JP2012058073A - Terahertz wave measuring apparatus and terahertz wave measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz wave measuring apparatus and a terahertz wave measuring method for measuring the physical property by using terahertz waves.SOLUTION: The terahertz wave measuring apparatus includes: a beam splitter for splitting a pulse laser into pump light and probe light; a terahertz wave generator for generating a terahertz wave by incidence of the pump light; a pulse train generation part for generating a plurality of pulses from the incident probe light; and a terahertz wave measurement part for making the pulse train incident to the terahertz wave detector and measuring a temporal change of an electric field of the terahertz wave incident to the terahertz detector on the basis of the plurality of the pulses.

Description

  The present invention relates to a terahertz wave measuring apparatus and a terahertz wave measuring method for measuring physical properties using terahertz waves.

  A terahertz wave is an electromagnetic wave having a frequency of 0.1 THz to 10 THz, transmits a soft material such as plastic, paper, and cloth, and has an absorption spectrum peculiar to a substance, a so-called fingerprint spectrum, so that the substance can be identified. It is.

  As one of terahertz wave measurement methods, terahertz time domain spectroscopy is known. The principle of measurement using this method is that the object to be measured is placed in the propagation path of the terahertz wave, the time waveform of the electric field of the terahertz wave transmitted through the object to be measured is measured, and nothing is placed in the propagation path of the terahertz wave. Similarly, measure the time waveform of the terahertz wave in the absence. Information on the amplitude and phase (time delay) of the terahertz wave, that is, the spectrum, is obtained by Fourier transforming the time waveform obtained by the two measurements. The object to be measured is identified by pattern matching this spectrum with the spectrum of a known substance.

  A general example of the configuration and operation of the terahertz wave measuring apparatus will be described next. First, an ultrashort pulse laser is used as a light source, this ultrashort pulse laser is divided into two by a beam splitter, one beam is used as pump light (excitation light), and the other is used as probe light (detection light). The pump light is incident on a photoconductive antenna for generating a terahertz wave (hereinafter also referred to as a terahertz wave generator or simply a generator) to generate a terahertz wave, which is condensed and irradiated onto the object to be measured. The terahertz wave transmitted through the object to be measured by the irradiation of the terahertz wave is incident on a terahertz wave detecting photoconductive antenna (hereinafter also referred to as a terahertz wave detector or simply a detector). On the other hand, the probe light enters the terahertz wave detector through the time delay stage. In the terahertz wave detector, a current proportional to the terahertz wave electric field at the timing when the probe light is incident flows, and the current is measured by a lock-in amplifier or the like. By gradually moving the time delay stage, the timing of the probe light with respect to the terahertz wave gradually changes, and the real time change (the above time waveform) of the electric field of the terahertz wave can be measured. Furthermore, the spectrum of the object to be measured can be acquired by Fourier transforming the time waveform.

  Since the frequency resolution of the spectral spectrum is the reciprocal of the time waveform measurement range, it is necessary to widen the time waveform measurement range in order to improve the frequency resolution. Therefore, it is necessary to move a long distance (for example, about 10 mm) on the time delay stage while measuring the detection signal with the lock-in amplifier, and there is a problem that the measurement time is long.

  To solve this problem, the time waveform measurement time is reduced by limiting the measurement range of the time waveform transmitted through the object to be measured only to the vicinity of the pulse and measuring only the amplitude and phase (time delay) information of the time waveform. The method is known.

  In addition, a plurality of terahertz wave detecting means including a terahertz wave detector and a detection signal measuring device are provided, and the terahertz wave detector is split into a plurality of terahertz wave detectors by splitting the probe light into a plurality of beams and changing the optical path difference. A method of simultaneously acquiring a plurality of points in a wave time waveform with one pulse is known. For example, by moving probe light with four different optical path differences into four sets of terahertz wave detection means, the movement distance of the time delay stage is reduced to 1/4, and the time waveform acquisition time is reduced to 1/4. It is.

  Furthermore, it is known that an optical delay element that causes probe light to enter a step-like mirror and cause a time delay due to a difference in reflection distance is applied to terahertz wave measurement. For example, probe light is incident on a 1000-stage optical delay element having a length of 5 μm and a width of 25 μm in each stage, thereby generating probe light with a spatial optical delay of 33 psec in 33 fsec increments. Each region with an optical delay in the beam is branched and incident on the terahertz wave detector.

Ch.Fattinger and D.Grischkowsky, “TeraHz Beams”, Applied Physics Letters, Vol.54, 490-492 (1989). Pradarutti, B .; Matthaus, G .; Riehemann, S .; Notni, G .; Note, s .; Tunnertrmann, A., “Advanced analysis concepts for terahertz time domain imaging”, Optics Communications, Vol. 278, 248〜 254 (2007).

Japanese Patent Application No. 2005-507238 (WO 2004/113858) JP 2010-127831 A

  As shown above, in terahertz wave measurement by terahertz time domain spectroscopy, it is necessary to increase the moving distance of the time delay stage in order to measure the time waveform of the terahertz wave with high resolution, and the measurement time is long There was a problem.

  In addition, the method for limiting the measurement range of the time waveform described above to the vicinity of the pulse is not problematic when the object to be measured is thin or has a low refractive index, but when measuring the time waveform when the thickness or refractive index is large. There is a problem that the range cannot be narrowed and this point is not taken into consideration.

  In addition, the method of branching the probe light into a plurality of beams and changing the optical path difference and providing a plurality of terahertz wave detection means can shorten the measurement time, but it is necessary to provide a plurality of expensive amplifiers such as lock-in amplifiers. There is a problem of increasing. Further, since the optical system becomes complicated, there is a problem that adjustment takes time.

  In addition, a method using an optical delay element that provides a step-like mirror and causes a time delay due to a difference in reflection distance is obtained by branching each region with an optical delay in a probe light beam to provide a separate terahertz detector. In the same manner as described above, a large number of expensive amplifiers such as a lock-in amplifier are required, which increases the cost and makes the optical system very complicated.

  The present invention has been devised to solve the above problems, and is a terahertz wave measuring apparatus and terahertz wave measuring method capable of measuring a time waveform of a terahertz wave in a short time with an inexpensive and simple configuration. The purpose is to provide.

  According to one aspect of the invention, a terahertz wave measuring apparatus according to the present invention includes a beam splitter that divides a pulse laser into pump light and probe light, a terahertz wave generator that generates a terahertz wave by incidence of the pump light, A pulse train generator that generates multiple pulses from the probe light, and the pulse train is incident on the terahertz wave detector, and the temporal change in the electric field of the terahertz wave incident on the terahertz wave detector is measured based on the multiple pulses There is provided a terahertz wave measuring device including a terahertz wave measuring unit.

  According to another aspect of the invention, a terahertz wave measuring method according to the present invention includes a beam splitting procedure for splitting a pulse laser into pump light and probe light, and a terahertz wave that generates terahertz waves upon incidence of the pump light. Wave generation procedure, pulse train generation procedure that makes probe light incident and generates multiple pulses from probe light, and pulse train is incident on terahertz wave detector, and electric field of terahertz wave incident on terahertz wave detector changes over time And a terahertz wave measuring procedure for measuring the terahertz wave based on a plurality of pulses.

  With the terahertz wave measuring apparatus and terahertz wave measuring method of the present invention, the time waveform of the terahertz wave can be acquired from a plurality of generated pulses at a time. It is possible to provide a terahertz wave measuring apparatus and a terahertz wave measuring method that can shorten the measurement time. Further, the terahertz wave measuring apparatus of the present invention only needs to newly add a pulse train generator to a general configuration, and thus can be provided at a low cost and with a simple configuration while enabling a short-time measurement.

1 is a configuration example of a terahertz wave measuring apparatus according to Embodiment 1. 2 is a structural example of a pulse train generation mechanism of the terahertz wave measuring apparatus according to the first embodiment. 3 is a time waveform measurement example in the terahertz wave measurement apparatus according to the first embodiment. 2 is a configuration example of a terahertz wave measuring apparatus according to a second embodiment. 10 is a structural example of a pulse train generation mechanism of the terahertz wave measuring apparatus according to Embodiment 3. 12 is a structural example of a variable pulse train generation block of the terahertz wave measuring apparatus according to the third embodiment. It is the example of a measurement by the pulse train of a different time interval in the terahertz wave measuring apparatus of Example 3. 10 is an example of a pulse train generation block of Example 4. It is a structural example of the terahertz wave time waveform measuring apparatus of a prior art. It is a structural example of a terahertz wave photoconductive antenna. It is an example of amplitude and phase (time delay) in a terahertz wave time waveform.

  In order to clarify the characteristics of the present invention, an example of a terahertz wave measuring apparatus having a general configuration will be described with reference to FIG.

  In FIG. 9, the light source of the terahertz wave measuring apparatus 100 uses an ultrashort pulse laser 1 having a wavelength of about 800 nm and a repetition frequency of 50 MHz at about 100 fsec or less. A beam emitted from the ultrashort pulse laser 1 is split into two beams of pump light 3 and probe light 11 by a beam splitter 2. The pump light 3 is condensed by a plastic condensing lens 8a through a mirror 4a and is incident on a terahertz wave generator 5, and a pulse terahertz wave 7 having a half-value width of about 1 psec or less (hereinafter simply referred to as a terahertz wave). Is generated). A bias voltage 6 is applied to the terahertz wave generator 5. The terahertz wave 7 generated by the terahertz wave generator 5 is condensed using a condensing lens 8b or a parabolic mirror (not shown) and enters the object 9 to be measured. The terahertz wave 7 that has passed through the object to be measured 9 is again collected and incident on the terahertz wave detector 10 via the condenser lens 8c.

  The probe light 11, which is the other beam divided by the beam splitter 2, enters a time delay mechanism 15 including a mirror 4 b, a folding mirror 12, a linear motion stage 13, a stage controller 14, and a mirror 4 c. The probe light 11 emitted from the time delay mechanism 15 enters the terahertz wave detector 10 via the mirror 4d, the mirror 4e, and the condenser lens 8d. In the terahertz wave detector 10, a current proportional to the electric field of the terahertz wave 7 at the same timing as the timing at which the ultrashort pulse laser that is the probe light 11 is incident flows. The current is a small signal of picoampere order, but is measured by the lock-in amplifier 16 and the measured value is recorded by the data recording unit 17. By gradually moving the linear motion stage 13 of the time delay mechanism 15, the waveform acquisition timing of the probe light 11 with respect to the terahertz wave 7 gradually changes, so that the actual time of the electric field of the terahertz wave 7 that has passed through the object to be measured 9 Change (time waveform) can be measured. Furthermore, the spectral spectrum of the device under test 9 in the terahertz wave region can be acquired by Fourier transforming the time waveform. Note that the time delay stage described in the above background art corresponds to the linear motion stage 13 of the time delay mechanism 15 described here.

  In the terahertz wave generator 5 described above, for example, as shown in FIG. 10, a low-temperature grown GaAs layer 5b is formed on a GaAs substrate 5a, and a dipole antenna electrode 5c is formed thereon by Ni. It has been done. The width of the dipole is 10 μm, the interval is 10 μm, and the electrode interval is 30 μm. On the back surface of the substrate, for example, a Si hemispherical lens 5d having a diameter of 10 mm is disposed so that the generated terahertz wave 7 can be efficiently radiated to free space. When a bias voltage is applied between the antenna electrodes 5c from the voltage source 6, a terahertz wave 7 having a pulse width of 1 psec or less is radiated as a spherical wave. The terahertz wave 7 is condensed using a condensing lens 8b (or a parabolic mirror) formed of plastic such as polyethylene and is incident on the object 9 to be measured. The structure of the terahertz wave detector 10 is the same as that of the terahertz wave generator 5.

  The terahertz wave detector 10 measures the time waveform of the terahertz wave when the object 9 is not placed in the propagation path of the terahertz wave 7 and when it is placed. The upper diagram of FIG. 11 shows the waveform when the device under test 9 is not placed, and the lower diagram shows the waveform when the device under test 9 is placed. As shown in the figure below, it can be seen that the amplitude and phase change by passing through the DUT 9.

Example 1
A terahertz wave measuring apparatus 101 of the present invention will be described with reference to FIG. FIG. 1 shows a configuration example of the terahertz wave measuring apparatus 101, and is a diagram drawn in accordance with the conventional terahertz wave measuring apparatus 100 shown in FIG. 1 is different from FIG. 9 in that a pulse train generation mechanism 20 is provided before the probe light 11 enters the terahertz wave detector 10, and the other configuration is the same as FIG. 9 (the same as FIG. 9). Components are given the same reference numbers). The pulse train generation mechanism 20 includes a beam expander 21 and a pulse train generation block 22.

  The pulse train generation mechanism 20 will be described in detail with reference to FIG. FIG. 2A is a diagram showing the shape of the pulse train generation block 22 in a side view (left figure) and a front view (right figure). The pulse train generation block 22 uses optical glass having a refractive index of 1.5 as a material, and forms six steps having a width of 3 mm and a height of 3 mm as shown in the side view. The front surface of the pulse train generation block 22 is an 18 mm square. Here, the probe light is incident on the step surface (front view of FIG. 2A) on which the staircase is formed. An anti-reflection coating is applied to the incident surface and the back surface on the opposite side to suppress reflection from this surface. In the method of forming the pulse train generation block 22, the glass block is optically polished after being ground, but a glass plate having a thickness of 3 mm with antireflection coating on both sides may be combined and adhered. Either method can be produced at low cost using a general glass processing technique.

  As shown in FIG. 2B, the pulse train is generated by arranging the beam expander 21 on the incident surface side of the pulse train generation block 22 and making the probe light 11 incident on the beam expander 21. More specifically, the probe light 11 incident on the beam expander 21 from the right side is enlarged and emitted by the beam expander 21 so as to have a beam diameter of 18 mm, and enters the pulse train generation block 22. The block light 11 incident on the pulse train generation block 22 passes through the medium of the pulse train generation block 22 having a refractive index n and a thickness T, and a time delay is generated by (n−1) · T / c (where c is the speed of light). is there). The beam transmitted through the pulse train generation block 22 is spatially and temporally divided into 6 pulse trains at intervals of 5 psec due to the difference in the path of the optical path from the front to the back (that is, the thickness T) in each staircase. become. The probe light 11 transmitted through the shortest path of the pulse train generation block 22 (the lower step in FIG. 2B) is delayed by 5 psec with respect to the incident probe light 11, and the longest path (FIG. 2B). The probe light 11 transmitted through the upper step) has a delay of 30 psec.

  FIG. 2C is a diagram schematically illustrating the generated pulse train. The left figure shows a pulse train generated for the incident probe light 11. Moreover, the right figure is the figure which showed typically the time delay in the beam spot of the probe light 11 radiate | emitted from the pulse train production | generation block 22. FIG. In the figure on the left, the reason why the intensity of each beam is strong in the central part is that the intensity distribution in the beam of the incident probe light 11 is Gaussian distribution, and the central part of the beam spot having the strongest intensity is on the right. As shown in the figure, it corresponds to the time delay of 15 psec or 20 psec in the beam spot.

  In the first embodiment, the probe light 11 enlarged from the surface on which the staircase of the pulse train generation block 22 is formed is incident. However, the probe light 11 may be incident on the surface opposite to the surface on which the staircase is formed.

Next, the operation of the terahertz wave measuring apparatus 101 according to the first embodiment will be described with reference to FIG. 3A shows a waveform of a terahertz wave when there is no device under test 9, and FIG. 3B shows a waveform when the device under test 9 is placed in a propagation path. FIG. 3C shows a pulse train incident on the terahertz wave detector 10. As described above, the terahertz wave detector 10 can measure the terahertz wave electric field at the same timing as the incident timing of the probe light 11. When six pulse trains are incident on the terahertz wave detector 10 at intervals of 5 psec, a current proportional to the sum of the electric fields of the terahertz wave 7 every 5 psec flows. Accordingly, the time delay mechanism 15 is set to 5 psec, that is, 0.75 mm (the distance for the reciprocation is increased by moving the mirror, so the moving distance becomes delay time × light speed × (1/2). To delay by 5 psec (5 × 10 ⁻¹² ) × (3 × 10 ⁸ ) /2=7.5×10 ⁻⁴ m) while moving the terahertz wave 7 by measuring the current signal of the terahertz wave detector 10. Can be obtained. Further, even if the time delay of the terahertz wave 7 generated by the device under test 9 is 30 psec, the distance for moving the time delay mechanism 15 is 5 psec, which is the time interval of the pulse train, and high speed can be realized.

  As described above, in the terahertz wave measuring apparatus 101 according to the first embodiment, the pulse train generating mechanism 20 including the beam expander 21 and the pulse train generating block 22 is simply added to the terahertz wave measuring apparatus 100 having a general configuration. With the configuration, it is possible to acquire a time waveform in a short time. Further, the pulse train generating mechanism 20 can be manufactured with an inexpensive material, and the cost is not significantly increased.

(Example 2)
In the first embodiment, the probe light 11 is “transmitted” to the stepped pulse train generation block 22 to generate an equidistant pulse train. In the second embodiment, the probe light 11 is sent to the stepped pulse train generation block 34 “ Reflected "to generate a pulse train of equal intervals.

  As in the first embodiment, the configuration of the terahertz wave measuring apparatus 102 in the second embodiment is shown in FIG. 4, but the configuration other than the pulse train generating mechanism 30 is the same as that in the first embodiment, and therefore only the pulse train generating mechanism 30 will be described. To do.

  As shown in FIG. 4, the pulse train generation mechanism 30 includes a beam expander 31, a mirror 32, a beam splitter 33, and a pulse train generation block 34. The beam expander 31 is the same as the beam expander 21 of the first embodiment. The pulse train generation block 34 may have a high reflection coating on the surface of the pulse train generation block 22 of the first embodiment, and the dimensions thereof are also the same. Next, the mechanism of pulse train generation will be described.

  First, as in the first embodiment, the laser beam of the probe light 11 is incident on the beam expander 31 and is expanded so as to have a beam diameter of 18 mm. The probe light 11 having an enlarged beam diameter is incident on the pulse train generation block 34 via the mirror 32 and the beam splitter 33. The beam of the probe light 11 reflected by the stepped pulse train generation block 34 has an optical path difference due to the difference in reflection position, and is spatially and temporally divided into 6 pulse trains at intervals of 5 psec. This pulse train is reflected by the beam splitter 33 and enters the terahertz wave detector 10. The operation of the terahertz wave measuring apparatus 102 according to the second embodiment of the present invention is the same as that of the first embodiment.

  As described above, the terahertz wave measuring apparatus 102 according to the second embodiment has a simple configuration in which only the pulse train generation mechanism 30 including the beam expander 31, the mirror 32, the beam splitter 33, and the pulse train generation block 34 is added. It is possible to acquire a waveform in a short time. Further, since the pulse train generating mechanism 20 can be manufactured at a low cost, the cost is not significantly increased. Furthermore, since the reflection optical system is used, there is no expansion of the pulse width due to the wavelength dispersion of the medium, so that the time waveform can be acquired more accurately.

Example 3
In the third embodiment, as in the second embodiment, the probe light 11 is reflected on the step-like reflecting surface to generate a pulse train. In the third embodiment, the optical path difference on the reflecting surface is changed by moving the steps. Two types of pulse trains having different equal intervals are generated.

  In the third embodiment, as shown in FIG. 5, the configuration of the pulse train generation mechanism 40 is the same as that of the second embodiment except for the configuration of the pulse train generation block 30 of the pulse train generation mechanism 30 of the second embodiment. Only the variable pulse train generation block 50 will be described.

  FIG. 6 shows the configuration of the variable pulse train generation block 50 in more detail. FIG. 6A shows a front view and FIG. 6B shows a side view. The surface that reflects the incident probe light 11 is the front surface. As shown in FIG. 6, the variable pulse train generation block 50 includes a reflector 51, a spring 53, a pressing plate 54, and a linear motion stepping motor stage 55. The reflector 51 is obtained by optically polishing one end face of a glass plate having a thickness of 3 mm and applying a reflective coat 52, and six reflectors are overlapped. The surface to which the reflective coat 52 is applied is the front. Each reflector 51 is urged by a spring 53 in the direction opposite to the front (the right direction in FIG. 6B). The back surface of the reflector 51 is pressed by a pressing plate 54. Then, the pressing plate 54 can move the reflector 51 to the left and right in FIG. 6B by using the linear stepping motor stage 55 with the lower end as a fulcrum. That is, by controlling the linear stepping motor stage 55, the step of the six reflectors can be changed, and the time interval of the generated pulse train can be changed. Since the mechanism of pulse train generation is the same as that in the second embodiment, a description thereof will be omitted.

  Next, the operation of the terahertz wave measuring apparatus according to the third embodiment will be described. Needless to say, the time waveform of the terahertz wave can be acquired in a short time, but it is possible to acquire the phase (time delay) information more accurately by acquiring and analyzing the terahertz wave time waveform at two kinds of pulse train intervals. is there. For example, consider an example in which a medium having a refractive index of 1.3 in the terahertz wave region and a thickness of 22 mm (time delay is 22 psec) is measured. FIG. 7 shows a terahertz wave waveform in FIG. 7A, a 5 psec pulse train and measurement waveform in FIG. 7B, and a 6 psec pulse train and measurement waveform in FIG. 7C. As shown in FIG. 7B, when the time interval of the pulse train is 5 psec, the peak of the terahertz wave pulse is measured at the position of 2 psec as shown in the right diagram of FIG. It can be estimated that the time delay of the object is 5N + 2 psec (N is an integer). On the other hand, when the time interval of the pulse train is measured at 6 psec, a peak is measured at a position of 4 psec as shown in the right diagram of FIG. ). From these two measurement results, it can be estimated that (N, M) = (4,3), (8,6), (12,9), etc., but terahertz wave pulses can be measured with six pulse trains. Can be determined as (N, M) = (4, 3), and it can be detected that the time delay is 22 psec. In the general method, it is necessary to move the time delay mechanism 15 by 22 psec. However, in the terahertz wave measuring apparatus according to the third embodiment of the present invention, it is only necessary to move the time delay mechanism 15 by 5 + 6 = 11 psec. Reduction of waveform acquisition time can be realized. Of course, when the number of pulse trains is large, the uncertainty of (the least common multiple of the interval between two pulse trains) × (integer) psec remains, but considering the refractive index and thickness of the medium that can transmit the terahertz wave, no problem.

  As described above, in the terahertz wave measuring apparatus according to the third embodiment of the present invention, pulse train generation that can be manufactured at a relatively low cost, such as the beam expander 31, the mirror 32, the variable pulse train generation block 50, the linear motion stepping motor stage 55, and the like. It is possible to acquire a time waveform in a short time with a simple configuration in which only the mechanism 40 is added. Furthermore, since the time interval of the pulse train can be changed, for example, phase (time delay) information can be acquired more accurately by acquiring and analyzing time waveforms at two types of time intervals.

Example 4
In the embodiment described above, the intensity of the generated pulse train is not constant based on the intensity distribution in the beam of the incident probe light 11. In the fourth embodiment, the intensity of the generated pulse train is constant. Here, the reflection type pulse train generation block 34 shown in the second embodiment is replaced with a pulse train generation block 60 to show a method of making the intensity of the pulse train constant.

  FIG. 8 shows an example of the pulse train generation block 60, FIG. 8 (a) shows the structure of the pulse train generation block 60 of the fourth embodiment in front and side views, and FIG. 8 (b) shows incident light (probe) on the reflecting surface. The intensity distribution of the light 11) and the intensity of each reflecting surface reflected based on the incident light are shown.

  As shown in FIG. 8A, the pulse train generation block 60 is obtained by applying a highly reflective coating 61 to an incident surface (front surface becomes the incident surface) obtained by grinding a cylindrical glass block in a stepped manner. Similar to Example 2, the height of the staircase is constant at 3 mm, but the width of the staircase is changed at each stage. Since the intensity of the laser beam of the incident probe light 11 is indicated by a Gaussian distribution as shown in FIG. 8B in the beam spot, it is divided spatially and temporally by adjusting the width of each stage. Each intensity of the pulse train is made constant. As for the radius of each cylinder, for example, considering that a beam having a beam diameter of 18 mm is divided into six intensities within the beam radius, d1 = 1.806 mm, d2 = 3.69 mm, d3 = 5.752 mm, d4 = 8. 176 mm, d5 = 11.418 mm, d6 = 18 mm may be set. By doing so, the intensity from the reflecting surface of each stage becomes constant. That is, P1 = P2 = P3... = P6.

  In the measurement of the time waveform of the terahertz wave measuring apparatus according to the fourth embodiment, the time waveform can be measured by moving the time delay stage by the interval by using a plurality of pulse trains as in the other embodiments. Furthermore, since the intensity of the pulse train is constant, not only phase information but also amplitude information can be acquired more accurately.

  In the terahertz wave measuring apparatus according to the fourth embodiment of the present invention, since the pulse train generating mechanism 20 can be manufactured at a low cost as in the second embodiment, the cost is not significantly increased. Furthermore, since the intensity of the pulse train is constant, not only phase information but also amplitude information can be obtained accurately.

  When the transmission type pulse train generation block 22 shown in the first embodiment is replaced, the high reflection coat 61 of FIG. 8 is not provided, and a non-reflective film is provided on the surface on which the stairs are formed and the opposite surface instead. Incident probe light 11 is incident from the surface on which the staircase is formed or the opposite surface.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations, conditions, and the like described in the embodiments, and various modifications can be made. For example, a terahertz wave photoconductive antenna is used as means for generating a terahertz wave, but a nonlinear optical crystal such as ZnTe or a semiconductor material such as Si or GaP can be used. Also, the means for detecting the terahertz wave may be a non-linear optical crystal such as ZnTe, and the terahertz electric field may be detected as the inclination of the polarization direction of the probe light. Further, here, a description has been given of a configuration for acquiring a time waveform of a terahertz wave that has passed through one point of the object to be measured, but terahertz is obtained by two-dimensionally scanning the object to be measured on a plane perpendicular to the optical axis of the terahertz wave. The two-dimensional distribution of wave characteristics can be known.

  The above-described embodiments are not limited to this, and can be carried out in various modes without departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 Ultrashort pulse laser 2 Beam splitter 3 Pump light 4a, 4b, 4c, 4d, 4e Mirror 5 Photoconductive antenna for terahertz wave generation (terahertz wave generator)
5a GaAs substrate 5b Low temperature growth GaAs layer 5c Antenna electrode 5d Si hemisphere lens 6 Bias voltage 7 Terahertz wave 8a, 8b, 8c, 8d Condensing lens 9 Measured object 10 Terahertz wave detection photoconductive antenna (terahertz wave detector)
DESCRIPTION OF SYMBOLS 11 Probe light 12 Folding mirror 13 Linear motion stage 14 Stage control apparatus 15 Time delay mechanism 16 Lock-in amplifier 17 Data recording part 20 Pulse train generation mechanism 21 Beam expander 22 Pulse train generation block 30 Pulse train generation mechanism 31 Beam expander 32 Mirror 33 Beam splitter 34 Pulse train generation block 40 Pulse train generation mechanism 50 Variable pulse train generation block 51 Reflector 52 Reflective coat 53 Spring 54 Pressing plate 55 Direct acting stepping motor stage 60 Pulse train generation block 61 High reflection coat 100 Terahertz wave measuring device 101 Terahertz wave measuring device 102 Terahertz Wave measuring device

Claims (7)

A beam splitter that divides the pulse laser into pump light and probe light;
A terahertz wave generator that generates a terahertz wave by incidence of the pump light;
A pulse train generator for generating a plurality of pulses from the incident probe light; and
A terahertz wave measuring unit that enters the pulse train into a terahertz wave detector and measures a temporal change in the electric field of the terahertz wave incident on the terahertz wave detector based on the plurality of pulses. Terahertz wave measuring device. The pulse train generator is
A beam diameter expanding section for expanding the beam diameter of the probe light;
A pulse train generation block including a first surface made of a medium having a predetermined refractive index and having a stepped shape with a constant step, and a second surface facing the first surface;
The beam diameter of the incident probe light is enlarged by the beam diameter enlargement unit, and the probe light having the enlarged beam diameter is transferred from the first surface of the pulse train generation block to the second surface or the second surface. The terahertz wave measuring apparatus according to claim 1, wherein the plurality of pulses are generated by being transmitted through the first surface. The pulse train generator is
A beam diameter expanding section for expanding the beam diameter of the probe light;
A pulse train generation block having a reflecting surface having a stepped shape with a constant step,
The beam diameter of the incident probe light is enlarged by the beam diameter enlargement unit, and the probe light having the enlarged beam diameter is reflected by the reflection surface of the pulse train generation block to generate the plurality of pulses. The terahertz wave measuring apparatus according to claim 1. The pulse train generator is
A beam diameter expanding section for expanding the beam diameter of the probe light;
A plurality of reflectors each having a reflecting surface on one surface are arranged in a stepped manner with the reflecting surface facing in the same direction, and the arranged stepped step is variable, a variable reflecting portion, and the variable reflecting portion A pulse train generation block comprising a step controller for controlling the step,
The variable reflecting portion in which the beam diameter of the incident probe light is expanded by the beam diameter expanding portion, and the probe light whose beam diameter has been expanded is controlled to the first step and the second step by the step control portion, respectively. The terahertz wave measurement unit makes each of the two types of pulses generated by the pulse train generation unit incident on the detector and detects the plurality of pulses. The terahertz wave measuring apparatus according to claim 1, wherein a temporal change in the electric field of the terahertz wave incident on the instrument is measured. The terahertz wave measuring apparatus according to claim 1, wherein the plurality of pulses are equally spaced in time. The terahertz wave measuring apparatus according to claim 1, wherein the plurality of pulses generated by the pulse train generator have a constant pulse intensity. A beam splitting procedure for splitting the pulse laser into pump light and probe light;
A terahertz wave generating procedure for generating terahertz waves upon incidence of the pump light;
A pulse train generation procedure for entering the probe light and generating a plurality of pulses from the probe light;
A terahertz wave measurement procedure for making the pulse train incident on a terahertz wave detector and measuring temporal changes in the electric field of the terahertz wave incident on the terahertz wave detector based on the plurality of pulses. Terahertz wave measurement method.

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