JP2000258363A - Millimeter and submillimeter wave spectrometer - Google Patents

Abstract

(57) [Problem] To provide a small, inexpensive, and stable millimeter-wave / sub-millimeter-wave spectrometer for electromagnetic wave spectroscopy in the millimeter-wave / sub-millimeter-wave region. SOLUTION: An optical switch element 4 is irradiated with an incoherent laser beam of, for example, a multi-mode semiconductor laser of an excitation light source 1 to generate a millimeter wave / sub-millimeter wave band electromagnetic wave. This is made parallel by the parabolic mirror 9 and guided to a Martin-Pullet interferometer. The electromagnetic wave coming out is sample 1
After passing through 0, detection is performed with a hot electron bolometer 11 using a parabolic mirror 9. Thereby, the transmittance of the sample can be obtained. In addition, the reflectivity of the sample can be obtained by measuring the reflected electromagnetic waves while changing the arrangement of the parabolic mirror 9 and the hot electron bolometer 11. By calculating these, the complex refractive index, carrier concentration, mobility, and the like of the sample in the millimeter-wave / sub-millimeter-wave band can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a millimeter-wave / sub-millimeter-wave spectrometer for radiating an electromagnetic wave having a frequency in a millimeter-wave / sub-millimeter-wave band to perform electromagnetic wave spectroscopy. Related to the device.

[0002]

2. Description of the Related Art Electronic elements such as gun diodes and electron tubes such as backward wave tubes oscillate in the millimeter and submillimeter wave bands to generate electromagnetic waves. The electromagnetic wave transmittance of a material is measured using the generated electromagnetic wave. In addition, the measurement of the refractive index and the dielectric constant of an electromagnetic wave using a radio wave interferometer is also performed. However, the frequencies of the electromagnetic waves of these light sources are discrete or limited to a narrow frequency range around a certain frequency. Therefore, a spectrometer using these light sources cannot measure the electromagnetic wave transmittance, the electromagnetic wave refractive index, the dielectric constant, and the like over a wide frequency range.

[0003] Therefore, utilizing the fact that a heated black body or mercury lamp generates electromagnetic waves in a wide frequency range of the millimeter wave and submillimeter wave regions by thermal radiation or quantum transition of atoms, an interferometer is used as a light source. Electromagnetic spectroscopy has been performed by the used Fourier spectroscopy.

On the other hand, apart from these, a spectroscopic method using an optical switch element has been developed since the 1980's, and an example thereof is described in M. Exter et al., Opt. Lett.
989) 2277 ”and“ D. Grischkowsky et al., J. Opt.
c. Am. B7 (1990) 2006 ”can be cited.
In the spectroscopy method using this optical switch element, electromagnetic wave radiation is performed by utilizing the fact that when the optical switch element is irradiated with a sub-picosecond ultrashort light pulse, it becomes instantaneously conductive due to the generation of an optical carrier. . In addition, the detection of radiated electromagnetic waves is also performed by utilizing the instantaneous conductivity.

The above-described spectroscopy will be described in detail with reference to the schematic configuration diagram of FIG. The conventional spectroscopy shown in FIG.
me Domain Spectroscopy). FIG.
In the TDS, an ultrashort pulse light 27 from a mode-locked titanium sapphire (Ti: Sapphire) laser or the like is split by a beam splitter 28, and one of the pulse lights 29 is transmitted through an optical chopper 40 and a lens 31. The light is applied to the electromagnetic wave emission optical switch element 32 to which a voltage is applied by the power supply 34.

[0006] Since the current instantaneously flows through the optical switch element 32 for radiating the electromagnetic wave by the irradiation of the pulse light 29, the electromagnetic wave pulse 36 is radiated by the current modulation. This is collimated by the first parabolic mirror 35a and irradiated onto the sample 37,
The electromagnetic pulse 36 passes through the sample 37.

Then, the transmitted electromagnetic wave pulse 36 is condensed by the second parabolic mirror 35b on the receiving side, and is irradiated to the electromagnetic wave receiving optical switch element 33. At this time, the other ultrashort pulsed light 30 split by the beam splitter 28 is irradiated to the electromagnetic wave receiving optical switch element 33 via the delay system, and the electromagnetic wave receiving optical switch element 33 becomes conductive only at the moment of irradiation. The electric field of the arrived electromagnetic wave pulse 36 is detected as a current. Furthermore, ultrashort pulse light 30
By changing the time from the beam splitter 28 to reach the electromagnetic wave receiving optical switch element 33 on the receiving side by the delay system, the time waveform of the electromagnetic wave 36 transmitted through the sample 37 is obtained.

Therefore, a semiconductor sample operating at a high frequency can be irradiated with a radiated electromagnetic wave to detect a transmitted electromagnetic wave of the semiconductor sample as a current, and a complex physical quantity can be obtained from the detected transmittance of the sample. In addition,
6, 38 is a current amplifier, 39 is a lock-in amplifier,
41 is a plane mirror, 42 is a retroreflector, 43 is a moving stage, and 44 is a computer.

[0009]

However, in such a conventional spectroscope, when the light source is an electronic element or an electron tube, data over a wide frequency range cannot be obtained, and a heated black body, a mercury lamp, etc. In this case, the output was low on the low frequency side, particularly at 150 GHz or less, and it was difficult to perform spectroscopy.

[0010] Further, in the TDS, although an optical switch element excited by an excitation pulse laser can emit electromagnetic waves over a wide frequency range, a large and extremely expensive apparatus is required as an excitation laser.

Therefore, the present invention can radiate an electromagnetic wave in the millimeter / submillimeter wave range from 0 to several THz, and has a small, inexpensive, stable electromagnetic wave excitation light source in the millimeter / submillimeter wave band in electromagnetic wave spectroscopy. It is intended to provide a device.

[0012]

In order to achieve the above object, a millimeter-wave / sub-millimeter-wave band spectrometer according to the first aspect of the present invention is produced by irradiating an incoherent light from an excitation light source to an optical switch element. In this configuration, the light is separated using electromagnetic waves in the millimeter and submillimeter wave bands. Furthermore, the invention according to claim 2 includes an excitation light source that emits incoherent light, and an optical switch element that generates a photocurrent that performs high-speed modulation based on the irradiation of incoherent light and emits millimeter-wave / sub-millimeter-wave electromagnetic waves. The apparatus is configured so that the emitted electromagnetic waves are separated into electromagnetic waves through a two-beam interferometer. The invention according to claim 3 is
In addition to the configuration described in claim 1 or 2, the pump light source is a multimode semiconductor laser. The invention according to claim 4 is characterized in that the excitation light source is a laser diode made of aluminum gallium arsenide. The invention according to claim 5 is characterized in that the optical switch element is a high-temperature superconducting thin-film element. The invention according to claim 6 is characterized in that the optical switching element is a semiconductor optical switching element. Further, the invention according to claim 7 is characterized in that the two-beam interferometer is a martin-applet interferometer.

In the millimeter-wave / sub-millimeter-wave spectrometer of the present invention having such a configuration, a high-speed modulation current flows through the optical switch element based on the intensity modulation of the incoherent light, and the optical switch element is driven in the millimeter-wave / submillimeter-wave band Emits electromagnetic waves.
The object to be measured is irradiated with the radiated electromagnetic wave via a two-beam interferometer that gives an optical path difference to the radiated electromagnetic wave to perform electromagnetic wave spectroscopy.

Although the coherent light having good monochromaticity has a constant intensity even when viewed on a short time scale, the incoherent light according to the present invention includes different wavelengths and frequencies and generates a beat due to the overlap of these lights. Includes noise over frequencies with amplitudes faster than quantum fluctuations in intensity. Therefore, when this incoherent light is applied to the optical switch element, a photocurrent accompanying fast intensity modulation flows, and an electromagnetic wave in the millimeter wave / submillimeter wave band can be emitted.

When the pumping light source is a multi-mode semiconductor laser, it is small, inexpensive and stable. In the case where the two-beam interferometer is a Martin-Pullet interferometer, there is almost no frequency dependency in the millimeter wave / submillimeter wave band, and the configuration is simplified.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 1 is a schematic configuration diagram showing a first embodiment according to the present invention. A first embodiment according to the present invention will be described with reference to FIG. 1. This spectroscopic device includes an excitation light source 1 that emits incoherent light,
An optical switch element 4 to which the incoherent light is irradiated by an optical system via, for example, a lens 2a, an optical chopper 3, and a lens 2b; a silicon hemispherical lens 5 provided with the optical switch element 4; A first parabolic mirror 9a is provided for converging and collimating the electromagnetic waves emitted by the switch element 4 via a hemispherical lens 5 made of silicon.

An optical switch element that follows current modulation at frequencies in the millimeter-wave and sub-millimeter-wave bands has a small light-irradiated portion.
A small and condensable light source is desirable as the excitation light source. As an excitation light source that emits such incoherent light, for example, a multi-mode semiconductor laser can be used. As the multimode semiconductor laser, a laser diode made of a compound semiconductor, for example, a laser diode made of aluminum gallium arsenide (AlGaAs) can be used. The multi-mode semiconductor laser is small and inexpensive, and has stable performance. In addition, maintenance is very easy because only laser light that does not require adjustment of cavity resonance is collected.

The optical switch element 4 is formed on the surface of the semiconductor chip, and a hemispherical silicon lens 5 is provided in close contact with the lower surface of the semiconductor chip with the optical switch element 4 on the upper surface. The optical switch element 4 may be any element as long as incoherent light is applied to generate photocarriers of electron-hole pairs and flow current. As the optical switch element, for example, a semiconductor optical switch element such as a compound semiconductor or a high-temperature superconducting thin film element can be used. Note that a voltage is applied to the optical switch element as needed.

Further, in the first embodiment, the sample 10 irradiated with the electromagnetic wave collimated by the first parabolic mirror 9a through, for example, a two-ray interferometer, and the electromagnetic wave transmitted through the sample 10 are collected. It has a second parabolic mirror 9b that emits light, and a hot electron bolometer 11 that detects electromagnetic waves collected by the second parabolic mirror 9b.

The two-beam interferometer having an output in the millimeter-wave / sub-millimeter-wave band and having no frequency dependence is optimal. For example, a multi-unit
A Martin-Puplet interferometer is available. This martin-applet interferometer is shown in FIG.
The first wire grid polarizer 6a and the second
, A fixed plane mirror 7a, and a movable plane mirror 7b disposed on the moving stage 8, and the optical path difference of the interferometer is controlled by the computer 13 in synchronization with the optical chopper 3. . In FIG. 1, reference numeral 12 denotes a lock-in amplifier.

As the two-beam interferometer, a Mach-Zehnder interferometer or a Michelson interferometer can be used.

Next, the operation of the present embodiment will be described. Laser light, which is incoherent light from, for example, a multi-mode semiconductor laser of the excitation light source 1, is supplied to an optical chopper 3
The light is focused on the optical switch element 4 in a rectangular shape by an optical system including. Since incoherent light includes modulation in intensity,
The current flowing through the optical switch element 4 undergoes high-speed modulation, and the optical switch element 4 emits an electromagnetic wave.

In particular, in a multi-mode semiconductor laser, since the emitted laser light includes many modes, the light intensity beats due to the superposition of the modes. When two modes are taken, the frequency is the difference frequency of the modes, and the spectrum width of the laser beam extends to about 30 cm ^-1 . Therefore, the modulation of the light intensity extends over several THz. Therefore, when the optical switch element is irradiated with the laser light of the multi-mode semiconductor laser, a photocurrent modulated at a high speed is generated, and the optical switch element irradiates the millimeter-wave / sub-millimeter-wave electromagnetic wave.

Next, the optical switch element 4 radiates electromagnetic waves almost inside the semiconductor chip. The silicon hemispherical lens 5 disposed closely to the semiconductor chip effectively takes out the radiated electromagnetic waves into the space, and the efficiency of taking out the electromagnetic waves is improved.

This radiated electromagnetic wave is condensed and collimated by a first parabolic mirror 9a, and is converted into a Martin pullet (Martin) composed of a first wire grid polarizer 6a, a fixed plane mirror 7a and a movable plane mirror 7b on a moving stage 8. -Pupl
ett) lead to the interferometer. The radiated electromagnetic waves having an optical path difference and emitted from the interferometer are collected by the second parabolic mirror 9b, and the collected radiated electromagnetic waves are detected by the hot electron bolometer 11.

The radiated electromagnetic wave is subjected to rectangular modulation based on the timing of the optical chopper 3. Only the detection signal synchronized with the rectangular modulation is detected by the lock-in amplifier 12 from the power spectrum of the hot electron bolometer. The computer 13 controls the optical path difference in synchronization with the optical chopper 3 and obtains a Fourier spectrum by Fourier spectroscopy from a change in the signal intensity of the lock-in amplifier 12 caused by changing the optical path difference.

Finally, if the sample 10 is inserted between the interferometer and the second parabolic mirror 9b and the optical path difference is changed to obtain the spectrum of the transmitted electromagnetic wave, the electromagnetic wave transmittance of the sample can be known. it can.

FIG. 2 shows transmittance data for an n-type Si wafer (10 Ω · cm, thickness 0.5 mm) in the first embodiment. As shown in FIG. 2, in the present embodiment, the transmittance of an n-type Si wafer up to 500 GHz can be measured. The data curve has a waveform structure, which is due to the effect of interference inside the sample. As described above, the transmittance of a semiconductor element such as a Si wafer can be obtained in the millimeter wave / submillimeter wave band.

The electrical characteristics of a low-concentration silicon semiconductor can be well described using a free-electron model, and the scattering time in the free-electron model can be considered to be constant irrespective of the concentration in a concentration range of 10 ¹⁵ cm ^-3. Therefore, the carrier concentration can be determined. Thus, the electrical characteristics of the Si wafer can be calculated from DC to RF.

Next, a second embodiment of the present invention will be described. FIG. 3 is a schematic configuration diagram showing a second embodiment according to the present invention. In the first embodiment, the arrangement is such that the transmitted electromagnetic wave of the sample is measured. In the second embodiment, the arrangement is such that the reflected electromagnetic wave of the sample is measured. That is, the electromagnetic wave reflected by the sample 23 is collected by the parabolic mirror, collected by the hot electron bolometer 24, and detected.

By the way, in this frequency region, metal can be regarded as having a reflectance of 100%. Therefore, by comparing the Fourier spectrum of the electromagnetic wave reflected by the sample 23 with the Fourier spectrum obtained by replacing the sample 23 with a plane mirror, The reflection spectrum of the sample 23 can be obtained. n-type Si wafer (10Ωcm,
FIG. 4 shows the data of the reflectance with respect to the thickness of 0.5 mm). As described above, the reflectance of a semiconductor element such as a Si wafer can be obtained in the millimeter wave / submillimeter wave band.

Further, as shown in FIG. 4, the reflection spectrum is data complementary to the transmission spectrum according to the first embodiment. Even when a model describing the electrical characteristics cannot be assumed, a complex physical quantity at each frequency, for example, a complex refractive index, a carrier concentration, a mobility, and the like can be obtained from these two data. As an example, FIG. 5 shows a complex refractive index calculated from the data of FIGS.

As described above, in order to evaluate the electrical characteristics of the millimeter-wave / sub-millimeter-wave band, the optical switch element is irradiated with incoherent light to generate millimeter-wave / sub-millimeter-wave electromagnetic waves. By irradiating an ultra-high-speed semiconductor device operating at a frequency in the sub-millimeter wave band and performing spectroscopy, electrical characteristics of the ultra-high-speed semiconductor device can be evaluated.

[0034]

As will be understood from the above description, the millimeter-wave / sub-millimeter-wave spectrometer of the present invention irradiates an optical switch element with incoherent light to generate a millimeter-wave / sub-millimeter-wave electromagnetic wave. Thus, there is an effect that electromagnetic wave spectroscopy can be performed using a light source that generates this millimeter wave / submillimeter wave band electromagnetic wave. Therefore, it is possible to provide a small, inexpensive, and stable millimeter-wave / sub-millimeter-wave spectrometer in the millimeter-wave / sub-millimeter-wave band range of 0 to several THz.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment according to the present invention.

FIG. 2 is a diagram showing frequency dependence of transmittance according to the first embodiment of the present invention.

FIG. 3 is a schematic configuration diagram showing a second embodiment according to the present invention.

FIG. 4 is a diagram showing frequency dependence of reflectance according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a complex refractive index calculated from the data shown in FIGS. 2 and 4;

FIG. 6 is a schematic configuration diagram of a conventional spectroscopy (TDS).

[Explanation of symbols]

 1,14 Excitation light source 2a, 2b, 15a, 15b Lens 3,16 Optical chopper 4,17 Optical switch 5,18 Silicon hemispherical lens 6a, 19a First wire grid polarizer 6b, 19b Second wire grid polarizer 7a, 20a Fixed plane mirror 7b, 20b Movable plane mirror 8, 21 Moving stage 9a, 22a, 35a First parabolic mirror 9b, 22b, 35b Second parabolic mirror 10, 23 Sample 11, 24 Hot electron bolometer 12, 25 Lock-in amplifier 13, 26 Computer 27 Optical pulse 28 Beam splitter 29 Pulse light (for electromagnetic wave radiation) 30 Pulse light (for electromagnetic wave reception) 31 Lens 32 Electromagnetic wave radiation optical switch element 33 Electromagnetic wave optical switch element 34 Power supply 36 Electromagnetic wave pulse 37 Sample 38 Current amplifier 39 Lock Quin amplifier 40 optical chopper 41 plane mirror 42 retroreflector 43 moving stage 44 computer

Claims (7)

[Claims] 1. A millimeter-wave / sub-millimeter-wave spectral device that irradiates an optical switch element with incoherent light from an excitation light source and performs spectroscopy using millimeter-wave / sub-millimeter-wave electromagnetic waves. 2. An excitation light source that emits incoherent light, and an optical switch element that generates a photocurrent that performs high-speed modulation based on irradiation of the excitation light source with incoherent light and emits an electromagnetic wave in a millimeter-wave or sub-millimeter-wave band. A millimeter-wave / sub-millimeter-wave spectrometer for spectrally separating the emitted electromagnetic waves through a two-beam interferometer. 3. The millimeter-wave / sub-millimeter-wave band spectrometer according to claim 1, wherein the excitation light source is a multimode semiconductor laser. 4. The millimeter-wave / sub-millimeter-wave spectrometer according to claim 1, wherein the excitation light source is a laser diode made of aluminum gallium arsenide. 5. The millimeter-wave / sub-millimeter-wave band spectrometer according to claim 1, wherein the optical switch element is a high-temperature superconducting thin-film element. 6. The millimeter wave device according to claim 1, wherein said optical switching device is a semiconductor optical switching device.
Submillimeter wave spectrometer. 7. The millimeter-wave / sub-millimeter-wave band spectrometer according to claim 2, wherein said two-beam interferometer is a martin-applet interferometer.