JP2012186430A - Terahertz wave generation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz wave generation device which is capable of generating a terahertz wave having less noise.SOLUTION: The terahertz wave generation device comprises: a photoconductive antenna 10 obtained by forming a pair of electrodes (a dipole antenna 102) on a GaAs substrate 101 with a gap G therebetween; a bias power supply 20 for applying a DC voltage to the dipole antenna 102; and light irradiation means for irradiating the gap G of the dipole antenna 102 with optical pulses having a wavelength of 1490-1640 nm.

Description

  The present invention relates to a terahertz wave generator.

  The characteristics of terahertz waves (electromagnetic waves with a frequency of 30 GHz to 12 THz including millimeter waves and submillimeter waves) are that they have high spatial resolution, and because they have longer wavelengths than infrared rays and visible light, they are caused by dust, smoke, flames, etc. during spatial propagation The scattering is small, and the absorption spectrum of toxic substances and toxic gases is in the submillimeter wave band, and these substances can be detected. Terahertz waves have been called electromagnetic waves in undeveloped areas so far, but they are expected to be applied in various fields such as industrial, medical, bio, security, etc., and there are active terahertz wave generation and detection methods. Technology development is underway.

  As a technique for generating terahertz waves, a method is known in which a dipole antenna is formed on a semiconductor substrate to form a photoconductive antenna, a bias electric field is applied to the photoconductive antenna, and a femtosecond optical pulse is irradiated. . According to this method, carriers generated in the semiconductor substrate by the light pulse move by the bias electric field, and an instantaneous current is formed. This instantaneous current becomes a source of terahertz waves.

  Here, as a femtosecond optical pulse used in the above method, a mode-locked titanium sapphire laser having a wavelength of about 0.8 μm was generally used, but recently, a 1.5 μm band femtosecond laser for communication is used. The femtosecond laser in the 1.5 μm band has been studied for the generation of terahertz waves. As an example, Non-Patent Document 1 reports that a terahertz wave can be generated by irradiating a photoconductive antenna formed of an InGaAs substrate with a femtosecond light pulse in a 1.5 μm band. Yes.

A. Takazato, et., "Detection of terahertz waves using low-temperature-grown InGaAs with 1.56um pulse excitation", APPLIED PHYSICS LETTERS, 90,101119 (2007)

  However, in the report example of Non-Patent Document 1, since an InGaAs substrate is used as the substrate constituting the photoconductive antenna, the electric resistance value of the substrate with respect to the carrier generated by the optical pulse is low, and as a result, the generated terahertz wave There is a problem that a lot of noise is included.

  The present invention has been made in view of the above points, and an object thereof is to provide a terahertz wave generator capable of generating a terahertz wave with less noise.

  The present invention has been made in order to solve the above-described problems. An antenna in which a pair of electrodes is formed on a GaAs substrate with a gap, and voltage application for applying a voltage between the pair of electrodes. And a light irradiation means for irradiating the gap of the antenna with a light pulse having a wavelength of 1490 to 1640 nm.

In the terahertz wave generator, the light irradiating means irradiates the light pulse only to the vicinity of both or one of the pair of electrodes.
In the terahertz wave generator, the beam diameter of the optical pulse is smaller than the gap distance.
In the terahertz wave generator, the antenna is a dipole antenna.
In the terahertz wave generator, the voltage is a DC voltage.

  The terahertz wave generator according to the present invention uses a GaAs substrate as a semiconductor substrate constituting the photoconductive antenna. A GaAs substrate has a higher electrical resistance than an InGaAs substrate. Therefore, the terahertz wave generator according to the present invention can generate terahertz waves with less noise.

It is a figure which shows the structure of the terahertz wave generator by embodiment of this invention. It is a figure which shows a mode that the optical pulse was irradiated to the gap of the dipole antenna. It is a figure which shows the example of the experimental result which generated the terahertz wave. It is a figure which shows the example of the experimental result which generated the terahertz wave. It is a figure which shows the modification of a dipole antenna. It is a figure which shows the modification of a terahertz wave generator.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram illustrating a configuration of a terahertz wave generation device 1 according to an embodiment of the present invention. The terahertz wave generator 1 includes a photoconductive antenna 10 and a bias power source 20.

  The photoconductive antenna 10 has a configuration in which a dipole antenna 102 is formed on one surface M1 of a GaAs substrate 101 on which a GaAs film is grown at a low temperature (about 300 ° C. or less). A bias power supply 20 is connected to the dipole antenna 102. A DC bias electric field is applied between the gaps G of the dipole antenna 102 by the bias power source 20. Further, a femtosecond light pulse is applied to the gap G of the dipole antenna 102 from the surface M1 side of the GaAs substrate 101 where the dipole antenna 102 is formed. Then, carriers (free electrons) having a lifetime of about 300 fs are generated in the GaAs substrate 101 by optical pulses, and the carriers are accelerated by a DC bias electric field, and a pulse-like instantaneous current having a time width of about ps. Occurs. By this instantaneous current, an electromagnetic wave having an intensity distribution proportional to the time derivative of the instantaneous current, that is, a terahertz wave is radiated. Since the GaAs substrate 101 has a higher electrical resistance value than a conventionally used InGaAs substrate, it is possible to obtain a terahertz wave with less noise.

  The optical pulse for generating carriers uses femtosecond pulsed laser light (pulse width: ˜100 fs) in a 1.5 μm band (1490 nm to 1640 nm). This light pulse is condensed to a spot smaller than the gap interval Lg of the dipole antenna 102 by a condensing means such as a lens (not shown) and irradiated to the gap G of the dipole antenna 102.

  FIG. 2 shows a state in which an optical pulse is applied to the gap G of the dipole antenna 102. As shown in the figure, the dipole antenna 102 has a dipole length Ld of, for example, 50 μm and a gap interval Lg between electrodes of, for example, 10 μm. Then, the light pulse is irradiated so that the spot is narrowed down to about 5 μm (<Lg), for example, so that a part of the spot overlaps the electrode 102A on the plus side of the dipole antenna 102.

  At this time, free electrons are generated as carriers in the region of the GaAs substrate 101 irradiated with the light pulse, that is, in the vicinity of the positive electrode 102A of the dipole antenna 102, and the generated free electrons are attracted to the positive electrode 102A by a DC bias electric field. Thus, an instantaneous current j which is a terahertz radiation source is generated. Thus, since the terahertz wave is generated by the local instantaneous current j in the vicinity of one electrode of the dipole antenna 102, the light pulse is not necessarily applied to the entire gap G of the dipole antenna 102 (both electrodes 102A, 102A, It is not necessary for the light pulse spot to overlap 102B), but only in the vicinity of the electrode where the carrier moves in the bias electric field.

Therefore, the optical pulse can be focused on a spot smaller than the gap G and irradiated to the photoconductive antenna 10 regardless of the size of the gap G of the dipole antenna 102, thereby improving the generation efficiency of the terahertz wave. Can be improved. The reason is that the band gap energy of GaAs is about twice the photon energy in the 1.5 μm wavelength band, and carriers (free electrons) are excited in the GaAs substrate 101 by the two-photon absorption process. This is because the higher the density is, the more carriers are generated.
When a light pulse having a photon energy of a wavelength band similar to the band gap energy of GaAs, for example, a wavelength of 780 nm, is used, a two-photon absorption process does not occur, so the number of carriers generated by light pulse irradiation is the number of light pulses. It becomes independent of the power density.

  3 and 4 show examples of experimental results in which terahertz waves are generated using an optical pulse with a wavelength of 1560 nm. The experiment was performed by condensing the light pulse spot (beam diameter φ) to 6.9 μm, 3.5 μm, and 3.0 μm and irradiating the photoconductive antenna 10 as shown in FIG. However, the power of the light pulse was constant. FIG. 3 shows the time waveform of the generated terahertz wave, where the horizontal axis represents time, and the vertical axis represents the current value obtained by measuring the terahertz wave with the terahertz wave detector (corresponding to the intensity of the terahertz wave). Represents. FIG. 4 shows the time waveform amplitude (difference between peaks) in FIG. 3 on the vertical axis and the beam diameter on the horizontal axis. 3 and 4 that the intensity of the generated terahertz wave can be increased as the spot of the optical pulse is reduced, that is, as the power density of the optical pulse is increased.

  5A and 5B show a modification of the dipole antenna applicable to the photoconductive antenna 10 of the terahertz wave generating apparatus 1 according to the present embodiment. FIG. 4A shows a bow-tie configuration, and FIG. 4B shows a spiral configuration. Note that the shape of the photoconductive antenna 10 is not limited to this example, and any shape having a gap between the electrodes in the region irradiated with the light pulse may be used.

FIG. 6 shows a modification of the terahertz wave generator. The light pulse is applied to the gap of the dipole antenna 102 on the GaAs substrate 101 through a single mode optical fiber, a GRIN collimator lens, and a GRIN condenser lens. The generated terahertz wave is extracted to the outside through a hemispherical lens.
In this way, the optical fiber, the lens, and the substrate are integrated, that is, the light pulse is incident using an in-line configuration different from the space system, so that a foreign object to the dipole antenna that is a very fine metal pattern. Can be prevented and problems such as short circuit can be prevented. In addition, since the light pulse applied to the substrate needs to be applied to a micron-order minute part on the substrate, a highly accurate and stable optical system is required. However, as in this configuration, each part is made of an adhesive or the like. By being firmly fixed, it is possible to suppress the positional deviation of the light pulse applied to the substrate.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to

  DESCRIPTION OF SYMBOLS 1 ... Terahertz wave generator 10 ... Photoconductive antenna 20 ... Bias power supply 101 ... GaAs substrate 102 ... Dipole antenna

Claims (5)

An antenna having a pair of electrodes formed on a GaAs substrate with a gap;
Voltage applying means for applying a voltage between the pair of electrodes;
A light irradiation means for irradiating the gap of the antenna with a light pulse having a wavelength of 1490 to 1640 nm;
A terahertz wave generator characterized by comprising:   2. The terahertz wave generation device according to claim 1, wherein the light irradiation unit irradiates the light pulse only to the vicinity of one or both of the pair of electrodes.   3. The terahertz wave generation device according to claim 1, wherein a beam diameter of the optical pulse is smaller than a separation distance of the gap.   The terahertz wave generation device according to any one of claims 1 to 3, wherein the antenna is a dipole antenna.   The terahertz wave generation device according to any one of claims 1 to 4, wherein the voltage is a DC voltage.

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