JP4756853B2 - Electro-optic crystal element - Google Patents

Description

  The present invention relates to an electro-optic crystal element (EO crystal element) that is effective for use in a terahertz electromagnetic wave detector, and in particular, a technique that is effective when applied to an electro-optic crystal element having a two-layer structure of a ZnTe-based compound semiconductor single crystal. About.

  In general, a frequency region (0.1 to 100 THz) including a submillimeter wave to a far infrared region is collectively referred to as a terahertz electromagnetic wave region, and is located at a boundary between a light wave and a radio wave. In recent years, a technology for generating terahertz electromagnetic waves by exciting electro-optic crystals (EO crystals) made of oxide single crystals or compound semiconductor single crystals and semiconductor photoconductive switch elements with femtosecond lasers, Technologies for detecting terahertz electromagnetic waves by utilizing the birefringence characteristics of EO crystals have been developed, and techniques related to terahertz electromagnetic waves have been remarkably advanced.

  The oxide single crystal used as the EO crystal has a large electro-optic coefficient (EO coefficient) and can detect terahertz electromagnetic waves efficiently. However, the refractive index of terahertz electromagnetic waves is much higher than that of visible light. Therefore, it is difficult to detect terahertz electromagnetic waves in combination with visible light. On the other hand, the compound semiconductor single crystal has a smaller EO coefficient than that of the oxide single crystal, but the difference between the refractive index of terahertz electromagnetic waves and the refractive index of visible light is small, so detection of terahertz electromagnetic waves by combination with visible light is possible. There is an advantage that it is suitable for.

  In particular, a ZnTe single crystal, which is one of II-VI group compound semiconductors among compound semiconductors, has a relatively large EO coefficient and a band gap of 2.26 eV at room temperature. Therefore, a laser in the visible light band was used. It can be said that this is the most promising material for an EO element substrate for a terahertz electromagnetic wave detector.

FIG. 2 is a schematic configuration diagram of a terahertz electromagnetic wave detector using a ZnTe single crystal as a substrate for an electro-optic element.
A terahertz electromagnetic wave detector 100 shown in FIG. 2 includes, for example, a laser light source 101 that oscillates laser light, optical delay (time delay) means 102, an emitter 103 that emits terahertz electromagnetic waves, and an EO element 106 made of a ZnTe single crystal. , Measuring the intensity of the probe light that has passed through the EO element, for example, a photodetector 105 made of a photodiode, and reflecting mirrors (reflectors) R1 to R1 that reflect excitation light or probe light to determine a required optical path. R6, beam splitters S1 and S2 that branch or combine light, and polarizers 104 and 107 that limit the vibration direction of light to one direction.

  The terahertz electromagnetic wave detector of FIG. 2 uses the same laser light source 101 as the excitation light source and the probe light source, branches the laser light oscillated from the laser light source 101 by the beam splitter S1, and each optical delay means 102 etc. The device configuration is such that the optical path length can be adjusted, and the timing at which the terahertz electromagnetic wave and the probe light enter the EO crystal 106 can be adjusted.

  First, the laser light oscillated from the excitation light source 101 is branched into excitation light and probe light by the beam splitter S1. The excitation light passes through the optical delay means 102 and then enters the GaAs substrate 108 as a photoconductive element. At this time, for example, a terahertz electromagnetic wave is generated by applying a bias voltage of about 5 kV, and the generated terahertz electron wave enters the beam splitter S2.

  On the other hand, the probe light enters the polarizer 107 through the reflectors R5 and R6 and is adjusted to linearly polarized light, and then enters the beam splitter S2. Next, the terahertz electromagnetic wave and the probe light are combined by the beam splitter S <b> 2 and enter the EO element (ZnTe single crystal) 106. At this time, since an electric field is generated in the ZnTe single crystal by the terahertz electromagnetic wave, birefringence is induced in the crystal, and the probe light is slightly changed to elliptically polarized light.

  The probe light transmitted through the EO element 106 is incident on the polarizer 104 having a polarization direction rotated by 90 ° from the polarization direction of the polarizer 107, and the intensity of the probe light leaking through the polarizer 104 is determined. Detection is performed by the photodetector 105.

  That is, since the change in the birefringence of the ZnTe single crystal 106 increases as the electric field generated by the terahertz electromagnetic wave increases, the ratio of elliptically polarized light increases. Therefore, the greater the intensity of the leaked probe light, the greater the electric field due to the terahertz electromagnetic wave.

  Thus, by utilizing the change in the birefringence of the ZnTe single crystal 106 due to the electric field, the terahertz electromagnetic wave can be detected by converting the magnitude of the electric field due to the terahertz electromagnetic wave into the intensity of the probe light and measuring it. it can. Further, a technique for imaging by combining visible light and a CCD camera has been proposed (Non-patent Document 1).

  By the way, in general, the ratio (electro-optic coefficient, EO coefficient) at which the birefringence of a crystal changes due to an electric field has a finite value in a plane orientation other than (100) in a zinc flash structure crystal such as a ZnTe single crystal. It is known that when the plane orientation is (100), it becomes 0 (birefringence does not change). A crystal whose plane orientation is (110), (111), or the vicinity thereof has a large electro-optic coefficient. Therefore, conventionally, a ZnTe single crystal having a plane orientation of (110) or (111) has been used as an EO element substrate of a terahertz electromagnetic wave detector.

In such a terahertz electromagnetic wave detector using a crystal substrate having an electro-optic effect, the terahertz electromagnetic wave incident on the crystal substrate is reflected on the opposite side (back side), and the detection efficiency of the terahertz electromagnetic wave is reduced. There is a problem of end. Therefore, in the case of a compound semiconductor having a single crystal substrate (first crystal substrate) on the incident side of the terahertz electromagnetic wave and a plane orientation with the same component composition as the first crystal substrate and a small electro-optic effect (zincblende structure) (100) plane) is joined to a single crystal substrate (second crystal substrate) to constitute an electro-optic crystal element (detection element) (for example, Patent Document 1). That is, since the refractive indexes of the two crystal substrates are equal, the first crystal substrate is not affected by the reflection of the terahertz electromagnetic wave from the back side, and the second crystal substrate is not efficiently affected by the electro-optic effect due to the terahertz electromagnetic wave. It becomes possible to detect terahertz electromagnetic waves.
JP 2003-270598 A Applied Physics Letter 69 (8) p.1996

  However, even if the above-described electro-optic crystal element having the two-layer structure is used, the terahertz electromagnetic wave is reflected on the back side of the second crystal substrate because there is a difference between the refractive index of the second crystal substrate and the refractive index of air. Cannot be avoided. Therefore, the present situation is that the influence of the reflection of the terahertz electromagnetic wave on the back side is reduced by making the thickness of the second crystal substrate relatively thick as several mm.

  Thus, when the thickness of the second crystal substrate is set to several mm, the electro-optic crystal element becomes large considering that the thickness of the first crystal substrate is about 0.01 to 1 mm and is very thin. In addition to hindering downsizing of the detection unit, there is a problem that the crystal substrate itself is large and the cost is increased.

  The present invention is an electro-optic crystal element that is used in a terahertz electromagnetic wave detector and is formed by joining a first crystal substrate (incident side) and a second crystal substrate (exit side), and the thickness of the second crystal substrate is reduced. An object of the present invention is to provide an electro-optic crystal element capable of reducing the influence of reflection of terahertz electromagnetic waves on the back side of the second crystal substrate without increasing the thickness.

The present invention has been made to solve the above-described problems, and includes a first crystal substrate (a terahertz electromagnetic wave incident side) made of a ZnTe-based compound semiconductor single crystal having an electro-optic effect, and lower than the first crystal substrate. resistance is and electro-optic effect is small, and a second crystal substrate having the same chemical composition as the first crystal substrate, Ri is Na are joined,
The second crystal substrate absorbs the terahertz electromagnetic wave transmitted through the first crystal substrate by free carriers in the second crystal substrate, and attenuates the terahertz electromagnetic wave reaching the back side surface of the second crystal substrate. The electro-optic crystal element is characterized in that no electro-optic effect is generated by the terahertz electromagnetic wave .

Specifically, the carrier concentration of the second crystal substrate was set to 1 × 10 ^{17 to} 5 × 10 ¹⁸ cm ⁻³ . Further, the first crystal is composed of a ZnTe single crystal whose plane orientation is other than (100), and the second crystal is composed of a ZnTe single crystal whose plane orientation is (100) or its vicinity. Thereby, in the 2nd crystal substrate, a terahertz electromagnetic wave can be absorbed efficiently, without reducing the transmittance | permeability of visible light.

  That is, the inventors have studied the second crystal substrate to be attached to the first crystal substrate having the electro-optic effect, and as a result, the single crystal substrate has the same component composition as the first crystal substrate and the carrier concentration is a predetermined concentration or more. Has found that the absorption coefficient in the terahertz band is large and the absorption coefficient in the visible light band is small, and by using this property, the first crystal substrate and the second crystal substrate are joined to form an electro-optic crystal element. The inventors have proposed that the second crystal substrate can be made as thin as the first crystal substrate without being affected by reflection from the back side of the second crystal substrate.

  Table 1 shows the measurement results of transmittance when a terahertz electromagnetic wave having a predetermined wavelength (frequency), far-infrared light, and visible light are incident on a ZnTe single crystal having a thickness of 1 mm. As shown in Table 1, when the carrier concentration of the second crystal substrate is increased, the absorption of terahertz electromagnetic waves by free carriers increases, and the terahertz electromagnetic waves reaching the backside surface can be sufficiently attenuated. The influence can be reduced.

  On the other hand, even if the free carriers increase, the visible light transmittance required for detection is only about half, which is smaller than the decrease in the transmittance of the terahertz wave. Therefore, since the detection intensity of the visible light that has been elliptically polarized by the EO crystal and passed through the polarizer is sufficiently high, it is possible to detect the terahertz electromagnetic wave efficiently based on the detection intensity of the visible light.

  According to the present invention, since the terahertz electromagnetic wave transmitted through the first crystal substrate and incident on the second crystal substrate is absorbed by the free carriers in the second crystal substrate, the second crystal is reduced even if the second crystal substrate is thinned. The influence of the reflection of the terahertz electromagnetic wave on the back side of the substrate is small. On the other hand, visible light is transmitted without being absorbed, so that the terahertz electromagnetic wave can be detected efficiently.

  Further, since the second crystal substrate can be made thin, it is possible to reduce the size of the electro-optic crystal element (detection unit) composed of the first crystal substrate and the second crystal substrate, and to reduce the cost of the electro-optic crystal. it can.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings.
FIG. 1 is an explanatory diagram showing the configuration of an electro-optic crystal element using a ZnTe single crystal according to the present invention.
First, a ZnTe single crystal (first ZnTe single crystal) having a high resistance (1 × 10 ⁶ Ω · cm) and a plane orientation of (110) and a ZnTe single crystal having a low resistance and a plane orientation of (100) ( A second ZnTe single crystal) was produced by the VGF method. At this time, the carrier concentration of the second ZnTe single crystal was 1 × 10 ¹⁷ cm ⁻³ . Thus, the first ZnTe single crystal has a large electro-optic effect, and the second ZnTe single crystal has a very small electro-optic effect.

  These crystals were cut to a thickness of 1 mm, and then mirror-polished so that the thicknesses were 0.6 mm and 0.6 mm, respectively, to produce ZnTe single crystal substrates 10 and 20. Here, the transmittance of the terahertz electromagnetic wave (wavelength 20 μm, frequency 15 THz) and visible light (wavelength 0.7 μm, frequency 430 THz) in the second ZnTe single crystal substrate was as shown in Table 2.

  Next, the bonded surfaces of the ZnTe single crystal substrates 10 and 20 were polished so that the surface irregularities were 1 μm or less. At this time, the polished surface (bonding surface) was in a state in which a work-affected layer was not formed.

Next, the polished surfaces of the ZnTe single crystal substrates 10 and 20 are bonded to each other, and are sandwiched and fixed by graphite parts having substantially the same size as the ZnTe single crystal substrates 10 and 20, and these are integrally molded. Placed in a quartz container. At this time, the ZnTe single crystal substrate 10, 20 was set to take the pressure of 500g heavy ^/ cm 2 to 1000 g heavy / cm ^2. And the quartz container in which the ZnTe single crystal substrate was arrange | positioned was arrange | positioned in a high temperature furnace, and heat processing was performed at 450 degreeC for 2 hours in inert gas atmosphere.

  Then, it cooled at the temperature decreasing rate of 1-10 degreeC / min, and grind | polished the surface S of the taken out ZnTe electro-optic crystal. At this time, polishing was performed so that the thickness of the ZnTe single crystal having a plane orientation of (110) was 0.5 mm.

  Since the ZnTe electro-optic crystal produced by the above-described method has a high carrier concentration in the second ZnTe single crystal substrate 20, absorption of terahertz electromagnetic waves by free carriers is increased. Thereby, the terahertz electromagnetic wave that reaches the back side surface can be sufficiently attenuated, so that the influence of the reflection of the terahertz electromagnetic wave on the back side surface can be reduced. On the other hand, since the visible light transmittance hardly changes in the second ZnTe single crystal substrate 20, the detection intensity of visible light does not decrease.

  Therefore, the terahertz electromagnetic wave detector using the above-described ZnTe single crystal as the electro-optic element substrate can efficiently detect the terahertz electromagnetic wave based on the detection intensity of visible light. In addition, since the second ZnTe single crystal substrate can be thinned, the electro-optic crystal element (detection unit) composed of the first ZnTe single crystal substrate 10 and the second ZnTe single crystal substrate 20 can be reduced in size. The cost of the electro-optic crystal can be reduced.

As mentioned above, although the invention made | formed by this inventor was concretely demonstrated based on embodiment, this invention is not limited to the said embodiment.
For example, the carrier concentration of the second ZnTe single crystal substrate 20 is not limited to the numerical value of the above embodiment, and the same effect can be obtained if the carrier concentration is in the range of 1 × 10 ^{17 to} 5 × 10 ¹⁸ cm ^−3. Obtainable.

  In the above embodiment, the case where the (110) plane ZnTe single crystal and the (100) plane ZnTe single crystal are bonded to each other has been described. However, the (111) plane ZnTe single crystal is used instead of the (110) plane. Even if it is used, the same effect can be obtained.

  In the above embodiment, the method for laminating single crystal substrates has been described as thermocompression bonding. However, the same effect can be obtained even when a transparent adhesive is used for the pressure bonding method.

  Moreover, although the case where visible light was used for detection light was demonstrated in the said embodiment, the same effect can be anticipated with the same structure even when near infrared light having a wavelength close to visible light is used as detection light.

It is explanatory drawing showing the structure of the ZnTe electro-optic crystal which concerns on this invention. It is a schematic block diagram of the terahertz electromagnetic wave detector using a ZnTe electro-optic crystal.

Explanation of symbols

10 (110) plane ZnTe single crystal (first ZnTe single crystal)
20 (100) plane ZnTe single crystal (second ZnTe single crystal)

Claims (3)

A first crystal substrate made of a ZnTe-based compound semiconductor single crystal having an electro-optic effect, and a second crystal having a resistance lower than that of the first crystal substrate and a smaller electro-optic effect, and having the same component composition as the first crystal substrate and the substrate, Ri is Na are joined,
The second crystal substrate absorbs the terahertz electromagnetic wave transmitted through the first crystal substrate by free carriers in the second crystal substrate, and attenuates the terahertz electromagnetic wave reaching the back side surface of the second crystal substrate. An electro-optic crystal element characterized in that no electro-optic effect is produced by the terahertz electromagnetic wave . 2. The electro-optic crystal element according to claim 1, wherein the second crystal substrate has a carrier concentration of 1 × 10 ^{17 to} 5 × 10 ¹⁸ cm ⁻³ .   The first crystal is a ZnTe single crystal having a plane orientation other than (100), and the second crystal is a ZnTe single crystal having a plane orientation of (100) or its vicinity. The electro-optic crystal element described in 1.

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