JP2016164974A - Photoconductive element, method for manufacturing the same, and measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a terahertz wave pulse nearer to a monopulse than conventional ones, in a photoconductive element using surface plasmon resonance.SOLUTION: A photoconductive element includes: a photoconductive layer 110; and a plurality of conductive units 112 disposed on the photoconductive layer. Each of the plurality of conductive units includes: an electrode 103; an antenna unit 102 electrically connected with the electrode; and a diffraction grating unit 101 which is in contact with the antenna unit and includes a plurality of periodically arranged gratings 150. The plurality of conductive units are disposed to face each other with a gap therebetween, between the respective diffraction grating units of the plurality of conductive units. The current reflection coefficient in a contact portion 160 between the antenna unit and the diffraction grating unit is ±5% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a photoconductive element, a manufacturing method thereof, and a measuring apparatus using the same.

  Terahertz waves are electromagnetic waves having a frequency of 30 GHz to 30 THz. There is a photoconductive element as an element for generating or detecting terahertz waves. The photoconductive element is composed of a special semiconductor having relatively high mobility and a carrier lifetime of picoseconds or less, and an antenna electrode (conductive portion) that also serves as an electrode provided on the semiconductor. When ultrashort pulse light is applied to the gap between the antenna electrodes with a voltage applied to the antenna electrodes, a current flows instantaneously between the electrodes by the excited optical carriers, and a terahertz wave is emitted. It is a mechanism to do.

  As a means of improving the generation output and detection sensitivity of the photoconductive element, the generation efficiency of the optical carrier is increased by increasing the light absorption amount in the semiconductor by enhancing the electric field of the excitation light on the semiconductor surface in the gap portion between the antenna electrodes. There are ways to improve. Specifically, the diffraction grating is arranged between the antenna electrodes with a material having a negative dielectric constant at a period of submicron order. Irradiation of ultrashort pulse light onto the diffraction grating causes surface plasmon resonance and diffraction. As a result, the generation efficiency of optical carriers is improved by utilizing the electric field enhancement near the interface between the diffraction grating and the semiconductor and the electric field concentration by diffraction.

  For example, Patent Document 1 and Non-Patent Document 1 disclose a photoconductive element provided with a diffraction grating that generates surface plasmon resonance between bowtie antennas. A comparison is made between the ability to generate terahertz waves and the ability to detect terahertz waves with and without the diffraction grating, and it is described that the generation output and detection sensitivity are improved by providing the diffraction grating.

US2014 / 0346357

Nature Communications 4, 4, 1622, (2013)

  As a measuring apparatus that performs measurement using a photoconductive element, there is a THz-TDS apparatus that measures a time waveform of a terahertz wave pulse by THz-Time-Domain Spectroscopy (THz-TDS). When measuring with a THz-TDS apparatus using the photoconductive element described in Patent Document 1, a terahertz wave pulse is transmitted and reflected under the influence of the physical properties and shape of the measurement object. It becomes. Since the plurality of pulses are detected at different times according to the optical path length through which each pulse has propagated, the obtained time waveform has a plurality of pulses. By separating these plural pulses on the time waveform and analyzing each of them, information on the measurement object can be acquired. Therefore, when the terahertz wave pulse is detected as it is without passing through the measurement object, it is desirable that the time waveform of the terahertz wave pulse has only a single pulse. In other words, it is desirable to make the terahertz wave pulse a monopulse.

  On the other hand, for example, a terahertz wave generated by the photoconductive element of Non-Patent Document 1 may obtain a time waveform having a plurality of pulses.

  The present invention has been made in view of such a problem, and an object of the present invention is to obtain a terahertz wave pulse closer to a monopulse than a conventional one in a photoconductive element using surface plasmon resonance.

  A photoconductive element according to an aspect of the present invention includes a photoconductive layer and a plurality of conductive portions disposed on the photoconductive layer, and each of the plurality of conductive portions includes an electrode, An antenna part that is electrically connected to the electrode; and a diffraction grating part that is in contact with the antenna part and includes a plurality of gratings that are periodically arranged. The plurality of conductive parts are arranged so as to face each other with a gap between the diffraction grating parts, and a current reflection coefficient at a contact part between the antenna part and the diffraction grating part is ± 5% or less. It is characterized by being.

  In addition, a photoconductive element as another aspect of the present invention includes a photoconductive layer and a plurality of conductive portions disposed on the photoconductive layer, and each of the plurality of conductive portions includes: An electrode, an antenna part electrically connected to the electrode, a diffraction grating part having a plurality of periodically arranged gratings, and the antenna part and the diffraction grating part. A connecting portion in contact with the diffraction grating portion, and the plurality of conductive portions are arranged to face each other with a gap between the diffraction grating portions of the plurality of conductive portions. The current reflection coefficient at the contact portion between the connection portion and the diffraction grating portion is ± 5% or less.

  According to the photoconductive element as one aspect of the present invention, in the photoconductive element using surface plasmons, a terahertz wave pulse closer to a monopulse than before can be obtained.

The schematic diagram for demonstrating the structure of the photoconductive element of 1st Embodiment. The schematic diagram for demonstrating the structure of the photoconductive element of 2nd Embodiment. The schematic diagram for demonstrating the structure of the photoconductive element of 3rd Embodiment. The schematic diagram for demonstrating the structure of the photoconductive element of 4th Embodiment. The schematic diagram for demonstrating the structure of the photoconductive element of 5th Embodiment. The flowchart which shows the manufacturing method of the photoconductive element in 6th Embodiment. The schematic diagram for demonstrating the structure of the measuring apparatus of 7th Embodiment. The schematic diagram for demonstrating the structure of the photoconductive element used for calculation of the characteristic impedance in 1st Embodiment.

(First embodiment)
A photoconductive element 100 (hereinafter referred to as “element 100”) of the first embodiment will be described with reference to FIG. 1A is an overall configuration diagram of the element 100, FIG. 1B is a cross-sectional view taken along the line AA in FIG. 1A, and FIG. 1C is a cross-sectional view taken along the line BB in FIG. It is. The element 100 includes a photoconductive layer 110, a substrate 111, and a plurality of conductive portions (antenna electrodes) 112.

  The photoconductive layer 110 is a semiconductor that generates carriers when ultrashort pulse light as excitation light is incident thereon. The “ultra-short pulse light” in this specification is light having a pulse width of several hundred fs or less, and in particular, ultra-short pulse light having a pulse width of 1 fs to 100 fs is referred to as femtosecond pulse light. As a material for the photoconductive layer 110, a material whose wavelength corresponding to the band gap energy of the photoconductive layer 110 is shorter than the wavelength of the excitation light may be selected, and carrier excitation by one-photon absorption may be used. Further, even if the wavelength corresponding to the band gap energy is longer than the wavelength of the excitation light, the excitation of carriers by multiphoton absorption, which is one of the nonlinear optical effects, can be used. For example, low temperature growth (hereinafter referred to as LT) GaAs is used when the wavelength of the excitation light is in the 0.8 μm band, and LT-InGaAs or LT is used when the wavelength of the excitation light is in the 1.5 μm band. -GaAs can be used.

  The photoconductive layer 110 is disposed on the substrate 111. In the case where the photoconductive layer 110 is epitaxially grown on the substrate 111, a substrate that is lattice-matched with the photoconductive layer 110 to be used may be selected as the substrate 111. Of course, the method is not limited to the method of epitaxially growing the photoconductive layer 110, and a technique of bonding the photoconductive layer 110 and the substrate 111 by transfer or the like may be used.

  The plurality of conductive portions 112 are arranged on the photoconductive layer (on the photoconductive layer 110) so as to have a gap portion (gap) between the diffraction grating portions (between the diffraction grating portions 101) included in each conductive portion 112. Yes. Each of the plurality of conductive portions 112 includes a diffraction grating portion 101, an antenna portion 102, and a plurality of electrodes 103. The antenna unit 102 electrically connects the diffraction grating unit 101 and the electrode 103. Each antenna portion 102 has a convex portion, and the convex portions are opposed to each other with an arbitrary interval. The antenna portion 102 has a function as an electrode, and a voltage can be applied between the convex portions of the antenna portion 102.

  The electrode 103 is electrically connected to the antenna unit 102 and is connected to a signal line (not shown). As a material for each of the antenna portion 102 and the electrode 103, it is desirable to use a metal that has high electrical conductivity and is not easily oxidized. Specifically, gold (Au) or the like is used. The shape of the antenna unit 102 is determined in consideration of the frequency and output of the terahertz wave to be generated or detected. In the present embodiment, a dipole antenna capable of generating and detecting terahertz over a wide band is taken as an example.

  As shown in FIG. 1B, the element 100 further includes a diffraction grating portion 101 at the tip of each convex portion of the two antenna portions 102. The diffraction grating unit 101 improves the light absorption efficiency of the excitation light in the photoconductive layer 110 by the surface plasmon resonance effect and the diffraction effect. The diffraction grating unit 101 includes a plurality of gratings 150 made of a material having high electrical conductivity.

  Each of the plurality of lattices 150 is a rectangular parallelepiped lattice. A plurality of adjacent gratings 150 are periodically arranged at a certain interval. In other words, in the diffraction grating portion 101, the conductive member disposed on the photoconductive layer 110 has a plurality of grooves, and the plurality of grooves are periodically formed.

  The gap between the diffraction grating portions 101 of each conductive portion 112 is preferably determined in consideration of the withstand voltage of the photoconductive layer 110. For example, if the photoconductive layer 110 is LT-GaAs, the gap between the diffraction grating portions 101 may be about 1.0 μm or more and several tens of μm or less. For example, in order to ensure a withstand voltage of about 100 V, the gap portion may be provided with a distance of about 10 μm.

  The area of each of the two diffraction grating portions 101 disposed so as to be opposed to each other between the two antenna portions 102 only needs to be more than the irradiation region of the excitation light irradiated in both the AA direction and the BB direction. Similarly to the antenna unit 102 and the electrode 103, it is desirable to use a material having high conductivity for the diffraction grating unit 101. Specifically, Au or the like is used.

  An antireflection film layer (not shown) that suppresses reflection of excitation light may be provided in a region including the photoconductive layer 110 and the diffraction grating portion 101 in the gap portion.

From here, the conditions under which surface plasmon resonance is excited will be described. Surface plasmon resonance can be excited most efficiently when the polarization direction of the excitation light is parallel to the longitudinal direction of the antenna portion 102 (the BB linear direction in FIG. 1A). The condition for surface plasmon resonance is expressed by equation (1). It is assumed that the speed of light is c, the wavelength of the excitation light is λ, the angular frequency of the excitation light is ω, and the incident angle of the excitation light with respect to the element 100 is θ. In addition, the real part of the dielectric constant of the diffraction grating part 101 at the wavelength of the excitation light is ε _g , the period of the diffraction grating part 101 is L, the area of the substance in contact with the diffraction grating part 101 and the effective complex dielectric constant determined by the complex dielectric constant. _Let ε _{p be} the real part of the rate.

  In the formula (1), m is a positive integer. Further, for simplification, when the incident angle is 0 °, equation (2) is obtained.

  In order to excite surface plasmon resonance, the real part of the dielectric constant of the material of the diffraction grating part 101 takes a negative value, and the absolute value is determined by the area of the substance in contact with the diffraction grating part 101 and the complex dielectric constant. Must not be higher than the effective complex permittivity. From the above, the diffraction grating unit 101 includes a material having a negative dielectric constant. Specifically, the diffraction grating unit 101 includes silver (Ag), gold (Au), copper (Cu), platinum (Pt), aluminum (Al), and the like as materials. Note that a part of the diffraction grating portion 101 may include a material having a positive dielectric constant, the real part of the dielectric constant of the diffraction grating portion 101 as a whole is negative, and the absolute value thereof is the diffraction grating portion 101. It should be higher than the effective complex dielectric constant determined by the area and the complex dielectric constant of the substance in contact with.

  In addition, the period L of the diffraction grating portion 101 that enables surface plasmon resonance includes the wavelength of the excitation light, the real part of the dielectric constant of the material of the grating 150 of the diffraction grating portion 101, the area of the substance in contact with the diffraction grating portion 101, and the complex It is uniquely determined by the effective complex dielectric constant determined by the dielectric constant.

  As an example, a case where the incident angle of the excitation light to the element 100 is 0 degree (0 °), LT-GaAs is selected as the photoconductive layer 110, and Au is selected as the material of the diffraction grating portion 101 will be described. In such a configuration, when the wavelength of the excitation light is 0.8 μm, the period L of the diffraction grating portion 101 where the surface plasmon resonance is excited is about 200 nm. When the wavelength of the excitation light is 1.55 μm, the period L of the diffraction grating portion 101 in which the surface plasmon resonance is excited is about 700 nm. Note that the diffraction grating unit 101 includes a plurality of gratings 150 having a constant period that can excite surface plasmon resonance with respect to excitation light. Therefore, it is desirable that the period L of the diffraction grating portion 101 is equal to the period in which the gratings 150 are arranged. Further, when the wavelength of the excitation light is broadband as in femtosecond pulsed light, it is desirable to set the period of the diffraction grating unit 101 in accordance with the wavelength with the highest output.

  The thickness of each grating 150 of the diffraction grating portion 101 is preferably about 1/10 of the wavelength of the excitation light, but may be at least 1/3 or less of the wavelength of the excitation light. If it becomes thicker than this, reflection of excitation light at the diffraction grating portion 101 becomes large, and efficient surface plasmon resonance becomes difficult. When an antireflection film is provided, the thickness of each of the gratings 150 may be 1 / 3n or less of the wavelength of the excitation light in consideration of the refractive index n of the antireflection film at the wavelength of the excitation light. .

  In addition to the dipole antenna shown in FIG. 1, the antenna unit 102 may use a stripline, bowtie, spiral antenna, or the like depending on the application.

  The inventor has found that in the prior art, there are a plurality of pulses on the time waveform, and one of the causes of the distortion of the shape is in the contact portion (connection surface) 160 between the grating 150 of the diffraction grating portion and the antenna portion 102. It was found that there was a mismatch in characteristic impedance. If there is a mismatch in characteristic impedance at the contact portion 160, the photocurrent flowing from the diffraction grating portion 101 is reflected and a reflected wave is generated, which may induce resonance or loss of terahertz waves having a frequency different from the desired frequency. . Thereby, a terahertz wave pulse having a plurality of peaks may be obtained, or the pulse shape may be distorted. In this embodiment, the characteristic impedance mismatch is reduced, and the reflected wave of the photocurrent at the contact portion 160 is reduced.

  The relationship between the difference in characteristic impedance and the reflected wave of the photocurrent when a surface having mismatched characteristic impedance exists between the diffraction grating unit 101 and the antenna unit 102 will be described. In this embodiment, since the diffraction grating unit 101 and the antenna unit 102 are in contact with each other and electrically connected, the difference in characteristic impedance at the contact part 160 between the diffraction grating unit 101 and the antenna unit 102 and the reflection of photocurrent. Describe the relationship of waves.

The plurality of gratings 150 of the diffraction grating unit 101 can be regarded as a plurality of parallel and periodically arranged transmission lines. Therefore, it can be considered that the diffraction grating part 101 having a plurality of parallel transmission lines (grating 150) forms a contact part 160 and a single transmission line called the antenna part 102 and is electrically connected. In this case, if the characteristic impedance of the entire diffraction grating part 101 is Z ₁ and the characteristic impedance of the antenna part 102 is Z ₂ , the current reflection coefficient R at the contact part 160 can be expressed by equation (3). In the expression (3), the direction from the antenna unit 102 toward the diffraction grating unit 101 is positive.

  In this specification, the “current reflection coefficient” means that an incident wave directed from the diffraction grating portion 101 to the antenna portion 102 is reflected toward the diffraction grating portion 101 at the contact portion of two members that are in contact and electrically connected. This is defined as the ratio of the amplitude of the reflected wave to the amplitude of the incident wave when the reflected wave is generated. This reflected wave is generated at the contact portion because the characteristic impedances of the two members are different.

  In the present specification, of the two members described above, the member on the antenna unit 102 side is the first member, and the member on the diffraction grating unit 101 side is the second member. The ratio of the amplitude of the reflected wave generated at the contact portion between the first member and the second member with respect to the amplitude of the incident wave is referred to as a current reflection coefficient at the contact portion between the first member and the second member. Moreover, although it may only call the electric current reflection coefficient of a 1st member, these are synonymous.

  In the case of this embodiment, the ratio of the amplitude of the reflected wave reflected toward the diffraction grating portion 101 by the contact portion 160 with respect to the amplitude of the incident wave from the diffraction grating portion 101 toward the antenna portion 102 is expressed as “the diffraction grating portion 101 and the antenna portion. The current reflection coefficient at the contact portion 160 with the contact 102 ”. The “current reflection coefficient at the contact portion 160 between the diffraction grating portion 101 and the antenna portion 102” may be referred to as “the current reflection coefficient at the contact portion 160” or “the current reflection coefficient at the contact portion 160”. In the present specification, the contact portion 160 refers to a contact portion between the entire diffraction grating portion 101 and the antenna portion 102, and the diffraction grating portion 101 is a surface where each of the plurality of gratings 150 and the antenna portion 102 are in contact with each other. Including.

  When the diffraction grating unit 101 and the antenna unit 102 are regarded as transmission lines, the respective characteristic impedances are mainly the frequency of the incident wave, the width, thickness, and arrangement of the diffraction grating unit 101 and the antenna unit 102. , And the dielectric constant of the photoconductive layer 110. Therefore, in this embodiment, the reflected wave of the photocurrent at the contact portion 160 is reduced by adjusting the width and thickness of the grating 150 in the diffraction grating portion 101 and the width and thickness of the antenna portion 102.

  Here, as an example, it is assumed that a time waveform of a terahertz wave pulse (reflected pulse) reflected by a measurement object is measured and the refractive index of the measurement object is acquired using the time waveform. In this case, consider how much the reflection of the photocurrent at the contact portion between the diffraction grating portion 101 and the antenna portion 102 affects the refractive index obtained by the measurement. The refractive index of the measurement object is obtained with reference to an amplitude spectrum obtained from a time waveform of a reflected pulse from a metal mirror having a reflectance of about 100%. If the reflectance is obtained from the ratio between the amplitude spectrum and the amplitude spectrum obtained from the time waveform of the reflected pulse from the measurement object, the refractive index can be obtained from the reflectance.

  In a reflective measurement system that measures a reflected pulse, the measured value of a reflected pulse from a measurement object is generally very small compared to the measured value of a reflected pulse from a metal mirror as a reference. Therefore, even if the reflected wave of the photocurrent can be confirmed in the time waveform of the reflected pulse from the metal mirror, the reflected wave signal of the photocurrent cannot be confirmed in the time waveform of the reflected pulse from the measurement object because it is below the noise. There is a case.

  If the terahertz wave is radiated or detected from the element 100 without loss due to the reflected wave of the photocurrent, when the refractive index of the measured object is about 1.5, the refractive index of the current reflection coefficient is ± 5%. The maximum measurement error is about 0.03. Similarly, when the current reflection coefficient R is ± 1%, the measurement error of the refractive index is 0.01 or less at maximum.

  The smaller the current reflection coefficient R, that is, the closer the characteristic impedance value of the diffraction grating unit 101 and the characteristic impedance value of the antenna unit 102 are, the smaller the reflected wave of the photocurrent contained in the acquired time waveform of the reflected pulse. . For this reason, it is desirable to make the current reflection coefficient R at the contact portion 160 as small as possible. Therefore, in order to reduce the measurement error to 0.03 or less, the element 100 desirably has a current reflection coefficient R at the contact portion 160 of ± 5% or less, and more preferably ± 1% or less. Configure.

  In the present embodiment, an example of a configuration in which the current reflection coefficient R of the antenna unit 102 satisfies ± 5% or less when the diffraction grating unit 101 and the antenna unit 102 are regarded as transmission lines is shown. In the present embodiment, it is assumed that the diffraction grating portion 101 and the antenna portion 102 have the same thickness as shown in FIG.

  Each of the diffraction grating unit 101 and the antenna unit 102 of the present embodiment is arranged on a GaAs substrate having a thickness of 500 μm. Note that the back surface of the GaAs substrate is grounded. The period L of the diffraction grating part 101 was fixed at 700 nm, the characteristic impedances of the diffraction grating part 101 and the antenna part 102 at 30 GHz were calculated independently, and the current reflection coefficient R of the antenna part 102 was calculated therefrom. The characteristic impedance of the diffraction grating unit 101 is a grounded coplanar line model, and the antenna unit 102 is a grounded microstrip line model.

  The characteristic impedance tends to increase as the width of the grating 150 is reduced, in other words, the distance between the gratings 150 increases. For example, when the width of the grating 150 is about 350 nm and the distance between the gratings 150 is also about 350 nm, the characteristic impedance of the diffraction grating portion 101 is about 60 [Ω]. On the other hand, when the width of the grating 150 is reduced to about 100 nm, the distance between the gratings 150 becomes about 600 nm, and the characteristic impedance increases to about 90 [Ω]. On the other hand, when the width of the antenna portion 102 is about 50 μm, the characteristic impedance of the antenna portion 102 is about 90 [Ω], whereas when the width of the antenna portion 102 is increased to 180 μm, the characteristic impedance is 60 [Ω. ] To a degree.

  Therefore, in order to satisfy the condition that the current reflection coefficient R of the antenna unit 102 is ± 5% or less, when the width of the grating 150 is 350 nm, the width of the antenna unit 102 is in a range of 170 μm or more and 290 μm or less. You can see that In addition, when the width of the grating 150 is reduced to 100 nm, the current reflection coefficient R of the antenna unit 102 can be reduced to ± 5% or less if the width of the antenna unit is set in a range of 30 μm to 70 μm. As described above, the current reflection coefficient R can be adjusted by adjusting the width of the grating 150 and the width of the antenna section 102 of the diffraction grating section 101, so that the current reflection coefficient R of the antenna section 102 can be ± 5% or less. .

  By adopting such a configuration, it is possible to reduce the proportion of components derived from the reflected wave of the photocurrent mixed in the amplitude spectrum obtained by Fourier transforming the time waveform of the reflected pulse using the measuring device including the element 100. Conceivable.

  Note that the present invention includes a case where a plurality of contact portions having different characteristic impedances are included. In other words, the diffraction grating unit 101 and the antenna unit 102 are not in contact with each other, and the diffraction grating unit 101 and the antenna unit 102 are connected to each other through another structure having a characteristic impedance different from that of the diffraction grating unit 101 or the antenna unit 102. A configuration in which is electrically connected is also included in the present invention. Another structure is, for example, a connection part that is disposed between the diffraction grating part 101 and the antenna part 102 and electrically connects the diffraction grating part 101 and the antenna part 102. As a specific example of the connection part, there is an extension part for extending the width of the grating 150 toward the antenna part 102.

  In this case, since there are a plurality of contact portions between the diffraction grating portion 101 and the antenna portion 102, impedance mismatch occurs at each contact portion, and a reflected wave is generated. Therefore, it is desirable to configure so that the current reflection coefficient R at each contact portion falls within the above range. That is, if the current reflection coefficient R at each contact portion is configured to fall within the above-described range, a configuration having a contact portion with different characteristic impedance between the diffraction grating portion 101 and the antenna portion 102 is also possible. include. Details of such a configuration will be described in a second embodiment.

  The characteristic impedance of the diffraction grating unit 101 can be estimated by calculation. As a method for calculating the characteristic impedance of the diffraction grating portion 101, a method for calculating the diffraction grating portion 101 as a uniform resistor will be described. Specifically, for example, each of the diffraction grating unit 101 and the antenna unit 102 is regarded as a structure that is sufficiently small with respect to the wavelength of the terahertz wave. Hereinafter, a calculation method assuming that each of the diffraction grating unit 101 and the antenna unit 102 is a resistor will be described with reference to FIG.

  Here, as an example, a photoconductive element 800 as shown in FIG. 8 is considered. The photoconductive element 800 includes a diffraction grating portion 801 and an antenna portion 102. The material of the diffraction grating unit 801 is the same as that of the diffraction grating unit 101. The diffraction grating unit 801 can also consider each of the gratings 150 in the diffraction grating unit 101 as a resistor and a lumped constant circuit in which a plurality of them are arranged in parallel.

  The ratio of the width of the grating 150 of the diffraction grating portion 801 to the interval between the gratings 150 is 1: 1, and the period L is 700 nm. In the diffraction grating section 801, it is assumed that the length of the grating 150 is 3 μm, the thickness is 100 nm, and five gratings 150 are periodically arranged. The material of the lattice 150 is Au. At this time, if the resistivity of the grating 150 is known, it can be regarded as a resistor having a plurality of gratings 150 in parallel, and the resistance of the diffraction grating unit 801 as a whole, that is, the characteristic impedance can be estimated from these resistance values. When Au is a thin film of about 100 to 300 nm, the resistivity is about 0.03 Ω · μm. That is, the resistance of the grating 150 is 2.57Ω, and the resistance of the entire diffraction grating portion is about 0.5Ω. Further, when the thickness of the antenna portion 102 is 300 nm, the width is 10 μm, and the length of the convex portion of the antenna portion 102 is 20 μm, the resistance value of the antenna portion 102 is about 0.2Ω.

  Based on such a calculation result, the structures of the diffraction grating portion 801 and the antenna portion 102 are determined so that the current reflection coefficient R at the contact portion 160 is ± 5% or less. A method of determining the structure of the diffraction grating portion 801 and the antenna portion 102 based on the calculation result and the measurement result obtained by actually measuring the characteristic impedance of the diffraction grating portion 801 and the antenna portion 102 is also effective. That is, the design of the element 100 does not necessarily need to use the above-described characteristic impedance calculation method assuming the transmission line. The calculation method in which the diffraction grating unit 101 is regarded as a lumped constant circuit, or actually measured measurement is used. Results etc. can be used.

  The plurality of electrodes 103 are connected to an external device via a signal line (not shown) in order to apply a voltage between the antenna units 102 or measure a current flowing between the antenna units 102. For electrical connection with a signal line, a metal wire used also as a signal line is often melted and joined to the electrode 103 by wire bonding or the like. FIG. 1A shows an example in which there are four electrodes 103. However, if two electrodes 103 and a signal line are connected so that a voltage can be applied to the gap or a current in the gap can be detected. Good.

  It is assumed that the diffraction grating unit 101 and the antenna unit 102 are formed of the same material and the same thickness. In this case, the melting point and thermal conductivity of the metal wire material and the diffraction grating portion 101 and the antenna portion 102 are different, the compatibility of the bonding is poor due to factors such as surface oxidation, or the thickness of the electrode 103 necessary for the bonding is small. It may be insufficient. In such a case, as shown in FIG. 1C, it is desirable to form the electrode 103 having a thickness necessary for bonding on the material of the antenna portion 102 using a material that can be easily bonded to a signal line (not shown). Due to the difference in thickness between the antenna portion 102 and the electrode 103, a photocurrent generated in the gap portion may flow to the electrode 103 to generate a reflected wave. Therefore, it is preferable that the propagation distance (transmission line length) of the photocurrent from the center of the gap portion to the electrode 103 is 1.5 mm or more. The reason will be described.

The effective relative dielectric constant determined by the speed of light c, the time from when the photocurrent is generated until the reflected wave is generated at the electrode 103 and returning to the gap, t, the structure of the photoconductive layer 110 and the conductive portion 112 and the frequency of the terahertz wave If the rate is ε _r , the transmission line length d is expressed by equation (4).

Here, for example, when LT-GaAs is used for the photoconductive layer 110, the effective relative permittivity ε _r is a value of about 9 or more although it depends on the structure such as the width and thickness of the antenna portion 102 and the frequency in the terahertz region. I take the. Therefore, if the transmission line length d is 1.5 mm or more, after the terahertz wave is generated or detected, the reflected wave of the photocurrent generated at the electrode 103 returns to the gap portion, and the reflected wave is generated or detected. The time difference until is 30 ps or more.

  If the time difference is 30 ps or more, it is a sufficiently long delay time on the time waveform when the form and physical properties of the measurement object are evaluated by the THz-TDS apparatus. For example, when the transmission line length d is 1.5 mm, the reflected wave of the photocurrent generated at the electrode 103 returns to the gap portion after the terahertz wave is generated or detected until the reflected wave is generated or detected. Is about 30 ps. Therefore, even if the time waveform including the reflected wave is Fourier transformed to obtain the frequency spectrum, the resonance frequency is around 15 GHz, and if the frequency resolution is about 30 GHz, the measurement result and the analysis result are not affected. It is done.

  From the above, the reflected wave of the photocurrent matches the characteristic impedance of the diffraction grating part 101 and the characteristic impedance of the antenna part 102, and further determines the transmission line length between the electrode 103 and the diffraction grating part 101 as a measurement result. It can be reduced by making it long enough not to affect it.

  As described above, according to the element 100 of this embodiment, the reflected wave of the photocurrent at the contact portion between the diffraction grating portion 101 and the antenna portion 102 can be reduced. As a result, in the time waveform of the terahertz wave generated by the element 100 or the terahertz wave detected by the element 100, it is possible to reduce the influence of the reflected wave at the contact portion and bring it closer to a monopulse than before. That is, in a photoconductive element using surface plasmon resonance, a terahertz wave pulse closer to a monopulse than before can be obtained.

(Second Embodiment)
A photoconductive element 200 (hereinafter referred to as “element 200”) of the second embodiment will be described with reference to FIG. FIG. 2 is a schematic diagram for explaining the configuration of the element 200. In the element 100 of the first embodiment, as shown in FIG. 1A, the diffraction grating portion 101 and the antenna portion 102 are arranged in contact with each other, and are directly electrically connected. On the other hand, the conductive portion 212 of the element 200 of the present embodiment has a connection portion 104 between the diffraction grating portion 101 and the antenna portion 102. That is, the diffraction grating unit 101 is not disposed in contact with the antenna unit 102. In addition, the same code | symbol is attached | subjected to the structure similar to 1st Embodiment, and description is abbreviate | omitted about the structure similar to 1st Embodiment.

  In the configuration in which the diffraction grating unit 101 and the antenna unit 102 are connected in contact as in the first embodiment, the reflected wave of the photocurrent can be reduced by adjusting the structures of the diffraction grating unit 101 and the antenna unit 102. It may not be easy. In such a case, as in this embodiment, another configuration such as the connection unit 104 is provided between the diffraction grating unit 101 and the antenna unit 102 to increase the number of connection points having different characteristic impedances. With such a configuration, it is possible to disperse the impedance mismatch points and reduce the reflected wave of the photocurrent at each connection point.

  The connection unit 104 is an extension unit that extends the line width from the width of the grating 150 toward the antenna unit 102. The connecting portion 104 is disposed between the diffraction grating portion 101 and the antenna portion 102, and the line width is changed from the grating 150 to the antenna portion 102 within a range larger than the width of the grating 150 and smaller than the width of the antenna portion 102. Expand toward multiple times.

  Specifically, the connecting portion 104 has a plurality of extending portions, and as shown in FIG. 2, two adjacent lattices 150 are connected in contact with a first extending portion that is wider than the lattice 150. . Further, two adjacent first extension portions are connected in contact with a second extension portion that is wider than the first extension portion. Two adjacent second extension portions are connected in contact with a third extension portion that is wider than the second extension portion. The connection part 104 is finally connected to the antenna part 102 by arranging the third extension part in contact with the antenna part 102.

  As described above, the element 200 electrically connects the grid 150 and the antenna unit 102 via the connection unit 104. And the connection part 104 of this embodiment expands a line | wire width in several steps so that the impedance of the antenna part 102 may approach from the impedance of the diffraction grating part 101. FIG.

  The connection part 104 of the element 200 includes a first contact part 170 in contact with the diffraction grating part 101, a second contact part 171 and a third contact part 172 in which lines of different line widths contact in the connection part 104, and And a fourth contact portion 173 where the connection portion 104 and the antenna portion 102 are in contact with each other. The connection section 104 extends the line width of each grid 150 in multiple times from the width of each grid 150, and finally connects to the antenna section 102, thereby mismatching the characteristic impedance at the section where the line width is expanded. Disperses the reflected wave of photocurrent due to. As a result, the magnitude of the reflected wave of the photocurrent due to the characteristic impedance mismatch at each of the contact portions 170 to 173, that is, the current reflection coefficient at the contact portion can be reduced.

  Depending on the structure of the diffraction grating part 101 and the antenna part 102 and various restrictions such as the line width of the processing limit of the connection part 104, the extension width per extension number in the connection part 104 should be as small as possible. Is good. Specifically, it is desirable to configure the connecting portion 104 so that the current reflection coefficient in each of the contact portions 170 to 173 is ± 5% or less, more preferably ± 1% or less.

  Here, an example of a specific configuration of the element 200 will be described. The element 200 is configured such that the current reflection coefficient at each of the contact portions 170 to 173 is ± 5% or less.

  The element 200 assumes a case where the thickness of the diffraction grating portion 101 and the thickness of the antenna portion 102 are the same as in the first embodiment. Further, it is assumed that the diffraction grating portion 101 and the antenna portion 102 are arranged on a GaAs substrate having a thickness of 500 μm whose back surface is grounded. The period L of the grating 150 of the diffraction grating unit 101 is 700 nm, and the width of each of the plurality of gratings 150 is fixed at 350 nm.

  In this case, the characteristic impedance of the diffraction grating portion 101 was about 56 [Ω]. If the portion (first extension portion) between the first contact portion 170 and the second contact portion 171 in the connection portion 104 has a periodic structure having a width of about 600 nm with a period of 1400 nm, the characteristic impedance is It will be about 61 [Ω]. Further, when the portion (second extension portion) between the second contact portion 171 and the third contact portion 172 of the connection portion 104 has a periodic structure having a width of about 1000 nm with a 2800 nm period, the characteristic impedance is It will be about 67 [Ω]. Furthermore, if the portion (third extended portion) between the third contact portion 172 and the fourth contact portion 173 of the connecting portion 104 has a periodic structure having a width of about 1600 nm with a period of 5600 nm, the characteristic impedance is It becomes about 73 [Ω].

  Therefore, all of the current reflection coefficients in each of the plurality of contact portions 170 to 173 can be ± 5% or less. In addition, by setting the width of the antenna unit 102 within the range of 80 μm to 140 μm, the characteristic impedance is within the range of 65 to 80 [Ω], and therefore the fourth contact portion between the antenna unit 102 and the connection unit 104. The current reflection coefficient at 173 can also be ± 5% or less.

  In the first embodiment described above, when the period of the diffraction grating portion 101 is 700 nm and the width of the grating 150 is 350 nm, the width of the antenna portion having a current reflection coefficient of ± 5% or less is within a range of 170 μm to 290 μm. Met. On the other hand, in the second embodiment, by providing the connection portion 104, the width of the antenna portion 102 having a current reflection coefficient of ± 5% or less can be reduced within a range of 80 μm to 140 μm. That is, by providing the connection portion 104, the degree of freedom of the structure such as the width of the grating 150 of the diffraction grating portion 101 and the width and thickness of the antenna portion 102 can be improved.

  With such a configuration, it is possible to reduce the mismatch of characteristic impedance in each of the contact portions 170 to 173 and reduce the reflected wave of the photocurrent.

  As described above, according to the element 200 of the present embodiment, the reflected wave of the photocurrent at the contact portion between the diffraction grating portion 101 and the antenna portion 102 can be reduced. In addition, the time waveform of the terahertz wave pulse generated by the element 200 and the terahertz wave pulse detected by the element 200 can be made closer to a monopulse than before. That is, in a photoconductive element using surface plasmon resonance, a terahertz wave pulse closer to a monopulse than before can be obtained.

  Further, by providing the connecting portion 104 between the diffraction grating portion 101 and the antenna portion 102, the reflected wave of the photocurrent is dispersed in the region of the tapered portion 105, so that the magnitude of the reflected wave at each contact portion is increased. That is, the current reflection coefficient can be further reduced. Furthermore, the degree of freedom of the structure such as the width of the grating 150 of the diffraction grating portion 101 and the width and thickness of the antenna portion 102 can be improved.

(Third embodiment)
A photoconductive element 300 (hereinafter referred to as “element 300”) according to a third embodiment will be described with reference to FIG. Descriptions of parts common to the above description are omitted. 3A is a diagram illustrating the configuration of the element 300, FIG. 3B is a cross-sectional view taken along line AA in FIG. 3A, and FIG. 3C is a diffraction grating portion 101 in FIG. 3A. It is an enlarged view of the vicinity.

  As shown in FIGS. 3B and 3C, the conductive portion 312 of the element 300 further includes a tapered portion 105 as a connecting portion between the diffraction grating portion 101 and the antenna portion 102. The diffraction grating part 101 and the antenna part 102 are electrically connected via a tapered part 105. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  The taper part 105 is an extension part that extends the line width from the width of the grating 150 toward the antenna part 102. The taper portion 105 has a taper structure configured such that the thickness and width gradually increase from the diffraction grating portion 101 toward the antenna portion 102. The tapered portion 105 is disposed between the diffraction grating portion 101 and the antenna portion 102. Therefore, the taper part 105 has a contact part 180 with the diffraction grating part 101 and a contact part 181 with the antenna part 102. The characteristic impedance in the taper part 105 changes continuously.

  In this case, the current reflection coefficient of the antenna portion 102 (current reflection coefficient at the interface between the antenna portion 102 and the tapered portion 105) is ± 5% or less, and more preferably ± 1% or less. In addition, the current reflection coefficient of the taper portion 105 (current reflection coefficient at the interface between the taper portion 105 and the diffraction grating portion 101) is ± 5% or less, more preferably ± 1% or less.

  The reflected wave of the photocurrent in the taper part 105 is reflected from the reflected wave of the photocurrent due to impedance mismatching compared to the case where the thickness and width are stepped as in the first and second embodiments described above. 105 can be distributed throughout. That is, the taper portion 105 can be regarded as an infinite number of transmission lines having different impedances electrically connected thereto. Therefore, by dispersing the reflected wave of the photocurrent generated between the diffraction grating part 101 and the antenna part 102 in the region of the taper part 105, the magnitude of the reflected wave at each connection surface, that is, the current reflection coefficient is further increased. Can be reduced. In addition, it is better to make the inclination of the taper portion in the thickness direction and the width direction as gentle as possible, and it is preferably in the range of 1 ° to 45 °, but is not limited to this range.

  As described above, according to the element 300, the reflected wave of the photocurrent generated by the difference in characteristic impedance between the diffraction grating unit 101 and the antenna unit 102 can be reduced. Then, the time waveform of the terahertz wave pulse generated by the element 300 and the terahertz wave pulse detected by the element 300 can be made closer to a monopulse than before. That is, in a photoconductive element using surface plasmon resonance, a terahertz wave pulse closer to a monopulse than before can be obtained.

  In addition, by using a tapered portion having a tapered structure as a connection portion, the reflected wave of the photocurrent is dispersed in the region of the tapered portion 105, so that the magnitude of the reflected wave at each contact portion, that is, the current reflection coefficient can be reduced. It can be further reduced. Furthermore, as in the second embodiment, the degree of freedom of the structure such as the width of the grating 150 of the diffraction grating unit 101 and the width and thickness of the antenna unit 102 can be improved.

(Fourth embodiment)
A photoconductive element 400 (hereinafter referred to as “element 400”) of the fourth embodiment will be described with reference to FIG. Descriptions of parts common to the above description are omitted. FIG. 4 is a diagram for explaining the configuration of the element 400. The conductive portion 412 of the element 400 has an arc portion 106 in which a part of the antenna portion 402 has an arc shape. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  When the electrode 103 is moved 1.5 mm or more away from the center of the gap portion between the antennas, the simplest configuration is that the antenna portion 102 and the electrode 103 are rectangular as shown in FIGS. 1 (a) and 3 (a). . In this case, the length of the long side portion of the rectangle may hinder miniaturization of the photoconductive element. In addition, since there is a portion with a right angle or the like between the antenna unit 102 and the electrode 103, a reflected wave of photocurrent is generated in the angled portion, and resonance of the terahertz wave, loss of detection current, etc. There is a risk of triggering. The element 400 includes the arc portion 106 to form an antenna portion 402 that does not have an angled portion, and at the same time, the antenna portion 402 realizes a reduction in size of the element 400.

  Even in this case, similarly to the first embodiment, it is preferable that the propagation length (transmission line length) of the photocurrent from the gap portion between the antennas to the electrode 103 is 1.5 mm or more. Note that the radius of curvature of the arc portion 106 may be equal to or greater than the width of the long side portion of the antenna portion 402, and can be formed in the range of several μm to several mm. Further, there may be a plurality of arc portions 106 between one electrode 103 and the diffraction grating portion 101.

  As described above, according to the element 400, the reflected wave of the photocurrent generated by the difference in characteristic impedance between the diffraction grating unit 101 and the antenna unit 402 can be reduced. Then, the time waveform of the terahertz wave pulse generated by the element 400 and the terahertz wave pulse detected by the element 400 can be made closer to a monopulse than before. That is, in a photoconductive element using surface plasmon resonance, a terahertz wave pulse closer to a monopulse than before can be obtained. Further, by forming the arc portion 106 in a part of the antenna portion 102, the element 400 can be downsized while the transmission line length is 1.5 mm or more.

(Fifth embodiment)
A photoconductive element 500 (hereinafter referred to as “element 500”) according to a fifth embodiment will be described with reference to FIG. FIG. 5 is a cross-sectional view illustrating the configuration of the element 500. The element 500 of this embodiment uses a Si substrate 511 as the substrate 111 in the first embodiment, and further has a lattice matching layer 501. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  In the element 100 of the first embodiment, when the generated terahertz wave is emitted to the substrate 111 side or the terahertz wave is detected from the substrate 111 side, the band is limited by the phonon absorption of the terahertz wave in the substrate 111. Therefore, in this embodiment, a Si substrate 511 having no phonon absorption in the terahertz wave frequency region is used as the substrate 111.

  The photoconductive layer 110 is formed on the lattice matching layer 501 after forming a lattice matching layer 501 that lattice matches with the photoconductive layer 110 on the Si substrate 511 first. For example, when LT-GaAs is used for the photoconductive layer 110, the lattice matching layer 501 is formed of a layer containing AlGaAs or AlAs, GaAs, and Ge. With such a configuration, lattice mismatch between the Si substrate 511 and the photoconductive layer 110 can be absorbed. Note that it is desirable to use a material having higher resistance than the Si substrate 511 for the lattice matching layer 501.

  As described above, according to the element 500, the reflected wave of the photocurrent generated by the difference in characteristic impedance between the diffraction grating unit 101 and the antenna unit 102 can be reduced. Then, the time waveform of the terahertz wave pulse generated by the element 500 and the terahertz wave pulse detected by the element 500 can be made closer to a monopulse than before. That is, in a photoconductive element using surface plasmon resonance, a terahertz wave pulse closer to a monopulse than before can be obtained. Further, by forming the lattice matching layer 501 between the substrate 111 and the photoconductive layer 110, the substrate 111 having no phonon absorption in the frequency region of the terahertz wave can be applied.

(Sixth embodiment)
As a sixth embodiment, a method for manufacturing the element 100 of the first embodiment will be described with reference to FIG. FIG. 6 is a flowchart showing a method for manufacturing the element 100. Descriptions of parts common to the above description are omitted. In this embodiment, a method for manufacturing the diffraction grating portion 101, the antenna portion 102, and the electrode 103 included in the conductive portion 112 by exposing them collectively will be described.

LT-GaAs formed on the Si substrate as shown in the fifth embodiment was selected as the photoconductive layer 110, and Au was selected as the material for the diffraction grating portion 101, the antenna portion 102, and the electrode 103. Further, SiO ₂ is selected as the antireflection film, the excitation light wavelength is 1.55 μm, and the carrier excitation method in LT-GaAs is multiphoton absorption.

  Carrier generation efficiency due to multiphoton absorption, which is a kind of nonlinear optical effect, is proportional to the square of the electric field generated in the photoconductive layer 110. Therefore, the effect of improving the carrier generation efficiency due to the electric field enhancement effect by surface plasmon resonance and the electric field concentration effect by diffraction is greater in the case of multiphoton absorption than in one-photon absorption.

At a pumping light wavelength of 1.55 μm, the dielectric constant of Au is −135 and the dielectric constant of LT-GaAs is 11.4 from the Drude model. In this case, the real part ε _p of the effective complex dielectric constant determined by the area of the material such as LT-GaAs in contact with the diffraction grating portion 101 and the complex dielectric constant was calculated to be about 6. Further, when the period L of the diffraction grating portion 101 is calculated from the equation (2), it is about 700 nm when m = 1. The thickness of the diffraction grating portion 101 is about 100 nm in consideration of the refractive index of the antireflection film SiO ₂ .

A method for manufacturing the element 100 will be described with reference to the flowchart of FIG. First, in order to improve the adhesion between LT-GaAs as the photoconductive layer 110 and Au as the diffraction grating portion 101, the antenna portion 102, and the electrode 103, a titanium (Ti) film is formed on the LT-GaAs. A 5 nm film is formed (S601). After forming the Ti film, an Au layer containing Au was formed to 100 nm on the Ti film (S602). Examples of the film forming method include EB vapor deposition and sputtering. The Ti film takes a positive dielectric constant at 1.55 μm, but its abundance ratio is lower than that of the Au layer. Therefore, the entire diffraction grating unit 101 has a negative dielectric constant, and its absolute value is higher than the real part ε _p of the effective complex dielectric constant, so that surface plasmon resonance can be excited.

Further, a SiN layer containing SiN used as a mask material when etching the Au layer is formed on the Au layer by using a plasma CVD method (S603). In addition to the SiN layer, a metal mask such as SiO _2, Ni, or Cr may be used. Thereafter, a resist is applied (S604) and exposed (S605). The exposure is not limited to an electron beam exposure apparatus, but may be an exposure of a fine line width of about 300 nm, such as an i-line stepper or a KrF stepper. After exposure, development is performed to form a resist pattern (S606), and then the SiN layer is dry-etched (S607). At this time, the etching gas for the SiN layer is preferably a CF-based gas having a high selectivity with respect to the resist or Au. In order to increase the anisotropy of etching, it is desirable to reduce the pressure during etching as much as possible. When the etching of the SiN layer is completed, the resist is removed by ashing with O ₂ (S608).

Next, the Au layer and the Ti film are etched using the SiN layer as a mask (S609). As an etching gas, argon (Ar) or a mixed gas of Ar and chlorine (Cl ₂ ) is used. Thereafter, the Ti film which is the diffraction grating portion 101 and the SiN layer which is a mask for the Au layer are dry-etched (S610). Finally, SiO ₂ is sputtered on the gap portion between the antennas to form an antireflection film (S611).

  By using such a manufacturing method, the element 100 can be manufactured. Furthermore, in order to reduce the manufacturing cost, it is desirable that the antenna unit 102 and the diffraction grating unit 101 are formed of the same material and the same thickness. For this purpose, it is desirable to manufacture the diffraction grating portion 101 and the antenna portion 102 at a time through a photolithography process or the like. In addition, as in the second and third embodiments, the element 200 and the element 300 in which the connection part such as the connection part 104 or the taper part 105 is provided between the diffraction grating part 101 and the antenna part 102 are also manufactured in a lump. May be. According to the manufacturing method of this embodiment, the diffraction grating part 101, the antenna part 102, and the electrode 103 can be produced at once.

(Seventh embodiment)
In the seventh embodiment, a measurement apparatus 700 (hereinafter referred to as “apparatus 700”) using any of the photoconductive elements described in the above-described embodiments will be described with reference to FIG. Descriptions of parts common to the above description are omitted. FIG. 7 is a schematic diagram for explaining the configuration of the apparatus 700. The apparatus 700 is a THz-TDS apparatus that acquires a time waveform of a terahertz wave pulse using a THz-TDS method. Since the THz-TDS apparatus itself is basically the same as that conventionally known, only the outline will be described.

  The apparatus 700 includes a light source 701 that outputs ultrashort pulse light as excitation light, a beam splitter 702, a delay unit 705, lenses 706 and 707, a generation unit 710, and a detection unit 711. Excitation light from the light source 701 is branched into pump light 703 and probe light 704 by a beam splitter 702. The pump light 703 enters the generation unit 710 through the lens 706. The probe light 704 enters the detection unit 711 via the delay unit 705 and the lens 707. In this embodiment, a fiber type laser that outputs femtosecond pulsed light in a 1.5 μm band is used as the light source 701.

  The generation unit 710 generates a terahertz wave pulse when the pump light is incident. The detection unit 711 detects the terahertz wave pulse by the incidence of the terahertz wave pulse and the probe light. In this embodiment, one of the photoconductive elements of the first to fifth embodiments is used for at least one of the generation unit 710 and the detection unit 711. Here, a case where the element 500 is used for both the generation unit 710 and the detection unit 711 will be described. In each of the generation unit 710 and the detection unit 711, the photoconductive layer 110 can use LT-GaAs, and can generate carriers due to the multiphoton absorption effect.

  The terahertz wave pulse generated from the generation unit 710 is guided to the measurement object 708 and reflected by the measurement object 708. The reflected terahertz wave pulse is detected by the detection unit 711. The terahertz wave pulse reflected by the measurement object 708 includes information such as an absorption spectrum of the measurement object 708. The delay unit 705 is an optical delay system that changes the optical path length of the probe light 704, and adjusts the optical path length difference between the pump light 703 and the probe light 704 by moving the delay unit 705. Thereby, the timing for detecting the terahertz wave pulse in the detection unit 711 can be controlled. By using the detection result of the detection unit 711, the time waveform of the terahertz wave pulse can be acquired.

  In the present embodiment, the terahertz wave pulse reflected by the measurement object 708 is detected. However, the detection unit 711 may detect the terahertz wave pulse transmitted through the measurement object 708.

  As described above, the apparatus 700 uses any one of the photoconductive elements of the first to fifth embodiments for at least one of the generation unit 710 and the detection unit 711. The photoconductive element of each embodiment can reduce the reflected wave of the photocurrent generated by the difference in characteristic impedance between the diffraction grating portion and the antenna portion. Therefore, the apparatus 700 can perform measurement using a terahertz wave pulse that is closer to a monopulse than before, and can improve measurement accuracy.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

  For example, it is possible to use a combination of the different connecting portions shown in the second embodiment and the third embodiment. Even in such a case, the current reflection coefficient at the contact portion where the characteristic impedance is mismatched may be ± 5% or less, more preferably ± 1% or less.

DESCRIPTION OF SYMBOLS 100 Photoconductive element 101 Diffraction grating part 102 Antenna part 103 Electrode 110 Photoconductive layer 112 Conductive part 150 Grating 160 Contact part

Claims (14)

A photoconductive layer;
A plurality of conductive portions disposed on the photoconductive layer,
Each of the plurality of conductive parts is
Electrodes,
An antenna portion electrically connected to the electrode;
A diffraction grating part that is in contact with the antenna part and includes a plurality of gratings periodically arranged;
The plurality of conductive portions are arranged to face each other with a gap between the diffraction grating portions of the plurality of conductive portions,
A photoconductive element, wherein a current reflection coefficient at a contact portion between the antenna portion and the diffraction grating portion is ± 5% or less. 2. The photoconductive element according to claim 1, wherein the current reflection coefficient at a contact portion between the antenna portion and the diffraction grating portion is ± 1% or less. A photoconductive layer;
A plurality of conductive portions disposed on the photoconductive layer,
Each of the plurality of conductive parts is
Electrodes,
An antenna portion electrically connected to the electrode;
A diffraction grating portion having a plurality of gratings arranged periodically;
A connecting portion disposed between the antenna portion and the diffraction grating portion, and in contact with the diffraction grating portion;
The plurality of conductive portions are arranged to face each other with a gap between the diffraction grating portions of the plurality of conductive portions,
The photoconductive element according to claim 1, wherein a current reflection coefficient at a contact portion between the connection portion and the diffraction grating portion is ± 5% or less. The photoconductive element according to claim 3, wherein the current reflection coefficient at a contact portion between the connection portion and the diffraction grating portion is ± 1% or less.   5. The photoconductive element according to claim 3, wherein the antenna portion and the connection portion are in contact with each other, and a current reflection coefficient at a contact portion between the antenna portion and the connection portion is ± 5% or less.   The photoconductive element according to claim 5, wherein the current reflection coefficient at a contact portion between the antenna portion and the connection portion is ± 1% or less. The connecting portion is in contact with a part of the plurality of lattices, and has a first extension portion having a width larger than the width of each of the plurality of lattices;
A second extension part in contact with the first extension part and having a width larger than the width of the first extension part,
The photoconductive element according to claim 3, wherein a current reflection coefficient at a contact portion between the second extension portion and the first extension portion is ± 5% or less. The photoconductive element according to claim 3, wherein the connection portion has a tapered portion whose width increases from the diffraction grating portion toward the antenna portion. 3. The photoconductive element according to claim 1, wherein a propagation distance of a photocurrent propagating between the center of the gap and the electrode is 1.5 mm or more. 10. The photoconductive element according to claim 1, wherein a thickness of the diffraction grating portion is the same as a thickness of the antenna portion. 11. The photoconductive element according to claim 1, wherein a thickness of the diffraction grating portion is 1/3 or less of a wavelength of the light. The period L in which the plurality of gratings are arranged is that the speed of light is c, the wavelength of light is λ, the angular frequency of light is ω, the incident angle of light is θ, and the dielectric of the diffraction grating portion at the wavelength of light. When the real part of the rate is ε _g , the real part of the effective complex dielectric constant determined by the area of the substance in contact with the diffraction grating part and the complex dielectric constant is ε _p , and an arbitrary integer is m,

The photoconductive element according to claim 1, wherein: A generator that generates a terahertz wave pulse by the incidence of light;
Detecting a terahertz wave that has passed through the measurement object or reflected by the measurement object, and
At least one of the said generation | occurrence | production part and the said detection part is a photoconductive element as described in any one of Claim 1 to 8, The measuring apparatus characterized by the above-mentioned. A method for producing a photoconductive element, comprising:
Forming a photoconductive layer;
Forming a first layer, a second layer, and a third layer on the photoconductive layer;
Etching the second layer using the first layer as a resist;
Removing the first layer;
Forming the antenna portion and the diffraction grating portion by etching the third layer using the second layer as a resist.

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