JP2014044365A - Pulse front inclined optical system and terahertz wave generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulse front inclined optical system capable of transmitting a laser pulse as an almost parallel light by continuously inclining a pulse front of the laser pulse.SOLUTION: A pulse front inclined optical system 20 for inclining a pulse front of a pulsatile electromagnetic wave includes: a first diffraction element 11 for diffracting the electromagnetic wave; transmission optical systems 12, 13 for transmitting the electromagnetic wave diffracted by the first diffraction element 11; and a second diffraction element 14 which has a different dispersion characteristic from that of the first diffraction element 11 and diffracts the electromagnetic wave transmitted by the transmission optical systems 12, 13, the electromagnetic wave diffracted by the second diffraction element 14 being an almost parallel light.

Description

  The present invention relates to a pulse front tilt optical system that tilts a pulse front of a laser pulse, and a terahertz wave generator including the pulse front tilt optical system.

  In recent years, a terahertz wave technique using a technique of tilting a pulse front of a laser pulse (a plane connecting peak positions of pulse intensities) has been studied. When a laser pulse is diffracted by a diffraction element such as a diffraction grating, the pulse front that is in a direction perpendicular to the optical axis direction is inclined before the diffraction element is incident, and a time difference is generated depending on the spatial position in the pulse.

  In terahertz time domain spectroscopy, which is a general spectroscopy of terahertz waves, terahertz waves are detected using a laser pulse (probe pulse) different from terahertz waves. Here, the probe pulse serves as a time window for temporally cutting out the terahertz wave, and by changing the relative time difference between the probe pulse and the terahertz wave, it is possible to measure the terahertz wave with time resolution. Become.

  Conventionally, as a technique for changing the time difference between a terahertz wave and a probe pulse, a technique of arranging a delayed optical path in the optical path of a laser that generates a probe pulse or a terahertz wave and scanning the delayed optical path has been widely used. However, this method has a problem that it takes time to scan the delay optical path. As a technique that can solve such problems, a technique has been proposed in which the time front of the terahertz wave is detected depending on the spatial position of the probe pulse without tilting the optical path by tilting the pulse front of the probe pulse. (For example, refer to Patent Document 1).

On the other hand, terahertz wave technology is expected to have large-diameter terahertz wave imaging and nonlinear phenomena due to terahertz waves. However, it has been difficult to perform them with the conventional generation intensity of terahertz waves. In order to solve such a problem, a high-power terahertz wave generation method using a technique of tilting the pulse front with a diffraction grating has been recently proposed (for example, see Non-Patent Document 1). This method uses difference frequency mixing, which is a kind of nonlinear optical effect, and irradiates a nonlinear optical element having a large second-order nonlinear optical coefficient (for example, LiNbO ₃ crystal) with a near-infrared laser pulse whose pulse front is inclined. Thus, a high-output terahertz wave is generated by satisfying the phase matching condition.

  Here, in order to cause difference frequency mixing, it is generally necessary to satisfy the following phase matching conditions.

However, the LiNbO ₃ crystal usually satisfies the following formula (2). Therefore, the phase matching condition that causes the difference frequency mixing cannot be satisfied, and a high-intensity terahertz wave cannot be generated as it is.

  Therefore, in Non-Patent Document 1, by tilting the pulse front of the near-infrared laser pulse by θc, the phase matching condition is satisfied and a high-output terahertz wave is generated as shown in the following equation (3). It is possible. In equation (3), θc is the pulse front tilt angle of the near-infrared laser pulse, and the optimum angle varies depending on the frequency of the generated terahertz wave.

FIG. 4 is a schematic configuration diagram of the terahertz wave generator disclosed in Non-Patent Document 1. In this terahertz wave generator, the near-infrared laser pulse is diffracted by the diffraction grating 100 to tilt the pulse front. A near-infrared laser pulse having a tilted pulse front is condensed on the LiNbO ₃ crystal 104 by the lens 103 through the reflection mirror 101 and the half-wave plate 102 to generate a terahertz wave.

In the configuration shown in FIG. 4, the tilt angle of the pulse front of the near-infrared laser pulse focused on the LiNbO ₃ crystal 104 depends on the magnification of the lens 103. Therefore, in order to generate a terahertz wave, it is necessary to construct an apparatus including the magnification of the lens 103 together with the diffraction grating 100. When the near-infrared laser pulse is diffracted by the diffraction grating 100, each wavelength component of the laser pulse is diffracted in different directions. In order to generate the terahertz wave by the difference frequency mixing of the diffracted light, different wavelength components of the near infrared laser pulse involved in the difference frequency mixing are spatially generated at the position where the terahertz wave is generated in the LiNbO ₃ crystal 104. Need to overlap. In the configuration shown in FIG. 4, when the near-infrared laser pulse is condensed in the LiNbO ₃ crystal 104 by the lens 103, terahertz waves are generated by spatially overlapping each wavelength component at the condensing position.

  Note that the difference frequency mixing here does not mean that two types of laser beams having different frequencies are incident to generate light of the difference frequency, but a frequency component having a finite width of a near-infrared laser pulse. In the above, difference frequency mixing is performed. In addition, there is a method of using a step mirror for the inclination of the pulse front. However, in this method, the pulse is intermittently divided and the phase matching condition cannot be satisfied in a wide region. Therefore, for the generation of terahertz waves, a technique using a diffraction element whose pulse front is continuously inclined is suitable.

JP 2011-158432 A

M. C. Hoffmann, K.-L. Yeh, J. Hebling, and K. A. Nelson, “Efficient terahertz generation by optical rectification at 1035 nm” Opt. Express 15 (2007) 11706.

However, in the pulse front tilt method disclosed in Patent Document 1 and Non-Patent Document 1, when the laser pulse is diffracted by the diffraction element, each wavelength component of the laser pulse is diffracted at different angles. The propagation directions of the components do not coincide, that is, they do not become parallel light. In the terahertz wave generator shown in FIG. 4, the near-infrared laser light diffracted by the diffraction grating 100 is condensed in the LiNbO ₃ crystal 104. Therefore, each wavelength component related to difference frequency mixing spatially overlaps only at the condensing position, and at a position away from the condensing point in the optical axis direction, each wavelength component spatially deviates, resulting in difference frequency mixing. The efficiency of this will be reduced. That is, in the configuration of FIG. 4, a region where the effect can be exhibited even though the LiNbO ₃ crystal 104 having a large nonlinear optical coefficient is used to increase the generation efficiency of the terahertz wave by utilizing the method of tilting the pulse front. Is limited.

  Accordingly, an object of the present invention made in view of such a viewpoint is to provide a pulse front tilt optical system capable of continuously tilting the pulse front of a laser pulse and propagating the laser pulse as substantially parallel light.

  Furthermore, another object of the present invention is to provide a terahertz wave generator that includes the above-described pulse front tilt optical system and can generate a terahertz wave with high efficiency in a wide area of a nonlinear optical crystal.

The pulse front tilt optical system according to the present invention that achieves the above object is a pulse front tilt optical system that tilts the pulse front of a pulsed electromagnetic wave,
A first diffraction element that diffracts the electromagnetic wave;
A propagation optical system for propagating the electromagnetic wave diffracted by the first diffraction element;
A second diffraction element having a dispersion characteristic different from that of the first diffraction element and diffracting the electromagnetic wave propagated by the propagation optical system,
The electromagnetic wave diffracted by the second diffraction element is configured to be substantially parallel light,
It is characterized by this.

  According to the present invention, when a pulsed electromagnetic wave is diffracted by the first diffraction element 1, the pulse front of the electromagnetic wave is inclined as compared with that before being diffracted. When the electromagnetic wave diffracted by the first diffraction element is diffracted by the second diffraction element via the propagation optical system, the pulse front is inclined again. Further, the electromagnetic wave that becomes divergent light when diffracted by the first diffraction element becomes substantially parallel light after passing through the propagation optical system and the second diffraction element. As a result, the pulsed electromagnetic wave incident on the first diffractive element can be emitted from the second diffractive element as a substantially parallel light electromagnetic wave with the pulse front inclined.

  In the above invention, a light beam diameter adjusting optical system for adjusting a light beam diameter of the electromagnetic wave diffracted by the second diffraction element may be further provided.

  Thus, by adjusting the beam diameter of the electromagnetic wave with the beam diameter adjusting optical system, the tilt angle of the pulse front of the electromagnetic wave can be changed.

Furthermore, the terahertz wave generator according to the present invention that achieves the above-described other object is as follows.
The pulse front tilt optical system according to claim 1 or 2,
A non-linear optical element that generates a terahertz wave by irradiation of the electromagnetic wave through the pulse front tilt optical system,
The pulse front tilt optical system tilts the pulse front of the electromagnetic wave in the nonlinear optical element so that the electromagnetic wave satisfies a phase matching condition for generating the terahertz wave by a nonlinear optical effect in the nonlinear optical element. ,
It is characterized by this.

  According to the present invention, it is possible to generate a terahertz wave by satisfying the phase matching condition of the above equation (2) by causing an electromagnetic wave having a tilted pulse front to enter the nonlinear optical element. In addition, since the electromagnetic wave having a tilted pulse front incident on the nonlinear optical element is substantially parallel light, it is possible to perform difference frequency mixing with high efficiency at any position in the nonlinear optical element. This makes it possible to generate a high-power terahertz wave.

In the above invention, the nonlinear optical element may be a LiNbO ₃ crystal or a LiTaO ₃ crystal.

As described above, when a LiNbO ₃ crystal or a LiTaO ₃ crystal is used as the nonlinear optical element, these crystals have a large second-order nonlinear optical coefficient, so that it is possible to generate a terahertz wave with a very high output.

  In the above invention, the electromagnetic wave may be near infrared light.

  Thus, by using near infrared light as the electromagnetic wave, a titanium sapphire laser often used as a femtosecond pulse light source can be used as the electromagnetic wave source. In addition, when the present invention is applied to terahertz wave time domain spectroscopy using a titanium sapphire laser to detect terahertz waves, the same light source can be used for terahertz wave detection and terahertz wave generation, thus saving the light source. It becomes possible to do.

  In the above invention, in the nonlinear optical element, the length of the region in the propagation direction of the electromagnetic wave that contributes to the generation of the terahertz wave may be longer than the length in the propagation direction of the pulse front of the electromagnetic wave.

  Accordingly, it is possible to generate a terahertz wave with high efficiency and high efficiency in a wider region in the propagation direction of the electromagnetic wave in the nonlinear optical element.

  ADVANTAGE OF THE INVENTION According to this invention, the pulse front inclination optical system which can incline the pulse front of a laser pulse continuously and can propagate a laser pulse as substantially parallel light can be provided. Further, according to the present invention, it is possible to provide a terahertz wave generating device that can generate a terahertz wave with high efficiency in a wide region of a nonlinear optical crystal.

1 is a schematic configuration diagram of a terahertz wave generation device according to a first embodiment. It is a schematic block diagram of the terahertz wave generator which concerns on 2nd Embodiment. It is a figure which shows the modification of a nonlinear optical element. It is a schematic block diagram of the conventional terahertz wave generator.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic configuration diagram of a terahertz wave generator according to a first embodiment of the present invention. The terahertz wave generator includes a near-infrared femtosecond pulse light source 10, a pulse front tilt optical system 20, and a nonlinear optical element 30. The near-infrared femtosecond pulse light source 10 constitutes a pulse electromagnetic wave source. For example, a titanium sapphire laser or a fiber laser that emits near-infrared light is used.

The pulse front tilt optical system 20 tilts the pulse front of the electromagnetic wave from the near-infrared femtosecond pulse light source 10, and includes a first diffraction grating 11, lenses 12 and 13, a second diffraction grating 14, A half-wave plate 15 and a beam expander 16 are provided. Here, the first diffraction grating 11 constitutes a first diffraction element. The lenses 12 and 13 constitute a propagation optical system. The second diffraction grating 14 constitutes a second diffraction element and has a dispersion characteristic different from that of the first diffraction grating 11. In the present embodiment, the first diffraction grating 11 and the second diffraction grating 14 are of a reflection type. The beam expander 16 constitutes a light beam diameter adjusting optical system. The nonlinear optical element 30 is made of LiNbO ₃ crystal or LiTaO ₃ crystal.

  In FIG. 1, a near-infrared pulse laser beam of a femtosecond pulse emitted from a near-infrared femtosecond pulse light source 10 enters a first diffraction grating 11 and is diffracted. At this time, the pulse front Pa of the near-infrared laser pulse light before being incident on the first diffraction grating 11 is perpendicular to the propagation direction of the near-infrared laser pulse light. However, after diffraction, the pulse front Pb of the near-infrared laser pulsed light is not perpendicular and tilts. In addition, since the diffraction angle of the first diffraction grating 11 varies depending on the wavelength, each wavelength component included in the near-infrared laser pulse light is diffracted in different directions, and as a result, is diffracted by the first diffraction grating 11. Near-infrared laser pulse light becomes divergent light.

  The near-infrared laser pulse light diffracted by the first diffraction grating 11 is imaged on the diffraction surface of the second diffraction grating 14 via the lens 12 and the lens 13. At this time, the lens 12 and the lens 13 are a telecentric optical system, and the propagation directions of the near-infrared laser pulse light imaged on the second diffraction grating 14 are parallel to each other at each image height.

  The first diffraction grating 11 and the second diffraction grating 14 are arranged so that the diffraction surfaces are optically conjugate. That is, the image plane on which the diffraction surface of the first diffraction grating 11 is imaged is tilted by the diffraction angle of the first diffraction grating 11 and the combined magnification of the lens 12 and the lens 13 (Shine-proof principle). ), The image plane and the diffraction plane of the second diffraction grating 14 are arranged to coincide with each other. Further, the pulse front Pc of the near-infrared laser pulse light incident on the second diffraction grating 14 through the lens 12 and the lens 13 is more than the pulse front Pb when diffracted by the first diffraction grating 11. Further, the tilt angle further changes depending on the combined magnification of the lens 13.

  Here, the second diffraction grating 14 differs from the first diffraction grating 11 in dispersion characteristics, that is, the number of engravings. Further, the number of engravings of the first diffraction grating 11, the incident angle of the near-infrared laser pulse light to the first diffraction grating 11, the combined magnification of the lens 12 and the lens 13, and the number of engravings of the second diffraction grating 14. Is set so that the diffraction angles of all wavelengths of the near-infrared laser pulsed light diffracted by the second diffraction grating 14 are substantially equal. Thereby, the near-infrared laser pulse light after being diffracted by the second diffraction grating 14 becomes substantially parallel light. Further, the pulse front Pd of the near-infrared laser pulse light after being diffracted by the second diffraction grating 14 is not perpendicular to the propagation direction but inclined at a certain angle. That is, the near-infrared laser pulse light of the pulse front Pa perpendicular to the propagation direction from the near-infrared femtosecond pulse light source 10 is the first diffraction grating 11, the lens 12 and the lens 13, and the second diffraction grating 14. Thus, the light is converted into near-infrared laser pulse light having a pulse front Pd inclined with respect to the propagation direction and substantially parallel light.

  The near-infrared laser pulse light diffracted by the second diffraction grating 14 enters the nonlinear optical element 30 through the half-wave plate 15 and the beam expander 16. Thereby, a pulsed terahertz wave is generated from the nonlinear optical element 30. Here, the half-wave plate 15 adjusts the polarization direction of the near-infrared laser pulse light so that the nonlinear optical element 30 is in an optimal polarization state in order to generate a terahertz wave by difference frequency mixing. Further, the beam expander 16 changes the beam diameter of the near-infrared laser pulse light, and changes the tilt angle of the pulse front Pd like the pulse front Pe accordingly.

  In this way, by adjusting the tilt angle of the pulse front of the near-infrared laser pulse light so as to satisfy the phase matching condition of the above expression (3), a terahertz wave can be efficiently generated in the nonlinear optical element 30. Thus, the terahertz wave pulse Tp can be emitted from the nonlinear optical element 30. If the pulse front Pd of the near-infrared laser pulse light diffracted by the second diffraction grating 14 is inclined at an angle that satisfies the phase matching condition, the beam expander 16 is not necessary. The beam expander 16 is used not only to expand the beam diameter but also to reduce the beam diameter, and by adjusting the magnification, the tilt angle of the pulse front can be adjusted to an arbitrary angle. It becomes.

  Note that the tilt angle of the pulse front Pf of the near-infrared laser pulsed light in the nonlinear optical element 30 varies with the refractive index of the nonlinear optical element 30 with respect to the pulse front Pe before entering the nonlinear optical element 30. Therefore, the conditions for each optical element are set in consideration of the refractive index of the nonlinear optical element 30. Also, in FIG. 1, the cross section of the nonlinear optical element 30 is cut into a trapezoidal shape so as to form the pulse front inclination angle θc of the above equation (3). This is to make the exit surface of the terahertz wave perpendicular to the propagation direction of the terahertz wave, and may be cut at other angles, and the shape of the nonlinear optical element 30 is not limited to the trapezoidal shape. .

  In the terahertz wave generator according to the present embodiment, the near-infrared laser pulse light is transmitted in a substantially parallel light state using the first diffraction grating 11 and the second diffraction grating 14 having different dispersion characteristics. Inclined and incident on the nonlinear optical element 30. Therefore, the region where the terahertz wave is generated in the optical axis direction is not limited as in the prior art, and the terahertz wave can be generated with high efficiency at an arbitrary position of the nonlinear optical element 30. In addition, it is not necessary to dispose the nonlinear optical element 30 at the position of the image plane of the diffraction grating, and the nonlinear optical element 30 can be disposed at an arbitrary position in the optical axis direction. There is also an advantage that it is simple.

  In the configuration shown in FIG. 1, the number of engravings of the first diffraction grating 11, the incident angle of the near-infrared laser pulse light to the first diffraction grating 11, the combined magnification of the lens 12 and the lens 13, the second The number of engravings of the diffraction grating 14 and the magnification of the beam expander 16 need to be set as appropriate, and various combinations are possible. Among them, the first diffraction grating 11 and / or the second diffraction grating 14 is arranged in a Littrow arrangement to increase the diffraction efficiency of the first diffraction grating 11 and / or the second diffraction grating 14, or the beam expander 16. Thus, the generation efficiency of terahertz waves can be increased by selecting conditions such as reducing the beam diameter of the near-infrared laser pulse light incident on the nonlinear optical element 30 and increasing the light density.

  Further, in the configuration shown in FIG. 1, the near-infrared laser pulse light diffracted by the first diffraction grating 11 is diffracted by the second diffraction grating 14 using the two lenses 12 and 13. The image is formed on the surface. However, the number of lenses constituting the propagation optical system is not limited to two, and the parallelism of the light diffracted by the second diffraction grating 14 may be improved by further improving the aberration by increasing the number of lenses. Is possible.

(Second Embodiment)
FIG. 2 is a schematic configuration diagram of the terahertz wave generation device according to the second embodiment of the present invention. This terahertz wave generating device is the same as the terahertz wave generating device according to the first embodiment, but instead of the reflective first diffraction grating 11 and the second diffraction grating 14, the transmissive first diffraction grating 18 and The second diffraction grating 19 is used. Since other configurations are the same as those in FIG. 1, the same elements as those in FIG.

  According to the present embodiment, the same effects as those of the first embodiment can be obtained. In particular, even when a lens 12 having a short focal length is used, the near-infrared laser pulse incident on the first diffraction grating 18 is used. There is an advantage that the degree of freedom of the configuration of the optical system is expanded, such as that the light is not blocked by the lens 12.

  In addition, this invention is not limited to the said embodiment, A various deformation | transformation or change is possible in the range which does not deviate from the meaning of invention. For example, one of the first diffraction grating and the second diffraction grating may be a reflection type and the other may be a transmission type. Further, the first diffractive element and the second diffractive element are not limited to the diffraction grating, and may be other dispersive elements such as a prism and a grism.

  The non-linear optical element has a cross section as shown in FIGS. 1 and 2 that is not limited to a trapezoidal shape but may have a shape as shown in FIG. The nonlinear optical element 31 shown in FIG. 3 has a shape that extends long in the optical axis direction (propagation direction) of near-infrared laser pulse light. That is, the length of the region R in the propagation direction of the near-infrared laser pulse light contributing to the generation of the terahertz wave is longer than the length of the pulse front Pf in the propagation direction of the near-infrared laser pulse light. Further, for example, a silicon prism 32 is bonded to the terahertz wave emission end face of the nonlinear optical element 31.

In the terahertz wave generation device of FIG. 4, in the LiNbO ₃ crystal 104, the generation efficiency of the terahertz wave is high only near the image plane position of the diffraction surface of the diffraction grating 100, and the generation efficiency of the terahertz wave is high at a position away from it. Lower. If the tilt angle of the image plane of the diffraction surface of the diffraction grating 100 is equal to the tilt angle of the pulse front, the region where the terahertz wave is efficiently generated in the LiNbO ₃ crystal 104 is the propagation direction of the near-infrared laser pulse light. When a crystal longer than that is used, and the crystal is longer than that, the generation efficiency of the terahertz wave is lowered due to absorption of the terahertz wave by the crystal. However, according to the configuration shown in FIG. 3, terahertz waves are generated in all regions R in which the near-infrared laser pulse light propagates in the nonlinear optical element 31, and therefore, a wide region in the propagation direction of the near-infrared laser pulse light. Can generate terahertz waves. In addition, the silicon prism 32 can prevent the terahertz wave generated at a specific angle in the nonlinear optical element 31 according to the phase matching condition of the above formula (3) from being totally reflected by the end face of the nonlinear optical element 31. . In addition, by cutting the prism in a direction (XY plane in FIG. 3) perpendicular to the propagation direction of the terahertz wave in the silicon prism 32 (Z direction in FIG. 3), it is perpendicular to the end face of the silicon prism 32. A terahertz wave pulse Tp can be generated, and alignment can be simplified.

  Further, the nonlinear optical element 31 has a part of the propagation distance of the terahertz wave in the nonlinear optical element 31 by making the end face from which the terahertz wave is emitted parallel to the optical axis (propagation direction) of the near-infrared laser pulse light. It can be made uniform in the area. As a result, the terahertz wave generated from the region having a uniform propagation distance (the terahertz wave generated at X1 to X2 in FIG. 3) has a uniform absorption amount of the terahertz wave in the nonlinear optical element 31, resulting in nonlinear optics. There is an advantage that the spatial distribution of the terahertz wave emitted from the element 31 is uniform.

  In addition, the silicon prism 32 may be comprised with another material, and a shape can also be made arbitrary. Further, the nonlinear optical element 31 omits the silicon prism 32 and cuts the end face on the side where the terahertz wave is generated in parallel with the optical axis of the near-infrared laser pulse light, thereby omitting the silicon prism 32. A terahertz wave pulse can also be emitted from 31. In addition, the nonlinear optical element 31 can have various shapes. In addition, the nonlinear optical element 31 and the silicon prism 32 are non-linear considering the phase mismatch of the difference frequency mixing and the light density by reducing the Fresnel reflection at the incident end face of the terahertz wave by adopting a coating or an antireflection structure. It is also possible to increase the output of the terahertz wave emitted from the nonlinear optical element 31 by, for example, optimizing the beam diameter of the near infrared laser pulse light incident on the optical element 31.

  The near-infrared laser pulse light propagating in the nonlinear optical element 31 has its generation efficiency gradually lowered due to the chirp of the near-infrared laser pulse light when propagating. For this reason, it is desirable that the length of the nonlinear optical element 31 in the propagation direction of the near-infrared laser pulse light is set to an appropriate length in consideration of the generation efficiency of the terahertz wave.

  Further, in the above embodiment, the lens 12 and the lens 13 constituting the propagation optical system can be cylindrical lenses, which can also spread the light beam of the terahertz wave pulse Tp in the Y direction of FIG. is there.

DESCRIPTION OF SYMBOLS 10 Near-infrared femtosecond pulse light source 11,18 1st diffraction grating 12,13 Lens 14,19 2nd diffraction grating 15 1/2 wavelength plate 16 Beam expander 20 Pulse front inclination optical system 30,31 Nonlinear optical element 32 Silicon Prism

Claims (6)

A pulse front tilt optical system for tilting a pulse front of a pulsed electromagnetic wave,
A first diffraction element that diffracts the electromagnetic wave;
A propagation optical system for propagating the electromagnetic wave diffracted by the first diffraction element;
A second diffraction element having a dispersion characteristic different from that of the first diffraction element and diffracting the electromagnetic wave propagated by the propagation optical system,
The electromagnetic wave diffracted by the second diffraction element is configured to be substantially parallel light,
A pulse front tilt optical system.   The pulse front tilt optical system according to claim 1, further comprising a light beam diameter adjusting optical system that adjusts a light beam diameter of the electromagnetic wave diffracted by the second diffraction element. The pulse front tilt optical system according to claim 1 or 2,
A non-linear optical element that generates a terahertz wave by irradiation of the electromagnetic wave through the pulse front tilt optical system,
The pulse front tilt optical system tilts the pulse front of the electromagnetic wave in the nonlinear optical element so that the electromagnetic wave satisfies a phase matching condition for generating the terahertz wave by a nonlinear optical effect in the nonlinear optical element. ,
The terahertz wave generator characterized by the above-mentioned. The terahertz wave generator according to claim 3, wherein the nonlinear optical element is made of LiNbO ₃ crystal or LiTaO ₃ crystal.   The terahertz wave generator according to claim 3 or 4, wherein the electromagnetic wave is near infrared light. 6. The nonlinear optical element according to claim 3, wherein a length of a region in a propagation direction of the electromagnetic wave contributing to generation of the terahertz wave is longer than a length in a propagation direction of a pulse front of the electromagnetic wave. The terahertz wave generator according to any one of the above.

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