JP2005315708A - Measuring apparatus for physical property using terahertz electromagnetic wave - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measuring apparatus for physical properties using terahertz electromagnetic waves for precisely measuring Hall coefficients, or the like by a change in the polarization of reflection waves from a material to be measured irradiated with terahertz electromagnetic waves without any contact and with high sensitivity. <P>SOLUTION: The measuring apparatus includes an incidence optical system 10 for generating terahertz electromagnetic waves, a polarizer 25 for controlling the polarization of terahertz electromagnetic waves entering a material 2 to be measured from the incidence optical system 10 and for controlling the polarization of terahertz electromagnetic waves reflected from the material 2 to be measured, and a detection optical system 30 for detecting the polarization of reflection terahertz electromagnetic waves from the material 2 to be measured, thus detecting a change in the polarization of reflection waves by the material 2 to be measured in the terahertz electromagnetic waves by direct measurement and measuring a complex index of refraction or complex conductivity that is a physical property constant of the material 2 to be measured as a function of terahertz electromagnetic waves. And by applying a magnetic field 28 to the object 2 to be observed, Hall coefficients can be measured precisely without any contact and with high sensitivity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention uses terahertz electromagnetic waves that can accurately measure electrical conduction characteristics and magnetic characteristics and their response performance to various materials such as semiconductors, metals, and magnetic materials using terahertz time domain spectroscopy. The present invention relates to a physical property measuring apparatus.

Recently, new functional materials are being developed to pioneer next-generation electronics. Determining the conductive properties of a new material over a wide frequency range is essential to assess the potential capabilities of the material. The new function is to control the conduction characteristics, magnetism, and optical properties by an electric field. As such new materials, various nanostructured materials, strongly correlated electronic materials typified by transition metal oxides, semiconductor quantum structures, functional glasses, and the like are attracting attention.
Examining the response characteristics of these materials to the electric field in the normal conduction measurement in the DC to picosecond region has difficult problems such as the bandwidth of the measuring instrument and the installation of terminal electrodes for measurement. There is no convenient method. As a method for solving this problem, it is known to use a measurement method applying Fourier transform spectroscopy.

Furthermore, several examples of measuring devices using far-infrared pulsed electromagnetic waves (hereinafter referred to as terahertz electromagnetic waves) generated by ultra-short pulse light as light sources have been reported (for example, see Patent Document 1 and Non-Patent Document 1). ). Non-Patent Document 1 reports a measurement method using an arrangement in which terahertz electromagnetic waves are transmitted through a material to be measured.
However, the transmission method cannot measure an opaque material to be measured such as a magnetic material or metal that does not transmit terahertz electromagnetic waves.

  Therefore, the present inventors have proposed a spectroscopic method for detecting the rotation of the polarization plane by the Kerr effect of reflected light, that is, the so-called Kerr rotation angle. It has been reported that a dielectric constant, a Hall coefficient, etc. can be measured (see Non-Patent Document 2).

FIG. 9 is a diagram schematically showing a Hall coefficient measuring device by a reflection method using a terahertz electromagnetic wave reported in Non-Patent Document 2. In FIG. 9, a Hall coefficient measurement apparatus 100 based on a reflection method using terahertz electromagnetic waves includes an incident optical system 110, a magnet 105 that applies a magnetic field to a material to be measured 102, and a detection optical system 120.
The incident optical system 110 generates a terahertz electromagnetic wave pulse by the optical rectification effect of the electro-optic crystal (ZnTe) 112 using the ultrashort light pulse laser 111 as a light source. The terahertz electromagnetic wave pulse is collected by the lenses 112 and 113, and irradiated to the material 102 to be measured through the input side polarizer 114.
Further, the terahertz electromagnetic wave pulse reflected from the surface of the material to be measured 102 is emitted to the detection optical system 120 through the emission side polarizer 121. In the exit side analyzer 121, the polarization can be switched between 45 ° and −45 °. The reflected terahertz electromagnetic wave pulse 122 that has passed through the emission-side analyzer 121 is collected by the lenses 123 and 124, enters the electro-optic crystal (ZnTe) 125, and the polarization of the reflected terahertz electromagnetic wave pulse 122 is detected.
At this time, the reference light 126 from the ultrashort optical pulse laser 111 is applied to the electro-optic crystal (ZnTe) 125, and a time-series signal related to the electric field of the reflected terahertz electromagnetic wave 12 2 is obtained by so-called electro-optic sampling (EO sampling). It is done. This signal is Fourier transformed to measure the Hall coefficient in the terahertz electromagnetic wave region of the material 102 to be measured.

Japanese Patent Laid-Open No. 2001-21503 S. Spielman and 7 others, "Observation of the Quasiparticle Hall Effect in Superconducting YBa2Cu3O7- δ" 1994, Physical Review Letters, vo1.73, pp.1537-1540 R. Shimano, Y. P. Svirko, M. Kuwata-Gonokami, "Teraherz frequency Hall measurement by magneto-optical Kerr spectroscopy in InAs", 2002, Applied Physics Letters, vol.81, No.2, pp.199-201

  However, when the transmission method is used in a conventional measuring device such as a complex dielectric constant using terahertz electromagnetic waves, only transparent materials and ultrathin films can be measured, and bulk materials and opaque materials that are important in application cannot be measured. There is a problem.

Further, the conventional reflection method has a problem that the polarization detection sensitivity is 10 ⁻² rad due to mechanical vibration of the polarizer and the analyzer, and sufficient sensitivity cannot be obtained.

  In view of the above problems, the present invention provides a terahertz electromagnetic wave that can be measured in a non-contact, highly sensitive and highly accurate manner, such as a Hall coefficient due to a change in polarization of a reflected wave from a measured material irradiated with the terahertz electromagnetic wave. It aims at providing the used physical property measuring apparatus.

In order to achieve the above object, the physical property measuring apparatus using the terahertz electromagnetic wave of the present invention controls the incident optical system that generates the terahertz electromagnetic wave, and the polarization of the terahertz electromagnetic wave that is incident on the measured material from the incident optical system, And a polarizer that controls the polarization of the terahertz electromagnetic wave reflected from the material to be measured and a detection optical system that detects the polarization of the reflected terahertz electromagnetic wave from the material to be measured. A change in the polarization state of the reflected wave is detected by direct measurement of an electric field, and a complex refractive index or a complex conductivity, which is a physical constant of the material to be measured, is measured as a function of a terahertz electromagnetic wave.
In the above configuration, the terahertz electromagnetic wave is preferably perpendicularly incident on the material to be measured. The terahertz electromagnetic wave is preferably a terahertz electromagnetic wave pulse generated by using an optical rectification method using a nonlinear optical crystal. Alternatively, the terahertz electromagnetic wave may be a terahertz electromagnetic wave pulse generated using an optical rectification method using a photoconductive element.
The terahertz electromagnetic wave detection optical system preferably includes a detector that uses the electro-optic effect of the nonlinear optical crystal. Alternatively, a detector using a photoconductive element may be provided.

According to the above configuration, by using the terahertz electromagnetic wave pulse and directly measuring the time waveform of the polarization of the reflected terahertz electromagnetic wave from the material to be measured, it is possible to perform measurement exceeding the limit in the case of electric field strength measurement by one digit or more. Moreover, by adding a reference measurement, the real part and imaginary part of the complex refractive index or complex conductivity can be measured simultaneously with high accuracy.
Further, by making the terahertz electromagnetic wave perpendicularly incident on the material to be measured, the same polarizer can be used as the incident-side polarizer and the reflecting-side analyzer. For this reason, the measurement error due to the mechanical vibration of the polarizer can be significantly reduced.

In the above configuration, the physical property measuring apparatus using the terahertz electromagnetic wave preferably further includes a magnetic field system that applies a magnetic field to the material to be measured in order to measure the hole conductivity.
The magnetic field system preferably includes means for reversing the direction of the magnetic field. In addition, it is preferable to measure only the polarization component due to the Hall effect of the material to be measured based on the difference between the electric fields of the reflected terahertz electromagnetic waves when the direction of the magnetic field is reversed and non-reversed.
According to the above configuration, the Hall coefficient can be measured with high accuracy by applying a magnetic field to the material to be measured. Moreover, the non-quenching component can be completely removed due to the incompleteness of the polarizing element by measuring the difference between the inversion and non-inversion of the direction of the magnetic field.

  The physical property measuring apparatus using the terahertz electromagnetic wave preferably further includes a computer system. According to this configuration, it is possible to detect a physical constant such as a Hall coefficient at high speed and with high sensitivity.

  According to the physical property measuring apparatus using the terahertz electromagnetic wave of the present invention, various conductive characteristics of the material to be measured can be measured over a wide band of the terahertz electromagnetic wave without contact with and without destructing the electrode. It is also possible to evaluate the spatial distribution of conduction characteristics over a wide area. In addition, it is possible to remotely measure materials to be measured placed in extreme environments (high temperature, high magnetic field, high pressure, etc.).

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same or corresponding members are denoted by the same reference numerals.
First, a physical property measuring apparatus using a terahertz electromagnetic wave according to the first embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing a configuration of a physical property measuring apparatus using a terahertz electromagnetic wave according to the first embodiment.
In FIG. 1, a physical property measuring apparatus 1 using a terahertz electromagnetic wave controls an incident optical system 10 that generates a terahertz electromagnetic wave, a polarization of the terahertz electromagnetic wave from the incident optical system 10 to a material 2 to be measured, and a material to be measured. 2, a polarizer 25 that controls the polarization of the terahertz electromagnetic wave reflected from the magnetic field 2, a magnetic field system 28 that applies a magnetic field to the material to be measured 2, a detection optical system 30 that detects the polarized terahertz electromagnetic wave, and a detection optical system 30. And a computer system 40 that performs calculation processing of measurement data from the computer.

The incident optical system 10 includes a pulse laser 11, mirrors 12, 13, 14, 15, 18, a beam splitter 16, an optical time delay circuit 17, an electro-optic crystal 19, a lens 21, a light intensity modulator 6, and the like. It is composed of
The pulse laser beam generated from the pulse laser 11 is incident on the optical time delay circuit 17 by the mirrors 12, 13, 14, 15 and the beam splitter 16, and the reference beam 39 to the detection optical system 30 described later. And demultiplexed.
The pulsed laser light that has passed through the optical time delay circuit 17 becomes pulsed laser light 11a whose light intensity is modulated by the light intensity modulator 6, and is irradiated to the electro-optic crystal 19 having a second-order nonlinear optical effect. The light intensity modulator 6 is driven by a light intensity modulator driver 7. Further, a part of the output of the light intensity modulator driver 7 is output as a reference signal 8 to an amplifier 37 in the detection optical system 30 described later.

A terahertz electromagnetic wave pulse 20 (hereinafter appropriately referred to as a terahertz electromagnetic wave) is generated by a transient photocurrent generated in the electro-optic crystal 19 which is a nonlinear optical crystal irradiated with the pulsed laser light, that is, a so-called photorectification effect. This terahertz electromagnetic wave generates a terahertz electromagnetic wave having a band approximately equal to the reciprocal of the pulse width of the pulsed laser beam.
This terahertz electromagnetic wave is condensed by the lens 21a, becomes parallel light, and further condensed by the lens 21b on the material 2 to be measured. Between these lenses 21 (21a, 21b), a filter 22 is inserted to block the pulse laser beam 11a, and only the terahertz electromagnetic wave pulse 20 is irradiated to the material 2 to be measured.
Here, as the pulse laser 11, for example, a light source capable of generating an ultrashort light pulse by a mode-synchronized titanium sapphire laser can be used.
In addition, the terahertz electromagnetic wave in this invention is an electromagnetic wave which is also called a far-infrared light from the short millimeter wave to a submillimeter wave with a wavelength of 3 mm or less, and is about 0.1-30 THz as a frequency.

The polarizer 25 is disposed in the optical path between the lens 21 b and the material to be measured 2. The polarizer 25 controls the polarization of the terahertz electromagnetic wave from the incident optical system 10 to the material to be measured 2, and controls the polarization of the terahertz electromagnetic wave reflected from the material to be measured 2 to be output to the detection optical system 30. .
The polarizer 25 is a polarizer that can be used in terahertz electromagnetic waves, so-called far infrared light or submillimeter waves, and a so-called wire grid polarizer can be used. The material to be measured 2 is irradiated with the vertical polarization component 23 of the incident terahertz electromagnetic wave by the polarizer 25. Then, the reflected wave of the terahertz electromagnetic wave from the material to be measured 2 is incident again on the polarizer 25, and the vertical polarization component 24 of the reflected terahertz electromagnetic wave is taken out and emitted to the detection optical system 30 (the polarization in FIG. 1). (See an enlarged view of the vicinity of the child 25).

Therefore, in the physical property measuring apparatus 1 using the terahertz electromagnetic wave of the present invention, the polarizer 25 also serves as an analyzer of the reflected terahertz electromagnetic wave from the material to be measured 2, so that the terahertz electromagnetic wave 23 is perpendicular to the material to be measured 2. An optical arrangement is adopted in which incident and reflected terahertz electromagnetic waves are emitted to the same polarizer 25.
For this reason, when magnetization is present in the material to be measured 2 or when a magnetic field is applied for Hall measurement, which will be described later, the reflected terahertz electromagnetic wave from the material to be measured 2 has its polarization plane inclined and elliptical due to the Hall effect. Only the vertical polarization component 24 is emitted to the detection optical system 30 by the wire grid polarizer 25.
On the other hand, when the material to be measured 2 is not magnetized or a magnetic field is not applied, the vertical polarization component of the incident terahertz electromagnetic wave 23 is reflected from the material to be measured, and only the vertical polarization component is extracted by the polarizer 25. The change in polarization does not theoretically occur, and only the non-quenching component exits from the polarizer 25.
Thus, in comparison with the case where polarizers are provided for the incident wave and the reflected wave of the terahertz electromagnetic wave, instability due to mechanical vibration of the polarizer is eliminated, and measurement errors can be drastically reduced.

  The magnetic field system 28 is provided to apply a magnetic field in order to perform hole measurement of the material 2 to be measured. The magnetic field system 28 is composed of an electromagnet, a permanent magnet, or the like, and includes means for applying a magnetic field perpendicular to the material 2 to be measured, that is, in the horizontal direction of the paper, and reversing the magnetic field (± B in FIG. 1). reference). The reversal of the magnetic field can be controlled by the computer system 40 by reversing the direct current applied to generate the magnetic field when an electromagnet is used.

Next, a detection optical system that detects reflected light of the terahertz electromagnetic wave from the material to be measured will be described.
The detection optical system 30 includes lenses 31, 32, a terahertz electromagnetic wave detector 33, a phase plate 34, an analyzer 35, a photodetector 36, and an amplifier 37. Then, the reference light 39 demultiplexed from the pulse laser 11 through the mirror 18 is incident on the terahertz electromagnetic wave detector 33.

Here, as the terahertz electromagnetic wave detector 33, an electro-optic crystal 33 having a second-order nonlinear optical effect can be used. Further, as the phase plate 34, a ¼ wavelength plate that converts linearly polarized light into circularly polarized light can be used. Further, as the analyzer 35, a Wollaston prism that decomposes the circularly polarized light from the quarter wave plate into polarized light components orthogonal to each other can be used. As a result, the polarization components 35a and 35b divided by the analyzer 35 enter the balanced photodiode 36, which is a photodetector of the reference light 39, and the difference between them is detected. This difference is amplified by an amplifier 37 such as a lock-in amplifier, A / D converted, and output to the computer system 40. At this time, the amplifier 37 such as a lock-in amplifier receives the reference signal 8 from the light intensity modulator driver 7 and performs signal detection based on the phase, so that the noise component is reduced.
The balanced photodiode 36 is a detector for the reference light 39 from the pulsed laser light, and includes two photodiodes 36a and 36b having two characteristics for the difference measurement.

The reflected terahertz electromagnetic wave 24 incident on the detection optical system 30 from the material to be measured 2 is a product of the incident electric field E (ω) to the material to be measured 2 and the complex reflectance r _sp (ω) of the material to be measured 2, That is, E (ω) r _sp (ω). Here, r _sp (ω) is the s-polarized component of the reflected terahertz electromagnetic wave with respect to p-polarized light incidence.
The electric field component of the reflected terahertz electromagnetic wave 24 is detected by so-called electro-optic sampling (EO sampling) by the electro-optic crystal 33 and the optical time delay circuit 17 in the incident optical system 10. This electro-optic sampling is performed, for example, by controlling the stage 17a for minutely displacing the lens in the optical time delay circuit 17 from the computer system 40 and changing the time from the beam splitter 16 until it enters the material 2 to be measured. Can do.

Next, the operation of the detection optical system 30 will be described.
When there is no material to be measured 2 and the reflected terahertz electromagnetic wave 24 does not enter the detection optical system 30, only the reference light 39 is incident on the detection optical system 30. In this case, the linearly polarized reference light 39 passes through the electro-optic crystal 33 and is converted into circularly polarized light by the quarter-wave plate 34. After that, the analyzer 35 spatially decomposes the polarization components 35a and 35b orthogonal to each other and enters each of the balanced photodiodes composed of two photodiodes 36. The outputs of the two photodiodes 36a and 36b are differentially detected. Thereby, when the reference light 39 is completely linearly polarized light, the outputs of the respective photodiodes 36a and 36b are equal, and the output obtained by differentially detecting them is zero.
On the other hand, when the pulse of the reflected terahertz electromagnetic wave 24 is incident on the detection optical system 30 and the electro-optic crystal 33 is irradiated with the pulse of the reflected terahertz electromagnetic wave 24, the reference is caused by the nonlinear effect due to the electric field component in the electro-optic crystal 33. The polarization of the light 39 changes from linear to elliptical polarization. As a result, the outputs of the two photodiodes 36a and 36b are not canceled out, and a signal proportional to the amplitude of the electric field of the reflected terahertz electromagnetic wave 24 is output.

  The computer system 40 includes a computer, a stage controller of the optical time delay circuit 17, a display device, a storage device, and the like. If a predetermined time of a time-series signal that is an electric field waveform of the reflected terahertz electromagnetic wave 24 is Fourier-transformed by a computer using a fast Fourier transform (FFT) algorithm, the calculation time can be shortened. The means for obtaining the Fourier spectrum may be a dedicated DSP (digital signal processor) or FFT device that does not rely on a computer.

According to the physical property measuring apparatus 1 using the terahertz electromagnetic wave of the present invention configured as described above, the electric time of the reflected terahertz electromagnetic wave 24 is measured while applying a time delay to the pulse laser beam by the optical time delay circuit 17. Thus, the electric field waveform of the reflected terahertz electromagnetic wave 24 with respect to this time delay, that is, a time-series signal is obtained.
The time series signal is digitally converted by A / D conversion, and Fourier-transformed by a computer, whereby the amplitude and phase in the frequency domain of the reflected terahertz electromagnetic wave 24 can be obtained. Then, the amplitude spectrum and phase spectrum of the reflected terahertz electromagnetic wave 24 by the material to be measured 2 can be obtained with reference to the reference light spectrum generated when there is no material to be measured 2.

Next, a physical property measuring apparatus using terahertz electromagnetic waves according to a second embodiment of the present invention will be described.
FIG. 2 is a schematic diagram showing a configuration of a physical property measuring apparatus using terahertz electromagnetic waves according to the second embodiment of the present invention. In FIG. 2, the physical property measuring device 50 using the terahertz electromagnetic wave according to the second embodiment is different from the physical property measuring device 1 using the terahertz electromagnetic wave of the first embodiment for generating the terahertz electromagnetic wave pulse of the incident optical system 10. The photoconductive element 26 and the photoconductive element 53 of the detection optical system 30 are used.

FIG. 3 is a diagram schematically showing the structure and operation of the photoconductive element 26 for generating a terahertz electromagnetic wave pulse according to the second embodiment. As shown in the drawing, the photoconductive element 26 is made of an insulating substrate such as GaAs as a semiconductor, and electrodes 26a and 26b having a minute gap 26c having a width of several μm at the center are provided on the surface thereof. A DC voltage 27 is applied to the electrodes 26a and 26b in the minute gap 26c. For this reason, a strong DC electric field is applied between the electrodes 26a and 26b having the minute gap 26c.
Thereby, when the pulse laser beam 11a which is an ultrashort light pulse is irradiated between the minute gaps 26c, an optical carrier is instantaneously generated and becomes conductive. Since a strong DC electric field is applied between the minute gaps 26c, a current flows instantaneously by light irradiation. Terahertz electromagnetic radiation is generated using this instantaneous current as a source. In this way, the terahertz electromagnetic wave pulse 20 is generated by using the optical rectification method by the photoconductive element 26 and is emitted from the back surface of the insulating substrate such as GaAs.
Note that the thickness of the insulating substrate such as GaAs may be made thicker than the penetration depth of the pulsed laser light 11 a and thinner than the penetration depth of the terahertz electromagnetic wave pulse 20.

FIG. 4 is a diagram schematically showing the structure and operation of the photoconductive element 53 for detecting a terahertz electromagnetic wave pulse according to the second embodiment.
As the photoconductive element 53, an element similar to the photoconductive element 26 can be used. The photoconductive element 53 is made of an insulating semiconductor GaAs or the like. As shown in the figure, electrodes 53a and 53b having a minute gap 53c with a width of several μm in the center are provided on the surface. The difference from the photoconductive element 29 is that, in this embodiment, an ammeter 54 for detecting a photocurrent described later is connected between the electrodes 53 a and 53 b and the current is output to the amplifier 37.
Further, as shown in FIG. 2, the pulse laser beam is generated as a reference beam 39 by the pulse laser 11 to the mirror 12, the beam splitter 16, and the mirror 18 (18a, 18b, 18c). It is incident on the minute gap 53c on the surface side. At this time, the pulse width of the pulsed laser light is shorter by about one digit or more than the vibration period of the terahertz electromagnetic wave pulse. Further, the vertical polarization component 24 of the reflected terahertz electromagnetic wave is simultaneously collected on the back surface of the photoconductive element 53 for detecting the terahertz electromagnetic wave pulse.

As a result, the photoconductive element 53 becomes conductive only when irradiated with the pulse laser beam by the carrier generated by the pulse laser beam. However, unlike the photoconductive element 26 for detecting the terahertz electromagnetic wave pulse, no DC bias electric field is applied. For this reason, the photocurrent is induced by the electric field of the vertical polarization component 24 of the reflected terahertz electromagnetic wave. This photocurrent signal is measured as a function of the difference between the time at which the vertical polarization component pulse of the reflected terahertz electromagnetic wave 24 and the reference light 39 from the pulsed laser light reach the photoconductive element 53, so that the vertical polarization of the reflected terahertz electromagnetic wave is measured. The time waveform of the component 24 can be measured. This difference in arrival time is generated by using an optical time delay circuit 17 installed in the incident optical system 10.
Note that the thickness of the insulating substrate such as GaAs may be thicker than the penetration depth of the pulsed laser beam and thinner than the penetration depth of the terahertz electromagnetic wave pulse.

  The photoconductive element 53 is configured to replace the electro-optic crystal 19 that is the terahertz electromagnetic wave pulse detector 36 of the physical property measuring apparatus 10 using the terahertz electromagnetic wave according to the first embodiment. 34, the analyzer 35, and the photodetector 36 are unnecessary. In the physical property measuring apparatus 50 using the terahertz electromagnetic wave according to the second embodiment, the configuration other than the photoconductive elements 26 and 53 for generating and detecting the terahertz electromagnetic wave is the same as the terahertz electromagnetic wave according to the first embodiment of the present invention. Since it is the same as the physical property measuring apparatus 10 using, the description is omitted.

According to the physical property measuring apparatus 50 using the terahertz electromagnetic wave of the present invention configured as described above, the electric time of the reflected terahertz electromagnetic wave 24 is measured while applying a time delay to the pulse laser beam by the optical time delay circuit 17. Thus, the electric field waveform of the reflected terahertz electromagnetic wave 24 with respect to this time delay, that is, a time-series signal can be obtained.
This time-series signal is digitally converted by A / D conversion and Fourier-transformed by a computer, whereby the amplitude and phase of the reflected terahertz electromagnetic wave 24 in the frequency domain can be obtained. Then, the amplitude spectrum and phase spectrum of the reflected terahertz electromagnetic wave 24 by the material to be measured 2 can be obtained with reference to the reference light spectrum generated when there is no material to be measured 2.
In the physical property measuring apparatuses 1 and 50 using the terahertz electromagnetic wave of the present invention, the electro-optic crystal 19 and the photoconductive element 26 are used for generating the terahertz electromagnetic wave pulse, and the electro-optic crystal 33 and the light are used as the terahertz electromagnetic wave detector. Although the conductive element 53 is used, these can be used in appropriate combination according to the measurement purpose.

ここで、ｒ（ω）は複素反射率、ｎ_iiは複素屈折率である。ｉ＝ｘ，ｙである。 Next, a method for deriving the conductivity tensor and Hall coefficient of the material to be measured by the physical property measuring apparatuses 1 and 50 using the terahertz electromagnetic wave of the present invention will be described.
The signal observed by the detection optical system 30 corresponds to r _sp (ω) E (ω) that is the product of the complex reflectance and the incident electric field. r _sp (ω) means the s-polarized component of the reflected wave with respect to the incidence of p-polarized light, and since terahertz time domain spectroscopy can simultaneously measure the amplitude and phase of the electric field, the complex reflectance r _sp (ω) Real part and imaginary part can be determined simultaneously. However, the incident wave E (ω) is separately measured for reference as will be described later.
First, the complex refractive index is calculated from the complex reflection coefficient by the following formula (1) by ordinary terahertz reflection measurement.

Here, r (ω) is a complex reflectance, and n _ii is a complex refractive index. i = x, y.

Next, a method for measuring the incident wave E (ω) will be described.
In the first method, the wire grid polarizer 25 is replaced with a beam splitter, and then the material to be measured 2 is replaced with a reference element (gold mirror, aluminum mirror, etc.). At this time, no magnetic field is applied. In this case, since there is no wire grid polarizer 25 and only a beam splitter is used, the incident terahertz electromagnetic wave is not polarized, and the terahertz electromagnetic wave is reflected at 100% intensity by the gold mirror, and the detection optical system 30 To exit.
By measuring this waveform in this manner, the time waveform of the incident wave E (ω) can be obtained without applying a magnetic field.
The second method is a method in which the non-quenching component spectral characteristics of the wire grid polarizer 25 are examined in advance. In this case, the incident wave E (ω) can be determined simply by replacing the material to be measured 2 with a known reference element (such as a gold mirror or an aluminum mirror).
The third method is a method of inserting a retardation plate (such as a crystal plate) having an appropriate thickness between the material to be measured 2 and the wire grid polarizer 25.
For example, when a phase plate having a phase difference of λ / 4 at a specific wavelength (λ) is used, the natural axis is inserted with an inclination of 45 degrees with respect to the polarization plane of the incident terahertz wave 23. The terahertz pulse incident on the material to be measured reciprocates through the phase plate to generate a phase difference of λ / 2 in the direction component orthogonal to the natural axis direction component of the phase plate, and the plane of polarization rotates 90 degrees. As a result, the reflected terahertz wave 24 that has been reflected back from the material to be measured 2 is reflected almost 100% by the wire grid polarizer 25 and is emitted to the detection optical system 30.
Thus, the incident wave E (ω) can be determined by switching the material to be measured 2 to a reference element (such as a gold mirror or an aluminum mirror) from which a reflected wave of 100% is obtained.
If the refractive index and thickness of the phase plate inserted between the material to be measured 2 and the wire grid polarizer 25 are known, it is not necessary to use a quarter wavelength plate, and calibration can be performed at any thickness. It is.

Next, a dielectric function (dielectric constant tensor) ε _ii is calculated from the complex refractive index by the following formula (2).

Since r _sp (ω) is a dielectric function represented by the following equation (3), the non-diagonal dielectric constant ε _xy (ω) is obtained using the dielectric constant tensor ε _ii (ω) obtained by the above equation (2). Ask for.

ここで、δ_ijはクロネッカのデルタであり、ε_b は下地の誘電率である。 Next, from the non-diagonal permittivity ε _xy (ω) obtained in the above (3), a conductivity tensor is uniquely calculated by the following equation (4), and the complex conductivity can be obtained.

Here, δ _ij is the Kronecker delta, and ε _b is the dielectric constant of the base.

Next, a method for calculating the Hall coefficient of the material to be measured 2 will be described.
A magnetic field B _z is applied to the material 2 to be measured, a conductivity tensor is obtained by the above equation (4) with respect to the strength of the magnetic field, and a Hall coefficient R _H (ω) is obtained by the following equation (5).

ここで、ｅは電子の単位電荷であり、１．６０２×１０^-19 Ｃ（クーロン）である。 When the material to be measured 2 is a semiconductor, the sign of the Hall coefficient differs depending on the conductivity type, so that the conductivity type of the material to be measured 2 can be determined. Further, from the following formulas (6) and (7), the n-type and p-type impurity densities can be calculated as n and p, respectively.

Here, e is a unit charge of electrons and is 1.602 × 10 ⁻¹⁹ C (Coulomb).

At this time, if the direction of the magnetic field is reversed, the sign of ε _{xy in} the above equation (3) is reversed. For this reason, when the direction of the magnetic field is reversed and non-reversed, in addition to the value of r _sp (ω) measured by the above equation (3), the observation is further performed due to the imperfection of the polarizer 25. Since the non-quenching component is the same, if the difference is taken, the non-quenching component is canceled out, and only the time series signal corresponding to the Hall coefficient R _H (ω) is doubled.
Thereby, the non-quenching component can be completely removed, and the measurement accuracy of the Hall coefficient R _H (ω) of the material to be measured 2 can be remarkably increased. Therefore, by measuring the Hall coefficient using the terahertz electromagnetic property measuring apparatus 1, 50 according to the present invention, instability due to mechanical vibration of the polarizer 25 and imperfections derived from the polarizer 25 are eliminated, and measurement is performed. The error can be drastically reduced. Further, the measurement accuracy can be remarkably increased by performing the difference measurement by reversal and non-reversal of the magnetic field.

Next, based on an Example, this invention is demonstrated further in detail.
As Example 1, a physical property measuring apparatus 1 using a terahertz electromagnetic wave of the present invention was manufactured. The main part will be described with reference to FIG.
As the pulse laser 11, a pulsed laser beam having a wavelength of 800 nm, a pulse width of 200 fs, and a repetition frequency of 200 kHz was generated using a reproduction amplification system using a mode-synchronized titanium sapphire laser (manufactured by Coherent, USA, ReqA9000). The pulse laser beam was applied with intensity modulation of 100 kHz by the light intensity modulator 6. The electro-optic crystals 19 and 33 for generating the terahertz electromagnetic wave pulse and for the terahertz electromagnetic wave detector both use ZnTe. The lenses 21, 31, and 32 for collecting the terahertz electromagnetic waves used parabolic mirrors. Further, as the polarizer 25, a wire grid type polarizer was used.

Using the terahertz electromagnetic property measuring apparatus 1 of this example, the Hall effect was detected at room temperature of a semiconductor InAs substrate as the material to be measured 2. As the InAs substrate 2, a commercially available non-doped single crystal substrate was used. The carrier (electron) concentration at room temperature was 2 × 10 ¹⁶ cm ⁻³ according to the direct current Hall measurement by the four-terminal method.

FIG. 5 shows an electric field waveform in the time domain due to the reflected terahertz electromagnetic wave measured in Example 2. In the figure, the horizontal axis represents the delay time (ps), and the vertical axis represents the terahertz electric field intensity (arbitrary scale) of the reflected terahertz electromagnetic wave 24. A case where a magnetic field is not applied to the material 2 to be measured (B = 0 Gauss) is indicated by a solid line, and a case where a magnetic field of 90 Gauss is applied is indicated by a dotted line.
As is apparent from the figure, the signal observed even in the absence of a magnetic field is a non-quenching component in which the terahertz electromagnetic wave 24 reflected from the InAs of the material to be measured is reflected by the imperfection of the wire grid polarizer 25. is there. It can also be seen that when a magnetic field is applied, an electric field component is generated due to the Hall effect.

FIG. 6 is a diagram illustrating the direction dependence of the applied magnetic field in the electric field waveform of the reflected terahertz electromagnetic wave measured in Example 2 in the time domain. In the figure, the horizontal axis represents the delay time (ps), and the vertical axis represents the terahertz electric field intensity (arbitrary scale) of the reflected terahertz electromagnetic wave. In the figure, a case where a 90 gauss magnetic field is applied to the material to be measured is indicated by a solid line, and a case where a −90 gauss magnetic field is applied is indicated by a dotted line.
As is apparent from the figure, in the waveform on the time axis of the polarization change component of the terahertz wave due to the Hall effect, it can be seen that the sign is inverted by the magnetic field.

FIG. 7 is a diagram illustrating waveforms obtained by inverting the sign of the magnetic field, measuring the reflected terahertz time waveform, and recording and integrating the difference in the second embodiment. In the figure, the horizontal axis represents the delay time (ps), and the vertical axis represents the terahertz electric field intensity (arbitrary scale) of the reflected terahertz electromagnetic wave. A magnetic field of 11 gauss and 22 gauss was applied as the magnetic field.
As can be seen from the figure, the non-quenching component is removed and only the Hall effect component is clearly detected. Despite the low magnetic field strength of 11 gauss, the amplitude of polarization rotation was 0.4 mrad from the amplitude, which was found to be about 20 times more sensitive than the measurement by the conventional reflected terahertz electromagnetic wave.
In this example, magnetic field inversion and non-inversion were alternately performed, and recording and integration of the terahertz time waveform in each case were repeated 50 times. Then, FFT was performed using the time waveform obtained by the average operation of these time waveforms as data, and the Hall coefficient described later and the dielectric function and conductivity necessary for its derivation were calculated.

Next, the reflection terahertz time waveform was measured by changing the magnetic field strength, and the Hall coefficient was measured from the observed Hall effect signal.
FIG. 8 is a diagram showing the frequency dependence of the Hall coefficient of InAs measured in Example 2. In FIG. In the figure, the horizontal axis indicates the frequency (THz), and the vertical axis indicates the real part (m ³ / C) of the Hall coefficient.
As is apparent from the figure, the real part of the Hall coefficient is substantially constant at −300 × 10 ⁻⁶ m ³ / C in the region of about 0.5 to 2 THz of the reflected terahertz electromagnetic wave. Is n-type. Further, it can be seen from the value of the Hall coefficient that the carrier density is about 2.1 × 10 ²² m ⁻³ , that is, about 2.1 × 10 ¹⁶ cm ⁻³ . This value is reasonable when compared with a normal Hall effect measurement by applying a direct current.

  From the above results, it can be seen that according to the physical property measuring apparatus using the terahertz electromagnetic wave of the present invention, the Hall coefficient can be measured with high sensitivity and high accuracy without contact with the material to be measured.

  The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the invention described in the claims, and it goes without saying that these are also included in the scope of the present invention. Absent. In the embodiment described above, the measurement of the Hall coefficient is mainly described, but the complex refractive index and the complex conductivity tensor can also be measured.

Industrial applicability

According to the physical property measuring apparatus using the terahertz electromagnetic wave of the present invention, the hole conductivity can be determined in a non-contact, high sensitivity and high accuracy in the terahertz frequency region. In addition, the hole conductivity and mobility of various substances including opaque materials can be evaluated in a non-contact manner in a high band. In addition, non-contact evaluation of a wide range of materials such as semiconductors where electrodes are difficult to form, various strongly correlated electronic materials, magnetic materials, metal materials, and organic materials becomes possible.
Moreover, according to the physical property measuring apparatus using the terahertz electromagnetic wave of the present invention, the physical property measurement can be performed at high speed without contacting the material to be measured. For this reason, the hole conductivity of industrially important semiconductor substrate materials can be efficiently and nondestructively performed.

It is a schematic diagram which shows the structure of the physical property measuring apparatus using the terahertz electromagnetic wave by 1st Embodiment based on this invention. It is a schematic diagram which shows the structure of the physical-property measuring apparatus using the terahertz electromagnetic wave by 2nd Embodiment which concerns on this invention. It is a figure which shows typically the structure and operation | movement of the photoconductive element for the terahertz electromagnetic wave pulse generation by 2nd Embodiment. It is a figure which shows typically the structure and operation | movement of the photoconductive element for terahertz electromagnetic wave pulse detection by 2nd Embodiment. 6 is a diagram showing an electric field waveform in a time domain by a reflected terahertz electromagnetic wave measured in Example 2. FIG. It is a figure which shows the direction dependence of the applied magnetic field in the electric field waveform in the time domain of the reflected terahertz electromagnetic wave measured in Example 2. FIG. In Example 2, it is a figure which shows the waveform which reversed the code | symbol of the magnetic field, measured each reflected terahertz time waveform, recorded the difference, and integrated | accumulated. It is a figure which shows the frequency dependence of the Hall coefficient of InAs measured in Example 2. FIG. It is a figure which shows typically the Hall coefficient measuring device by the reflection method using the terahertz electromagnetic wave reported by the nonpatent literature 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,50: Physical property measuring apparatus using terahertz electromagnetic wave 2: Material to be measured 6: Light intensity modulator 7: Light intensity modulator driver 8: Reference signal 10: Incident optical system 11: Pulse laser 11a: Pulse laser beam 12, 13, 14, 15, 18: mirror 16: beam splitter 17: optical time delay circuit 17a: stage 19: electro-optic crystal 20: terahertz electromagnetic wave pulse 21, 31, 32: lens 22: filter 23: vertical polarization of incident terahertz electromagnetic wave Component 24: Vertically polarized light of reflected terahertz electromagnetic wave 25: Polarizer 26: Photoconductive elements 26a and 26b for generating electromagnetic wave pulses: Electroconductive element electrode 26c: Micro gap 27 between photoconductive element electrodes 27: DC voltage 28: Magnetic field System 30: detection optical system 33: terahertz electromagnetic wave detector (electro-optic crystal)
34: Phase plate (1/4 wavelength plate)
35: Analyzer (Wollaston prism)
35a, 35b: Polarization component 36: Photo detectors 36a, 36b: Photo detector output 37: Amplifier (lock-in amplifier)
39: Reference beam 40: Computer system 53: Photoconductive elements 53a, 53b: Electroconductive element electrodes 53c: Micro gap 54 between photoconductive element electrodes: Ammeter

Claims (10)

An incident optical system that generates terahertz electromagnetic waves;
A polarizer for controlling the polarization of the terahertz electromagnetic wave incident on the material to be measured from the incident optical system, and for controlling the polarization of the terahertz electromagnetic wave reflected from the material to be measured;
A detection optical system for detecting the polarization of the reflected terahertz electromagnetic wave from the material to be measured,
A change in the polarization state of the reflected wave of the terahertz electromagnetic wave by the measured material is detected by direct measurement of an electric field, and a complex refractive index or a complex conductivity, which is a physical constant of the measured material, is measured as a function of the terahertz electromagnetic wave. An apparatus for measuring physical properties using terahertz electromagnetic waves.   The physical property measuring apparatus using terahertz electromagnetic waves according to claim 1, wherein the terahertz electromagnetic waves are perpendicularly incident on the material to be measured.   The physical property measuring apparatus using the terahertz electromagnetic wave according to claim 1, wherein the terahertz electromagnetic wave is a terahertz electromagnetic wave pulse generated using an optical rectification method using a nonlinear optical crystal.   The physical property measuring apparatus using the terahertz electromagnetic wave according to claim 1, wherein the terahertz electromagnetic wave is a terahertz electromagnetic wave pulse generated using an optical rectification method using a photoconductive element.   The physical property measuring apparatus using the terahertz electromagnetic wave according to claim 1, wherein the terahertz electromagnetic wave detecting optical system includes a detector using an electro-optic effect by a nonlinear optical crystal.   The physical property measuring apparatus using terahertz electromagnetic waves according to claim 1, wherein the terahertz electromagnetic wave detection optical system includes a detector using a photoconductive element.   7. The terahertz according to claim 1, wherein the physical property measuring apparatus using the terahertz electromagnetic wave further includes a magnetic field system that applies a magnetic field to the material to be measured in order to measure hole conductivity. An apparatus for measuring physical properties using electromagnetic waves.   The physical property measuring apparatus using terahertz electromagnetic waves according to claim 7, wherein the magnetic field system includes means for reversing the direction of the magnetic field.   9. The polarization component due to the Hall effect of the material to be measured is measured based on the difference in electric field of the reflected terahertz electromagnetic wave when the direction of the magnetic field is inverted and non-inverted. Physical property measurement device using terahertz electromagnetic waves.   The physical property measuring apparatus using the terahertz electromagnetic wave according to claim 1, wherein the physical property measuring apparatus using the terahertz electromagnetic wave further includes a computer system.

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