JP4950769B2 - Terahertz wave neutralization filter - Google Patents

Description

  The present invention relates to a neutral density filter for terahertz waves, a terahertz wave measuring apparatus including the neutral density filter, and a method for dimming terahertz waves.

  The terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 100 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 transmitted or reflected by the measurement object has been studied (Patent Document 1 and (Refer nonpatent literature 1).

  A technique for measuring information on a measurement object using a terahertz wave is generally as follows. That is, the pulsed light output from the light source (for example, femtosecond laser light source) is bifurcated by the branching unit into pump light and probe light. Among them, the pump light is input to a terahertz wave generating element (for example, a nonlinear optical crystal or a photoconductive antenna element), and thereby a pulsed terahertz wave is generated from the terahertz wave generating element. 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 the terahertz wave detection element at substantially the same timing as the probe light. (E.g., an electro-optic crystal or a photoconductive antenna element).

  In the terahertz wave detecting element to which the terahertz wave and the probe light are input, the correlation between both lights is detected. For example, when an electro-optic crystal is used as a terahertz wave detecting element, the terahertz wave and the probe light are combined by a multiplexing unit and incident on the electro-optic crystal, and birefringence occurs along with the propagation of the terahertz wave in the electro-optic crystal. Is induced, and the polarization state of the probe light changes due to the birefringence. A change in the polarization state of the probe light in the electro-optic crystal is detected, and consequently, the electric field amplitude of the terahertz wave is detected, and information on the measurement object is obtained.

  In such a measurement using a terahertz wave, a signal value obtained from the terahertz wave detection element (for example, when an electro-optic crystal is used as the terahertz wave detection element, the amount of change in the polarization state of the probe light in the electro-optic crystal And the relationship between the electric field amplitude of the terahertz wave represented by the signal value is not necessarily a linear relationship. Therefore, in order to calibrate the relationship between the two, a neutral density filter for terahertz waves is required.

  In general, as an optical neutral density filter, a glass substrate coated with a chromium metal film and having a transmittance according to the thickness of the metal film (hereinafter referred to as “type 1”), a glass substrate. A mixture of a light absorbing material and a material having a transmittance corresponding to the thickness of the substrate (hereinafter referred to as “type 2”), and a glass substrate mixed with a light absorbing material And what has the transmittance | permeability according to the said mixing ratio (henceforth "type 3"), etc. are known.

  In addition, as an optical neutral density filter, a transmission region that transmits light and a blocking region that blocks light are distributed, and has a transmittance according to a ratio occupied by the transmission region (hereinafter, “ Type 4 ”is also known (see, for example, Patent Document 2 and Non-Patent Document 2).

The polka dot beam splitter described in Non-Patent Document 2 is originally a beam splitter that divides light into two, but if attention is paid to transmitted light, it can also be used as a neutral density filter. This neutral density filter (polka dot beam splitter) is made by selectively depositing an aluminum film on a plurality of predetermined areas discretely arranged on a synthetic quartz substrate. The light can be reflected in a region (blocking region) where light is applied, and the light can be transmitted in a region (transmission region) where an aluminum film is not deposited. The neutral density filter described in Non-Patent Document 1 is said to have a certain transmittance when light having a wavelength included in the wavelength range of 250 nm to 2000 nm is incident at an incident angle of 0 to 45 degrees.
JP 2004-354246 A JP 2007-027206 A Sakai Kiyomi, “Terahertz Time Domain Spectroscopy”, Spectroscopic Research, Vol. 50, No. 6 (2001), pp. 261-273 Edmund Optics Japan's 2007 catalog of optical parts and products, J074A, page 54

  By the way, all of the above-described type 1-4 neutral density filters are for optics in the ultraviolet region, the visible region, and the infrared region. Terahertz wave neutralization filters, including those at the proposal or prototype level, are not yet known and at least not commercially available.

  As described above, the terahertz wave is an electromagnetic wave having a frequency 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. From this, it can be considered that the terahertz wave is attenuated by using the optical neutralization filters of types 1 to 4 described above. However, the present inventors have found that the following problems occur in this case.

  In the type 1 to 3 neutral density filters, the optical path length also changes when the transmittance is changed. That is, in the type 1 neutral density filter, when the thickness of the metal film coated on the glass substrate is changed, not only the transmittance is changed, but also the optical path length is changed. In the type 2 neutral density filter, when the thickness of the glass substrate mixed with the light absorbing material is changed, not only the transmittance is changed, but also the optical path length is changed. Further, in the type 3 neutral density filter, when the mixing ratio of the light absorbing material in the glass substrate is changed, not only the refractive index is changed and the transmittance is changed, but also the optical path length is changed.

  In measurement of information on a measurement object using a terahertz wave, the time waveform of the electric field amplitude of the terahertz wave is measured by scanning the timing of the terahertz wave incident on the terahertz wave detecting element and the probe light. In such a measurement, if the optical path length of the terahertz wave changes due to the insertion of the neutral density filter, accurate measurement cannot be performed. Therefore, the type 1 to 3 neutral density filters are inappropriate for use in measurements using terahertz waves.

  On the other hand, the transmittance of the type 4 neutral density filter can be changed by changing the ratio of the transmission region. Since the optical path length can be made constant if the thickness and material of the substrate are made constant, it is expected that the type 4 neutral density filter can solve the above-mentioned problem of change in optical path length. However, when the present inventor made a prototype of a type 4 neutral density filter and tried to attenuate the terahertz wave using the neutral density filter, the transmittance of the neutral density filter depends on the frequency of the terahertz wave. I found out.

  The present invention has been made to solve the above problems, and for terahertz waves that can make the optical path length constant even when the transmittance is changed and can reduce the wavelength dependency of the transmittance. An object is to provide a neutral density filter and a terahertz wave attenuation method. It is another object of the present invention to provide a terahertz wave measuring apparatus that includes such a neutral density filter and can perform accurate terahertz wave measurement.

In the terahertz wave neutralizing filter according to the present invention, the transmission region transmitting the terahertz wave and the blocking region blocking the terahertz wave are distributed so as not to have a periodic structure at the wavelength of the terahertz wave, and the transmission region is shaped The terahertz wave incident from one side is transmitted to the other side at a transmittance corresponding to the ratio occupied by the transmission region. The neutral density filter configured as described above can make the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

  The terahertz wave measuring apparatus according to the present invention includes (1) a light source that outputs light, and (2) two branches of light output from the light source, and one of the two branched lights is used as pump light and the other as a probe. (3) a terahertz wave generating element that generates and outputs a terahertz wave by inputting pump light output from the branching part, and (4) a measurement object output from the terahertz wave generating element. A terahertz wave detecting element that receives the terahertz wave transmitted or reflected by the probe and the probe light output from the branching section and detects a correlation between the terahertz wave and the probe light; and (5) a terahertz wave generating element. From the terahertz wave to the terahertz wave detecting element, which is inserted into the optical path where only the terahertz wave propagates out of the terahertz wave and the probe light, and the incident terahertz wave is attenuated and transmitted. A neutral density filter according to the present invention described above that, characterized in that it comprises a.

In the terahertz wave attenuation method according to the present invention, the transmission region that transmits the terahertz wave and the blocking region that blocks the terahertz wave are distributed so as not to have a periodic structure at the wavelength of the terahertz wave, and the transmission region has a shape and size. For any of the above, a neutral density filter is used, and the terahertz wave incident from one side of the neutral density filter is transmitted to the other side at a transmittance corresponding to the ratio of the transmission region. Thus, it is characterized by dimming the terahertz wave. Thus, the method of dimming the terahertz wave can make the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

  According to the present invention, the optical path length can be made constant even if the transmittance is changed, and the wavelength dependency of the transmittance can be reduced.

  The best mode for carrying out the present invention will be described below 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.

  First, the configuration of the terahertz wave measuring apparatus 1 according to the present embodiment will be described. FIG. 1 is a configuration diagram of a terahertz wave measuring apparatus 1 according to the present embodiment. The terahertz wave measuring apparatus 1 shown in this figure acquires information on the measuring object S by a transmission measurement method using terahertz waves, and includes a light source 11, a branching unit 12, a chopper 13, and an optical path length difference adjusting unit. 14, a polarizer 15, a multiplexing unit 16, a terahertz wave generating element 20, a neutral density filter 30, a terahertz wave detecting element 40, a quarter wavelength plate 51, a polarization separating element 52, a photodetector 53A, a photodetector 53B, A differential amplifier 54 and a lock-in amplifier 55 are provided.

  The light source 11 is a femtosecond pulse laser light source that outputs pulsed light with a constant repetition period and preferably outputs pulsed laser light having a pulse width of about femtoseconds. The branching unit 12 is, for example, a beam splitter, splits the pulsed light output from the light source 11 into two, outputs one of the two branched pulsed light to the mirror M1 as pump light, and the other as the probe light to mirror. Output to M4.

  The chopper 13 is provided on the optical path of the pump light between the branching section 12 and the mirror M1, and alternately repeats the passage and blocking of the pump light at a constant cycle. The pump light output from the branching unit 12 and passing through the chopper 13 is sequentially reflected by the mirrors M <b> 1 to M <b> 3 and input to the terahertz wave generating element 20. The optical system of pump light from the branching section 12 to the terahertz wave generating element 20 is hereinafter referred to as “pump optical system”.

  The terahertz wave generating element 20 generates and outputs a pulsed terahertz wave by inputting pump light. For example, a nonlinear optical crystal (for example, ZnTe), a photoconductive antenna element (for example, an optical switch using GaAs), It is configured to include either a semiconductor (for example, InAs) or a superconductor. When the terahertz wave generating element 20 includes a non-linear optical crystal, the terahertz wave generating element 20 can generate a terahertz wave by a non-linear optical phenomenon that occurs as pump light enters.

  The terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 100 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. Further, the pulse terahertz wave is generated at a constant repetition period, and the pulse width is about several picoseconds. The terahertz wave output from the terahertz wave generation element 20 acquires information (for example, an absorption coefficient and a refractive index) of the measurement object S by passing through the measurement object S, and then is input to the multiplexing unit 16. The The terahertz wave optical system from the terahertz wave generating element 20 to the multiplexing unit 16 is hereinafter referred to as a “terahertz wave optical system”.

  On the other hand, the probe light output from the branching unit 12 is sequentially reflected by the mirrors M4 to M8, passes through the polarizer 15, and is input to the multiplexing unit 16. The optical system of the probe light from the branching unit 12 to the multiplexing unit 16 is hereinafter referred to as “probe optical system”. The four mirrors M4 to M7 constitute an optical path length difference adjusting unit 14. 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 optical system is adjusted. As a result, the optical path length difference adjusting unit 14 is configured so that the optical path lengths of the pump optical system and the terahertz wave optical system from the branching unit 12 to the multiplexing unit 16 and the branching unit 12 to the multiplexing unit 16 are reached. The difference from the optical path length of the probe optical system can be adjusted.

  The multiplexing unit 16 inputs the terahertz wave output from the terahertz wave generation element 20 and transmitted through the measurement object S, and the probe light output from the branching unit 12 and arrived, and the terahertz wave and the probe light are coaxial with each other. Are combined and output to the terahertz wave detecting element 40. The combining unit 16 is preferably a pellicle that is a film-like mirror that is bonded to a rigid support frame and stretched thinly.

  The terahertz wave detection element 40 detects a correlation between the terahertz wave and the probe light. When the terahertz wave detecting element 40 includes an electro-optic crystal, the terahertz wave detecting element 40 receives the terahertz wave and the probe light output from the multiplexing unit 16, and birefringence is caused by the Pockels effect as the terahertz wave propagates. The probe light is induced and the polarization state of the probe light is changed by the birefringence, and the probe light is output. Since the amount of birefringence at this time depends on the electric field strength of the terahertz wave, the amount of change in the polarization state of the probe light in the terahertz wave detecting element 40 depends on the electric field strength of the terahertz wave.

  The polarization separation element 52 is, for example, a Wollaston prism, and receives the probe light output from the terahertz wave detection element 40 and passed through the quarter wavelength plate 51, and separates the input probe light into two orthogonal polarization components. And output. The photodetectors 53A and 53B include, for example, photodiodes, detect the powers of the two polarization components of the probe light polarized and separated by the polarization separation element 52, and differentiate the electric signals having values corresponding to the detected powers. Output to the dynamic amplifier 54.

  The differential amplifier 54 receives the electric signal output from each of the photodetectors 53A and 53B, and outputs an electric signal having a value corresponding to the difference between the two electric signals to the lock-in amplifier 55. The lock-in amplifier 55 synchronously detects the electrical signal output from the differential amplifier 54 at the repetition frequency of passing and blocking the pump light in the chopper 13. The signal output from the lock-in amplifier 55 has a value that depends on the electric field strength of the terahertz wave. In this manner, the correlation between the terahertz wave transmitted through the measurement object S and the probe light is detected, and the electric field amplitude of the terahertz wave is detected, so that information on the measurement object S can be obtained.

  In the terahertz wave measuring apparatus 1, the neutral density filter 30 is inserted on the optical path of the terahertz wave optical system from the terahertz wave generating element 20 to the multiplexing unit 16. The neutral density filter 30 may be inserted on the optical path between the terahertz wave generating element 20 and the measuring object S, or may be inserted on the optical path between the measuring object S and the multiplexing unit 16. Good. The neutral density filter 30 transmits a terahertz wave incident from one side to the other side with a certain transmittance, and can attenuate the terahertz wave.

  The terahertz wave measuring apparatus 1 operates as follows. The pulsed light output from the light source 11 is bifurcated by the branching unit 12 to be pump light and probe light. The pump light output from the branching unit 12 is sequentially reflected by the mirrors M <b> 1 to M <b> 3 and input to the terahertz wave generating element 20. The terahertz wave generating element 20 generates and outputs a terahertz wave in response to the input of pump light. The terahertz wave output from the terahertz wave generating element 20 passes through the measurement target portion S and is input to the multiplexing portion 16. At this time, the intensity of the terahertz wave input to the multiplexing unit 16 is adjusted by the neutral density filter 30. On the other hand, the probe light output from the branching unit 12 is sequentially reflected by the mirrors M <b> 4 to M <b> 8, is linearly polarized by the polarizer 15, and is input to the multiplexing unit 16.

  The terahertz wave and the probe light input to the multiplexing unit 16 are combined by the multiplexing unit 16 so as to be coaxial with each other, and input to the terahertz wave detecting element 40 at substantially the same timing. In the terahertz wave detecting element 40 to which the terahertz wave and the probe light are input, birefringence is induced as the terahertz wave propagates, and the polarization state of the probe light changes due to the birefringence. The polarization state of the probe light in the terahertz wave detection element 40 is detected by the quarter wavelength plate 51, the polarization separation element 52, the photodetector 53A, the photodetector 53B, the differential amplifier 54, and the lock-in amplifier 55. The In this way, a change in the polarization state of the probe light in the terahertz wave detection element 40 is detected, and consequently, the electric field amplitude of the terahertz wave is detected, and the characteristic of the measurement object S is obtained.

  The optical path length difference adjustment unit 14 adjusts the optical path length between the mirrors M4 and M7 and the mirrors M5 and M6, and adjusts the optical path length of the probe optical system, so that the terahertz wave input to the terahertz wave detecting element 40 is obtained. The timing difference between the probe light and the probe light is adjusted. As described above, the pulse width of the terahertz wave is generally about picoseconds, whereas the pulse width of the probe light is about femtoseconds, and the pulse width of the probe light 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 to the terahertz wave detecting element 40 by the optical path length difference adjusting unit 14.

  Further, the amount of change in the polarization state of the probe light in the terahertz wave detection element 40 is obtained by inserting the neutral density filter 30 on the optical path of the terahertz wave optical system and making the transmittance of the neutral density filter 30 various. The relationship between the output value of the lock-in amplifier 55 representing the value and the electric field amplitude of the terahertz wave represented by the value can be calibrated. Further, as will be described later, the neutral density filter 30 according to the present embodiment can keep the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance. is there. From this, the terahertz wave measuring apparatus 1 including the neutral density filter 30 can perform accurate terahertz wave measurement.

  In the terahertz wave measuring apparatus 1 shown in FIG. 1, the terahertz wave transmitted through the measurement object S is measured. However, the terahertz wave reflected by the measurement object S (including total reflection) is measured. Also good. Regardless of the transmission and reflection of the terahertz wave, the time waveform of the electric field amplitude of the pulsed terahertz wave is obtained in the same manner as described above, and the characteristics of the measuring object S are obtained.

  Next, an embodiment of the neutral density filter according to the present invention will be described. The neutral density filters 31 to 34 of the embodiments described below can be suitably used as the neutral density filter 30 included in the terahertz wave measuring apparatus 1 described above.

  FIG. 2 is a perspective view of the neutral density filter 31 according to the first embodiment. The neutral density filter 31 shown in this figure is provided with a plurality of through holes 312 penetrating between both main surfaces in a flat plate 311 capable of blocking terahertz waves and having a constant thickness. The entire flat plate 311 may be made of a material that blocks terahertz waves, or a constant thickness portion near the main surface of the flat plate 311 may be made of a material that blocks terahertz waves. Examples of the material that blocks terahertz waves include metals.

  In the neutral density filter 31, the region where the plurality of through holes 312 are formed is a transmission region that transmits the terahertz wave. Further, a region where the through hole 312 is not formed in the flat plate 311 is a blocking region that blocks the terahertz wave. The neutral density filter 31 can selectively emit a portion of the terahertz wave incident on one main surface that is incident on the through hole 312 from the other main surface.

  Further, in the neutral density filter 31, the transmission region (the region where the through hole 312 is formed) and the blocking region (the region where the through hole 312 is not formed) do not have a periodic structure at the wavelength of the terahertz wave. It is distributed as follows. Each of the plurality of through holes 312 may be randomly arranged with respect to any of the shape, size, and position. In the neutral density filter 31 shown in FIG. 2, each of the plurality of through holes 312 has the same shape and the same size, but is disposed at random positions, and thus has a periodic structure at the wavelength of the terahertz wave. It is supposed not to. Further, it is desirable that the neutral density filter 31 has a structure in which the transmission region or the cutoff region is smaller than the wavelength of the terahertz wave so that the intensity of the transmitted terahertz wave is spatially uniform.

  FIG. 3 is a diagram showing a time waveform of the electric field amplitude of the pulse terahertz wave that is transmitted through the neutral density filter 31 and emitted. Here, four types of neutral density filters 31 are prepared, and the ratio of the plurality of through holes 312 that are transmission regions in the flat plate 311 is 30%, 60%, 70%, and 90%. The time waveform of an amplitude is shown. Further, even when this ratio is 100% (that is, when the neutral density filter is not disposed), the time waveform of the electric field amplitude of the pulse terahertz wave is shown. The time waveform of the electric field amplitude of the pulse terahertz wave is measured in a state where the measurement object S is not inserted in the terahertz wave measuring apparatus 1 shown in FIG.

  As shown in this figure, the transmittance of the terahertz wave in the neutral density filter 31 is in accordance with the ratio of the plurality of through holes 312 that are transmission regions in the flat plate 311. Further, the time position of the peak of the pulse terahertz wave transmitted through the neutral density filter 31 is constant regardless of the transmittance.

  FIG. 4 is a diagram showing the wavelength dependence of the transmittance of the neutral density filter 31. Here, the wavelength dependence of the transmittance is shown for each of the three types of neutral density filters 31A, 31B, and 31C having different ratios occupied by the plurality of through holes 312 that are transmission regions in the flat plate 311. Among these, the neutral density filters 31 </ b> A and 31 </ b> B are those of the present embodiment in which the plurality of through holes 312 are arranged in the flat plate 311 so as not to have a periodic structure at the wavelength of the terahertz wave. On the other hand, the neutral density filter 31C is a comparative example in which a plurality of through holes 312 having the same shape are periodically arranged at regular intervals in a flat plate 311 in a square lattice pattern.

  As shown in this figure, in the neutral density filter 31C of the comparative example, the wavelength dependency of the transmittance is not flat, and the transmittance increases in a certain wavelength region. On the other hand, in the neutral density filters 31A and 31B of the present embodiment, the wavelength dependency of the transmittance is flat. This difference is considered to occur for the following reasons.

  That is, in the neutral density filter 31C of the comparative example, the plurality of through holes 312 are periodically arranged in a square lattice pattern at regular intervals in the flat plate 311, so that when a terahertz wave with a certain wavelength is incident, surface plasmon Polaritons are generated. In the neutral density filter 31C of the comparative example, due to the influence of the surface plasmon polariton, the wavelength dependence of the transmittance is not flat, and the transmittance increases in the wavelength region of the terahertz wave corresponding to the resonance frequency of the surface plasmon polariton. Conceivable.

  On the other hand, in the neutral density filters 31A and 31B of the present embodiment, the plurality of through holes 312 in the flat plate 311 have a terahertz wave wavelength so that the resonance frequency of the surface plasmon polariton does not appear strongly in the terahertz wave wavelength region. It arrange | positions so that it may not have a periodic structure, and, thereby, the wavelength dependence of the transmittance | permeability is flat.

  As described above, in the neutral density filter 31 according to the first embodiment, the transmission region (the region where the through-hole 312 is formed) and the blocking region (the region where the through-hole 312 is not formed) have a terahertz wave wavelength. The terahertz waves are transmitted with a transmittance corresponding to the ratio occupied by the transmission region. The neutral density filter 31 can keep the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

  FIG. 5 is a perspective view of the neutral density filter 32 according to the second embodiment. The neutral density filter 32 shown in this figure is capable of transmitting a terahertz wave and has a constant thickness, and a second flat plate 321 that can block a terahertz wave and has a constant thickness. A plurality of through-holes 322 penetrating between the two main surfaces are provided in the second flat plate 321.

  The second flat plate 321 in the second embodiment is the same as the flat plate 311 in the first embodiment. The plurality of through-holes 322 in the second embodiment are the same as the plurality of through-holes 312 in the first embodiment, and any one of shape, size, and position so as not to have a periodic structure at the wavelength of the terahertz wave. Arranged randomly. In the neutral density filter 32 shown in FIG. 5, each of the plurality of through holes 322 has the same shape and the same size, but is arranged at random positions, and thus has a periodic structure at the wavelength of the terahertz wave. It is supposed not to.

  Examples of the material of the first flat plate 320 that can transmit the terahertz wave include plastic, PTFE (polytetrafluoroethylene), paper, and an OHP sheet. The neutral density filter 32 can be manufactured, for example, by thermally transferring a metal material as the first flat plate 321 in a pattern having predetermined discrete openings on one main surface of the first flat plate 320. The discrete openings serve as through holes 322 that transmit the terahertz waves. For this thermal transfer, for example, a printer for a personal computer can be used.

  Here, when the first flat plate 320 is thin like paper or the like, paper or the like is attached to a support plate such as PTFE transparent to terahertz waves with an adhesive, and this attached is used as the first flat plate 320. Also good. The adhesive used at this time also needs to be transparent with terahertz waves. Alternatively, two support plates such as PTFE may be used, and paper or the like may be sandwiched between these two support plates. In this case, there is no need to use an adhesive, which is preferable.

  The neutral density filter 32 according to the second embodiment has a transmission region (a region where the through hole 322 is formed) and a blocking region (a region where the through hole 322 is not formed) having a periodic structure at the wavelength of the terahertz wave. The terahertz waves are transmitted with a transmittance corresponding to the ratio occupied by the transmission region. The neutral density filter 32 can keep the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

  FIG. 6 is a perspective view of the neutral density filter 33 according to the third embodiment. The neutral density filter 33 shown in this figure has a plurality of blocking materials 331 capable of blocking terahertz waves on one main surface of a flat plate 330 capable of transmitting terahertz waves and having a constant thickness. It is what was pasted together.

  The flat plate 330 in the third embodiment is the same as the first flat plate 320 in the second embodiment. Each of the plurality of blocking members 331 in the third embodiment has the same shape and the same size, but is arranged at random positions, and thereby has no periodic structure at the wavelength of the terahertz wave. Yes. The neutral density filter 33 can be manufactured, for example, by thermally transferring a metal material as a blocking material 331 in a predetermined discrete pattern onto one main surface of the flat plate 330.

  The neutral density filter 33 according to the third embodiment has a transmission region (a region where the blocking material 331 is not formed) and a blocking region (a region where the blocking material 331 is formed) having a periodic structure at the wavelength of the terahertz wave. The terahertz waves are transmitted with a transmittance corresponding to the ratio occupied by the transmission region. The neutral density filter 33 can keep the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

  FIG. 7 is a plan view of the neutral density filter 34 according to the fourth embodiment. The neutral density filter 34 shown in this figure is provided with a plurality of through holes 342 penetrating between both main surfaces in a flat plate 341 capable of blocking terahertz waves and having a constant thickness.

  The flat plate 341 in the fourth embodiment is the same as the flat plate 311 in the first embodiment. The plurality of through holes 342 in the fourth embodiment are randomly arranged with respect to any shape, size, and position.

  In the neutral density filter 34 according to the fourth embodiment, a transmission region (a region where the through hole 342 is formed) and a blocking region (a region where the through hole 342 is not formed) have a periodic structure at the wavelength of the terahertz wave. The terahertz waves are transmitted with a transmittance corresponding to the ratio occupied by the transmission region. The neutral density filter 34 can keep the optical path length constant even if the transmittance is changed, and can reduce the wavelength dependency of the transmittance.

It is a lineblock diagram of terahertz wave measuring device 1 concerning this embodiment. It is a perspective view of the neutral density filter 31 which concerns on 1st Embodiment. It is a figure which shows the time waveform of the electric field amplitude of the pulse terahertz wave which permeate | transmits and is radiate | emitted by the neutral density filter 31 which concerns on 1st Embodiment. It is a figure which shows the wavelength dependence of the transmittance | permeability of the neutral density filter 31 which concerns on 1st Embodiment. It is a perspective view of the neutral density filter 32 concerning a 2nd embodiment. It is a perspective view of the neutral density filter 33 concerning a 3rd embodiment. It is a top view of the neutral density filter 34 which concerns on 4th Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Terahertz wave measuring apparatus, 11 ... Light source, 12 ... Branch part, 13 ... Chopper, 14 ... Optical path length difference adjustment part, 15 ... Polarizer, 16 ... Multiplexing part, 20 ... Terahertz wave generating element, 30-34 ... Neutral density filter, 40 ... terahertz wave detection element, 51 ... 1/4 wavelength plate, 52 ... polarization separation element, 53A, 53B ... photodetector, 54 ... differential amplifier, 55 ... lock-in amplifier, M1-M8 ... mirror , S: Measurement object.

Claims (3)

The transmission region that transmits the terahertz wave and the blocking region that blocks the terahertz wave are distributed so as not to have a periodic structure at the wavelength of the terahertz wave, and the transmission region is randomly arranged in any shape and size. , neutral density filter for terahertz wave, wherein a terahertz wave incident from one side to the other, and transmits a transmission rate corresponding to the rate of the transmission region occupied. A light source that outputs light;
A branching unit that splits the light output from the light source into two, and outputs one of the two branched light as pump light and the other as probe light;
A terahertz wave generating element that generates and outputs a terahertz wave by inputting the pump light output from the branch unit; and
The terahertz wave output from the terahertz wave generating element and transmitted or reflected by the measurement object and the probe light output from the branching unit are input, and the terahertz for detecting the correlation between the terahertz wave and the probe light A wave detection element;
An optical path from the terahertz wave generating element to the terahertz wave detecting element, which is inserted into an optical path through which only the terahertz wave of the terahertz wave and the probe light propagates, and the incident terahertz wave is dimmed and transmitted. The neutral density filter according to claim 1, wherein
A terahertz wave measuring device comprising: A method for dimming terahertz waves,
The transmission region that transmits the terahertz wave and the blocking region that blocks the terahertz wave are distributed so as not to have a periodic structure at the wavelength of the terahertz wave, and the transmission region is randomly arranged in any shape and size . Using a neutral density filter
By transmitting the terahertz wave incident from one side of the neutral density filter to the other side at a transmittance according to the ratio of the transmission region, the terahertz wave is attenuated.
A terahertz wave attenuation method characterized by the above.

Images