JP2016075864A - Terahertz light generation device and terahertz spectroscopic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz light generation device capable of freely changing an excitation wavelength and a terahertz radiation frequency, without drastically breaking an optical system and to provide a terahertz spectroscopic device using the terahertz light generation device.SOLUTION: The terahertz light generation device includes an excitation pulse light source 21 which generates excitation pulse light, a first movable mirror 22 which has a function for reflecting the excitation pulse light and can adjust a reflection angle, a diffraction grating 23 which diffracts the excitation pulse light reflected by the first movable mirror, a second movable mirror 24 which has the function for reflecting the diffracted excitation pulse light and can adjust the reflection angle, a nonlinear crystal 25 which generates terahertz pulse light on the basis of the excitation pulse light reflected by the second movable mirror, a sample installation member 31 on which a sample to be measured M can be installed and which irradiates the sample to be measured with the generated terahertz pulse light, a polarization prism 32 on which light transmitted through the sample is made incident, and a detector 33 which detects the terahertz pulse light transmitted through the polarization prism.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a terahertz light generator and a terahertz spectrometer.

  A method of generating a terahertz light having a low frequency by irradiating a nonlinear optical crystal having a large nonlinear optical constant with a strong visible or infrared laser pulse is known as optical rectification in the nonlinear optical effect.

  In order to efficiently obtain this nonlinear optical effect, the wave speeds of incident pulse light and terahertz radiation in the nonlinear crystal must be the same, which is known as a phase matching condition.

  By the way, the speed of light in the crystal is inversely proportional to the refractive index, and in many nonlinear crystals, the refractive index differs greatly between the visible / infrared region and the terahertz band. It could not be satisfied, and the generation efficiency of terahertz radiation was limited.

  On the other hand, Hebling et al. In Non-Patent Document 1 below, the refractive index of incident pulse light and terahertz light are different by appropriately tilting the pulse surface of the incident pulse light with respect to the light propagation direction inside the nonlinear crystal. Even so, a technology that satisfies the phase matching condition was proposed. This technique is called “pulse surface tilt method”.

  As another technique related to the pulse plane tilt method, for example, in Non-Patent Document 2 below, light is diffracted by a diffraction grating, the pulse plane is tilted, and the angle of the tilted pulse plane is matched with the cut angle of the nonlinear crystal. A technique for producing a large electric field amplitude is disclosed.

Hebling et al. , Opt. Express 10,1161 (2002) H. Hirori et al. , Appl. Phys. Lett. 98,091106 (2011)

  However, the high-intensity terahertz pulse generation method using the pulse surface tilt method is an extremely effective method free from phase mismatching, but can be performed only under limited conditions. More specifically, once the wavelength and output frequency of the incident pulse are determined, the corresponding pulse surface tilt angle, crystal prism angle, and terahertz pulse radiation direction are also determined. As a result, mechanical restrictions become severe, and it is not easy to change the incident wavelength and the terahertz radiation frequency. In particular, changing the radiation direction of the terahertz pulse is equally impossible to move the detection optical system including the parabolic mirror accordingly. Limiting the excitation wavelength and the terahertz radiation frequency means that it cannot be applied to a terahertz spectrometer and cannot be used for purposes other than some research purposes such as prosecution property evaluation.

  Therefore, in view of the above problems, the present invention provides a terahertz light generator capable of freely changing the excitation wavelength and the terahertz radiation frequency without greatly destroying the optical system, and further provides a terahertz spectrometer using the terahertz spectrometer. With the goal.

  Therefore, a terahertz light generation device according to one aspect of solving the above-described problem is an excitation pulse light source that generates excitation pulse light, and a first movable that has a function of reflecting excitation pulse light and can adjust a reflection angle. A mirror, a diffraction grating for diffracting the excitation pulsed light reflected by the first movable mirror, a second movable mirror having a function of reflecting the diffracted excitation pulsed light and capable of adjusting a reflection angle, and a second And a nonlinear crystal that generates terahertz pulsed light based on the excitation pulsed light reflected by the movable mirror.

  Moreover, although not necessarily limited in this viewpoint, it is preferable that the diffraction grating is rotatable.

  Moreover, although not necessarily limited in this viewpoint, it is preferable that the position of at least one of the first movable mirror and the second movable mirror is also movable.

  A terahertz spectrometer according to another aspect that solves the above problem includes an excitation pulse light source that generates excitation pulse light, and a first movable mirror that has a function of reflecting the excitation pulse light and is capable of adjusting a reflection angle. A diffraction grating that diffracts the excitation pulsed light reflected by the first movable mirror, a second movable mirror that has a function of reflecting the diffracted excitation pulsed light and can adjust the reflection angle, and a second movable A nonlinear crystal that generates terahertz pulsed light based on excitation pulsed light reflected by a mirror, a sample setting member that can set a sample to be measured and irradiate the terahertz pulsed light generated to the sample to be measured, and a sample A polarizing prism that receives the incident light transmitted through the polarizing prism, and a detector that detects the terahertz pulse light transmitted through the polarizing prism; Characterized in that it has a.

  As described above, according to the present invention, it is possible to provide a terahertz light generating apparatus that can freely change the excitation wavelength and the terahertz radiation frequency without greatly destroying the optical system, and a terahertz spectroscopic apparatus using the terahertz light generating apparatus.

It is a figure which shows the outline of the terahertz spectroscopy apparatus which concerns on embodiment. It is a figure which shows the outline of the optical system of the terahertz light generator which concerns on embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention can be implemented in many different forms, and is not limited to the examples mentioned in the following embodiments.

  FIG. 1 is a diagram showing an outline of a terahertz terahertz spectrometer (hereinafter referred to as “the present spectrometer”) 1 according to the present embodiment, and FIG. 2 shows a terahertz light generator (hereinafter referred to as a “terahertz light generator”). 2 is a diagram showing an outline of an optical system and an optical path of 2). As shown in these drawings, the spectroscopic device 1 includes a main light generating device 2 and a spectroscopic unit 3.

  The light generator 2 also includes an excitation pulse light source 21 that generates excitation pulse light, a first movable mirror 22 that has a function of reflecting the excitation pulse light and can adjust the reflection angle, and a first movable mirror. The diffraction grating 23 that diffracts the excitation pulse light reflected by the light source 22, the second movable mirror 24 that has a function of reflecting the diffracted excitation pulse light and can adjust the reflection angle, and the second movable mirror 24 reflects the light. And a nonlinear crystal 25 that generates terahertz pulsed light based on the excited pulsed light.

  In addition, the spectroscopic unit 3 in the spectroscopic device 1 can set the measurement target sample M, and has transmitted the sample setting member 31 for irradiating the measurement target sample M with the generated terahertz pulse light and the measurement target sample. A polarizing prism 32 that receives light and a detector 33 that detects the terahertz pulse light transmitted through the polarizing prism 32 are provided.

  Moreover, the excitation pulse light source 21 in this light generator 2 literally generates excitation pulse light. The excitation pulse light is not limited as long as terahertz pulse light can be generated in combination with other components of the light generation device 2, but, for example, light from the visible region to the infrared region is more specific. Specifically, it is a preferable example that the light source can emit light having a wavelength of 300 nm to 3000 nm, preferably 400 nm to 2000 nm. In particular, according to the present embodiment, generation of terahertz pulses can be realized with almost no change in the optical system even when excitation is performed in a communication wavelength band that is significantly different from the light in the vicinity of 800 nm that is normally used for terahertz pulse utterance.

  The first movable mirror 22 in the light generator 2 has a function of reflecting the excitation pulse light and can adjust the reflection angle. As a result, it is possible to change the reflection angle of the excitation pulse light and control the incident angle α and the emission angle β to the diffraction grating 23 in combination with the rotation of the diffraction grating 23. The configuration of the first movable mirror 22 is not limited as long as it has the above function, and a commercially available mirror can be adopted.

  The first movable mirror 22 in the light generating device 2 can adjust the reflection angle, and can move its position, specifically, the moving direction of the excitation pulse light. preferable. As a result, the first movable mirror 22 can move the reflection position with respect to the diffraction grating, and can be reflected at a predetermined position of the diffraction grating within the rotatable range of the diffraction grating. become.

  The diffraction grating 23 in the light generating device 2 diffracts the excitation pulse light reflected by the first movable mirror 22 as described above. Specifically, the excitation pulse light incident at the incident angle α can be diffracted at the emission angle β, and the pulse surface can be inclined by the inclination angle γ.

  In the present embodiment, the diffraction grating 23 is preferably rotatable. In this way, not only the inclination angle γ of the pulse surface of the diffracted excitation pulse light but also the light traveling direction itself can be adjusted.

  Moreover, as a structure of the diffraction grating 23 in this embodiment, as long as it has the said function, it is not necessarily limited and what is generally marketed can be used. For example, it is a preferable example that linear minute irregularities (grooves) are formed and the cross section has a sawtooth shape.

  In the present embodiment, the diffraction grating 23 is preferably movable in parallel to the traveling direction of the excitation pulse light. In this way, light can be diffracted at a desired position of the diffraction grating.

  Further, as described above, the second movable mirror 24 in the light generating apparatus 2 has a function of reflecting the diffracted excitation pulse light and can adjust the reflection angle. As a result, in the present embodiment, it is possible to accurately match the surface of the nonlinear crystal 25 from which light is emitted and the tilt angle γ of the pulse surface. The configuration of the second movable mirror 24 is the same as that of the first mirror 22, and a commercially available mirror can be adopted.

  In addition, the second movable mirror 24 in the light generating device 2 can be further adjusted in position, and more specifically, can be moved in a direction parallel to the traveling direction of incident light. It is preferable that By doing so, it is possible to reflect light and make it incident on a desired position of the non-war crystal without moving the position of the nonlinear crystal 25.

The nonlinear crystal 25 in the light generating device 2 generates terahertz pulse light based on the excitation pulse light reflected by the second movable mirror 24. The nonlinear crystal 25 of the present embodiment has a flat surface at the position where the excitation pulse light is incident and the position where the excitation pulse light is emitted. In particular, at the position where the excitation pulse light is incident, the non-linear crystal 25 The normal line is substantially parallel (for example, an error within 1 degree), and at the position where the excitation pulse light is emitted, the inclination angle γ of the pulse surface of the excitation pulse light and the surface are substantially parallel (for example, within 1 degree). Of the error). This makes it possible to satisfy the phase matching condition, and the light emitted from the nonlinear crystal 25 is a terahertz pulse. In addition, as a material example of the nonlinear crystal, for example, LiNbO ₃ , GaP, ZnTe, or the like can be used, but is not limited thereto.

  As described above, with the above configuration, the present light generation device 2 becomes a terahertz light generation device that can freely change the excitation wavelength and the terahertz radiation frequency without greatly damaging the optical system. More specifically, in this embodiment, the first movable mirror is provided between the light source that generates the excitation pulse light and the diffraction grating, and the second movable mirror is provided between the diffraction grating and the nonlinear crystal, respectively. The optical system is M-shaped. As a result, the incident angle of the incident light to the diffraction grating can be changed without moving the position of the excitation pulse light source, and the pulse surface tilt angle can be controlled without significantly breaking the optical system. Furthermore, in order to maintain the terahertz radiation angle, it is necessary to control the angle of light incident on the nonlinear crystal independently of the pulse plane tilt angle, and this can be realized by providing a second movable mirror.

  Moreover, in this light generator 2, as long as damage does not enter a nonlinear crystal, it can be excited with a strong pulse laser. In the conventional terahertz spectroscopy incorporating a photoconductive switch, a strong excitation laser cannot be used. However, the present light generator is not limited to this, and a highly sensitive terahertz spectroscopy using a strong terahertz light source is possible.

  In the present embodiment, the sample setting member 31 of the spectroscopic unit 3 can set the measurement target sample M, and can set the measurement target sample M at a position where the generated terahertz pulse light is irradiated.

  In the present embodiment, the measurement target sample M is not particularly limited, and may be a liquid, a solid, or even a gas. When the measurement target sample M is a liquid or a gas, it is preferably a container that contains the liquid or the gas and is made of a member that is not easily absorbed by the terahertz pulse light. In addition, when the measurement target sample M is a solid, in addition to the container, it is preferable that the measurement target sample M is a table for installing the solid or a pinching member for holding the sample. By doing so, the measurement target sample M can be stably held. Of course, the sample installation member 31 is preferably provided with a window for transmitting light.

  In the present embodiment, the sample setting member 31 is sandwiched between a pair of parabolic mirrors 34 and 35. Then, the terahertz pulse generated by the light generator 2 is once collected at the focal point from the parallel light state by one parabolic mirror 34 and is incident on the measurement target sample M, and then the other parabolic mirror 35. Due to this, it becomes a parallel light state again. This light is further collected at the focal point by the parabolic mirror 36, is incident on the detecting element 37 together with the probe pulse shown below, and is incident on the polarizing prism 32 and the detector 33 described later.

  Therefore, in the spectroscopic device 1, the excitation pulse light is first incident on the first movable mirror 21, but is separated in advance by the beam splitter BS, and one is incident on the first movable mirror 21 as described above. The pump pulse goes on, the other becomes the probe pulse. Since the optical path lengths of the pump pulse and the probe pulse from the beam splitter need to be equal, a mirror and a retroreflector 38 for changing the path are provided on the optical path of the probe pulse so that the optical path can be adjusted. Made.

  The polarizing prism 32 of the spectroscopic unit 3 receives light (and probe pulses) transmitted through the measurement target sample and separates the light according to the polarization state. An example of the polarizing prism 32 is not limited as long as it has the above-described function. For example, it is preferably a Wollaston prism that separates into two light beams having a polarization plane orthogonal using birefringence.

  The detector 33 of the spectroscopic unit 3 is for detecting the terahertz pulse light transmitted through the polarizing prism 32. In the spectroscopic unit 3, the detector 33 can detect each light separated by the polarizing prism 32 and perform a predetermined process to perform spectroscopic analysis. This configuration is not particularly limited, and a normal THz-TDS analysis method can be used.

  As described above, according to the present embodiment, it is possible to provide a terahertz light generating apparatus that can freely change the excitation wavelength and the terahertz radiation frequency without greatly destroying the optical system, and a terahertz spectroscopic device using the terahertz light generating apparatus. In particular, according to the present embodiment, the configuration is simple, and the apparatus is small and inexpensive.

  INDUSTRIAL APPLICABILITY The present invention has industrial applicability as a terahertz light generator, and further as a terahertz spectrometer using the terahertz light generator.

Claims (4)

An excitation pulse light source for generating excitation pulse light;
A first movable mirror having a function of reflecting the excitation pulse light and capable of adjusting a reflection angle;
A diffraction grating that diffracts the excitation pulse light reflected by the first movable mirror;
A second movable mirror having a function of reflecting the diffracted excitation pulse light and capable of adjusting a reflection angle;
A terahertz light generation device, comprising: a nonlinear crystal that generates terahertz pulse light based on the excitation pulse light reflected by the second movable mirror.   The terahertz light generation device according to claim 1, wherein the diffraction grating is rotatable.   The terahertz light generation device according to claim 1, wherein the position of at least one of the first movable mirror and the second movable mirror is also movable. An excitation pulse light source for generating excitation pulse light;
A first movable mirror having a function of reflecting the excitation pulse light and capable of adjusting a reflection angle;
A diffraction grating that diffracts the excitation pulse light reflected by the first movable mirror;
A second movable mirror having a function of reflecting the diffracted excitation pulse light and capable of adjusting a reflection angle;
A nonlinear crystal that generates terahertz pulsed light based on the excitation pulsed light reflected by the second movable mirror;
A sample setting member for irradiating the terahertz pulse light generated on the measurement target sample, the measurement target sample being settable,
A polarizing prism that receives incident light transmitted through the sample;
A terahertz spectrometer comprising: a detector that detects the terahertz pulse light transmitted through the polarizing prism.