JP5469474B2 - Single terahertz wave time waveform measurement device - Google Patents

Description

  The present invention relates to an apparatus for measuring a time waveform of a terahertz wave generated, transmitted, or reflected from a measurement object and evaluating characteristics of the measurement object.

  The terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 1000 THz corresponding to an intermediate region between the light wave and the radio wave, and has an intermediate property between the light wave and the radio wave. As an application of such a terahertz wave, a technique for acquiring information on the measurement object by measuring a time waveform of the electric field amplitude of the terahertz wave generated, transmitted, or reflected by the measurement object has been studied.

  A technique for measuring information on a measurement object using a terahertz wave is generally as follows. That is, an optical pulse output from a light source (for example, a femtosecond laser light source) is bifurcated by a branching unit into a pump light pulse and a probe light pulse. Among them, the pump light pulse is input to a nonlinear optical crystal for generating a terahertz wave, and thereby a pulsed terahertz wave is generated from the nonlinear optical crystal. The generated terahertz wave is transmitted or reflected by the measurement target unit to acquire information (for example, absorption coefficient, refractive index) of the measurement target, and then combined with the probe light pulse by the multiplexing unit. Then, it is incident on the nonlinear optical crystal for detecting the terahertz wave at substantially the same timing as the probe light pulse.

  In a nonlinear optical crystal to which a terahertz wave and a probe light pulse are input, birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse changes due to the birefringence. Therefore, a polarizer is provided on the optical path of the probe light pulse between the branching portion and the multiplexing portion, and an analyzer is provided on the output side of the nonlinear optical crystal, so that the probe light pulse transmitted through the analyzer is transmitted. By detecting the intensity, a change in the polarization state of the probe light pulse in the nonlinear optical crystal is detected, and consequently, the electric field amplitude of the terahertz wave is detected, and the characteristics of the measurement object are obtained. As described above, the measurement technique for the information on the measurement object using the terahertz wave utilizes the electro-optic effect caused by the pulsed terahertz wave on the nonlinear optical crystal for detecting the terahertz wave.

  In general, the pulse width of the terahertz wave is about picoseconds, whereas the pulse width of the probe light pulse is about femtoseconds, and the pulse width of the probe light pulse is several orders of magnitude narrower than that of the terahertz wave. From this, the time waveform of the electric field amplitude of the pulsed terahertz wave is obtained by sweeping the incident timing of the probe light pulse to the nonlinear optical crystal for detecting the terahertz wave.

  However, in the measurement technique of information on the measurement object described above, in order to obtain the time waveform of the electric field amplitude of the pulse terahertz wave, a large number of optical pulses are output from the light source, and the probe optical pulse to the nonlinear optical crystal for each pulse. Is required to be set, so the required time is long. An invention intended to solve such problems is disclosed in Patent Document 1.

  The invention disclosed in Patent Document 1 tilts the pulse surface of the probe light pulse so that the pulse surfaces of the terahertz wave and the probe light pulse when input to the nonlinear optical crystal for terahertz wave detection are non-parallel to each other. By detecting the distribution of the polarization state change in the beam cross section of the probe light pulse output from the nonlinear optical crystal with a photodetector, a time waveform of the electric field amplitude of the pulse terahertz wave is obtained by a single light pulse from the light source Is. At this time, the time waveform of the electric field amplitude of the pulse terahertz wave is obtained as a spatial image by the photodetector.

JP 2008-096210 A

  In the invention disclosed in Patent Document 1, in order to widen the measurement time range of the time waveform of the electric field amplitude of the pulsed terahertz wave, the beam diameter of the probe light pulse is increased and the nonlinear optical crystal for detecting the terahertz wave is large. It is necessary to use something.

  However, when the beam diameter of the probe light pulse is increased, the intensity of the probe light pulse per unit area in the nonlinear optical crystal for detecting the terahertz wave is lowered, and the detection sensitivity of the terahertz wave is lowered. Further, when the beam diameter of the probe light pulse is increased, it is necessary to use a large non-linear optical crystal for terahertz wave detection accordingly.

  A non-linear optical crystal for detecting terahertz waves (for example, ZeTe crystal) having good crystallinity is difficult to obtain and expensive. In particular, non-linear optical crystals that are large and have good crystallinity are still difficult to obtain and expensive. Further, when a large nonlinear optical crystal for detecting terahertz waves is used, the detection sensitivity of terahertz waves varies due to poor crystallinity, and the result obtained by the photodetector is distorted.

  The present invention has been made to solve the above-mentioned problems, and even when a nonlinear optical crystal for detecting a small terahertz wave is used, the time waveform of the electric field amplitude of a pulsed terahertz wave obtained by a single optical pulse is obtained. An object of the present invention is to provide a single terahertz wave time waveform measuring apparatus capable of widening the measurement time range.

The single terahertz wave time waveform measuring apparatus of the present invention is (1) a terahertz wave and a probe light pulse are input, and birefringence is induced along with the propagation of the terahertz wave, and the polarization state of the probe light pulse is changed by the birefringence. The nonlinear optical crystal that outputs the probe light pulse and (2) the pulse surface of the probe light pulse is tilted so that the terahertz wave and the pulse surface of the probe light pulse that are input to the nonlinear optical crystal (3) a beam of probe light pulses provided on the optical path of the probe light pulse between the pulse surface inclined part and the nonlinear optical crystal, the pulse surface being inclined by the pulse surface inclined part. Beam diameter adjusting optical system that reduces the diameter, and (4) Detecting the distribution of polarization state changes in the beam cross section of the probe light pulse output from the nonlinear optical crystal And a photodetector.
In the single terahertz wave time waveform measuring apparatus of the present invention, the pulse surface tilting unit inputs a probe light pulse whose beam diameter is enlarged after being output from the light source, and tilts the pulse surface of the probe light pulse. Good.

In the single terahertz wave time waveform measuring apparatus of the present invention, the probe light pulse is tilted by the pulse surface tilting part, and then the beam diameter is reduced by the beam diameter adjusting optical system to be converted into a nonlinear optical crystal together with the terahertz wave. Entered. The pulse surfaces of the terahertz wave and the probe light pulse when input to the nonlinear optical crystal are non-parallel to each other. In a nonlinear optical crystal to which a terahertz wave and a probe light pulse are input, birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse changes due to the birefringence. The distribution of the polarization state change in the beam cross section of the probe light pulse output from the nonlinear optical crystal is detected by the photodetector.

  The single terahertz wave time waveform measuring apparatus of the present invention further includes a multiplexing unit that multiplexes the terahertz wave and the probe light pulse so as to be coaxial with each other and outputs them to the nonlinear optical crystal, and the beam diameter adjusting optical system has a pulse surface. It may be provided on the optical path of the probe light pulse between the inclined part and the multiplexing part. The single terahertz wave time waveform measuring apparatus of the present invention further includes a multiplexing unit that multiplexes the terahertz wave and the probe light pulse so as to be coaxial with each other and outputs them to the nonlinear optical crystal, and the beam diameter adjusting optical system includes It may be provided on the optical path of the probe light pulse between the multiplexing part and the nonlinear optical crystal. In the latter case, the beam diameter adjusting optical system not only can adjust the beam diameter of the probe light pulse output from the combining unit, but also adjusts the beam diameter of the terahertz wave output from the combining unit. Can do. Further, the beam diameter adjusting optical system may have an imaging relationship between the pulse surface inclined portion and the nonlinear optical crystal.

  The single terahertz wave time waveform measuring apparatus of the present invention is provided on the optical path of the probe light pulse between the nonlinear optical crystal and the photodetector, and changes the beam diameter of the probe light pulse. A system may be further provided. This probe light pulse beam diameter changing optical system may have an imaging relationship between the nonlinear optical crystal and the photodetector.

  The single terahertz wave time waveform measuring apparatus of the present invention includes a terahertz wave generation unit that generates and outputs a terahertz wave by inputting a pump light pulse, and a terahertz wave that changes the beam diameter of the terahertz wave output from the terahertz wave generation unit And a wave beam diameter changing optical system. This terahertz wave beam diameter changing optical system may have an imaging relationship between a measurement object provided on the optical path of the terahertz wave output from the terahertz wave generation unit and the nonlinear optical crystal.

  The single terahertz wave time waveform measuring apparatus of the present invention generates a terahertz wave by inputting a pump light pulse, and changes the beam diameter of the pump light pulse input to the terahertz wave generating part. And a pump light pulse beam diameter changing optical system. The image plane formed by the pump light pulse beam diameter changing optical system may be located at the terahertz wave generation unit.

  The single terahertz wave time waveform measuring apparatus of the present invention further comprises (a) a pump light pulse irradiating unit that condenses and irradiates a pump light pulse onto the measurement object and scans the light collection irradiation position on the measurement object. (B) The pump light pulse irradiation unit collects and irradiates the pump light pulse onto the measurement object to generate a terahertz wave at the measurement object, and (c) inputs the terahertz wave and the probe light pulse to the nonlinear optical crystal. (D) the photodetector detects the distribution of the polarization state change in the beam cross section of the probe light pulse output from the nonlinear optical crystal at each focused irradiation position on the measurement object by the pump light pulse irradiation unit. May be.

  According to the present invention, even when a nonlinear optical crystal for detecting a small terahertz wave is used, the measurement time range of the time waveform of the electric field amplitude of the pulsed terahertz wave obtained by a single light pulse can be widened.

It is a block diagram of the single terahertz wave time waveform measuring device 101 of a comparative example. FIG. 3 is a diagram illustrating a configuration example of a pulse surface inclined portion 32. It is a figure which shows the mode of propagation of the terahertz wave and probe light pulse in the nonlinear optical crystal. It is a principal part block diagram of the single terahertz wave time waveform measuring device 1 of 1st Embodiment. It is a figure explaining the change of the inclination of the pulse surface of the probe light pulse by the beam diameter adjustment optical system. It is a principal part block diagram of the single terahertz wave time waveform measuring device 2 of 2nd Embodiment. It is a principal part block diagram of the single terahertz wave time waveform measuring device 3 of 3rd Embodiment. It is a principal part block diagram of the single terahertz wave time waveform measuring device 4 of 4th Embodiment. It is a principal part block diagram of the single terahertz wave time waveform measuring device 5 of 5th Embodiment. It is a principal part block diagram of the single terahertz wave time waveform measuring device 6 of 6th Embodiment. It is a principal part block diagram of the single terahertz wave time waveform measuring device 7 of 7th Embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Below, the structure of a comparative example is demonstrated first, and the structure of embodiment is demonstrated after that, contrasting with the structure of this comparative example.

  (Comparative example)

  FIG. 1 is a configuration diagram of a single terahertz wave time waveform measuring apparatus 101 of a comparative example. A single terahertz wave time waveform measuring apparatus 101 shown in this figure acquires information on a measurement object 9 using a terahertz wave, and includes a light source 11, a beam diameter adjusting unit 12, a branching unit 13, and terahertz wave generation. Unit 21, optical path length adjustment unit 31, pulse surface tilting unit 32, polarizer 33, combining unit 41, nonlinear optical crystal 42, analyzer 43, photodetector 44, and mirrors M1 to M8.

  The light source 11 outputs an optical pulse, and is preferably a femtosecond pulse laser light source that outputs a pulse laser beam having a pulse width of about femtoseconds. The beam diameter adjusting unit 12 receives the light pulse output from the light source 11, and expands and outputs the beam diameter of the light pulse. In this beam diameter expansion adjustment, the beam diameters of the terahertz wave and the probe light pulse are made equal to each other, or the beam diameter of the terahertz wave is made larger than the beam diameter of the probe light pulse. In addition, these beam diameters are expanded with the size of the terahertz wave generation unit 21 and the nonlinear optical crystal 42 as a guide.

  The branching unit 13 is, for example, a beam splitter, splits the optical pulse output from the beam diameter adjusting unit 12 into two, outputs one of the two branched optical pulses to the mirror M1 as a pump light pulse, and outputs the other to the mirror M1. The probe light pulse is output to the mirror M4.

  The pump light pulses output from the branch unit 13 are sequentially reflected by the mirrors M <b> 1 to M <b> 3 and input to the terahertz wave generation unit 21. The optical system of the pump light pulse from the branching unit 13 to the terahertz wave generation unit 21 is hereinafter referred to as “pump light pulse optical system”.

  The terahertz wave generation unit 21 generates and outputs a pulsed terahertz wave by inputting a pump light pulse, and includes, for example, any one of a nonlinear optical crystal, an optical antenna element, a semiconductor, and a superconductor. The In the case where the terahertz wave generation unit 21 includes a nonlinear optical crystal, the terahertz wave generation unit 21 can generate a terahertz wave by a nonlinear optical phenomenon that occurs as the pump light pulse is incident.

  The terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 1000 THz corresponding to an intermediate region between the light wave and the radio wave, and has an intermediate property between the light wave and the radio wave. The pulse width of the pulse terahertz wave is about several picoseconds. The terahertz wave output from the terahertz wave generation unit 21 passes through the measurement object 9 placed on the sample table 91 to acquire information (for example, absorption coefficient, refractive index) of the measurement object 9, and then Are input to the multiplexing unit 41. The measurement object 9 is placed on the sample table 91, and the pulse terahertz wave incident position can be adjusted by the movement of the sample table 91. The terahertz wave optical system from the terahertz wave generation unit 21 to the multiplexing unit 41 is hereinafter referred to as a “terahertz wave optical system”.

  On the other hand, the probe light pulses output from the branching unit 13 are sequentially reflected by the mirrors M4 to M8, pass through the pulse surface inclined unit 32 and the polarizer 33, and are input to the multiplexing unit 41. The polarizer 33 may be provided at the subsequent stage of the pulse surface inclined portion 32. The pulse surface tilting unit 32 inputs the probe light pulse that is reflected and arrived at by the mirror M8, tilts the pulse surface of the probe light pulse, and inputs the terahertz wave and the probe light pulse that are input to the nonlinear optical crystal 42. Each pulse surface is made non-parallel to each other. The pulse plane is a plane connecting positions that show the maximum output on the beam line of an optical pulse at a certain moment. On the other hand, the wavefront means an equiphase surface of light. The optical system of the probe light pulse from the branching unit 13 to the multiplexing unit 41 is hereinafter referred to as “probe light pulse optical system”.

  The four mirrors M4 to M7 constitute an optical path length difference adjusting unit 31. That is, when the mirrors M5 and M6 move, the optical path length between the mirrors M4 and M7 and the mirrors M5 and M6 is adjusted, and the optical path length of the probe light pulse optical system is adjusted. Thereby, the optical path length difference adjusting unit 31 reaches the optical path length of the pump light pulse optical system and the terahertz wave optical system from the branching unit 13 to the multiplexing unit 41, and reaches the multiplexing unit 41 from the branching unit 13. The difference from the optical path length of the probe light pulse optical system up to can be adjusted.

  The multiplexing unit 41 inputs the terahertz wave output from the terahertz wave generation unit 21 and transmitted through the measurement object 9 and the probe light pulse output from the branching unit 13 and arrived, and the terahertz wave and the probe light pulse are input. The signals are combined so as to be coaxial with each other and output to the nonlinear optical crystal 42. The combining unit 41 is preferably a pellicle.

  The nonlinear optical crystal 42 receives the terahertz wave and the probe light pulse output from the multiplexing unit 41, and birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse is changed by the birefringence. The probe light pulse is output to the analyzer 43. The photodetector 44 receives the probe light pulse output from the nonlinear optical crystal 42 and passed through the analyzer 43, and detects the intensity distribution of the received probe light pulse. The polarizer 33, the analyzer 43, and the photodetector 44 detect a one-dimensional distribution or a two-dimensional distribution of the polarization state change in the beam cross section of the probe light pulse output from the nonlinear optical crystal.

  FIG. 2 is a diagram illustrating an example of the configuration of the pulse surface inclined portion 32. The pulse surface inclined portion 32 shown in this figure includes a prism, and after the probe light pulse is input to the first surface of the prism and the input probe light pulse is propagated inside the prism, the second surface of the prism is To the outside. Thereby, the pulse surface of the probe light pulse before being input to the first surface of the prism was perpendicular to the principal ray, whereas the pulse surface of the probe light pulse after being output from the second surface of the prism Is inclined with respect to a plane perpendicular to the principal ray. Note that the pulse surface of the probe light pulse can be inclined by using, for example, a grism, a reflection diffraction grating, a transmission diffraction grating, or a spatial light modulator instead of the prism.

  The degree of temporal inclination of the pulse surface by the pulse surface inclined part 32 is a measurement time range when detecting a pulse terahertz wave. Therefore, since the time width of the pulse terahertz wave is about several picoseconds, it is preferable that the pulse surface of the probe light pulse has a slope of several picoseconds or more in time.

  FIG. 3 is a diagram illustrating the propagation state of the terahertz wave and the probe light pulse in the nonlinear optical crystal 42. As shown in this figure, the pulse width of the terahertz wave is about picoseconds, whereas the pulse width of the probe light pulse is about femtoseconds, and the pulse width of the probe light pulse is several times that of the terahertz wave. The pulse plane of the probe light pulse is inclined with respect to the pulse plane of the terahertz wave.

  The single terahertz wave time waveform measuring apparatus 101 of this comparative example operates as follows. The light pulse output from the light source 11 is input to the branching unit 13 after the beam diameter is expanded by the beam diameter adjusting unit 12, and is branched into two by the branching unit 13 to be a pump light pulse and a probe light pulse. The pump light pulses output from the branch unit 13 are sequentially reflected by the mirrors M <b> 1 to M <b> 3 and input to the terahertz wave generation unit 21. The terahertz wave generation unit 21 generates and outputs a terahertz wave in response to the input of the pump light pulse. The terahertz wave output from the terahertz wave generation unit 21 passes through the measurement target unit 9 and is input to the multiplexing unit 41. On the other hand, the probe light pulses output from the branching unit 13 are sequentially reflected by the mirrors M4 to M8, the pulse surface is inclined by the pulse surface inclined unit 32, and is linearly polarized by the polarizer 33. Entered.

  The terahertz wave and the probe light pulse input to the combining unit 41 are combined so as to be coaxial with each other by the combining unit 41 and input to the nonlinear optical crystal 42 at substantially the same timing. In the nonlinear optical crystal 42 to which the terahertz wave and the probe light pulse are input, birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse changes due to the birefringence. The polarization state of the probe light pulse in the nonlinear optical crystal 42 includes a polarizer 33 provided on the optical path of the probe light pulse optical system, an analyzer 43 provided on the output side of the nonlinear optical crystal 42, and this The light intensity is detected by a light detector 44 that detects the intensity of the probe light pulse transmitted through the analyzer 43. In this way, a change in the polarization state of the probe light pulse in the nonlinear optical crystal 42 is detected, and consequently, the electric field amplitude of the terahertz wave is detected, and the characteristics of the measurement object 9 are obtained.

  In the single terahertz wave time waveform measuring apparatus 101 of this comparative example, the pulsed terahertz wave output from the terahertz wave generating unit 21 usually has a beam diameter of centimeter to millimeter. The probe light pulse for detecting this also has the same beam diameter. When the pulse surface of the probe light pulse has an inclination similar to the time width of the pulse terahertz wave, the probe light pulse overlaps the pulse terahertz wave in a spatially oblique manner as shown in FIG. That is, in the propagation direction of the probe light pulse and the pulse terahertz wave (that is, in the time direction), the probe light pulse overlaps at all points of the pulse terahertz wave due to the inclination of the pulse surface, but the beam is perpendicular to the propagation direction. The position is different in the diameter range. From this, by using the photodetector 44 capable of detecting a one-dimensional or two-dimensional light intensity distribution, the time change (time information) of the electric field amplitude of the pulse terahertz wave is converted into the light intensity distribution in the beam cross section of the probe light pulse. It is possible to detect this spatial information by replacing it with (spatial information).

  As described above, the single terahertz wave time waveform measuring apparatus 101 of the comparative example converts time information into spatial information, detects the spatial information with the photodetector 44, and changes the time variation (time of the electric field amplitude of the pulse terahertz wave). Information) can be measured.

  In the single terahertz wave time waveform measuring apparatus 101, in order to widen the measurement time range of the time waveform of the electric field amplitude of the pulse terahertz wave, it is necessary to increase the beam diameter of the probe light pulse and use a large nonlinear optical crystal 42. is necessary. However, when the beam diameter of the probe light pulse is increased, the intensity of the probe light pulse per unit area in the nonlinear optical crystal 42 is lowered, and the terahertz wave detection sensitivity is lowered. Further, when the beam diameter of the probe light pulse is increased, it is necessary to use a large non-linear optical crystal 42 correspondingly. Those having good crystallinity of the nonlinear optical crystal 42 are difficult to obtain and expensive. In particular, the nonlinear optical crystal 42 that is large and has good crystallinity is still difficult to obtain and expensive. Further, when the large nonlinear optical crystal 42 is used, the detection sensitivity of the terahertz wave varies due to poor crystallinity, and the result obtained by the photodetector is distorted. The single-shot terahertz wave time waveform measuring apparatus according to the embodiment described below can solve such problems.

  (First embodiment)

  FIG. 4 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 1 according to the first embodiment. In this figure, the optical system after the terahertz wave generation unit 21 is shown for the terahertz wave, and the optical system after the pulse plane inclined unit 32 is shown for the probe light pulse.

  In the single terahertz wave time waveform measuring apparatus 1 according to the first embodiment, a multiplexing unit that combines the probe light pulse and the terahertz wave is not provided. The probe light pulse and the terahertz wave are incident on different surfaces with respect to the nonlinear optical crystal 42.

  The single terahertz wave time waveform measuring apparatus 1 includes a beam diameter adjusting optical system 34 that adjusts the beam diameter of the probe light pulse on the optical path of the probe light pulse between the pulse surface inclined portion 32 and the nonlinear optical crystal 42. Yes. The beam diameter adjusting optical system 34 includes a lens 34A and a lens 34B. The rear focal position of the front lens 34A and the front focal position of the rear lens 34B coincide with each other. The probe output from the beam diameter adjusting optical system 34 is made smaller than the beam diameter of the probe light pulse input to the beam diameter adjusting optical system 34 by shortening the focal length of the subsequent lens 34B from the focal length of the front lens 34A. The beam diameter of the light pulse can be reduced.

  FIG. 5 is a diagram for explaining a change in the inclination of the pulse surface of the probe light pulse by the beam diameter adjusting optical system 34. As shown in this figure, the pulse surface S1 of the probe light pulse input to the pulse surface inclined portion 32 is perpendicular to the optical axis, but the pulse surface S2 of the probe light pulse output from the pulse surface inclined portion 32 is Inclined with respect to a plane perpendicular to the optical axis. When the beam diameter of the output light changes with respect to the beam diameter of the input light of the beam diameter adjusting optical system 34, the beam diameter with respect to the tilt angle of the pulse surface S2 of the probe light pulse input to the beam diameter adjusting optical system 34. The tilt angle of the pulse surface S3 of the probe light pulse output from the adjustment optical system 34 also changes, but the time direction does not change. The measurement time range T2 by the probe light pulse output from the beam diameter adjusting optical system 34 is not different from the measurement time range T1 by the probe light pulse input to the beam diameter adjusting optical system 34. That is, the terahertz wave measurement time range in the single terahertz wave time waveform measurement does not change depending on the presence or absence of the beam diameter adjusting optical system 34 in the probe light pulse optical system.

  Therefore, the single terahertz wave time waveform measuring apparatus 1 having the configuration shown in FIG. 4 includes the beam diameter adjusting optical system 34 and reduces the beam diameter of the probe light pulse input to the nonlinear optical crystal 42 for detecting the terahertz wave. By doing so, a smaller nonlinear optical crystal 42 can be used while maintaining the measurement time range of the terahertz wave. By using the small non-linear optical crystal 42, it is possible to suppress measurement errors in single terahertz wave time waveform measurement due to variations in terahertz wave detection sensitivity due to poor crystallinity. In addition, by using the small nonlinear optical crystal 42, the apparatus using the single terahertz wave time waveform measurement can be made small and inexpensive.

  The beam diameter adjusting optical system 34 may be constituted by a lens pair or a curved mirror pair. The polarizer 33 may be provided in the previous stage of the pulse surface inclined part 32 or may be provided in the subsequent stage of the pulse surface inclined part 32. Further, the probe light pulse and the terahertz wave may be incident on the common surface of the nonlinear optical crystal 42 non-coaxially.

  The beam diameter adjusting optical system 34 preferably has an imaging relationship between the pulse surface inclined portion 32 and the nonlinear optical crystal 42. The beam diameter adjusting optical system 34 is more preferably an optical system that removes image distortion caused by aberrations such as image plane distortion, and for this purpose, for example, a 4f optical system is preferable.

  Further, the beam diameter adjusting optical system 34 is a zoom lens type optical system capable of arbitrarily changing the enlargement / reduction ratio while maintaining the positional relationship between the object plane and the image plane in the imaging relation. Is preferred. By doing in this way, the optimal beam diameter of the probe light pulse can be set according to the beam diameter of the terahertz wave.

  For example, when the terahertz wave reaches the nonlinear optical crystal 42 with the intensity distribution in the beam plane as in the case where the terahertz wave is a Gaussian type, the beam diameter of the probe light pulse is smaller than the beam diameter of the terahertz wave. As a result, only the central portion where the terahertz wave intensity is strong can be detected by the probe light pulse, and single terahertz wave time waveform measurement with high sensitivity and high quantitativeness becomes possible.

  (Second Embodiment)

  FIG. 6 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 2 according to the second embodiment. Also in this figure, the optical system after the terahertz wave generation unit 21 is shown for the terahertz wave, and the optical system after the pulse plane inclined unit 32 is shown for the probe light pulse.

  In the single terahertz wave time waveform measuring apparatus 2 of the second embodiment, a multiplexing unit 41 that multiplexes the probe light pulse and the terahertz wave so as to be coaxial with each other is provided. The probe light pulse and the terahertz wave are coaxial with each other and enter the nonlinear optical crystal 42.

  The single-shot terahertz wave time waveform measuring apparatus 2 includes a beam diameter adjusting optical system 34 that adjusts the beam diameter of the probe light pulse on the optical path of the probe light pulse between the pulse surface tilting section 32 and the combining section 41. Yes.

  The single terahertz wave time waveform measuring apparatus 2 of the second embodiment operates in substantially the same manner as the single terahertz wave time waveform measuring apparatus 1 of the first embodiment, and can provide the same effects. Also in the single terahertz wave time waveform measuring apparatus 2 of the second embodiment, it is preferable that the beam diameter adjusting optical system 34 has an imaging relationship between the pulse surface inclined portion 32 and the nonlinear optical crystal 42. The beam diameter adjusting optical system 34 is more preferably an optical system that removes image distortion caused by aberrations such as image plane distortion. Further, the beam diameter adjusting optical system 34 is a zoom lens type optical system capable of arbitrarily changing the enlargement / reduction ratio while maintaining the positional relationship between the object plane and the image plane in the imaging relation. Is preferred.

  (Third embodiment)

  FIG. 7 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 3 according to the third embodiment. Also in this figure, the optical system after the terahertz wave generation unit 21 is shown for the terahertz wave, and the optical system after the pulse plane inclined unit 32 is shown for the probe light pulse.

  The single terahertz wave time waveform measuring apparatus 3 according to the third embodiment shown in FIG. 7 includes a probe light pulse beam in addition to the configuration of the single terahertz wave time waveform measuring apparatus 2 according to the second embodiment shown in FIG. A diameter changing optical system 45 is further provided.

  The probe light pulse beam diameter changing optical system 45 is provided on the optical path of the probe light pulse between the nonlinear optical crystal 42 and the photodetector 44, and changes (enlarges or reduces) the beam diameter of the probe light pulse. The probe light pulse beam diameter changing optical system 45 includes a lens 45A and a lens 45B. The rear focal position of the front lens 45A and the front focal position of the rear lens 45B coincide with each other. The probe light pulse beam diameter changing optical system 45 can change the beam diameter of the probe light pulse according to the ratio of the focal length of the front lens 45A and the focal length of the rear lens 45B.

  The number of pixels of the photodetector 44 corresponds to the time resolution of the terahertz wave time waveform to be measured. Since the number of pixels and the pixel pitch differ depending on the photodetector, the probe light pulse beam diameter changing optical system 45 can be provided to perform measurement with a desired time resolution. The probe light pulse beam diameter changing optical system 45 may be an independent beam diameter change in the vertical and horizontal directions in the beam cross section of the probe light pulse. The analyzer 43 may be provided before the probe light pulse beam diameter changing optical system 45 or may be provided after the probe light pulse beam diameter changing optical system 45.

  The probe light pulse beam diameter changing optical system 45 preferably has an imaging relationship between the nonlinear optical crystal 42 and the photodetector 44. The probe light pulse beam diameter changing optical system 45 is more preferably an optical system that removes image distortion caused by aberrations such as image plane distortion, and for this purpose, for example, a 4f optical system is preferable. .

  Further, the probe light pulse beam diameter changing optical system 45 is a zoom lens type optical system that can arbitrarily change the enlargement / reduction ratio while maintaining the positional relationship between the object plane and the image plane in the imaging relation. It is preferable. By doing so, the optimum beam diameter of the probe light pulse can be set in accordance with the size of the light receiving surface of the photodetector 44.

  (Fourth embodiment)

  FIG. 8 is a configuration diagram of a main part of the single terahertz wave time waveform measuring device 4 according to the fourth embodiment. Also in this figure, the optical system after the terahertz wave generation unit 21 is shown for the terahertz wave, and the optical system after the pulse plane inclined unit 32 is shown for the probe light pulse.

  The single terahertz wave time waveform measuring apparatus 4 of the fourth embodiment shown in FIG. 8 has a terahertz wave beam diameter in addition to the configuration of the single terahertz wave time waveform measuring apparatus 3 of the third embodiment shown in FIG. A change optical system 22 is further provided.

  The terahertz wave beam diameter changing optical system 22 is provided on the optical path of the terahertz wave between the measuring object 9 and the multiplexing unit 41 and changes the beam diameter of the terahertz wave output from the terahertz wave generating unit 21. The terahertz wave beam diameter changing optical system 22 includes a lens 22A and a lens 22B. The rear focal position of the front lens 22A and the front focal position of the rear lens 22B coincide with each other. The terahertz wave beam diameter changing optical system 22 can change the beam diameter of the terahertz wave according to the ratio of the focal length of the front lens 22A and the focal length of the rear lens 22B.

  When the beam diameter of the terahertz wave is reduced by the terahertz wave beam diameter changing optical system 22, the intensity of the terahertz wave on the nonlinear optical crystal 42 is increased and the intensity of the detected signal is increased. Conversely, when the beam diameter of the terahertz wave is enlarged by the terahertz wave beam diameter changing optical system 22, only the center portion in the terahertz wave beam on the nonlinear optical crystal 42 is detected, and the intensity of the terahertz wave in the beam plane is detected. Distribution effects can be eliminated. Whether the terahertz beam diameter changing optical system 22 is a reduction optical system or an expansion optical system is determined by the application.

  The terahertz wave beam diameter changing optical system 22 preferably has an imaging relationship between the measurement object 9 and the nonlinear optical crystal 42. The terahertz wave beam diameter changing optical system 22 is more preferably an optical system that removes image distortion caused by aberrations such as image plane distortion, and for this purpose, for example, a 4f optical system is preferable.

  Further, the terahertz wave beam diameter changing optical system 22 is a zoom lens type optical system that can arbitrarily change the enlargement / reduction ratio while maintaining the positional relationship between the object plane and the image plane in the imaging relation. It is preferable. By doing so, the optimum beam diameter of the terahertz wave can be set in accordance with the beam diameter of the probe light pulse.

  (Fifth embodiment)

  FIG. 9 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 5 of the fifth embodiment. Also in this figure, the optical system after the terahertz wave generation unit 21 is shown for the terahertz wave, and the optical system after the pulse plane inclined unit 32 is shown for the probe light pulse.

  The single terahertz wave time waveform measuring apparatus 5 of the fifth embodiment shown in FIG. 9 is added to the beam diameter adjusting optical system 34 in the configuration of the single terahertz wave time waveform measuring apparatus 3 of the third embodiment shown in FIG. Instead, a beam diameter adjusting optical system 46 is provided.

  The beam diameter adjusting optical system 46 is provided on the optical path of the probe light pulse between the multiplexing unit 41 and the nonlinear optical crystal 42, and the probe light pulse output from the multiplexing unit 41 coaxially with each other. Adjust the beam diameter of each terahertz wave. The beam diameter adjusting optical system 46 includes a lens 46A and a lens 46B. The rear focal position of the front lens 46A and the front focal position of the rear lens 46B coincide with each other. The probe output from the beam diameter adjusting optical system 46 is made smaller than the beam diameter of the probe light pulse input to the beam diameter adjusting optical system 46 by shortening the focal length of the latter lens 46B from the focal length of the preceding lens 46A. The beam diameter of the optical pulse can be reduced, and the beam diameter of the terahertz wave output from the beam diameter adjusting optical system 46 can be made smaller than the beam diameter of the terahertz wave input to the beam diameter adjusting optical system 46. Can do.

  The beam diameter adjusting optical system 46 corresponds to a combination of the beam diameter adjusting optical system 34 and the terahertz wave beam diameter changing optical system 22 in the fourth embodiment. Each of the lens 46A and the lens 46B constituting the beam diameter adjusting optical system 46 is transparent to both the terahertz wave and the probe light pulse, and is made of a material having a refractive index substantially equal to both. Is preferred.

  (Sixth embodiment)

  FIG. 10 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 6 according to the sixth embodiment. In this figure, for the terahertz wave, the optical system after the preceding stage of the terahertz wave generation unit 21 is shown, and for the probe light pulse, the optical system after the pulse plane inclined unit 32 is shown.

  The single terahertz wave time waveform measuring apparatus 6 of the sixth embodiment shown in FIG. 10 includes a pump light pulse beam in addition to the configuration of the single terahertz wave time waveform measuring apparatus 3 of the third embodiment shown in FIG. A diameter changing optical system 23 is provided.

  The pump light pulse beam diameter changing optical system 23 is provided in the middle of the pump light pulse optical system between the branching unit 13 and the terahertz wave generating unit 21, and the beam diameter of the pump light pulse input to the terahertz wave generating unit 21. To change. The pump light pulse beam diameter changing optical system 23 includes a lens 23A and a lens 23B. The rear focal position of the front lens 23A and the front focal position of the rear lens 23B coincide with each other.

  The pump light pulse beam diameter changing optical system 23 can change the beam diameter of the pump light pulse in accordance with the ratio of the focal length of the front lens 23A and the focal length of the rear lens 23B. The beam diameter of the terahertz wave output from the terahertz wave generation unit 21 can be changed by changing the beam diameter of the pump light pulse input to the terahertz wave generation unit 21.

  The image plane formed by the pump light pulse beam diameter changing optical system 23 is preferably located at the terahertz wave generation unit 21. The pump light pulse beam diameter changing optical system 23 may be used together with the terahertz wave beam diameter changing optical system 22 in the fourth embodiment.

  (Seventh embodiment)

  FIG. 11 is a main part configuration diagram of the single terahertz wave time waveform measuring apparatus 7 of the seventh embodiment. In this figure, the optical system after the front part of the measuring object 9 as the terahertz wave generating unit is shown for the pump light pulse and the terahertz wave, and the optical system after the pulse surface tilting part 32 is shown for the probe light pulse. The system is shown.

  The single terahertz wave time waveform measuring apparatus 7 according to the seventh embodiment shown in FIG. 11 is the same as the single terahertz wave time waveform measuring apparatus 4 according to the fourth embodiment shown in FIG. It also serves as the generating unit 21 and includes a lens 24, an optical plate 25 with an ITO film, and an objective lens 26.

  The lens 24, the ITO film-coated optical plate 25, and the objective lens 26 provided in the pump light pulse optical system constitute a pump light pulse irradiation unit that collects and irradiates the pump light pulse to the measurement object 9. The focused irradiation position on the measurement object 9 is two-dimensionally scanned. The condensing irradiation position may be scanned by scanning the measurement object 9 or by scanning the principal ray of the pump light pulse incident on the lens 24.

  The measurement object 9 is a semiconductor device, for example, and generates a terahertz wave when irradiated with a pump light pulse. The terahertz wave is input to the multiplexing unit 41 through the objective lens 26, the optical plate 25 with the ITO film, and the terahertz wave beam diameter changing optical system 22.

  The multiplexing unit 41 inputs the terahertz wave generated in the measurement target 9 by the pump light pulse being focused and irradiated on the measurement target 9 and the probe light pulse output from the beam diameter adjusting optical system 34. These are combined so as to be coaxial with each other and output to the nonlinear optical crystal 42.

  The nonlinear optical crystal 42 receives the terahertz wave and the probe light pulse output from the multiplexing unit 41, and birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light pulse is changed by the birefringence. The probe light pulse is output to the analyzer 43.

  The photodetector 44 receives the probe light pulse output from the nonlinear optical crystal 42 and passed through the analyzer 43 and the probe light pulse beam diameter changing optical system 45, and detects the intensity distribution of the received probe light pulse. The polarizer 33, the analyzer 43, and the photodetector 44 change the polarization state in the beam cross section of the probe light pulse output from the nonlinear optical crystal 42 at each focused irradiation position of the pump light pulse to the measurement object 9. A one-dimensional distribution or a two-dimensional distribution is detected.

  The single-shot terahertz wave time waveform measuring apparatus 7 of the seventh embodiment constitutes a so-called laser terahertz emission microscope (LTEM). LTEM can measure the electric field distribution of an object to be measured, which is a semiconductor device, for example, in a non-contact manner.

  The present invention can be applied to LTEM. Thereby, the distortion of the terahertz wave time waveform in the single terahertz wave time waveform measurement can be suppressed, and a highly accurate LTEM system can be constructed.

DESCRIPTION OF SYMBOLS 1-7 ... Single terahertz wave time waveform measuring device, 9 ... Measuring object, 11 ... Light source, 12 ... Beam diameter adjustment part, 13 ... Branch part, 21 ... Terahertz wave generation part, 22 ... Terahertz wave beam diameter change optical system , 23 ... Pump light pulse beam diameter changing optical system, 31 ... Optical path length difference adjusting section, 32 ... Pulse surface tilting section, 33 ... Polarizer, 34 ... Beam diameter adjusting optical system, 41 ... Multiplexing section, 42 ... Nonlinear optics Crystal, 43 ... Analyzer, 44 ... Photodetector, 45 ... Probe light pulse beam diameter changing optical system, 46 ... Beam diameter adjusting optical system, M1-M8 ... Mirror.

Claims (12)

A nonlinear optical crystal that inputs a terahertz wave and a probe light pulse, induces birefringence as the terahertz wave propagates, changes the polarization state of the probe light pulse by the birefringence, and outputs the probe light pulse;
Inclining the pulse surface of the probe light pulse, a pulse surface inclined portion that makes the pulse surfaces of the terahertz wave and the probe light pulse non-parallel to each other when input to the nonlinear optical crystal,
A beam diameter adjusting optical system which is provided on the optical path of the probe light pulse between the pulse surface inclined portion and the nonlinear optical crystal and reduces the beam diameter of the probe light pulse whose pulse surface is inclined by the pulse surface inclined portion. When,
A photodetector for detecting the distribution of polarization state changes in the beam cross section of the probe light pulse output from the nonlinear optical crystal;
A single terahertz wave time waveform measuring apparatus comprising: The pulse surface inclined part inputs a probe light pulse whose beam diameter is expanded after being output from a light source, and inclines the pulse surface of the probe light pulse.
The single terahertz wave time waveform measuring apparatus according to claim 1. Further comprising a combining unit for combining the terahertz wave and the probe light pulse so as to be coaxial with each other and outputting the combined light to the nonlinear optical crystal;
The beam diameter adjusting optical system is provided on the optical path of the probe light pulse between the pulse surface inclined portion and the multiplexing portion,
The single terahertz wave time waveform measuring apparatus according to claim 1. Further comprising a combining unit for combining the terahertz wave and the probe light pulse so as to be coaxial with each other and outputting the combined light to the nonlinear optical crystal;
The beam diameter adjusting optical system is provided on an optical path of a probe light pulse between the multiplexing unit and the nonlinear optical crystal;
The single terahertz wave time waveform measuring apparatus according to claim 1. The beam diameter adjusting optical system has an imaging relationship between the pulse surface inclined portion and the nonlinear optical crystal;
The single terahertz wave time waveform measuring apparatus according to claim 1. A probe light pulse beam diameter changing optical system which is provided on the optical path of the probe light pulse between the nonlinear optical crystal and the photodetector and changes the beam diameter of the probe light pulse;
The single terahertz wave time waveform measuring apparatus according to claim 1. The probe light pulse beam diameter changing optical system has an imaging relationship between the nonlinear optical crystal and the photodetector;
The single terahertz wave time waveform measuring apparatus according to claim 6 . A terahertz wave generator that generates and outputs a terahertz wave by inputting a pump light pulse;
A terahertz wave beam diameter changing optical system that changes the beam diameter of the terahertz wave output from the terahertz wave generating unit;
The single terahertz wave time waveform measuring device according to claim 1, further comprising: The terahertz wave beam diameter changing optical system has an imaging relationship between the measurement object provided on the optical path of the terahertz wave output from the terahertz wave generation unit and the nonlinear optical crystal.
The single terahertz wave time waveform measuring apparatus according to claim 8 . A terahertz wave generator that generates and outputs a terahertz wave by inputting a pump light pulse;
A pump light pulse beam diameter changing optical system for changing the beam diameter of the pump light pulse input to the terahertz wave generation unit;
The single terahertz wave time waveform measuring device according to claim 1, further comprising: An image plane by the pump light pulse beam diameter changing optical system is located in the terahertz wave generation unit,
The single terahertz wave time waveform measuring apparatus according to claim 10 . Further comprising a pump light pulse irradiating unit for condensing and irradiating the measurement object with a pump light pulse and scanning the condensing irradiation position in the measurement object,
Terahertz waves are generated in the measurement object by condensing and irradiating the measurement object with the pump light pulse by the pump light pulse irradiation unit,
Input the terahertz wave and the probe light pulse to the nonlinear optical crystal,
The photodetector detects the distribution of polarization state changes in the beam cross section of the probe light pulse output from the nonlinear optical crystal for each focused irradiation position on the measurement object by the pump light pulse irradiation unit,
The single terahertz wave time waveform measuring apparatus according to claim 1.