on the applied voltage and the injected current. By using the present invention, it is possible to accurately measure the absorption spectrum and phase spectrum of the terahertz wave band.

Since the measured value of the distance between the sample to be measured and the terahertz wave source can be obtained, by using this measured value, or by artificially adjusting the above distance to a predetermined value or an optimal value through a distance adjusting means, Stable spectroscopic measurement results can be obtained.
As shown in FIG. 3B, since the time origin of the reflected terahertz wave is obtained, the measurement waveforms can be easily superimposed and the signal-to-noise ratio can be easily improved by averaging. .
Further, the housing of FIG. 7A is easy to downsize, and includes a laser light source that is an optical pulse source and a signal processing unit for detection output with two optical fibers and a code for detection output not depicted in the drawing. Since they are connected, a probe that is easy to handle, such as easy handling, can be created, and terahertz measurements of various materials can be easily performed.
In the configuration shown in FIG. 6C, a three-dimensional terahertz image of the sample to be measured can be easily obtained by using well-known software for tomography.

DESCRIPTION OF SYMBOLS 1 Optical crystal 2, 3 Optical fiber isolator 4 Sample to be measured 5, 6 Optical fiber 7 High reflection film 8, 9 Optical crystal 10 Spacer 11 Nonlinear optical crystal 12, 13 Lens 14 Spacer 15, 16 Optical fiber array 17 Housing 18 Terahertz guide Wave body 19 Pulse light source 20 Optical splitter 21 Optical delay device 22 Optical circulator 23 Polarizing beam splitter 24 Balanced photodetector 25 Polarizing beam splitter 26 Faraday rotator 27 Polarizing beam splitter 28 Optical path switch 29, 30 Antenna 31 Voltage source 32 Current Total 33, 34 layers 50 Signal processing unit 51 Spectrum calculation unit 52 Signal extraction unit 100, 111, 112, 113, 120 Terahertz wave generation / detection unit 150 Terahertz wave generation / detection probe

Claims (9)

A terahertz wave source that generates a terahertz wave by irradiation with the first light pulse, a configuration that irradiates the sample to be measured with the generated terahertz wave, and a terahertz transmission reflected by the sample to be measured or reflecting means provided on the sample to be measured Detection means for inputting and detecting a wave or a reflected wave, and the detection means outputs a terahertz transmitted wave or reflected wave intensity at the irradiation time by irradiation of the second light pulse, and outputs a second intensity with respect to the irradiation time of the first light pulse. In the terahertz time-resolved spectroscopy system that can acquire the waveform of the terahertz wave from the output obtained by changing the time at the time of light pulse irradiation,
The terahertz wave source has an optical pulse-terahertz wave converter provided on the first optical crystal region, and the detection means has a terahertz wave detector provided at least partially on the second optical crystal region. , Having a first path capable of propagating a direct terahertz wave that propagates directly from the optical pulse-terahertz wave conversion unit to the terahertz wave detection unit without reflection,
The arrival time of the second optical pulse at the terahertz wave detection unit is from the arrival time of the direct terahertz wave to the terahertz wave detection unit to the arrival time of the terahertz transmitted wave or reflected wave at the terahertz wave detection unit. The terahertz spectroscopy system is characterized in that it is a plurality of time points where discrete time sweeps are included.   2. The terahertz spectroscopy system according to claim 1, wherein a reflection means for the second light pulse is provided on a back surface of the second optical crystal region on the incident side of the second light pulse.   3. The terahertz spectroscopy system according to claim 1, wherein the first optical crystal region and the second optical crystal region are regions of an optical crystal having the same composition. 4.   The terahertz wave condensing system is provided on a terahertz wave propagation path that is reflected from the terahertz wave source and reflected by the sample to be measured to reach the terahertz wave detection unit. Terahertz spectroscopy system.   A signal extraction unit for separating and extracting the direct terahertz wave and the transmitted terahertz wave or reflected wave from the terahertz wave waveform, and an amplitude spectrum or phase spectrum of the terahertz transmitted wave or reflected wave from the extracted terahertz wave waveform The terahertz spectroscopy system according to any one of claims 1 to 4, further comprising: a spectrum calculation unit that calculates   The spectrum calculation unit for calculating an amplitude and a phase spectrum of each reflected wave with respect to the direct terahertz wave of the reflected terahertz wave at the stack interface when the sample to be measured is a laminate of a plurality of layers. 5. The terahertz spectroscopy system according to 5. The terahertz wave source includes a plurality of optical pulse-terahertz wave conversion units arranged in a one-dimensional or two-dimensional distribution,
The first light pulse is a plurality of light pulses incident on each of the plurality of light pulse-terahertz wave converters in a predetermined order,
The time period from the time when the direct terahertz wave by each light pulse of the first light pulse enters the terahertz wave detection unit to the time when the terahertz transmission wave or reflected wave enters the terahertz wave detection unit overlaps. The terahertz spectroscopy system according to any one of claims 1 to 6, wherein the terahertz spectroscopy system is not provided. Each of the terahertz detection means has a plurality of terahertz wave detection units provided at least partially on the second optical crystal region,
The second light pulse is a light pulse incident on each of the plurality of terahertz wave detection units,
Respective time zones from the time when the direct terahertz wave by the first light pulse is incident on the terahertz wave detection unit to the time when the terahertz transmitted wave or reflected wave is incident on the terahertz wave detection unit are each terahertz. The terahertz spectroscopy system according to any one of claims 1 to 7, wherein the wave detection units overlap each other.   9. The terahertz wave propagation path that is reflected from the terahertz wave source and reflected by the sample to be measured to reach the terahertz wave detection unit is provided with protection means for protecting from an external failure. The terahertz spectroscopy system according to one.

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