Adjustable strong field terahertz source based on quartz crystal and adjusting method thereof

Abstract

The application discloses a quartz crystal-based adjustable strong-field terahertz light source and an adjusting method thereof, and belongs to the optical field. The terahertz light source is designed by using the high damage threshold and low absorption characteristics of the quartz crystal in the terahertz wave band. In addition to the femtosecond laser, the beam-reducing system, the grating, a pair of X-direction achromatic column lenses, the 1 / 2 wave plate and the quartz crystal required by the general inclined wave front, the terahertz light source further comprises a pair of Y-direction column lenses. One of the X-direction achromatic column lenses is replaced by a double-lens combined lens. The focal length of the combined lens is finely adjusted by adjusting the lens spacing of the combined lens. The grating front mirror is placed on a translation stage to adjust the grating incidence angle. The inclination angle of the quartz crystal is 43 degrees, and the terahertz light is emitted along the inclined surface of the quartz crystal. The application adjusts the THz center frequency from 2 to 4 THz and the terahertz spectrum width from 0 to 6 THz by simultaneously adjusting the focal length of the combined lens, the grating incidence angle and the emission angle, and realizes the terahertz generation efficiency of nearly 0.04%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optics, in particular to a quartz crystal-based adjustable strong-field terahertz light source and an adjusting method thereof. BACKGROUND

[0002] The terahertz wave band is a frequency range between the optical wave band and the microwave wave band, and the frequency range is 0.1 THz-10 THz. In the field of basic physics, because the resonance frequencies of many solid material modes of excitation, such as phonons and antiferromagnetic spin waves, are in the THz wave band, the THz wave band is widely used in the regulation and detection of solid materials; in the field of applied physics, because the frequency of the terahertz is higher than that of the communication wave band, the terahertz is considered as the basis of 6G technology because of its higher data transmission rate and wider frequency band resources; and the terahertz has higher precision in imaging because of its shorter wavelength than that of the microwave.

[0003] However, compared with the microwave band generated from electronic circuits and the optical band generated by semiconductor devices and the like, the THz band has always been relatively lacking in efficient light sources. At present, the commonly used method for generating strong-field terahertz pulses with a relatively concentrated energy spectrum is the tilted pulse front method based on the phase matching of optical rectification in a femtosecond laser nonlinear crystal.

[0004] In the tilted pulse front method, the most commonly used nonlinear crystal is a LiNbO3 crystal, but because the lithium niobate crystal has strong absorption above 2 THz, it can only generate terahertz pulse light in the range of 0-2 THz, which limits the application of the terahertz light source. Although optical rectification using organic crystals can generate pulses in the range of 0-5 THz, the requirement for the pump light source is relatively high and the quality of the organic crystal is not high, so the spot pattern generated is not good, which makes the method of generating terahertz pulses using organic crystals not widely used.

[0005] It can be seen that the existing terahertz light source system either has the problem of low frequency of the generated terahertz pulses or has the problem of high requirement for the pump light source and low quality of the crystal, so it is necessary to develop a light source that can generate terahertz pulses with better frequency and good crystal quality. The inorganic alpha-quartz crystal has the characteristics of low absorption and high damage threshold in the range of 0-6 THz, so it can be used to generate terahertz pulses in a higher wave band. SUMMARY

[0006] In view of the problems of low frequency of the terahertz pulses generated by the existing terahertz light source and small frequency spectrum range, the purpose of the present application is to provide a quartz crystal-based adjustable strong-field terahertz light source and an adjusting method thereof.

[0007] To achieve the above object, the technical scheme of the present application is as follows:

[0008] In the first aspect, the present application provides a quartz crystal-based adjustable strong-field terahertz light source, comprising a femtosecond laser, a mirror, a grating, an imaging system and a triangular prism quartz crystal arranged in sequence along an optical path; the femtosecond laser is used to generate pulsed pump laser; the mirror is used to reflect the pulsed pump laser to the grating; the grating is used to make the pulsed pump laser pulse wavefront tilt to generate a tilted wavefront pulsed laser A; the imaging system is used to control the wavefront tilt angle of the tilted wavefront pulsed laser A by adjusting the magnification, and to converge the generated tilted wavefront pulsed laser B into the triangular prism quartz crystal; the triangular prism quartz crystal is used to generate terahertz pulsed light on its inclined surface; a half-wave plate for adjusting the polarization direction of the pulsed laser is further arranged between the grating and the triangular prism quartz crystal;

[0009] The imaging system comprises an X-direction telescope assembly and a Y-direction telescope assembly arranged along the Z-direction, the X-direction, the Y-direction and the Z-direction are perpendicular to each other, and the Z-direction is the propagation direction of the tilted wavefront pulsed laser A; the X-direction telescope assembly is used to adjust the X-direction focusing, comprising a combination lens and an achromatic cylindrical lens B, the combination lens comprises a long-focus cylindrical lens and an achromatic cylindrical lens A, and the long-focus cylindrical lens and the achromatic cylindrical lens A are movably mounted along the Z-direction; the Y-direction telescope assembly is used to adjust the Y-direction focusing, comprising a normal cylindrical lens A and a normal cylindrical lens B; wherein the grating, the combination lens, the achromatic cylindrical lens B and the triangular prism quartz crystal constitute a 4f telescope system;

[0010] Further comprising an angle adjusting device for adjusting the incident angle and the exit angle of the grating.

[0011] In a preferred embodiment, the X-direction telescope assembly further comprises a first moving platform moving along the Z-direction, and the long-focus cylindrical lens and the achromatic cylindrical lens A are movably mounted on the first moving platform along the Z-direction; or the first moving platform has two, and the long-focus cylindrical lens and the achromatic cylindrical lens A are fixedly installed on the two first moving platforms respectively.

[0012] In a preferred embodiment, a beam shrinking system is further arranged between the femtosecond laser and the mirror, and the beam shrinking system comprises a concave mirror and a convex mirror arranged in sequence.

[0013] In a preferred embodiment, the angle adjusting device comprises a first rotating platform, a second rotating platform and a second moving platform; the mirror is mounted on the first rotating platform, the first rotating platform is fixedly mounted on the second moving platform, the second moving platform is used to drive the mirror to move along the propagation direction of the pulsed pump laser, and the first rotating platform is used to control the incident angle of the reflected pulsed pump laser on the grating; the grating is mounted on the second rotating platform, and the second rotating platform is used to control the exit angle of the tilted wavefront pulsed laser A on the grating.

[0014] In a second aspect, the present application also provides a terahertz pulse frequency adjusting method applied to the terahertz light source as described above, the method being used to adjust the frequency of the terahertz pulse light generated by the terahertz light source, and the method comprising the following steps:

[0015] S1, obtaining the frequency of the required terahertz pulse light, and determining the phase velocity refractive index of the required terahertz pulse light in the three-prism quartz crystal according to a preset mapping relationship table

[0016] S2, according to the refractive index phase matching condition of the three-prism quartz crystal , calculating the wavefront tilt angle γ of the tilted wavefront pulsed laser B incident into the three-prism quartz crystal, wherein n g is the group velocity refractive index of the pulsed pump laser;

[0017] S3, making the wavefront tilt angle λ1 of the tilted wavefront pulsed laser A emitted by the grating the same as the wavefront tilt angle γ of the tilted wavefront pulsed laser B, so as to improve the light intensity of the tilted wavefront pulsed laser B in the three-prism quartz crystal;

[0018] S4, according to the imaging relationship of the imaging system the imaging relationship of the grating the magnification formula β = f1 / f2 (formula 4) and the grating equation of the grating simultaneously calculating the distance d of the long-focus cylindrical lens and the achromatic cylindrical lens A, the incident angle θ i and the exit angle θ d of the grating

[0019] of the pulsed pump laser, β is the magnification of the imaging system, p is the ruling density of the grating, θ i and θ drespectively, n is a linear refractive index of the pulse-pumped laser, f0 is a focal length of the long-focus cylindrical lens, f is a focal length of the achromatic cylindrical lens A, f2 is a focal length of the achromatic cylindrical lens B, and f1 is a focal length of a combined lens composed of the long-focus cylindrical lens and the achromatic cylindrical lens A;

[0020] S5, adjusting a distance d between the long-focus cylindrical lens and the achromatic cylindrical lens A, an incident angle θ i and an exit angle θ d of the grating according to the calculation result in S4, and starting the femtosecond laser, so that the terahertz pulse light generated by the three-prism quartz crystal can be adjusted to a required frequency.

[0021] In a preferred embodiment, the pulse-pumped laser emitted by the femtosecond laser is horizontally-propagating linearly-polarized light with a wavelength of 800 nm and a pulse width of 30-100 fs; the grating has a ruling density p of 1500 lines / mm; the magnification β of the imaging system is 1.0-1.05; the incident angle θ i of the grating is 31-34 degrees, and the exit angle θ d is 43-40 degrees; when β = 1.01, the phase-matching center frequency of the terahertz pulse light generated by the three-prism quartz crystal is 0 THz, and when β = 1.04, the phase-matching center frequency of the terahertz pulse light generated by the three-prism quartz crystal is 4 THz.

[0022] In a preferred embodiment, the focal length f2 of the achromatic cylindrical lens B is 200 mm, the focal length f of the achromatic cylindrical lens A is 250 mm, and the focal length f0 of the long-focus cylindrical lens is 1000 mm; when the distance d between the long-focus cylindrical lens and the achromatic cylindrical lens A is 1 cm-5 cm, the magnification β of the imaging system corresponds to 1.0-1.05.

[0023] In a preferred embodiment, in the 4f telescope system composed of the grating, the combined lens, the achromatic cylindrical lens B, and the three-prism quartz crystal, the spacing between the grating and the combined lens, the spacing between the combined lens and the achromatic cylindrical lens B, and the spacing between the achromatic cylindrical lens B and the three-prism quartz crystal are 200 mm, 400 mm, and 200 mm, respectively.

[0024] In a preferred embodiment, the focal lengths of the ordinary cylindrical lens A and the ordinary cylindrical lens B in the imaging light path are 500 mm and -50 mm, respectively, and the spacing between the ordinary cylindrical lens A and the ordinary cylindrical lens B is 450-500 mm.

[0025] By using the above technical solution, the present application has the following advantages:

[0026] 1、The application utilizes femtosecond laser inclined pulse wave front technology, and realizes a wide spectrum terahertz light source with adjustable frequency spectrum range in a three-prism quartz crystal, utilizes the characteristics that the three-prism quartz crystal has almost no absorption and dispersion in the terahertz wave band, and the three-prism quartz crystal has a very high damage threshold, can realize that the terahertz center frequency can be adjusted from 2THz to 4THz, the frequency spectrum width can reach 0-6THz, and the conversion efficiency is basically unchanged, and is about 0.04%; while the lithium niobate crystal generates terahertz with a center frequency of about 0.5THz, a frequency spectrum width of 0-1THz, and almost no energy higher than 2THz;

[0027] 2、The application successfully realizes the fine adjustment of the wave front inclination angle in the three-prism quartz crystal by adjusting the incidence angle and the exit angle of the grating and changing the magnification of the imaging system, realizes the phase matching of 0-5THz through the phase matching condition; and the optical rectification based on lithium niobate or zinc antimony crystal has a more strict phase matching relationship due to absorption limitation, and the frequency spectrum range cannot be adjusted;

[0028] 3、The group velocity character dispersion GVD of the three-prism quartz crystal is smaller than that of lithium niobate by one order of magnitude, so that the effective length of 800nm femtosecond pump light in the quartz crystal can be increased; and the damage threshold of the quartz crystal is about two orders of magnitude higher than that of lithium niobate, so that the fact that the second-order nonlinear coefficient of the quartz crystal is much smaller than that of the lithium niobate crystal can be compensated; in addition, the three-prism quartz crystal has a lower cost, only 1 / 10 of the price of lithium niobate, and the quality of the three-prism quartz crystal is higher than that of an organic crystal. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a structure schematic view of an adjustable strong field terahertz light source based on a quartz crystal disclosed by the application;

[0030] Figure 2 It is a schematic view of terahertz radiation generated by a nonlinear crystal through an inclined wave front;

[0031] Figure 3 It is a magnification schematic view of an imaging system corresponding to different terahertz center frequencies;

[0032] Figure 4 It is a grating incidence angle and grating exit angle schematic view corresponding to different terahertz center frequencies;

[0033] Figure 5 It is an imaging system magnification influence on terahertz spectrum schematic view.

[0034] In the diagram: 1-Femtosecond laser, 2-Mirror, 3-Grating, 4-Imaging system, 41-Long focal length cylindrical lens, 42-Achromatic cylindrical lens A, 43-Achromatic cylindrical lens B, 44-Ordinary cylindrical lens A, 45-Ordinary cylindrical lens B, 5-Triangular prism quartz crystal, 6-Half-wave plate. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "front", "rear", etc., indicate the orientation or positional relationship based on the description of the structure of this invention shown in the accompanying drawings. They are only for the convenience of describing this invention and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0037] The terms "first" and "second" in this technical solution are merely designations for corresponding structures that are identical or similar, or that perform similar functions. They do not represent an arrangement of the importance of these structures, nor do they imply any ranking, comparison of size, or other meaning.

[0038] Furthermore, unless otherwise explicitly specified and limited, the terms "installation" and "connection" should be interpreted broadly. For example, a connection can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two structures. Those skilled in the art can understand the specific meaning of the above terms in this invention by considering the overall concept of the invention and the specific context of the solution.

[0039] Example 1

[0040] A tunable high-field terahertz light source based on quartz crystal, such as Figure 1 As shown, it includes a femtosecond laser 1, a mirror 2, a grating 3, an imaging system 4, and a triangular prism quartz crystal 5 arranged sequentially along the optical path.

[0041] The femtosecond laser 1 is used to generate a pulsed pump laser; the mirror 2 is used to reflect the pulsed pump laser to the grating 3; the grating 3 is used to make the pulsed pump laser pulse wavefront tilt to generate a tilted wavefront pulsed laser A; the imaging system 4 is used to control the wavefront tilt angle of the tilted wavefront pulsed laser A by adjusting the magnification thereof, and to converge the generated tilted wavefront pulsed laser B into the three-prism quartz crystal 5; and the three-prism quartz crystal 5 is used to generate a terahertz pulsed light on the inclined surface thereof.

[0042] The beam-reducing system is further arranged between the mirror 2 and the femtosecond laser 1. The beam-reducing system is used to perform beam-reducing processing on the pulsed pump laser generated by the femtosecond laser 1. The beam-reducing system comprises a concave mirror and a convex mirror arranged in sequence. After the two mirrors are used in cooperation, the pulsed pump laser can be reduced to a 1 / e 2 The mirror 2 is preferably configured with two mirrors, i.e., a first mirror (M1) and a second mirror (M2). The first mirror (M1) is used to reflect the pulsed pump laser after the beam-reducing processing to the second mirror (M2), and the second mirror (M2) is used to reflect the pulsed pump laser after the beam-reducing processing to the grating 3. Through the cooperation of the first mirror (M1) and the second mirror (M2) and by increasing one reflection, the arrangement of the femtosecond laser 1 and the grating 3 in the spatial position is more flexible. For example, the pulsed pump laser emitted by the femtosecond laser 1 and the tilted wavefront pulsed laser A emitted by the grating 3 are parallel to each other, which is referred to as the Z direction, and the propagation direction of the pulsed pump laser after the beam-reducing processing between the first mirror (M1) and the second mirror (M2) is defined as the X direction.

[0043] The imaging system 4 is configured to comprise an X-direction telescope assembly and a Y-direction telescope assembly arranged along the above-mentioned Z direction, and the Y direction is perpendicular to both the X direction and the Z direction. The X-direction telescope assembly is used to adjust the X-direction focusing, and the Y-direction telescope assembly is used to adjust the Y-direction focusing. The X-direction telescope assembly specifically comprises a combination lens and an achromatic cylindrical lens B43, and the combination lens further comprises a long-focus cylindrical lens 41 (a general cylindrical lens with a long focal length) and an achromatic cylindrical lens A42. The Y-direction telescope assembly specifically comprises a general cylindrical lens A44 and a general cylindrical lens B45. The general cylindrical lens is relative to the achromatic cylindrical lens.

[0044] The long-focus cylindrical lens 41 and the achromatic cylindrical lens A 42 are movably arranged along the Z direction. The X-direction telescopic assembly further comprises a first moving platform (not shown) moving along the Z direction. The long-focus cylindrical lens 41 and the achromatic cylindrical lens A 42 are movably arranged on the first moving platform along the Z direction. In this way, the distance between the long-focus cylindrical lens 41 and the achromatic cylindrical lens A 42 is adjustable, and the position of the combined lens along the Z direction is also adjustable. Alternatively, in another preferred embodiment, two first moving platforms are arranged. The long-focus cylindrical lens 41 and the achromatic cylindrical lens A 42 are fixedly arranged on the two first moving platforms, respectively. In this way, the distance between the long-focus cylindrical lens 41 and the achromatic cylindrical lens A 42 is adjustable, and the position of the combined lens along the Z direction is also adjustable. In addition, the achromatic cylindrical lens B 43 is arranged between the ordinary cylindrical lens A 44 and the ordinary cylindrical lens B 45.

[0045] The length of the triangular prism quartz crystal 5 is arranged along the Y direction. The inclined surface of the triangular prism quartz crystal 5 faces away from the imaging system 4, and the side of the triangular prism quartz crystal 5 facing the imaging system 4 is perpendicular to the Z direction. In addition, a half-wave plate 6 is arranged between the grating 3 and the triangular prism quartz crystal 5. The half-wave plate 6 is used to adjust the polarization direction of the inclined wavefront pulsed laser A or the inclined wavefront pulsed laser B, so as to ensure that the polarization of the light incident into the triangular prism quartz crystal is along the Y direction. In this embodiment, the half-wave plate 6 is arranged between the ordinary cylindrical lens A 44 and the achromatic cylindrical lens B 43.

[0046] The angle adjusting device for adjusting the incident angle and the exit angle of the grating 3 comprises a first rotating platform, a second rotating platform and a second moving platform. The second mirror (M2) is arranged on the first rotating platform, and the first rotating platform is arranged on the second moving platform. The second moving platform is used to drive the second mirror (M2) to move along the X direction. The first rotating platform is used to control the pulsed pump laser reflected by the second mirror (M2) to be incident on the grating 3, so as to adjust the incident angle. The grating 3 is arranged on the second rotating platform. The second rotating platform is used to control the exit angle of the inclined wavefront pulsed laser A on the grating 3. Since the optical path plane is in the XZ plane, the rotation axes of the first rotating platform and the second rotating platform are parallel to the Y direction. In this way, the incident angle and the exit angle of the grating 3 are adjusted by the cooperation of the first rotating platform, the second rotating platform and the second moving platform.

[0047] In this embodiment, the pulsed pump laser emitted by the femtosecond laser 1 has a time length of 33 fs, a repetition frequency of 1 KHz, a center wavelength of 800 nm, a polarization direction parallel to the optical plane, and a spot 1 / e 2 diameter of 18 mm. After being shrunk by the shrinking system, the spot 1 / e2 The diameter of the three-prism quartz crystal 5 is 9.6 mm, and the input energy of the pulsed pump laser in front of the three-prism quartz crystal 5 is 2.5 W. The grating 3 is configured with a ruling density p of 1500 lines / mm. In terms of lens parameters, the focal length f2 of the achromatic cylindrical lens B43 is 200 mm, the focal length f of the achromatic cylindrical lens A42 is 250 mm, and the focal length f0 of the long-focus cylindrical lens 41 is 1000 mm. The general cylindrical lens A44 is configured as a cylindrical convex lens with a focal length of 500 mm, the general cylindrical lens B45 is configured as a cylindrical concave mirror with a focal length of -50 mm, and the distance between the general cylindrical lens A44 and the general cylindrical lens B45 is 450-500 mm. Within this distance range, increasing the distance can increase the light intensity and efficiency.

[0048] In this embodiment, the grating 3, the combined lens, the achromatic cylindrical lens B43, and the three-prism quartz crystal 5 are configured in a 4f telescope system, and the distances between the grating 3 and the combined lens, the combined lens and the achromatic cylindrical lens B43, and the achromatic cylindrical lens B43 and the three-prism quartz crystal 5 are 200 mm, 400 mm, and 200 mm, respectively.

[0049] On the basis of the above parameters, the THz is mainly generated by using the method of optical rectification, as shown in FIG. 2, which is a non-collinear wave vector matching condition. Figure 2

[0050]

[0051] And when the angle between k(ω0)-k(ω0+Ω THz is relatively small, it can be projected to the k(ω0) direction for calculation, and the left side of the equation is obtained:

[0052]

[0053] The group velocity phase matching condition and the corresponding refractive index phase matching condition are obtained:

[0054]

[0055] That is, the wavefront inclination angle γ of the inclined wavefront pulsed laser B incident into the three-prism quartz crystal 5 can be calculated according to the refractive index phase matching condition of the three-prism quartz crystal 5 is the phase velocity refractive index of the terahertz pulsed light in the three-prism quartz crystal 5, which changes with the corresponding THz frequency, and the two have a corresponding mapping relationship. In this embodiment, the value is selected as 2.11, and n g ​​The group velocity refractive index for the 800nm ​​band (i.e., the center wavelength of the pulsed pump laser emitted by the femtosecond laser) is 1.55, which remains almost constant. Therefore, according to Equation 1, γ can be calculated to be 43°.

[0056] This embodiment has the following formula:

[0057] Imaging relationship of imaging system 4

[0058] Imaging relationship of grating 3

[0059] The magnification calculation formula for imaging system 4 is β=f1 / f2 (Equation 4) and

[0060] Grating equation for grating 3

[0061] In the above formula, λ0 is the wavelength of the pulsed pump laser, β is the magnification of the imaging system 4, and n g ρ is the group velocity refractive index at the pulse-pumped laser band, p is the grating 3's line density, and θ is... i and θ d These are the incident and exit angles of the center wavelength of grating 3, respectively; n is the linear refractive index of the pulse-pumped laser; f0 is the focal length of the long focal length cylindrical lens 41; f is the focal length of the achromatic cylindrical lens A41; f2 is the focal length of the achromatic cylindrical lens B42; and f1 is the focal length of the combined lens formed by the long focal length cylindrical lens 41 and the achromatic cylindrical lens A42.

[0062] In this system, grating 3 is used to spatially separate wave vectors of different frequencies, which are then imaged into the triangular prism quartz crystal 5. It can be understood that the wavefront tilt angle γ1 of the tilted wavefront pulse laser A emitted from grating 3 is typically equal to the wavefront tilt angle γ1 of the tilted wavefront pulse laser B incident on the triangular prism quartz crystal 5, to ensure that the center wavelength of the tilted wavefront pulse laser A emitted from grating 3 is aligned with the wavefront of the prism quartz crystal 5. Figure 1 The optical axis is shown by the dashed line, thus ensuring the highest possible input energy. That is: γ = γ1 (Equation 7).

[0063] By combining equations (2) to (7) to form a system of equations, the distance d between the telephoto cylindrical lens 41 and the achromatic cylindrical lens A42, and the incident angle θ of the grating 3 can be calculated. i and the angle of departure θ d .

[0064] At this time, Figure 1 The terahertz light source shown is adjusted (to satisfy the distance d between the telephoto cylindrical lens 41 and the achromatic cylindrical lens A42, and the incident angle θ of the grating 3). i and the angle of departure θ d(The value is the same as the calculated value above). After starting the femtosecond laser, the terahertz pulse light generated by the triangular prism quartz crystal 5 can be adjusted to the required frequency.

[0065] It is understood that, within the capability range of the terahertz light source disclosed in this embodiment, any frequency of terahertz pulse light corresponds to a set of d, θ values. i θ d In use, the magnification β of the imaging system 4 in the terahertz light source and the emission angle θ of the grating 3 can be controlled by adjusting the distance d between the telephoto cylindrical lens 41 and the achromatic cylindrical lens A42, the positions of the first and second moving platforms, and the orientation of the first steering platform and the second rotating platform. d In turn, terahertz pulsed light with corresponding frequency and spectrum can be obtained.

[0066] In this embodiment, the χ of a quartz crystal is used. xxx As an effective second-order nonlinear coefficient, the crystal axis direction of the triangular prism quartz crystal 5 is chosen to be along... Figure 2 Substituting the parameters mentioned above into the simultaneous equations along the Z-axis, and considering the dispersion of the triangular prism quartz crystal 5 in the terahertz band, the magnification β of the imaging system 4 and the incident angle θ of the grating 3 corresponding to the phase-matching center frequencies of different terahertz pulses are calculated. i and the angle of departure θ d The relationship is as follows: Figure 3 and Figure 4 As shown.

[0067] pass Figure 3 It can be seen that when the magnification β is adjusted from 1 to 1.04, the terahertz light phase matching center frequency can be adjusted from 0THz to 4THz (Note: the phase matching center frequency is different from the actual spectrum center frequency). Correspondingly, by adjusting the distance d between the telephoto cylindrical lens 41 and the achromatic cylindrical lens A42 from 0 to 5cm, the magnification β can be adjusted from 1 to 1.05.

[0068] like Figure 5 As shown, this diagram displays the measured spectra of terahertz pulsed light with β values ​​of 1.01, 1.03, and 1.04, and terahertz phase-matching center frequencies of 1 THz, 3 THz, and 4 THz, respectively. It can be seen that when the magnification β of the imaging system is fine-tuned, the center frequency of the terahertz pulsed light spectrum shifts significantly, from 1.9 THz to 4 THz, and the pulse width can reach 0-6 THz, while the total energy remains approximately unchanged. This shift is due to the presence of linear absorption and χ² near 3.8 THz. (2) Resonance, therefore a resonance absorption peak and χ² are present near 3.8 THz. (2) Resonance peak.

[0069] Therefore, the scheme provided by the embodiment of the present application can generate 1mW terahertz pulsed light when 2.5W femtosecond laser with a center wavelength of 800nm is incident on the three-prism quartz crystal, and the generation efficiency is approximately 0.04% under the highest pump light intensity in the experimental implementation, which is close to the generation efficiency of the conventional lithium niobate crystal at room temperature.

[0070] Embodiment two

[0071] A terahertz pulsed light adjusting method applied to the terahertz light source disclosed in the above embodiments, the method being used to adjust the frequency of the terahertz pulsed light generated by the terahertz light source, and the method comprising the following steps:

[0072] S1, obtaining the frequency of the required terahertz pulsed light, and determining the phase velocity refractive index of the required terahertz pulsed light in the three-prism quartz crystal according to a preset mapping relationship table

[0073] S2, calculating the refractive index of the three-prism quartz crystal according to the refractive index phase matching condition of the three-prism quartz crystal and obtaining the wavefront tilt angle γ of the tilted wavefront pulsed laser B incident into the three-prism quartz crystal, wherein n g is the group velocity refractive index of the pulsed pump laser emitted by the femtosecond laser;

[0074] S3, making the wavefront tilt angle λ1 of the tilted wavefront pulsed laser A emitted by the grating the same as the wavefront tilt angle γ of the tilted wavefront pulsed laser B, so as to improve the light intensity of the tilted wavefront pulsed laser B in the three-prism quartz crystal;

[0075] S4, calculating the magnification β of the imaging system according to the imaging relationship of the imaging system the imaging relationship of the grating the magnification calculation formula β = f1 / f2 (formula 4) and the grating equation of the grating simultaneously calculating the distance d of the long-focus cylindrical lens and the achromatic cylindrical lens A, the incident angle θ i and the exit angle θ d of the grating

[0076] wherein λ0 is the wavelength of the pulsed pump laser, β is the magnification of the imaging system, p is the ruling density of the grating, θ i and θ d are the incident angle and the exit angle of the central wavelength of the grating, n is the linear refractive index of the pulsed pump laser, f0 is the focal length of the long-focus cylindrical lens, f is the focal length of the achromatic cylindrical lens A, f2 is the focal length of the achromatic cylindrical lens B, and f1 is the focal length of the combined lens composed of the long-focus cylindrical lens and the achromatic cylindrical lens A

[0077] S5, based on the calculation results in S4, adjust the distance d between the telephoto cylindrical lens and the achromatic cylindrical lens A, and the incident angle θ of the grating. i and the angle of departure θ d Once the femtosecond laser is activated, the terahertz pulse light generated by the triangular prism quartz crystal can be adjusted to the required frequency.

[0078] In this embodiment, the pulsed pump laser emitted by the femtosecond laser is specifically configured with a duration of 33 fs, a repetition rate of 1 kHz, a center wavelength of 800 nm, a polarization direction parallel to the optical plane, and a spot size of 1 / e. 2 The diameter is 18mm, and the spot size is 1 / e after beam reduction by the beam reduction system. 2 The diameter is 9.6 mm, and the input energy of the pulsed pump laser in front of the triangular prism quartz crystal is 2.5 W. The grating line density p is configured to be 1500 lines / mm. Regarding lens parameters, the achromatic cylindrical lens B has a focal length f2 of 200 mm, the achromatic cylindrical lens A has a focal length f of 250 mm, and the telephoto cylindrical lens has a focal length f0 of 1000 mm. Ordinary cylindrical lens A is configured as a convex cylindrical lens with a focal length of 500 mm, and ordinary cylindrical lens B is configured as a concave cylindrical lens with a focal length of -50 mm. The distance between ordinary cylindrical lenses A and B is 450-500 mm; within this range, increasing the distance enhances light intensity and increases efficiency. In a 4f telescope system configured with a grating, a combination lens, achromatic cylindrical lens B, and a triangular prism quartz crystal, the specific spacing between the grating and the combination lens, the spacing between the combination lens and the achromatic cylindrical lens B, and the spacing between the achromatic cylindrical lens B and the triangular prism quartz crystal are 200mm, 400mm, and 200mm, respectively.

[0079] In actual adjustment, the magnification β of the imaging system is 1.0-1.05, and the incident angle θ of the grating is... i 31-34 degrees, exit angle θ d The magnification is 43-40 degrees; when β = 1.01, the phase-matching center frequency of the terahertz pulse light generated by the triangular prism quartz crystal is 0 THz, while when β = 1.04, the phase-matching center frequency of the terahertz pulse light generated by the triangular prism quartz crystal is 4 THz. When the distance d between the telephoto cylindrical lens and the achromatic cylindrical lens A is 1cm-5cm, the magnification β of the imaging system corresponds to 1.0-1.05.

[0080] like Figure 5As shown, it shows the measured terahertz pulse light spectrum when β is 1.01, 1.03 and 1.04 respectively, and the terahertz phase matching center frequency is 1 THz, 3 THz and 4 THz respectively. It can be seen that when the magnification β of the fine-tuning imaging system is adjusted, the center frequency of the terahertz pulse light spectrum moves obviously, the center frequency moves from 1.9 THz to 4 THz, the pulse width can reach 0-6 THz, and the total energy is approximately unchanged. Among them, due to the existence of linear absorption and χ (2) resonance near 3.8 THz, there is a resonance absorption peak and χ (2) resonance peak near 3.8 THz.

[0081] Therefore, the scheme provided by the embodiment of the present application can generate 1 mW of terahertz pulse light when 2.5 W of 1 kHz femtosecond laser with a center wavelength of 800 nm is incident on the three-prism quartz crystal. Under the highest pump light intensity realized in the experiment, the generation efficiency is approximately 0.04%, which is close to the generation efficiency of the conventional lithium niobate crystal at room temperature.

[0082] The embodiments of the present application are described in detail above with reference to the drawings, but the present application is not limited to the described embodiments. For those skilled in the art, various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the present application, and still fall within the protection scope of the present application.

Claims

1. A tunable high-intensity terahertz light source based on quartz crystal, characterized in that: The system includes a femtosecond laser, a mirror, a grating, an imaging system, and a triangular prism quartz crystal arranged sequentially along the optical path. The femtosecond laser is used to generate a pulsed pump laser. The mirror is used to reflect the pulsed pump laser onto the grating. The grating is used to tilt the wavefront of the pulsed pump laser to generate a tilted wavefront pulsed laser A. The imaging system is used to control the wavefront tilt angle of the tilted wavefront pulsed laser A by adjusting the magnification, and to converge the generated tilted wavefront pulsed laser B into the triangular prism quartz crystal. The triangular prism quartz crystal is used to generate terahertz pulsed light on its inclined surface. A half-waveplate for adjusting the polarization direction of the pulsed laser is also provided between the grating and the triangular prism quartz crystal. The imaging system includes an X-axis telescope assembly and a Y-axis telescope assembly arranged along the Z-axis. The X, Y, and Z axes are mutually perpendicular, with the Z-axis being the propagation direction of the tilted wavefront pulsed laser A. The X-axis telescope assembly is used to adjust the X-axis focusing and includes a combined lens and an achromatic cylindrical lens B. The combined lens includes a telephoto cylindrical lens and an achromatic cylindrical lens A, both of which are movably mounted along the Z-axis. The Y-axis telescope assembly is used to adjust the Y-axis focusing and includes a regular cylindrical lens A and a regular cylindrical lens B. The grating, the combined lens, the achromatic cylindrical lens B, and the triangular prism quartz crystal constitute a 4f telescope system. The X-axis telescope assembly further includes a first moving platform that moves along the Z-axis, wherein the telephoto cylindrical lens and the achromatic cylindrical lens A are both movably mounted on the first moving platform along the Z-axis; or, there are two first moving platforms, wherein the telephoto cylindrical lens and the achromatic cylindrical lens A are respectively fixedly mounted on two of the first moving platforms. The system also includes an angle adjustment device for adjusting the incident and exit angles of the grating; the angle adjustment device includes a first rotating platform, a second rotating platform, and a second moving platform; the reflector is mounted on the first rotating platform, the first rotating platform is fixedly mounted on the second moving platform, the second moving platform is used to drive the reflector to move along the propagation direction of the pulsed pump laser, and the first rotating platform is used to control the incident angle of the reflected pulsed pump laser on the grating; the grating is mounted on the second rotating platform, and the second rotating platform is used to control the exit angle of the tilted wavefront pulsed laser A on the grating.

2. The terahertz light source according to claim 1, characterized in that: It also includes a beam-shrinking system disposed between the femtosecond laser and the reflector, the beam-shrinking system comprising a concave reflector and a convex reflector arranged sequentially.

3. A terahertz pulse frequency adjustment method, applied to a terahertz light source as described in any one of claims 1-2, wherein the method is used to adjust the frequency of the terahertz pulse light generated by the terahertz light source, characterized in that: The method includes the following steps: S1. Obtain the desired frequency of the terahertz pulse light, and determine the phase velocity refractive index of the desired terahertz pulse light in the triangular prism quartz crystal according to the preset mapping table. ; S2, based on the refractive index phase matching condition of the triangular prism quartz crystal. (Equation 1) Calculate the wavefront tilt angle of the tilted wavefront pulse laser B incident on the triangular prism quartz crystal. , where n g The group velocity refractive index of the pulsed pump laser; S3, let the wavefront tilt angle λ1 of the tilted wavefront pulse laser A emitted from the grating be the same as the wavefront tilt angle of the tilted wavefront pulse laser B. The same principle applies, thereby increasing the intensity of the tilted wavefront pulsed laser B in the triangular prism quartz crystal; S4. Based on the imaging relationship of the imaging system (Equation 2), the imaging relationship of the grating (Equation 3) Calculation formula for the magnification of the imaging system (Equation 4) and (Equation 5), the grating equation of the grating (Equation 6) By simultaneously calculating the distance d between the telephoto cylindrical lens and the achromatic cylindrical lens A, and the incident angle θ of the center wavelength of the grating, we can obtain the following: i and the angle of departure θ d ; Where λ0 is the wavelength of the pulsed pump laser, β is the magnification of the imaging system, p is the grating line density, and θ i and θ d The incident and exit angles are respectively the center wavelength of the grating, and n is the linear refractive index of the pulsed pump laser. The focal length of the telephoto cylindrical lens, The focal length of the achromatic cylindrical lens A, The focal length of the achromatic cylindrical lens B, The focal length of the combined lens formed by the telephoto cylindrical lens and the achromatic cylindrical lens A; S5, according to the calculation results in S4, adjusting the distance d between the long-focus cylindrical lens and the achromatic cylindrical lens A, the incident angle θ of the center wavelength of the grating i and the exit angle θ d After starting the femtosecond laser, the terahertz pulse light generated by the three-prism quartz crystal can be adjusted to the required frequency.

4. The method according to claim 3, characterized in that: The pulse pumped laser emitted by the femtosecond laser is linearly polarized light with a wavelength of 800 nm and a pulse width of 30-100 fs and propagating horizontally; the grating has a ruling density p of 1500 lines / mm, the magnification β of the imaging system is 1.0-1.05, the incidence angle θ i of the grating is 31-34 degrees, and the exit angle θ d of the grating is 43-40 degrees; wherein when β=1.01, the phase matching center frequency of the terahertz pulse light generated by the three-prism quartz crystal is 0 THz, and when β=1.04, the phase matching center frequency of the terahertz pulse light generated by the three-prism quartz crystal is 4 THz.

5. The method according to claim 4, characterized in that: The focal length of the achromatic cylindrical lens B The focal length of the achromatic cylindrical lens A is 200mm. The focal length of the telephoto lens is 250mm. The magnification β of the imaging system is 1.0-1.05 when the distance d between the telephoto cylindrical lens and the achromatic cylindrical lens A is 1cm-5cm.

6. The method according to claim 5, characterized in that: In the 4f telescope system composed of the grating, the combined lens, the achromatic cylindrical lens B, and the triangular prism quartz crystal, the spacing between the grating and the combined lens, the spacing between the combined lens and the achromatic cylindrical lens B, and the spacing between the achromatic cylindrical lens B and the triangular prism quartz crystal are 200mm, 400mm, and 200mm, respectively.

7. The method according to claim 4, characterized in that: The focal lengths of ordinary cylindrical lens A and ordinary cylindrical lens B in the imaging optical path are 500mm and -50mm, respectively, and the distance between ordinary cylindrical lens A and ordinary cylindrical lens B is 450-500mm.

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