JP2006313140A - Terahertz wave generating apparatus and method or spectroscopic measurement apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz wave generating apparatus and a spectroscopic measurement apparatus which are compact, highly efficient, wideband, high in resolution and excellent in temperature stability, and can simultaneously measure imaginary and real parts of a refractive index and also measure a polarization property. <P>SOLUTION: In the apparatus, a grating and a beam expander are disposed in a resonator of a laser light source for exciting a GaP crystal, thereby making a linewidth narrow without any damage, and an optical path for simultaneously measuring transmission and reflection of a measured sample is made up, and the GaP crystal is rotated in order to obtain the polarization property. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a coherent terahertz wave generator.

Nonlinear optics for generating terahertz waves using semiconductors such as GaP and dielectrics such as LiNbO ₃ as frequency-swept monochromatic terahertz light sources and spectrometers for identifying large organic molecules that form biological materials and cancer cells, or general organic molecules Those used as crystals are known. In particular, the method of exciting the phonon polariton mode of GaP with two excitation laser beams, that is, pump light and signal light, can sweep the frequency of terahertz waves in a very wide range from 0.3 THz to 7 THz, so that the biological material It is shown that the entire terahertz spectrum such as can be known, and is effective in detecting cancer. When an optical parametric oscillator (OPO) is used as pump light (or signal light), if the triple wave (355 nm) of a YAG laser is used to excite OPO, the line width of the OPO output becomes extremely narrow. The line width of the terahertz wave is reduced to 1.5 GHz. As a terahertz light source / spectrometer, it has a frequency purity of 1.5 GHz, so a high-resolution terahertz transmission or reflection spectrum can be obtained, and structural defects in organic molecules that could not be observed in the past can be detected. It is shown that there is. The absorption coefficient of the substance is obtained from the transmission spectrum of the monochromatic terahertz electromagnetic wave, and this corresponds to the imaginary part of the complex dielectric constant or the complex refractive index.

  The method of generating a frequency-swept terahertz wave from a GaP crystal using OPO as an excitation light source is wideband (0.3 THz-7 THz), high resolution (line width 1.5 GHz), and high output (100 mW-800 mW). A fine terahertz spectrum of biomolecules can be obtained. However, since the triple wave of the YAG laser is used as the pumping source for the OPO, the efficiency is extremely low, and therefore the YAG laser must have a high output. Since a YAG laser pulse energy of 1000 mJ is required, it is difficult to reduce the size of the apparatus to a desktop type or smaller. Since the temperature dependence of the tripler is large, the characteristics change due to the influence of ambient temperature changes. The object of the present invention is to provide a small-sized, high-efficiency, high-temperature-stable, broadband (0.3 THz-7 THz), high-resolution, high-output frequency-swept terahertz light source / spectroscopic measurement apparatus and method. is there. In the past, when a monochromatic terahertz light source was used, only the absorption coefficient corresponding to the imaginary part of the refractive index or dielectric constant was measured, so it was difficult to measure the dielectric dispersion of the substance in detail. . The present invention provides a method for simultaneously measuring the real part of the dielectric constant or refractive index, and further a method for measuring the terahertz electromagnetic wave polarization direction dependency.

  In order to generate a variable frequency telehertz wave, the phonon polariton of the GaP crystal is excited, and two Cr-added forsterite lasers (Cr: Forsterite) are used as a pump light source and a signal light source for excitation. Since the Cr: Forsterite laser is excited by the fundamental wave (1064 nm) of the YAG laser, an extremely efficient oscillation of 15% can be obtained. Since the oscillation wavelength is variable between 1200 nm and 1300 nm, the frequency between 0.3 THz and 7 THz can be swept in the same manner as OPO.

  However, since Cr: Forsterite has a wide oscillating wavelength band, the line width is wide and is 0.1 nm or more. The line width of the terahertz wave output is 30 GHz or more, and it is impossible to obtain high resolution. This is because the wavelength dispersion is small because a glass prism is used for wavelength selection in the Cr: Forsterite laser. If a grating is used as a wavelength selector instead of a glass prism, the wavelength dispersion can be 100 times that of the prism, and thus a narrow line width can be obtained. However, as a material for the grating, a metal film or an organic compound is used. Holographic gratings are used, but because the oscillation output is high, the material is liable to evaporate and cannot operate stably. Therefore, means for enlarging the aperture of the beam incident on the grating is inserted into the optical path of the Cr: Forsterite laser, and the beam surface is reduced to an appropriate beam power density. If the insertion loss of such means is small enough, a practical increase in the oscillation threshold can be avoided, so there is no need to increase the power of the YAG laser.

This will be described with reference to FIG. The Cr: Forsterite lasers 1 and 2 are lasers that generate pump light and signal light for generating terahertz waves, and are excited by the fundamental wave wavelength 1064 nm of the YAG laser 3 and oscillate at wavelengths from 1200 nm to 1300 nm. Either one of 1 and 2 may be variable in wavelength, but a case where 2 is variable in wavelength will be described. The wavelength of the Cr: Forsterite laser 1 is fixed at 1200 nm, for example. In order to generate a terahertz wave of 0.1 THz to 7 THz, the wavelength of the Cr: Forsterite laser 2 is changed in the range of 1204 nm to 1235 nm. Next, the outputs of the Cr: Forsterite lasers 1 and 2 are combined into a nearly parallel beam by the polarization beam splitter 4 and incident on the incident surface 6 of the GaP single crystal 5 at a predetermined crossing angle θ _in ^ext and incident angle α ^ext. . A terahertz wave output is extracted from the output surface 7 at a predetermined angle depending on the terahertz wave frequency, and is detected by using a non-parabolic mirror 8, 9, a detector 10 such as a DTGS (deuterium triglycine sulfide) or a Si bolometer. . In order to increase the spectral S / N ratio, a terahertz wave is divided by a wedge-type beam splitter before being incident on the sample, and a reference optical path for detection by another detector is provided. Yes.

  As shown in FIG. 2, the wavelength selecting grating 11 constituting the resonator of the Cr: Forsterite laser 2 is rotated by a grating rotating mechanism in accordance with the selected wavelength. The grating pitch is 1200 line / mm or 600 line / mm. Reference numeral 12 denotes a beam expander for enlarging the parallel beam diameter of the oscillation light. 13 is a Cr: Forsterite crystal, 14 is a laser output mirror, 15 is an incident beam from a YAG laser, and 16 is a laser output beam.

As shown in FIG. 3A, the beam expander 12 includes a concave lens 17 having a focal point f ₁ and a convex lens 18 having a focal point f ₂ , and widens the diameter of a parallel beam incident on the grating. The beam diameter enlargement ratio is given by f ₂ / f ₁ . Since the influence of the edge is reduced by widening the beam diameter, the line width of the oscillation light is also narrowed by this, and a line width of 0.003 nm or less can be obtained.

In order to obtain a narrow line width, the Cr: Forsterite laser 1 includes a grating and a beam expander, as in 2, or includes several prisms and an etalon plate in the resonator. With the filter effect of the etalon plate, almost the same resolution can be obtained as when using a grating. If an etalon is used, the oscillation frequency is limited to a discrete value that is the natural frequency of the etalon. Therefore, this method cannot be used for the Cr: Forsterite laser 2 because the wavelength cannot be changed continuously. The grating can be prepared by holographic technology using an organic polymer as a material. The surface is coated with a film such as gold to increase the reflectivity. The beam diameter expansion rate is designed so that the pulse beam energy density is such that the grating is not damaged. For example, when the pulse width is 20 nm, it is desirable to keep the pulse beam energy density at 3 mJ / mm ² or less so as not to be damaged. On the other hand, in order to obtain a 100 mW terahertz pulse necessary for spectroscopic measurement, both the Cr: Forsterite lasers 1 and 2 require a beam having an output of 6 mJ and a beam diameter of about 1 mm × 1 mm. Therefore, in order not to be damaged, the expansion ratio of the beam area must be twice or more, and f ₂ / f ₁ is 1.4 or more. The expander lens for expanding the beam diameter is anti-reflective coated in the 1200 nm to 1300 nm region, and insertion loss hardly poses a problem. When the line width of the Cr: Forsterite 1 and 2 outputs is 0.003 nm, the line width of the terahertz wave output is 0.6 GHz or less.

For example, the GaP crystal has a crystal length, width, and height of 10 mm, 10 mm, 3 mm, or 10 mm, 5 mm, 5 mm, and the incident surface faces the <110> axis direction. The output increases as the crystal length increases, but if the length is increased to a certain extent due to absorption of terahertz waves, the output is saturated or decreased. Accordingly, the crystal length is in the range of 5 mm to 20 mm, but is most preferably in the range of 7 mm to 13 mm. Since terahertz wave absorption is caused by free carriers, the carrier density needs to be 10 ¹² cm ⁻³ or less.

  In the above description, the excitation method using the Cr: Forsterite laser has been described, but a similar method can be added to the OPO excitation method. That is, the efficiency is increased by using a double wave (532 nm) instead of the triple wave of the YAG laser as an excitation light source for OPO. The line width of the OPO should be widened by doubling, but is narrowed by the effect of inserting the grating, and the line width of the terahertz wave output is 1 GHz or less.

  In order to maintain the stability of the laser oscillation, even if the optical components in the resonator are changed in the environment in which they are installed, their geometrical positional relationship must be fixed with high accuracy. In FIG. 3 (b), at least the reflecting mirror M and the grating 10 in the resonator are accurately controlled in all directions so as to maintain a stable laser oscillation state by a control means not shown. Of course, a ceramic material or metal that is difficult to be deformed by an external force such as temperature change or gravity is selected as the material of the structure constituting the resonator.

  For controlling the diffraction angle of the grating, for example, if a terahertz wave frequency of 10 THz is controlled with a resolution of 100 MHz, it is necessary to put 5 digits in the control range. Or a piezoelectric actuator. Further, it is desirable that the temperature in the resonator C is controlled within +/− 0.02 ° C. by temperature control means including an environment surrounded by a heat insulating material (not shown) and a PID controller. If it is not controlled within at least + -0.5 ° C., the high resolution effect by the grating cannot be exhibited. The same applies when a prism is used without using a grating.

The pump, the signal light laser part, the terahertz wave generator, and the reference optical path are the same as those in the first embodiment. However, as shown in FIG. 4, the output terahertz wave 19 is reflected by the transmitted optical path that passes through the sample 20 and the surface of the sample. The light is guided to the reflected light path for detecting the terahertz wave and detected by the detectors 10 and 10 '. In the figure, reference numerals 21 and 22 denote a planar mirror and a non-parabolic mirror that form a reflection optical path. The transmission spectrum and the reflection spectrum are simultaneously measured by two detectors, and both the real part and the imaginary part of the refractive index can be obtained. For simplicity, in the case of normal incidence, the reflectance R is expressed as follows using the complex refractive index n ^* = n−ik.

Since the imaginary part k of the refractive index has the following direct relationship with the absorption coefficient α, it is obtained immediately from the transmittance T. d is the thickness of the sample. T ₀ is the transmittance of standard polyethylene pellets. Note that the standard pellets are molded in a wedge shape to avoid multiple reflections. A typical measurement sample is obtained by mixing and diffusing a powdery substance in polyethylene powder, and the surface reflectance is almost the same as that of a standard pellet.

Number 3

T = T ₀ exp (−αd)

  On the other hand, the reflectivity depends on both the real part and the imaginary part, but the effect of the change in the real part is greater. The change in reflectance approximately represents the change in the real part of the refractive index. When the effect of the imaginary part k cannot be ignored, the refractive index can be obtained approximately and sequentially so as to match the measured value of reflectance using k obtained by the above equation. The measurement shows the frequency dependence of the complex refractive index of the substance. These calculations are automatically performed by a program built in the computer. Thus, for example, information on molecular behavior such as diffusion and vibration of biological molecules and water molecules in the solution can be obtained from the frequency dependence of the complex refractive index of the aqueous solution.

Depending on the crystal anisotropy of biomolecules and the flashiness in solution, the terahertz transmittance and reflectivity depend on the polarization direction. Therefore, knowledge about the material state can be obtained by obtaining the polarization direction dependency of the terahertz transmittance or reflectance. When there is a sample in the cryostat, rotating and measuring the sample is complicated and undesirable. Since the sample is formed in a wedge shape in order to avoid transmittance fluctuation due to the etalon effect, an error also occurs when the beam direction changes. Therefore, as shown in FIG. 5, the terahertz wave generating GaP crystal is rotated 90 degrees around the crystal length direction. Since the polarization direction of the terahertz wave is substantially <1-10> direction after the rotation, the polarization direction e _THz is rotated by 90 degrees with the crystal rotation. In the figure, e ₁ and e ₂ represent the polarization directions of the Cr: Forsterite lasers 1 and 2. If the spectrum is taken for each polarization direction of the terahertz wave and the ratio between the two is taken, the frequency dependence of the polarization dependence of the material can be found. In addition, if the GaP crystal is rotated at 1.25 times 10 Hz which is the generation repetition period of the terahertz wave pulse, that is, 12.5 Hz, the crystal can be held alternately at the positions of 0 degree and 90 degrees, so the polarization direction is almost the same. Can be obtained.

  FIG. 6 shows that the object P to be measured uses irradiation position variable means that does not show the position of the terahertz wave that irradiates the object to be measured when the cancer cell K is adjacent to the normal cell N with a different moisture distribution. This is an example of measuring changes in polarization characteristics before and after the change. A is a polarizing plate, B is a detector such as a silicon bolometer, and 19T is a terahertz wave obtained by changing the position of K after irradiation. As A, a metal mesh polarizing plate or an obliquely incident reflecting plate (not shown) is used.

  In Embodiment 1-3, a DTGS or Si bolometer is used as a terahertz wave detector. In this embodiment, a mechanical filter 23 (electromechanical vibrator) such as crystal or silicon as shown in FIG. 6 is used as a detection element. At least a part of the mechanical vibrator is coated with powdered carbon 24. When the terahertz wave 25 is absorbed, the temperature rises and heat distortion occurs, resulting in a change in the resonance frequency. Reference numeral 26 denotes a measurement terminal. The terahertz wave intensity is measured by detecting a change in frequency or phase, or detecting a change in filter characteristics such as a change in the voltage transmittance of the filter. Since the pulse width of the terahertz wave is 10 ns, the change does not follow this and fluctuates slowly. However, since the terahertz wave follows the pulse repetition frequency of 10 Hz, the terahertz wave can be detected. Unlike the Si bolometer, it operates at room temperature and is small. In addition, unlike DTGS, sensitivity deterioration due to a change in initial polling does not occur. Further, the sensitivity of DTGS rapidly decreases when the frequency is 0.6 THz or less, and there is almost no sensitivity at 0.1 THz, but this vibrator has no decrease in sensitivity.

  A terahertz wave imaging apparatus can also be configured by forming a one-dimensional array or two-dimensional matrix of silicon mechanical filters by silicon process technology.

The invention's effect

  According to the present invention, a frequency-swept terahertz wave generating light source and a spectroscopic measurement device in the range of 0.1 THz to 7 THz, in which the line width is extremely narrow, the real part and the imaginary part of the refractive index can be measured simultaneously, and the polarization characteristics can also be measured. Is obtained.

  It is a figure which shows the structure of the terahertz wave generator in Example 1. FIG.   FIG. 2 is a diagram illustrating a configuration of an excitation laser in Example 1.   (A) is a figure which shows the structure in the excitation laser resonator in Example 1. FIG. (B) is a figure which shows the control structure of the grating and reflecting mirror in the excitation laser resonator in Example 1. FIG.   It is a figure which shows the structure for measuring the terahertz wave transmission and reflection spectrum in Example 2. FIG.   6 is a diagram showing the arrangement of GaP crystals in Example 3. FIG.   10 is a diagram illustrating a configuration of a polarization characteristic change measurement system in Example 4. FIG.   Terahertz wave detector in embodiment 5

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... 1st Cr: Forsterite laser 2 ... 2nd Cr: Forsterite laser 3 ... YAG laser 4 ... Polarizing beam splitter 5 ... GaP crystal 6 ... GaP incident surface 7 ... GaP terahertz wave output surface 9 ... Non-parabolic Surface mirror 10 ... Non-parabolic mirror 11 ... Grating 12 ... Beam expander 13 ... Cr: Forsterite crystal 14 ... Output mirror 15 ... Incident YAG laser light 16 ... Cr: Forsterite laser output light 17 ... Concave lens 18 ... Convex lens 19 ... Output terahertz Wave beam 20 ... Measurement sample 21 ... Plane mirror 22 ... Non-parabolic mirror 23 ... Mechanical filter 24 ... Coated carbon 25 ... Terahertz wave 26 ... Measurement terminal A ... Polarized plate B ... Terahertz wave detector C ... Intracavity M ... Reflector P ... DUT K ... Cancer cell N ... Normal cell 19T ... Terahe Tsu wave

Claims (7)

  In a terahertz wave generator or a terahertz wave spectrometer that emits a pump laser beam into a GaP crystal and generates a terahertz electromagnetic wave in a frequency range of 0.1 THz to 7 THz, the frequency is in the resonator of at least one pump laser. A frequency-swept terahertz wave generating apparatus and method characterized by generating a terahertz electromagnetic wave having a narrow line width by arranging a selection grating and a beam expander that expands the beam diameter of oscillation light incident on the grating, or Spectroscopic measurement apparatus and method.   Means for controlling the temperature in the resonator within + -0.5 ° C. so that the positions of the grating and the reflector constituting the resonator of the excitation laser do not deviate, and the position of each of the grating and the reflector 2. The frequency swept terahertz wave generating apparatus and method or spectroscopic measuring apparatus and method according to claim 1, further comprising means for controlling the position of the laser so as not to deviate from the laser oscillation condition.   3. The frequency swept terahertz wave generating apparatus and method or the spectroscopic measuring apparatus and method according to claim 1, wherein at least one of the excitation lasers is a Cr: forsterite laser.   In a terahertz wave spectroscopic measurement apparatus that emits a monochromatic terahertz electromagnetic wave within a frequency range of 0.1 THz to 7 THz by entering excitation laser light into a GaP crystal, the transmittance and reflectance of the measurement sample in the terahertz frequency region are measured. Means for obtaining a real part and an imaginary part of the complex refractive index from the measured transmittance and reflectance, and obtaining a spectrum of each of the real part and the imaginary part, and a frequency swept terahertz wave generator, Method or spectroscopic measurement apparatus and method.   In a terahertz wave spectroscopic measurement apparatus that emits a monochromatic terahertz electromagnetic wave within a frequency range of 0.1 THz to 7 THz by making excitation laser light incident on a GaP crystal, the GaP crystal differs by 90 degrees around an axis perpendicular to the incident surface. A frequency-swept terahertz wave generating apparatus and method or a spectroscopic measurement apparatus and method characterized by having means for obtaining a terahertz transmission or reflection spectrum in each arrangement.   In a terahertz wave spectrometer that emits monochromatic terahertz electromagnetic waves in a frequency range of 0.1 THz to 7 THz by entering excitation laser light into a GaP crystal, the electromagnetic field change, temperature change, stress change, position change, etc. on the object to be measured Means for causing a change in refractive index by applying a terahertz electromagnetic wave that is transmitted or reflected by the object to be measured, and means for measuring polarization characteristics before and after the change in refractive index. A frequency-swept terahertz wave generation apparatus and method or a spectroscopic measurement apparatus and method characterized by comprising:   In a terahertz wave spectrometer that emits monochromatic terahertz electromagnetic waves in a frequency range of 0.1 THz to 7 THz by entering excitation laser light into a GaP crystal, the terahertz wave detector is a mechanical filter whose filter characteristics change due to thermal strain. A frequency-swept terahertz wave generation apparatus and method or a spectroscopic measurement apparatus and method, characterized by:

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