Strong-field terahertz generation device

Abstract

The application relates to the technical field of terahertz generation, in particular to a strong-field terahertz generation device, which comprises a square vacuum cavity, a cylindrical vacuum cavity and a cooling mirror frame provided with a lithium niobate crystal; the square vacuum cavity is internally provided with a first reflector and a second reflector; the cylindrical vacuum cavity is in communication with the square vacuum cavity and is internally provided with a third reflector, a fourth reflector, a grating, a first cylindrical lens, a half-wave plate, a second cylindrical lens and the cooling mirror frame provided with the lithium niobate crystal; incident laser enters the square vacuum cavity, passes through the first reflector and the second reflector to enter the cylindrical vacuum cavity, and sequentially passes through the third reflector, the fourth reflector, the grating, the first cylindrical lens, the half-wave plate and the second cylindrical lens to irradiate the lithium niobate crystal to generate a terahertz signal; the strong-field terahertz generation device can generate a terahertz pulse with high efficiency, high beam quality and high stability.

Description

Technical Field

[0001] This invention relates to the field of terahertz generation technology, and more specifically to a strong-field terahertz generation device. Background Technology

[0002] The terahertz (THz) electromagnetic band lies between microwaves and infrared, covering a frequency range of 0.1-10 THz. It possesses excellent penetration and a wide bandwidth, making it valuable for applications in material identification, non-destructive testing, and security imaging. In some forward-looking applications in physics, high-efficiency, high-beam-quality, and high-stability THz sources are crucial. However, extreme-field terahertz sources with these key characteristics are currently lacking. Over the past few decades, numerous femtosecond laser-driven strong terahertz sources have been established and studied, greatly stimulating the development of extreme terahertz science and applications.

[0003] The main technological approaches to generating THz radiation include electrical and optical methods. Optical THz sources are crucial for material characterization and device measurement, and have gradually moved from laboratory research to industrial applications. Optical THz radiation generation can be further divided into pulsed waves and continuous waves. The former is primarily based on femtosecond laser pumping, while the latter is achieved through semiconductor laser beat frequency generation. THz pulses generated by methods such as femtosecond laser pulse excitation of nonlinear crystals, photoconductive antennas, and plasma exhibit ultrafast time resolution and ultrawide spectral distribution, and have been widely applied in THz time-domain spectroscopy for characterizing and measuring the dielectric response of materials, as well as in sensing and imaging. Continuous THz waves generated by semiconductor DFB lasers acting on photoconductive antennas through beat frequency technology have also been widely used in gas sensing and THz communication. However, the low radiation efficiency and low energy of these THz sources directly limit cutting-edge scientific and applied research in areas such as the nonlinear effects of THz-matter interaction, novel quantum state manipulation, electron acceleration, and biomedical applications. Because of its large nonlinear coefficient, mature manufacturing process, and high damage threshold, lithium niobate crystal has become increasingly recognized as a promising candidate for generating high-energy, high-field THz radiation using femtosecond lasers. The development of tilted wavefront technology solved the phase mismatch problem, enabling the rapid development of lithium niobate THz sources. Through years of continuous exploration, significant breakthroughs have been achieved in generating terahertz pulses using lithium niobate tilted wavefront technology, increasing terahertz energy from tens of microjoules to hundreds of microjoules. These important advancements have given people hope for generating millijoule-level high-energy, high-field THz radiation.

[0004] However, limited by parameters such as energy, spectrum, and pulse width of most desktop lasers, the generation efficiency and terahertz pulse energy of common lithium niobate strong-field terahertz sources are not high. Due to the limited laser damage threshold of lithium niobate, the energy flux density incident on the lithium niobate crystal is typically restricted. Furthermore, due to the absorption of terahertz waves by lithium niobate itself, the generation efficiency of lithium niobate terahertz sources is difficult to improve. With the rapid development of ultrafast laser technology, terahertz-level and even paver-level high-power laser systems with wavelengths of 800 nm have been established. These large-scale laser facilities are expected to provide powerful pump intensities to generate strong-field terahertz radiation. However, large-scale and expensive high-energy laser systems are not primarily designed for terahertz generation experiments. Under such experimental conditions, the terahertz radiation energy and optical-to-terahertz conversion efficiency may decrease. All of these factors limit the generation of terahertz waves by lithium niobate crystals and the development of further extreme terahertz applications. To overcome this predicament, it is necessary to make lithium niobate-based terahertz sources compatible with giant pump laser systems, achieve efficient terahertz output, and further explore the limiting terahertz radiation energy. Summary of the Invention

[0005] In view of the above problems, the present invention provides a strong field terahertz generating device, which solves the problems of low radiation efficiency and low energy of traditional THz light sources in the prior art.

[0006] This invention provides a strong-field terahertz generating device, characterized in that the device comprises: a square vacuum cavity, wherein a first reflecting mirror and a second reflecting mirror are disposed within the square vacuum cavity; a cylindrical vacuum cavity, wherein the cylindrical vacuum cavity is connected to the square vacuum cavity, and wherein a third reflecting mirror, a fourth reflecting mirror, a grating, a first cylindrical lens, a half-wave plate, a second cylindrical lens, and a cooling frame on which a lithium niobate crystal is mounted are disposed within the cylindrical vacuum cavity; an incident laser enters the square vacuum cavity, passes through the first reflecting mirror and the second reflecting mirror, enters the cylindrical vacuum cavity, and sequentially passes through the third reflecting mirror, the fourth reflecting mirror, the grating, the first cylindrical lens, the half-wave plate, and the second cylindrical lens, irradiating the lithium niobate crystal to generate a terahertz signal.

[0007] Preferably, the cooling frame is an electrically adjustable frame with a cooling function, which cools the lithium niobate crystal by externally introduced liquid nitrogen.

[0008] Preferably, the cylindrical vacuum cavity is further provided with an off-axis parabolic mirror and a detection module. The terahertz signal is focused by the off-axis parabolic mirror and the terahertz waveform is detected by the detection module.

[0009] Preferably, the incident laser is generated by a femtosecond laser with a center frequency of 800nm. The laser source used is a 1PW femtosecond laser source with a single pulse energy of 2-15J, a repetition frequency of 0.1-1Hz, and a spot diameter >200mm.

[0010] Preferably, the lithium niobate crystal is composed of multiple lithium niobate crystal sub-blocks stacked together. The height of the lithium niobate crystal sub-blocks is 30-70mm. Six to seven lithium niobate crystal sub-blocks are stacked together to obtain the complete lithium niobate crystal.

[0011] Preferably, the first and second reflectors are electrically controlled reflectors, which adjust the position and direction of the pump light entering the cylindrical vacuum cavity by electrically controlling the pitch and left and right rotation directions; the third and fourth reflectors are electrically controlled reflectors, which control the laser incident angle by adjusting the direction and position, so that the laser is incident on the grating at the designed incident angle.

[0012] Preferably, the parameters of the grating tilted wavefront system are calculated to determine the parameters used in the equipment. The calculation method is as follows:

[0013]

[0014] Where γ is the tilted wavefront angle, m is the diffraction order (usually -1st order), λ0 is the center wavelength of the pump laser, p is the grating line density (number of lines per unit length), and β is the scaling ratio of the two-lens imaging system in the horizontal direction. θ is the pump laser group velocity refractive index, and θ is the diffraction angle.

[0015] Preferably, the grating has a grating density of 1480 lines / mm, the focal lengths of the first cylindrical lens and the second cylindrical lens are F1 = 380.0 mm and F2 = 190.0 mm, respectively, the incident angle of the pump light on the grating surface is 19.8°, and the -1st order diffraction angle is 57.8°.

[0016] Preferably, the half-wave plate is used to convert the horizontally polarized light generated by the laser into vertically polarized light, so that the polarization direction matches the direction of maximum crystal efficiency, and the optical axis of the half-wave plate is 45°.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects:

[0018] (1) The present invention uses a 1PW femtosecond laser with a super intense pulse (with a large spot) and tilted wavefront technology to excite a lithium niobate crystal to generate a high-energy terahertz pulse, which can realize a terahertz source with a center frequency of ~0.2THz, a single pulse energy of >15mJ, and a repetition frequency of 0.1Hz.

[0019] (2) The present invention can adjust parameters such as laser spectrum, chirp and pulse width to obtain higher terahertz generation efficiency; it expands the laser receiving area of ​​lithium niobate crystal by stacking multiple smaller lithium niobate crystals, and at the same time, it cools down the lithium niobate crystal to reduce the absorption of terahertz by the lithium niobate crystal.

[0020] (3) The optical path adjustment of the present invention adopts an all-electric design, and all optical devices can be remotely controlled via USB cable and network cable. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0022] Figure 1 A schematic diagram of a tilted wavefront generation method based on gratings and tiered mirrors provided in an embodiment of the present invention;

[0023] Figure 2 A schematic diagram of a square cavity structure provided in an embodiment of the present invention;

[0024] Figure 3 A schematic diagram of the cylindrical cavity optical path containing a lithium niobate tilted wavefront generation platform provided in an embodiment of the present invention;

[0025] Figure 4 A schematic diagram of a typical terahertz pulse signal restored according to an embodiment of the present invention;

[0026] Figure 5 This is a schematic diagram of the terahertz light spot obtained by measurement according to an embodiment of the present invention.

[0027] Reference numerals: 101-grating; 102-diagram line of laser light incident on the grating; 103-first lens of the grating imaging system; 104-second lens of the grating imaging system; 105-lithium niobate crystal; 106-diagram of the generated terahertz; 107-image of the grating in the crystal; 108-step mirror; 109-image of the step mirror in the crystal; 201-first reflecting mirror; 202-second reflecting mirror; 203-square vacuum cavity; 301-third reflecting mirror; 302-fourth reflecting mirror; 303-cylindrical cavity; 304-grating; 305-first cylindrical lens; 306-half-wave plate; 307-second cylindrical lens; 308-detection module; 309-off-axis parabolic mirror; 310-lithium niobate crystal. Detailed Implementation

[0028] To better understand the above-described objectives, features, and advantages of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other. Furthermore, the present invention can be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0029] This invention employs a 1PW femtosecond laser, using a 10J-level laser with an 800nm ​​center wavelength to excite a lithium niobate crystal to generate terahertz waves. The terahertz generation efficiency and pulse energy are improved by adjusting parameters such as the laser spectrum, chirp, and pulse width. To limit the laser energy flux density, for high-energy lasers, the incident light spot on the lithium niobate crystal is also relatively large.

[0030] To address the requirements of ultra-intense pulses (>10J), ultra-large spot size (approximately 200mm in diameter) and usage of 1PW femtosecond lasers, this invention places the entire system in a vacuum cavity and uses electric motors to control the adjustment of each optical component.

[0031] Furthermore, due to the processing limitations of lithium niobate crystals, a single lithium niobate crystal cannot meet the approximately 200mm spot diameter of a 1PW laser. Considering the characteristics of tilted wavefront technology, the incident light spot on the crystal is elliptical. This invention employs a stacking of multiple smaller lithium niobate crystals to expand the laser receiving area of ​​the lithium niobate crystal. Given that the laser spot height is approximately 200mm and the lithium niobate crystal size is between 30mm and 70mm, 6 to 7 stacks of lithium niobate crystals are required to receive all the light spots.

[0032] Simultaneously, the lithium niobate crystal is cooled to reduce its absorption of terahertz waves. This invention employs liquid nitrogen to cool the lithium niobate crystal, thereby reducing its absorption of terahertz waves. To cool the lithium niobate crystal, a crystal mounting bracket with cooling capabilities needs to be designed, the system needs to be evacuated, and liquid nitrogen needs to be introduced from the outside for cooling.

[0033] To illustrate the effectiveness of the method proposed in this invention, the following detailed description of the above technical solution of this invention is provided through a specific embodiment.

[0034] This invention employs a femtosecond laser-pumped nonlinear crystal to generate terahertz pulse radiation using an optical rectification method. The selected nonlinear crystal is a lithium niobate crystal, and a tilted wavefront technique is used to overcome the mismatch between terahertz and laser wave velocity, thereby improving terahertz radiation efficiency. The tilted wavefront angle is calculated using the following formula, and this angle also determines the size of the edge angles of the lithium niobate crystal.

[0035]

[0036] Where γ is the tilt wavefront angle. n is the group velocity refractive index of lithium niobate crystal in the laser band. THz It is the refractive index of lithium niobate crystal in the terahertz band.

[0037] Commonly used methods for generating tilted wavefronts are mainly based on two types of optical devices: gratings (usually reflective gratings) and escalator mirrors. For example... Figure 1As shown in the figure above, it is a schematic diagram of generating terahertz pulses based on a tilted wavefront using a grating. Figure 1 The image below is a schematic diagram of terahertz pulses generated by the tilted wavefront of a step mirror.

[0038] The grating uses a short-pulse scattering imaging system to generate a tilted wavefront, similar to projecting an image (tilted wavefront) of the grating surface into a crystal using an imaging system. The wavefront tilt angle calculated using the aforementioned formula can be used to determine various parameters of the grating tilted wavefront system. The specific parameters are calculated as follows:

[0039]

[0040] Where γ is the tilted wavefront angle, m is the diffraction order (usually -1st order), λ0 is the center wavelength of the pump laser, p is the grating line density (number of lines per unit length), and β is the scaling ratio of the two-lens imaging system in the horizontal direction. θ is the pump laser group velocity refractive index, and θ is the diffraction angle.

[0041] A staircase mirror (usually a reflective staircase mirror) is used to generate a series of small pulses. Each pulse has a different time delay and is imaged onto a lithium niobate crystal. Each pulse, acting as a near-line source, can generate Cherenkov sheet-like terahertz radiation. The spacing of the imaging steps is very small compared to the terahertz wavelength, therefore the discrete tilted pulse leading edges are equivalently continuous over the terahertz bandwidth. The system design parameters of the staircase mirror can be calculated using the following formula:

[0042]

[0043] Where γ is the tilt wavefront angle. Here, M is the group velocity of the laser in lithium niobate, H is the scaling factor, H is the step height of the step mirror, and W is the step width. The size of the step mirror is mainly determined by the laser spot size.

[0044] like Figure 2 , Figure 3 As shown, the present invention provides a strong-field terahertz generator, the device comprising:

[0045] A front chamber and a square vacuum chamber are connected to the front chamber. A first reflector and a second reflector are provided inside the square vacuum chamber.

[0046] A cylindrical vacuum cavity is connected to a square vacuum cavity. The cylindrical vacuum cavity is equipped with a third reflecting mirror, a fourth reflecting mirror, a grating, a first cylindrical lens, a half-wave plate, a second cylindrical lens, and a cooling frame on which a lithium niobate crystal is mounted.

[0047] The incident laser enters the square vacuum cavity from the pre-cavity chamber, passes through the first and second reflecting mirrors and enters the cylindrical vacuum cavity, and then passes through the third and fourth reflecting mirrors, the grating, the first cylindrical lens, the half-wave plate, and the second cylindrical lens in sequence, irradiating the lithium niobate crystal to generate a terahertz signal.

[0048] In some embodiments, the incident laser is generated by a femtosecond laser with a center frequency of 800 nm. The laser source used is a 1 PW femtosecond laser, a high-power laser device with a maximum single pulse energy of 15 J (the energy can be adjusted during the experiment) and a repetition frequency of 0.1 or 1 Hz (the repetition frequency can be increased to 1 Hz when the energy is reduced during the experiment), and its spot diameter can reach >200 mm.

[0049] In some embodiments, the first and second reflectors are electrically controlled reflectors, which can adjust the position and direction of the pump light entering the cylindrical cavity by electrically controlling the pitch and left and right rotation directions.

[0050] The third and fourth reflectors are electrically controlled reflectors, which can adjust their direction and position to control the laser incident angle, so that the laser is incident on the grating at the designed incident angle.

[0051] In some embodiments, for large-sized light spots, the present invention uses a grating as the main component of the tilted wavefront.

[0052] In some embodiments, the present invention uses a stepped mirror as the main component of the tilted wavefront.

[0053] In some embodiments, device parameters are obtained by calculating various parameters of the determined grating tilted wavefront system. The grating has a line density of 1480 lines / mm, the focal lengths of the first and second cylindrical lenses are F1 = 380.0 mm and F2 = 190.0 mm, respectively, the incident angle of the pump light on the grating surface is approximately 19.8°, and the -1st order diffraction angle is 57.8°.

[0054] In some embodiments, a half-wave plate is used to convert horizontally polarized light generated by a laser into vertically polarized light, so that the polarization direction matches the direction of maximum crystal efficiency, and its optical axis is 45°.

[0055] In some embodiments, the cooling frame is an electrically adjustable frame with a cooling function, which cools the lithium niobate crystal by externally introduced liquid nitrogen.

[0056] In some embodiments, the size of the lithium niobate crystal sub-blocks is between 30 mm and 70 mm, and 6 to 7 lithium niobate crystal sub-blocks are stacked together to obtain the complete lithium niobate crystal.

[0057] In some embodiments, the cylindrical cavity is further provided with an off-axis parabolic mirror and a detection module. The terahertz signal is focused by the off-axis parabolic mirror and detected by the detection module. Terahertz energy probes or electro-optic sampling technology can be used to detect the terahertz energy and electrical signal waveform, respectively.

[0058] The specific optical path of the strong-field terahertz generation device provided by this invention is as follows: A laser beam first enters a first square vacuum cavity through a pre-cavity chamber. The incident laser beam is guided by two mirrors within the square cavity and then enters the cylindrical cavity target chamber containing the lithium niobate tilted wavefront experimental platform. Two electrically controlled mirrors within the square cavity can be electrically controlled to adjust the position and direction of the pump light entering the cylindrical cavity via pitch and rotation. After entering the cylindrical cavity, the pump light passes through two more mirrors, and after its direction and position are adjusted, it is incident on the grating. The diffracted light is collected and focused onto the lithium niobate crystal by an imaging system composed of two cylindrical lenses; in other words, the image of the laser-irradiated grating is projected onto the lithium niobate crystal by the dual-lens imaging system. The lithium niobate crystal is mounted on an electrically controlled, cooled frame. The generated terahertz signal is designed to be collected and focused onto an energy probe using a large-aperture off-axis parabolic mirror to detect terahertz energy. Alternatively, an energy probe can be used to scan the terahertz emission surface of the crystal, and the terahertz energy emitted by the crystal can be calculated by accumulating the signals. To obtain the pulse waveform, the terahertz pulse needs to be focused onto the electro-optic crystal, and the terahertz waveform needs to be detected using electro-optic sampling technology. The measured terahertz pulse signal and terahertz spot are shown below. Figure 4 , Figure 5 As shown.

[0059] To address the issues of low radiation efficiency and low energy of traditional THz light sources, and considering the limitations and requirements of lasers with ultra-high energy and ultra-large spot sizes on system environment, system design, and the design and fabrication of system components, this invention solves the following technical challenges: using a 1PW femtosecond laser to generate high-efficiency and high-pulse-energy terahertz pulses; stacking crystals to expand the incident surface; and cooling the crystals to further improve the generation efficiency of terahertz pulses, thereby generating high-efficiency, high-beam-quality, and high-stability terahertz pulses.

[0060] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0061] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0062] In this invention, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. The term "multiple" refers to two or more unless otherwise expressly defined.

[0063] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A strong-field terahertz generator, characterized in that, The device includes: A square vacuum cavity, wherein a first reflector and a second reflector are provided inside the square vacuum cavity; A cylindrical vacuum cavity is connected to a square vacuum cavity. The cylindrical vacuum cavity is equipped with a third reflecting mirror, a fourth reflecting mirror, a grating, a first cylindrical lens, a half-wave plate, a second cylindrical lens, and a cooling frame on which a lithium niobate crystal is mounted. The first and second reflectors are electrically controlled reflectors, which adjust the position and direction of the pump light entering the cylindrical vacuum cavity by electrically controlling the pitch and left and right rotation directions; The third and fourth reflectors are electrically controlled reflectors. By adjusting their direction and position, the laser incident angle is controlled so that the laser is incident on the grating at the designed incident angle. The cooling frame is an electrically adjustable frame with a cooling function, which cools the lithium niobate crystal by introducing liquid nitrogen from the outside. All optical components can be remotely controlled via USB and Ethernet cables; The incident laser enters the square vacuum cavity, passes through the first and second reflecting mirrors and enters the cylindrical vacuum cavity, and then passes through the third and fourth reflecting mirrors, the grating, the first cylindrical lens, the half-wave plate, and the second cylindrical lens in sequence, irradiating the lithium niobate crystal to generate a terahertz signal. The incident laser is generated by a femtosecond laser with a center frequency of 800nm. The laser source used is a 1PW femtosecond laser source with a single pulse energy of 2~15J, a repetition frequency of 0.1~1Hz, and a spot diameter >200mm. The lithium niobate crystal is composed of multiple lithium niobate crystal sub-blocks stacked together. The height of each lithium niobate crystal sub-block is 30~70mm. Six to seven lithium niobate crystal sub-blocks are stacked together to obtain the complete lithium niobate crystal. The parameters of the grating tilted wavefront system are calculated to determine the parameters to be used in the equipment. The calculation method is as follows: in It is the tilted wavefront angle. For diffraction orders, The center wavelength of the pump laser, The grating line density, This represents the scaling ratio of the dual-lens imaging system in the horizontal direction. The refractive index of the pump laser group velocity, It is the diffraction angle; The grating has a line density of 1480 lines / mm, and the focal lengths of the first cylindrical lens and the second cylindrical lens are respectively... =380.0mm and =190.0mm, the incident angle of the pump light on the grating surface is 19.8°, and the -1st order diffraction angle is 57.8°.

2. The strong-field terahertz generating device according to claim 1, characterized in that, The cylindrical vacuum cavity is also equipped with an off-axis parabolic mirror and a detection module. The terahertz signal is focused by the off-axis parabolic mirror and the terahertz waveform is detected by the detection module.

3. The strong-field terahertz generating device according to claim 2, characterized in that: A half-wave plate is used to convert horizontally polarized light generated by a laser into vertically polarized light, so that the polarization direction matches the direction of maximum crystal efficiency. The optical axis of the half-wave plate is 45°.

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