JP2006145372A - Terahertz electromagnetic wave generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a teraheltz electromagnetic wave generator of high efficiency having a high output and a wide band. <P>SOLUTION: An energy gap of a radiation antenna material is made to be 1.42 eV or more provided in LT-GaAs to enhance a breakdown voltage of a photoconductive element for detection and to enhance a breakdown voltage, and a laser wavelength shorter than 800nm is used conformed to the material. An applied voltage is brought thereby into 15 V or more, and a laser beam intensity is brought thereby into 15 mW or more to emit an electromagnetic wave. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a terahertz electromagnetic wave generator that can generate a terahertz electromagnetic wave having a high intensity and can be applied to high-speed and large-capacity wireless communication, or is used for evaluating a material such as a semiconductor or a dielectric, and particularly, a terahertz band. The present invention can also be applied as a complex dielectric constant measuring apparatus for measuring a complex refractive index of a material constituting an element used in the above.

  Conventionally, industrial applications such as the terahertz band have not progressed, and a backward wave tube, a molecular laser, or the like has been used as an electromagnetic wave source for a frequency region that can be said to be undeveloped. (The terahertz band referred to here is a band from 0.1 THz to 5 THz, which is the longest wavelength region having light wave straightness and particle property, and at the same time having the shortest wavelength region having radio wave transparency. On the other hand, a hot electron bolometer that measures resistance change due to electromagnetic wave absorption of an electron gas in a solid such as InSb is frequently used for detection.

  However, these electromagnetic wave sources have a narrow frequency range even if the frequency is discrete or variable, and the obtained electromagnetic wave intensity is as weak as 1 microwatt (μW) or less, so terahertz band electromagnetic waves are in the range of 1 m to 10 m. It was difficult to send and receive.

  In order to solve these problems, a spectroscopic method using a photoconductive element excited by a pulse laser was developed around 1985.

  In this spectroscopic method using a photoconductive element, if a photoconductive element is irradiated with an ultrashort optical pulse of subpicosecond, it becomes conductive due to the generation of a photocarrier and the current flows transiently. Conducting electromagnetic radiation. In addition, detection of radiated electromagnetic waves has also been performed by utilizing the fact that it becomes instantaneously conductive when irradiated with light pulses.

  In an example using an optical pulse, there is an apparatus called TDS (Time Domain Spectroscopy) (see Patent Document 1) as an apparatus for measuring the response of a sample to a high-frequency electromagnetic wave in the vicinity of 1 THz.

  FIG. 8 is a schematic configuration diagram of a conventional TDS. As shown in this figure, in TDS, an ultrashort pulse light 23 from a femtosecond laser 22 made of a mode-locked Ti: Sappire laser or the like is split by a beam splitter 24, and one ultrashort pulse light 25 (ultrashort pulse light) is split. In order to discriminate between them, the ultrashort pulse light 25 is sometimes referred to as “first ultrashort pulse light”), and is irradiated to the electromagnetic wave radiation antenna 28 formed of a plate-like photoconductive element. As shown in FIG. 2, a coplanar counter electrode made of AuGe / Ni / Au is provided on the surface of the flat plate photoconductive element used as the electromagnetic wave radiation antenna 28, and a power source is provided between them. A voltage is applied from 30. When the first ultrashort pulse light 25 is irradiated between the electrodes of the electromagnetic wave radiation antenna 28, an electron-hole pair is instantaneously formed, whereby a current flows instantaneously in the electromagnetic wave radiation antenna 28. Sometimes a pulse electromagnetic wave 32 is emitted. The pulsed electromagnetic wave 32 is collimated by the first parabolic mirror 31 and transmitted through the sample 34, and collected by the second parabolic mirror 36 on the back surface of the receiving antenna 29 for electromagnetic wave detection.

  On the surface of the receiving antenna 29, another ultrashort pulse light 26 divided by the beam splitter 24 (in order to distinguish the ultrashort pulse light strictly, this ultrashort pulse light 26 is referred to as “second ultrashort pulse light”). ), And becomes conductive only at that moment (that is, the moment when the ultrashort pulse light 26 is irradiated). At this time, since no electric field is applied between the electrodes of the receiving antenna 29, no current is generated. When electromagnetic waves radiated from the radiation antenna enter while exhibiting electrical conductivity, the state is the same as when an electric field possessed by the electromagnetic waves is applied between the electrodes of the reception antenna 29. On the other hand, the amount of correlation between the light delayed by the optical delay stages 41 and 42 and the electromagnetic wave 35 transmitted through the sample 34 varies depending on the amount of delay. This correlation amount is observed as an amount of current corresponding to the electric field strength of the electromagnetic wave. That is, a current amount corresponding to the pulse shape possessed by the electric field strength of the electromagnetic wave is obtained. This is the principle of electromagnetic pulse detection by the cross correlation method. As described above, the cross-correlation signal between the electromagnetic wave 35 transmitted through the sample 34 and the ultrashort pulsed light 26 that has passed through the optical delay stages 41 and 42 can be detected as a current. Even high-speed electromagnetic waves that cannot be fully responded by electronic devices can be detected.

  At this time, the electromagnetic radiation antenna 28 and the electromagnetic wave detection antenna 29 are made of GaAs (which may be referred to as low temperature GaAs or LT-GaAs) having a substrate temperature of 300 ° C. to 450 ° C., and a femtosecond laser wavelength. For example, 800 nm having a shorter wavelength than the absorption edge (873 nm) of GaAs is used.

  In FIG. 8, 13 is a lens, 30 is a power supply, 34 is a sample, 37 is a current amplifier, 38 is a lock-in amplifier, 33 is an optical chopper, 40 is a plane mirror, 41 is a retroreflector, and 42 is a movable optical delay stage. , 43 indicates a computer.

  In this terahertz TDS method, since the electromagnetic wave used is a short pulse, it is excellent in coherency. Therefore, important information can be obtained from the intensity and phase of the electromagnetic wave, and it can be applied as a device for measuring physical properties. By comparing the electromagnetic wave waveform transmitted through the sample 34 with the electromagnetic wave waveform when the sample 34 is not inserted, the transmittance and phase delay of the electromagnetic wave over a wide frequency can be calculated. The actual data obtained by this method can be obtained a time-resolved spectrum with the horizontal axis as shown in FIG. 7A, and this is Fourier transformed as shown in FIG. 7B. A frequency spectrum in which the horizontal axis is the frequency component can be obtained.

  By the way, the photoconductive element for detection detects the current driven by the pulsed electromagnetic field that is incident during the light pulse irradiation, and the time width of the light pulse is about one tenth of the time width of the pulse electromagnetic wave. Pretty short. For example, the time width of an optical pulse is about 0.1 picoseconds, and the time width of a pulse electromagnetic wave is about several picoseconds due to antenna efficiency.

  Therefore, although both the light pulse and the pulse electromagnetic wave repeatedly enter the detection photoconductive element at the speed of light, the irradiation time of the light pulse is shorter than the time from the first part to the last part of the pulse electromagnetic wave at each time. Therefore, the current flowing through the detection photoconductive element during irradiation with the light pulse is due to a very short portion of the electric field of the pulse electromagnetic wave, and the timing at which the light pulse and the pulse electromagnetic wave reach the detection photoconductive element. Is fixed by time delay.

When the repetition frequency of the light pulse is about 100 MHz, for example, the light pulse and the pulse electromagnetic wave are incident on the detection photoconductive element about 108 times per second. This is a portion determined by the time delay, and the same current flows about 108 times per second. Since an actual ammeter cannot follow such a fast current change, an average value of about 108 pulse currents per second is measured. Therefore, the part determined by the time delay in the pulse electromagnetic wave waveform is measured as a current, and the other part of the pulse electromagnetic wave waveform can be measured by further shifting the time delay. Such a terahertz electromagnetic wave generation / detection device is today one of the most excellent terahertz electromagnetic wave generation methods and detection methods in terms of both output characteristics and broadband characteristics.
JP 2001-21503 A JP-A-62-281477 JP-A-62-196876 JP 63-012120 A Physical Review Letters vol.55 (1985), pp.2152-2155 InfraredPhysics vol.26 (1986), pp.23-27

  The TDS method described above is the simplest method that can generate a coherent and wideband electromagnetic wave at present, but it can only emit up to about 10 μW in intensity. Further, although the band reaches up to about 1.5 THz, the maximum value is around 0.3 THz, and the intensity at 1.5 THz is only about 0.1 μW.

  In order to use terahertz waves for space transmission such as communication applications, it is necessary to radiate stronger electromagnetic waves. Also, when used as a measuring instrument, if the electromagnetic wave intensity is not sufficient in the region of 1 to 3 THz, the S / N cannot be obtained and the reliability of the measured value is compromised.

  Therefore, a terahertz band electromagnetic wave generator having a higher output and a wider band is required.

  On the other hand, if the applied voltage or incident light intensity is set to 15 V or 15 mW or higher as used in the present situation even if more intense electromagnetic wave radiation is actually performed, the antenna material breaks down and the antenna is broken. In practice, this value is almost the limit in the generation method of the terahertz electromagnetic wave using the prior art.

  In view of the above-described problems, an object of the present invention is to provide a terahertz electromagnetic wave generator having a high output, a broadband, and a high efficiency.

In order to achieve the object, first, it is necessary to increase the breakdown voltage of the device to improve the breakdown voltage. For this purpose, the radiation antenna material is made of a material larger than the energy band gap value of 1.42 eV of LT-GaAs, and a pulse laser wavelength used is shorter than 800 nm in accordance with the material. As a result, the applied voltage can be set to 15 V or more, that is, the electric field strength can be made larger than 3 × 10 ⁴ V / cm.

  In addition, a nonlinear optical material was used to shorten the wavelength, thereby applying a compression effect to the pulse laser width and expanding the band of generated electromagnetic waves.

  Furthermore, in order to generate a terahertz electromagnetic wave more efficiently, it is necessary to increase the carrier concentration. For this purpose, the electromagnetic wave is emitted with the laser light intensity set to 15 mW or more.

Specifically, a terahertz wave generation device including a light source that emits ultrashort pulse light and an electromagnetic wave radiation antenna,
The antenna for electromagnetic wave radiation has a semi-insulating substrate, a photoconductive thin film formed on the semi-insulating substrate and having a band gap energy of 1.42 eV or more, and a coplanar transmission line,
The coplanar transmission line has a pair of transmission line electrode bodies, and protruding electrodes respectively protruding from the transmission line electrode bodies,
The pair of transmission line main bodies are arranged in parallel,
The protruding electrodes face each other,
The applied electric field strength between the electrodes is an electric field larger than 3 × 10 ⁴ V / cm,
Electromagnetic waves are generated by irradiating ultrashort pulse light having a wavelength of less than 800 nm and an average intensity of more than 20 mW between the protruding electrodes.

  According to the second aspect of the present invention, the pulse time width of the ultrashort pulse light of such a terahertz electromagnetic generator is 100 femtoseconds or less, and the pulse time width of the pulse electromagnetic wave is 5 picoseconds or less.

In the third aspect of the present invention, the ultrashort pulse light of such a terahertz electromagnetic wave generator has a femtosecond laser having a wavelength of 800 nm and a pulse width of 100 femtoseconds or less as a main light source,
Pulse light emitted from the main light source is transmitted through an element that can compress the pulse width at the same time as the wavelength is less than half,
An electromagnetic wave is generated by irradiating the electromagnetic wave radiation antenna with the pulsed light.

  This makes it possible to generate a larger electromagnetic wave than before, and the following measures are required to realize it.

That is, according to the fourth aspect of the present invention, the antenna material for electromagnetic wave radiation of such a terahertz electromagnetic wave generator is In _x Ga _1-x grown on any one of an Al ₂ O ₃ substrate, a SiC substrate, a GaN substrate, and a ZnO substrate. N (0.05 <x <0.4) film, In _x Al _1-x N (0.5 <x <0.8) film grown on BAs substrate, Al _x Ga _1-x on GaAs substrate As film (0 <x <0.8), Al _x Ga _1-x P (0 <x <0.8) film on GaP substrate, ZnSe _x S _1-x (0 < x <0.3) film or a CuGaSe ₂ film on a quartz substrate.

  This provides a means for solving the problem. Further, by using this terahertz electromagnetic wave generator, it is possible to accurately measure the dielectric constant of a sample in a band exceeding 1.5 THz.

That is, the fifth aspect of the present invention uses a terahertz electromagnetic wave generating / detecting device including a terahertz electromagnetic wave generating device.
A method for measuring a dielectric constant of a sample, comprising:
A light source that emits ultrashort pulse light, a beam splitter, an antenna for electromagnetic wave emission, an antenna for electromagnetic wave detection, and an optical delay system,
The ultrashort pulse light emitted from the light source is divided into a first ultrashort pulse light and a second ultrashort pulse light by the beam splitter,
The first ultrashort pulse light is transmitted through an element capable of compressing the pulse width at the same time as the wavelength is less than half, and then irradiated to an electromagnetic wave radiation antenna composed of a flat photoconductive element. , A pulse electromagnetic wave is radiated from the electromagnetic wave radiation antenna,
The pulse electromagnetic wave radiated from the electromagnetic wave radiation antenna reaches the back surface of the electromagnetic wave detection antenna,
The second ultrashort pulse light reaches the surface of the electromagnetic wave detection antenna through the optical delay system,
The electromagnetic wave radiation antenna includes a semi-insulating substrate, a photoconductive thin film formed on the semi-insulating substrate, having a band gap energy of 1.42 eV or more, and a coplanar transmission line.
The coplanar transmission line has a pair of transmission line electrode bodies, and protruding electrodes respectively protruding from the transmission line electrode bodies,
The pair of transmission line main bodies are arranged in parallel,
The protruding electrodes face each other,
The measurement method is:
A sample setting step of setting the sample between the antenna for electromagnetic wave radiation and the antenna for electromagnetic wave detection, and a pulse transmitted through the sample by radiating a pulse electromagnetic wave from the antenna for electromagnetic wave radiation toward the sample An electromagnetic wave is made to reach the surface of the electromagnetic wave detection antenna, and a cross-correlation signal between the pulse electromagnetic wave that has reached the surface of the electromagnetic wave detection antenna and the second ultrashort pulse light that has reached the back surface of the electromagnetic wave detection antenna is obtained. Pulse electromagnetic wave emission process to detect as current,
Is included.

  According to a sixth aspect of the present invention, there is provided a dielectric constant measurement method using a terahertz electromagnetic wave generating / detecting device including such a terahertz electromagnetic wave generating device, wherein the material for the electromagnetic wave detecting antenna is crystal grown at a substrate temperature of 300 ° C. to 450 ° C. The GaAs is used, the antenna shape is a dipole type, and the electrode shape of the radiation antenna is a bow tie type.

  According to the terahertz band electromagnetic wave radiation antenna of the present invention, a terahertz band electromagnetic wave having a maximum peak intensity of 10 μW or more can be generated, and an output of 1 μW or more can be obtained even in a region exceeding 1.5 THz. By using such an electromagnetic wave radiation antenna, a high-power and high-efficiency terahertz-band electromagnetic wave generator is provided, which can detect the electromagnetic wave intensity with sensitivity up to the 5 THz band and can observe the dielectric constant of the substance. Can be used.

(Embodiment 1)
The problem in the present invention is to increase the intensity of the generated terahertz electromagnetic wave. In order to overcome this problem, the electromagnetic wave is radiated in the form shown in FIG. That is, the ultrashort pulse light source 11 is configured by the combination of the femtosecond laser 22 having a pulse width of 100 fs or less, a wavelength of 800 nm, and a repetition frequency of 84 MHz and the SHG element 39 for wavelength conversion. It is emitted from the pulse light source 11. The ultrashort pulse light 12 of less than 800 nm is collected by the lens 13 and irradiated to a radiation antenna 28 made of a material having a band gap energy larger than 1.42 eV, whereby a terahertz electromagnetic wave 32 is obtained with high intensity. . A voltage is applied to the radiation antenna 28 by a voltage source 30.

  Next, each part shown in FIG. 1 will be described.

  FIG. 2 shows an external view of the electromagnetic wave radiation antenna 10 used in the present invention. This photoconductive element is incorporated in the terahertz electromagnetic wave generator of the present invention as an electromagnetic wave radiation antenna indicated by reference numeral 10 in FIG.

  This photoconductive element is flat, and a 6H-SiC film is epitaxially grown on a semi-insulating SiC substrate by a CVD method to a thickness of 1 to 5 μm to form a photoconductive thin film. Thereafter, AuGe / Ni / Au A coplanar transmission line made of electrodes is provided. The coplanar transmission line includes a pair of transmission line electrode bodies and protruding electrodes that protrude from the transmission line electrode bodies. The pair of transmission line electrode bodies are arranged so as to be parallel, and the protruding electrodes respectively protruding from the pair of transmission line electrode bodies are formed to face each other. These two opposing protruding electrodes form a minute dipole antenna.

  FIG. 3 (a) shows the dimensions of the minute dipole antenna. The interval between the coplanar transmission lines is set to 20 μm, and the interval between the protruding electrodes is set to 5 μm.

  In FIG. 2, the radiating antenna structure is shown with a dipole electrode configuration, but a bow tie electrode configuration as shown in FIG. 3B is also often used. In the bow tie type shown in FIG. 3B, the shape of the protruding electrode is different. In the dipole type of FIG. 3A, the protruding electrode is rectangular, and in the bow tie type of FIG. 3B, the protruding electrode is trapezoidal. In the case of the bow tie type, the distance between the coplanar transmission lines is 2 mm, the distance between the protruding electrodes is 5 μm, and the upper base length of the trapezoid is 5 μm.

  In any electrode configuration, the minimum distance between the two protruding electrodes facing each other is 5 μm, and the ultrashort light pulse 12 incident on the antenna via the lens 13 (see FIG. 1) has the minimum distance between the two protruding electrodes facing each other. The light is irradiated toward the portion (hereinafter, this portion is referred to as “current passing region”).

At this time, as the electromagnetic radiation antenna 28, a 6H-SiC103 film crystal-grown on the SiC substrate 101 by the MOCVD method is used, and the ultrashort pulsed light 23 having a wavelength of 800 nm emitted from the femtosecond laser 22 is LiB ₃ which is an SHG element. The electromagnetic radiation antenna 28 is irradiated with the component of the ultrashort pulsed light 12 having a wavelength of 400 nm converted into the second harmonic generated by passing through the O ₅ (LBO) crystal 39.

  The band gap energy of 6H—SiC is 3.0 eV, and the ultrashort pulse light 12 with a wavelength of 400 nm is 3.1 eV when converted to energy. Under this condition, the antenna material can absorb light, and as described above in the description of the prior art, the ultrashort pulse light 12 converted into the second harmonic is interposed between the electrodes of the electromagnetic wave radiation antenna 28. When irradiated, an electron-hole pair is formed instantaneously, whereby an electric current flows instantaneously and a pulse electromagnetic wave 32 is emitted.

  In order to explain that a high-intensity terahertz wave is generated in the above configuration, first, knowledge about the radiation principle of electromagnetic waves is necessary. Based on the experimental results and simulation results obtained so far, the radiation principle of electromagnetic waves will be described.

  In the electromagnetic wave radiation using the above-mentioned photoconductor switch, the intensity of the obtained electromagnetic wave is expressed by the following equation.

  At this time, I is an electromagnetic wave intensity, P is a dipole moment due to electrons and holes generated in the photoconductor, and i is a photocurrent. That is, an electromagnetic wave having an intensity proportional to the time change of the photocurrent is obtained. When this is expressed in more detail using the elementary charge amount e, the carrier concentration n, the mobility μ, and the applied electric field E, it becomes like the right side.

  As can be seen from the right side of this equation, I largely depends on the amount of time change in carrier concentration and the amount of time change in (carrier velocity) = (mobility μ) × (electric field E). Since the applied electric field, that is, the applied electric field between the electrode gaps of the antenna is constant, the amount of change in carrier velocity with time depends on the amount of change in mobility of the carrier. As shown in FIG. 4, the mobility μ in the semiconductor material depends on the applied electric field. The dependency varies greatly depending on the material, but the amount of change also varies greatly depending on the applied electric field.

  What is important here is that the factor that determines the electromagnetic wave intensity is the amount of change in the carrier concentration, and that the absolute value of the mobility of the carrier itself does not work, but the amount of change governs the intensity. It is. This indicates that the antenna material can be used even if it is not a high-speed (ie, high mobility) material with a low electric field.

  FIG. 5A shows the current attenuation characteristics with respect to the response speed of the antenna. The time change of the current generated when light having a pulse width of 75 fs is incident on the antenna with an electric field applied between the electrode gaps, using the response speed (time constant) τ of the antenna as a parameter. It is. Since almost no time lag occurs in the generation of carriers, the rising edge is the same as the pulse width of the pulse laser beam regardless of the magnitude of τ (negative side in terms of time axis). On the other hand, the generated carriers have a time constant depending on the material, so even if the pulse laser irradiation is completed, they do not disappear instantaneously, and electron-hole pairs recombine at a certain time constant τ. And disappear. This process defines what is called the carrier lifetime, which is a property that depends greatly on the material. In this result, it can be seen that the current decay time increases with the value of τ.

  FIG. 5B shows the radiated electromagnetic wave intensity obtained from the current attenuation characteristic. This is a result obtained by expressing the current attenuation characteristic shown in FIG. From the results obtained here, the time constant τ and the electromagnetic wave intensity I of the antenna material are independent of each other, and the electromagnetic wave intensity obtained is the same if the amount of photocurrent generated is the same for any time constant. I understood.

  As described above in the description of the prior art, the terahertz wave is measured by obtaining data as a time domain spectrum and then performing Fourier transform to convert it into a frequency domain spectrum. As a feature of the Fourier transform, the narrower the width of the time domain spectrum, the wider the frequency domain. Therefore, the terahertz electromagnetic wave intensity I is determined by the amount of change in current, and the band of the terahertz electromagnetic wave is determined by the width of the change. FIG. 5B shows that the time constant τ and the electromagnetic wave intensity I are independent of each other and do not affect the band at all. Conversely, if the line width is narrowed by increasing the electromagnetic wave intensity, it is suggested that the band can be widened at the same time.

Based on the above matters, the following four points are necessary for increasing the intensity of the terahertz electromagnetic wave and expanding the band.
1) Increasing the amount of change in carrier concentration over time.
2) To increase the electric field strength applied between the electrodes.
3) Increase the amount of time change in mobility.
4) Compress the pulse width of the incident light pulse laser.

  Returning to FIG. 1 again, the above four conditions are examined.

  In order to increase the time variation of the carrier concentration, the peak intensity of the irradiated laser light pulse may be increased. For this purpose, the material used for the radiation antenna 28 is changed to 1.42 eV which is the band gap energy of LT-GaAs. A material having a larger band gap energy (for example, 6H—SiC. The band gap energy is 3.0 eV) is used. By increasing the band gap energy, the breakdown voltage is improved and the strength against the applied electric field is increased, so that the strength of the electric field applied between the electrodes can be increased. When the electric field strength applied between the electrodes is increased, the absolute value of the saturation electron velocity can be increased when a material having a band gap energy larger than 1.42 eV which is the band gap energy of LT-GaAs is used. As a result, the amount of time change in mobility associated with pulse laser irradiation increases.

  Here, in order to use a material having a band gap energy larger than 1.42 eV, the wavelength of the pulse laser beam is shortened by the SHG element 39. However, in the nonlinear optical effect, the output light is increased to the square of the input optical power. Since the power is proportional, the pulse width is compressed at that time, and the pulse width becomes narrower than that during normal use.

  In summary, an antenna material having a band gap energy greater than 1.42 eV is irradiated with ultrashort pulse light having a higher energy, and a second harmonic generation element such as LBO is used for wavelength conversion. The effect of compressing is also obtained, and all of the above four conditions can be achieved simultaneously, increasing the intensity of the terahertz electromagnetic wave and expanding the band.

  From the above, a terahertz electromagnetic wave generator capable of generating a terahertz band electromagnetic wave having a maximum peak intensity of 10 μW or more according to the present invention and obtaining an output of 1 μW or more even in a region exceeding 1.5 THz is realized.

Note that the material used for the antenna can have the above effects as long as it is a semiconductor having a band gap larger than 1.42 eV. Therefore, an Al ₂ O ₃ substrate, a 6H—SiC substrate, a 4H—SiC substrate, and a GaN substrate. In _x Ga _1-x N (0.05 <x <0.4) film or 4H—SiC film grown on any of the ZnO substrates, In _x Al _1-x N grown on the BAs substrate ( 0.5 <x <0.8) film, Al _x Ga _1-x As film on GaAs substrate (0 <x <0.8), Al _x Ga _1-x P on GaP substrate (0 <x < 0.8) film, ZnSe _x S _1-x (0 <x <0.3) film on Si substrate or GaP substrate, CuGaSe ₂ film on quartz substrate, and the like.

  The ultrashort pulse light 12 converted to the second harmonic does not necessarily need to be shortened by the second harmonic, and the pulse light shortened by the wavelength conversion function of the pulse laser. It is also possible to use. When a titanium sapphire laser is used, wavelength conversion up to 700 nm is possible.

  At the same time, since the pulse laser wavelength can be used with a wavelength conversion function of less than 800 nm, the optimum phase matching angle of the SHG crystal may be selected accordingly, and the wavelength of the femtosecond laser 22 is not limited to 800 nm. The phase matching angle of the SHG element 39 is not used in a combination that corresponds to 400 nm.

  That is, the wavelength of the ultrashort pulse light 12 converted into the second harmonic can be less than 400 nm and up to 350 nm.

  Since the present invention can realize the above four effects at the same time, it cannot be simply estimated to what extent the radiated electromagnetic wave intensity is increased, but at least the breakdown voltage of the element is 1 of the band gap energy. When the antenna is made of a material proportional to .5 power and 3.0 eV, approximately three times as much applied voltage is possible. Actually, in addition to this, the pulse light intensity can be increased, and the pulse width is shortened by compression, so that the time change of the generated carrier concentration and the change of the carrier speed can be increased simultaneously.

  Furthermore, the relative dielectric constant of the material used for the substrate is about 13 for a GaAs substrate, but can be as low as about 9 for a SiC substrate. Loss when passing through the substrate can be reduced. Considering the use of a material other than SiC, the radiation efficiency can be further improved by appropriately selecting a combination of antenna materials.

(Example)
Experimental results in the case of using a photoconductive element actually made of a material having a band gap energy of 1.42 eV or more according to the embodiment will be shown. SiC was used as the photoconductive element material. This SiC photoconductive element is formed by epitaxially growing a SiC film to a thickness of 3 μm on a semi-insulating SiC substrate at a substrate temperature of about 450 ° C. to 650 ° C. by a thermal CVD method, and then forming the surface of the surface. In this configuration, a coplanar transmission line made of AuGe / Ni / Au electrodes is provided. As this electrode structure, a bow tie type shown in FIG.

  The obtained electromagnetic wave includes an ultra-wideband wave of 1.5 THz or more, and if it is not considered to take a wide band at the time of detection, it is guided to a waveguide by a band cut-off antenna such as a horn antenna, a photonic mixer, Detection of down-conversion with a harmonic synthesizer and detection with a power meter using a calorimeter is possible up to about 2 THz, but in this experiment, the above-mentioned terahertz TDS method is adopted in consideration of sensitivity, resolution, and broadband characteristics. did. The optical system is shown in FIG.

  As described above, the terahertz TDS method can simultaneously handle generation and detection of terahertz waves using an ultrashort pulse laser, and is a simple and highly sensitive detection method particularly in detection. The configuration used in this experiment is shown in FIG.

  As shown in this figure, in TDS, an ultrashort pulse light 23 from a femtosecond laser 22 is divided by a beam splitter 24, and the wavelength of the first ultrashort pulse light 25 is converted by an SHG element 39 made of a BLO crystal. The electromagnetic wave radiation antenna 28 composed of a photoconductive element is irradiated. As shown in FIG. 2, a coplanar counter electrode made of AuGe / Ni / Au is provided on the surface of the flat plate photoconductive element used as the electromagnetic wave radiation antenna 28, and a power source is provided between them. A voltage is applied from 30. When the ultrashort pulse light 12 that has been subjected to wavelength conversion is irradiated between the electrodes of the electromagnetic wave radiation antenna 28, an electron-hole pair is instantaneously formed, so that an electric current is instantaneously applied to the electromagnetic wave radiation antenna 28. At this time, a pulse electromagnetic wave 32 corresponding to the equation described in (Equation 1) is emitted. The pulse electromagnetic wave 32 is collimated by the first parabolic mirror 31 and collected on the back surface of the electromagnetic wave detection antenna 29 by the second parabolic mirror 36.

  The surface of the electromagnetic wave detection antenna 29 is irradiated with the second ultrashort pulse light divided by the beam splitter 24 and becomes conductive only at that moment (that is, the moment when the ultrashort pulse light 26 is irradiated). At this time, since no electric field is applied between the electrodes of the receiving antenna 29, no current is generated. When electromagnetic waves radiated from the radiation antenna enter while exhibiting electrical conductivity, the state is the same as when an electric field possessed by the electromagnetic waves is applied between the electrodes of the reception antenna 29. On the other hand, the amount of correlation between the light delayed by the optical delay stages 41 and 42 and the electromagnetic wave 32 collected by the second parabolic mirror 36 varies depending on the amount of delay. This correlation amount is observed as an amount of current corresponding to the electric field strength of the electromagnetic wave.

  At this time, 6H—SiC is used for the photoconductive element material of the electromagnetic wave radiation antenna 28, and LT-GaAs obtained by crystal growth at a low substrate temperature of 300 ° C. to 450 ° C. is used for the electromagnetic wave detection antenna 29. The wavelength of the femtosecond laser 22 is 800 nm, and the wavelength of the ultrashort pulse light 12 converted by the SHG element 39 made of LBO crystal is 400 nm.

  In FIG. 6, 13 is a lens, 30 is a power supply, 37 is a current amplifier, 38 is a lock-in amplifier, 33 is an optical chopper, 40 is a plane mirror, 41 is a retroreflector, 42 is a movable optical delay stage, and 43 is a computer. Indicates.

  In this experiment, importance is attached to wideband characteristics. Therefore, the electrode structure of the receiving antenna is a dipole type (see FIG. 3 (a)) with better broadband characteristics than the bow-tie type. As a material, LT-GaAs was used as a photoconductive element, and the shape of the antenna electrode of the radiation antenna was a bow-tie type in order to further increase the radiation intensity.

  At this time, the experiment is performed with the voltage applied to the radiation antenna being 50 V (the electric field strength is 100 kV / cm) and the average incident light intensity is 40 mW.

  FIG. 7 shows the experimental results. In FIG. 7 (a), the black dots and the white squares in FIG. 7 (b) show the experimental results when an LT-GaAs photoconductive element (conventional one) is used as the radiation antenna material. The result when the antenna material is 6H-SiC according to the present invention is shown by a solid line. The result obtained when the vertical axis represents the electromagnetic wave intensity and the horizontal axis represents the time axis has a spectrum shape as shown in FIG. 7A. The antenna material is 6H-SiC, and the wavelength of the pulsed light is 400 nm. It can be seen that the spectral line width on the time axis is narrowed by converting to and irradiating. This means that the intensity in the high frequency region is increased when converted to frequency display.

  The result obtained in FIG. 7B can be converted into a spectrum distribution display of the electromagnetic wave generated by performing Fourier transform on the vertical axis: intensity, and the horizontal axis: frequency, as shown in FIG. 7B. Spectral distribution is obtained. That is, when an LT-GaAs photoconductive element is used, the peak of the intensity of the pulsed electromagnetic wave occurs at a frequency of about 0.1 THz, whereas the antenna material is SiC and the wavelength of the pulsed light is 400 nm. When converted and irradiated, a peak of the intensity of the pulse electromagnetic wave occurs at a frequency of about 0.5 THz.

  As shown in FIG. 7B, the antenna material is SiC, the wavelength of the pulsed light is converted to 400 nm, and the spectrum intensity is increased in the frequency region of 1.5 THz or higher, resulting in high output and high efficiency terahertz. Generation and detection of electromagnetic waves can be expected.

  According to the present invention, a high-output and high-efficiency terahertz electromagnetic wave generating / detecting device can be obtained.

Schematic configuration diagram of an embodiment according to a terahertz band electromagnetic wave generator of the present invention The figure which showed the dipole type photoconductive element concerning the terahertz band electromagnetic wave generator Schematic configuration diagram of antenna electrode according to terahertz band electromagnetic wave generator (a) Diagram showing dipole electrode (b) Diagram showing bowtie electrode Diagram showing the relationship between electron drift velocity and electric field strength Relationship diagram between current generated in photoconductive element and terahertz wave intensity generated at that time (a) Diagram showing time change of generated current (b) Diagram showing time change of terahertz electromagnetic wave intensity calculated from current change The schematic block diagram of the Example which concerns on the terahertz band electromagnetic wave generator of this invention, The figure which showed the measurement system which detects the electromagnetic wave generated by the terahertz band electromagnetic wave generator of this invention Comparison diagram between experimental results obtained in embodiments of terahertz band electromagnetic wave generator of the present invention and conventional experimental results (a) From graphs (b) and (a) showing current signals with respect to delay time according to the present invention A graph showing the spectrum distribution that has been subjected to Frye transform Schematic configuration diagram of a conventional terahertz TDS

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Antenna for electromagnetic wave radiation 11 Ultra-short pulse light source 12 Wavelength-converted ultra-short pulse light 13 Lens 24 Beam splitter 25 First ultra-short pulse light 26 Second ultra-short pulse light 28 Electromagnetic radiation photoconductive element, radiation antenna DESCRIPTION OF SYMBOLS 29 Photoconductive element for electromagnetic wave detection, receiving antenna 30 Power supply 31, 36 Parabolic mirror 32 Radiated electromagnetic wave 34 Sample 35 Terahertz electromagnetic wave after sample transmission 38 Lock-in amplifier 39 SHG element composed of LBO crystal 40 Plane mirror 41 Retroreflector 42 Moving stage 43 Computer

Claims (6)

A terahertz wave generator including a light source that emits ultrashort pulse light and an antenna for electromagnetic wave radiation,
The electromagnetic wave radiation antenna includes a semi-insulating substrate, a photoconductive thin film formed on the semi-insulating substrate, having a band gap energy of 1.42 eV or more, and a coplanar transmission line.
The coplanar transmission line has a pair of transmission line electrode bodies, and protruding electrodes respectively protruding from the transmission line electrode bodies,
The pair of transmission line main bodies are arranged in parallel,
The protruding electrodes face each other,
The applied electric field strength between the electrodes is an electric field larger than 3 × 10 ⁴ V / cm,
A terahertz electromagnetic wave generator characterized in that an electromagnetic wave is generated by irradiating from a light source ultra-short pulse light having a wavelength of less than 800 nm and having an average intensity of more than 20 mW between the protruding electrodes. The terahertz according to claim 1, wherein the pulse time width of the ultrashort pulse light described as the light source in claim 1 is 100 femtoseconds or less, and the pulse time width of the pulse electromagnetic wave is 5 picoseconds or less. Electromagnetic wave generator. The light source according to claim 1 is:
A femtosecond laser having a wavelength of 800 nm and a pulse width of 100 femtoseconds or less;
An element capable of compressing the pulse width at the same time as making the wavelength half or less,
2. The terahertz electromagnetic wave generator according to claim 1, wherein the electromagnetic wave is generated by irradiating the electromagnetic wave radiation antenna according to claim 1 with pulsed light emitted from the light source. The antenna material for electromagnetic wave radiation according to any one of claims 1 to 4, wherein In _x Ga _1-x N (0.05 <x <) grown on any one of an Al ₂ O ₃ substrate, a SiC substrate, a GaN substrate, and a ZnO substrate. 0.4) _{film, in x Al 1-x N} (0.5 grown BAs substrate <x <0.8) film, _Al x _{Ga 1-x} As layer (0 on a GaAs substrate <x <0 .8), Al _x Ga _1-x P (0 <x <0.8) film on GaP substrate, ZnSe _x S _1-x (0 <x <0.3) film on Si substrate or GaP substrate, 5. The terahertz electromagnetic wave generation device according to claim 1, wherein the terahertz electromagnetic wave generation device is any one of CuGaSe ₂ films on a quartz substrate. Using the terahertz electromagnetic wave generation / detection device including the terahertz electromagnetic wave generation device,
A method for measuring a dielectric constant of a sample, comprising:
A light source that emits ultrashort pulse light, a beam splitter, an antenna for electromagnetic wave emission, an antenna for electromagnetic wave detection, and an optical delay system,
The ultrashort pulse light emitted from the light source is divided into a first ultrashort pulse light and a second ultrashort pulse light by the beam splitter,
The first ultrashort pulse light is transmitted through an element capable of compressing the pulse width at the same time as the wavelength is less than half, and then irradiated to an electromagnetic wave radiation antenna composed of a flat photoconductive element. , A pulse electromagnetic wave is radiated from the electromagnetic wave radiation antenna,
The pulse electromagnetic wave radiated from the electromagnetic wave radiation antenna reaches the back surface of the electromagnetic wave detection antenna,
The second ultrashort pulse light reaches the surface of the electromagnetic wave detection antenna through the optical delay system,
The antenna for electromagnetic wave radiation has a semi-insulating substrate, a photoconductive thin film formed on the semi-insulating substrate and having a band gap energy of 1.42 eV or more, and a coplanar transmission line,
The coplanar transmission line has a pair of transmission line electrode bodies, and protruding electrodes respectively protruding from the transmission line electrode bodies,
The pair of transmission line main bodies are arranged in parallel,
The protruding electrodes face each other,
The measurement method is:
A sample setting step of setting the sample between the antenna for electromagnetic wave radiation and the antenna for electromagnetic wave detection, and a pulse transmitted through the sample by radiating a pulse electromagnetic wave from the antenna for electromagnetic wave radiation toward the sample An electromagnetic wave is made to reach the surface of the electromagnetic wave detection antenna, and a cross-correlation signal between the pulse electromagnetic wave that has reached the surface of the electromagnetic wave detection antenna and the second ultrashort pulse light that has reached the back surface of the electromagnetic wave detection antenna is obtained. Pulse electromagnetic wave emission process to detect as current,
Is included. 7. The dielectric constant measurement method according to claim 6, wherein the material for the electromagnetic wave detecting antenna is GaAs crystal-grown at a substrate temperature of 300 ° C. to 450 ° C., and the antenna shape is a dipole type. A terahertz electromagnetic wave generator / detector characterized by a bow-tie shape.

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