Integrated transmission-reflection high-repetition-rate strong-field terahertz time-domain spectroscopy system

Abstract

This invention relates to the field of terahertz time-domain spectroscopy, specifically to an integrated high-repetition-rate, high-field terahertz time-domain spectroscopy system, comprising a femtosecond laser source, a laser beam expander module, a terahertz strong source generation module, a terahertz transmission-reflection focusing and collimation module, a terahertz detection module, and a mechanical delay module. The femtosecond laser beam is expanded by the laser beam expander module and then incident on the terahertz strong source generation module to generate a strong-field terahertz pulse. This strong-field terahertz pulse propagates to the terahertz transmission-reflection focusing and collimation module, is focused onto the sample, and interacts with the sample. The interaction with the sample includes transmission through the sample and reflection from the sample surface. The mechanical delay module delays the detection light. The terahertz detection module focuses and processes the detection light and the terahertz pulse after interaction with the sample to finally obtain the terahertz time-domain spectral waveform. This invention can improve the terahertz spectroscopy testing effect.

Description

Technical Field

[0001] This invention relates to the field of terahertz time-domain spectroscopy, specifically to an integrated transmission-reflection high repetition rate strong field terahertz time-domain spectroscopy system. Background Technology

[0002] Terahertz waves typically refer to electromagnetic waves with frequencies ranging from 0.1 to 10 THz. They lie between low-energy electronics and high-energy photonics, forming a unique electromagnetic radiation region. Terahertz waves possess many excellent properties, including strong penetrability of nonpolar materials and sensitivity to polar molecules (such as water molecules); their photon energy is low, preventing ionizing damage to biological tissues. These properties endow terahertz waves with significant application value in numerous fields such as materials science, biomedicine, food safety, non-destructive testing, and security inspection.

[0003] Terahertz time-domain spectroscopy systems utilize the time-domain signal resulting from the interaction of terahertz waves with matter. Through Fourier transform, frequency-domain spectral information is obtained to analyze the physical structure, chemical composition, and optical properties of matter, finding wide applications in component identification, electromagnetic parameter measurement, and imaging. A typical system consists of terahertz generation, detection, transmission, and data processing modules, with the terahertz source being the core component. Currently, mainstream terahertz pulse generation methods include optical guide antennas, femtosecond laser-pumped nonlinear crystals, and lithium niobate-based tilted wavefront pumping technology. The first two are the mainstream integrated solutions, but are limited by power and penetration; the latter can generate strong-field terahertz waves, improving system testing capabilities and signal-to-noise ratio. System structures are divided into transmission and reflection types; most systems require separate testing of these two types of signals, impacting testing efficiency.

[0004] To simplify the testing system, existing technologies offer the following solutions: Chinese utility model patent CN221326333U provides a transmission and reflection integrated testing scheme using a GaAs low-temperature optical guide antenna as a terahertz transmitting and receiving device. However, the power and single-pulse energy of the terahertz source used in this scheme are relatively low; and the use of a parallel beam to irradiate the sample under test results in a very low terahertz electric field intensity at the sample, leading to weak interaction between the terahertz wave and the sample under test, thus limiting the testing effect for some samples.

[0005] For most terahertz spectroscopy systems that currently use lithium niobate strong-field terahertz radiation sources, such as the lithium niobate terahertz radiation source used in Chinese invention patent application CN118464831A, a femtosecond laser with a repetition frequency of 1 kHz and a single pulse energy in the mJ range is typically used to pump and generate terahertz radiation with a repetition frequency of 1 kHz (consistent with the pump laser) and a single pulse energy in the μJ range. This method is limited by the damage threshold of lithium niobate crystals and cannot generate terahertz radiation with a repetition frequency higher than 1 kHz, resulting in low test speed and efficiency. Summary of the Invention

[0006] In view of the above problems, the present invention provides an integrated high repetition rate and strong field terahertz time-domain spectroscopy system that can solve the technical problems of the prior art, which cannot achieve simultaneous testing of terahertz time-domain signals transmitted and reflected at 45 degrees, resulting in low testing efficiency, low power and repetition frequency leading to low testing speed and efficiency, and low degree of freedom in power and parameter adjustment, thus failing to meet different testing requirements.

[0007] This invention provides an integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system, comprising: a femtosecond laser source, a laser beam expansion module, a terahertz strong source generation module, a terahertz transmission and reflection focusing and collimation module, a terahertz detection module, and a mechanical delay module;

[0008] The femtosecond laser source outputs femtosecond laser and probe light. The single pulse energy range of the femtosecond laser is 0-1mJ, and the repetition frequency range is 1Hz-50kHz.

[0009] The femtosecond laser, after being expanded by the laser beam expansion module, is injected into the terahertz strong source generation module to generate a strong field terahertz pulse. The strong field terahertz pulse propagates to the terahertz transmission and reflection focusing collimation module, is focused on the sample, and interacts with the sample. The interaction with the sample includes transmission through the sample and reflection by the sample surface.

[0010] The mechanical delay module delays the probe light, and the terahertz detection module focuses the probe light and the terahertz pulse after it interacts with the sample and processes them to finally obtain the terahertz time-domain spectral waveform.

[0011] Preferably, the femtosecond laser is a laser with a center wavelength of 1030nm and a pulse width of 570fs; the probe light is a laser with a wavelength of 800nm ​​and a pulse width of less than 70fs.

[0012] Preferably, the laser beam expander module includes:

[0013] A lens group consisting of a concave lens and a first convex lens, and a precision displacement stage. The concave lens and the first convex lens are mounted on the precision displacement stage. The size and direction of the laser spot emitted by the laser spot beam expanding module are finely adjusted by the precision displacement stage.

[0014] The laser beam expander module expands the input laser beam to 1.5 times its original size, forming an expanded laser beam.

[0015] Preferably, the terahertz strong source generation module includes:

[0016] The system comprises a transmission diffraction grating, a first gold mirror, a second gold mirror, a 4-f standard imaging module, a lithium niobate crystal, and a manually operated multi-dimensional displacement stage. The manually operated multi-dimensional displacement stage is used to adjust the relative positions of the lithium niobate crystal and the 4-f standard imaging module. The 4-f standard imaging module includes a second convex lens and a third convex lens. The front surfaces of the transmission diffraction grating and the lithium niobate crystal are located on the object plane and image plane of the 4-f standard imaging module, respectively. The lithium niobate crystal is a triangular prism cut with a base angle of 63 degrees to accommodate the tilted wavefront angle.

[0017] The expanded laser beam passes sequentially through a transmission diffraction grating, a first gold reflector, a second gold reflector, and a 4-f standard imaging module and is incident on a lithium niobate crystal to generate a strong field terahertz pulse.

[0018] The peak frequency of the strong-field terahertz pulse is 0.6 THz, and the spectral range is 0.1-2.5 THz.

[0019] Preferably, the terahertz transmission-reflection focusing collimation module includes:

[0020] The system comprises a first off-axis parabolic mirror, a first ITO flat glass, a second off-axis parabolic mirror, an electric rotary table, a third off-axis parabolic mirror, a fourth off-axis parabolic mirror, and a bundled silicon wafer.

[0021] The strong-field terahertz pulse passes sequentially through the first off-axis parabolic mirror, the first ITO flat glass, and the second off-axis parabolic mirror, and is finally focused onto the sample on the motorized rotating stage. After being transmitted through the sample and reflected at a 45-degree angle by the sample surface, the terahertz pulse is reflected by the third and fourth off-axis parabolic mirrors and then combined by the silicon wafer.

[0022] Preferably, a temporary metal plate can be installed in front of either the third off-axis parabolic reflector or the fourth off-axis parabolic reflector to block the transmitted or reflected terahertz pulses, respectively.

[0023] Preferably, the mechanical delay module includes a mechanical delay line, which comprises a sixth ultrafast mirror, a seventh ultrafast mirror, and a one-dimensional electric translation stage; the mechanical delay line is used to delay the probe light.

[0024] Preferably, the terahertz detection module includes a second ITO flat glass, a third off-axis parabolic mirror, and an electro-optic sampling module. The electro-optic sampling module includes a zinc telluride electro-optic crystal, a fourth convex lens, a quarter-wave plate, a fifth convex lens, a Wollaston prism, a first photodetector, and a second photodetector.

[0025] The probe light passes through the second ITO flat glass to the third off-axis parabolic mirror. The terahertz pulse after interacting with the sample is reflected by the second ITO flat glass to the third off-axis parabolic mirror. The probe light and the terahertz pulse are focused together onto the zinc telluride electro-optic crystal. After passing through the fourth convex lens, the quarter-wave plate, the fifth convex lens, and the Wollaston prism in sequence, the light intensity is detected by the first photodetector and the second photodetector.

[0026] Preferably, the light intensity detected by the first photoelectric probe and the second photoelectric probe is sent to a differential amplifier, and the resulting differential signal is sent to a lock-in amplifier for further amplification to obtain the terahertz electric field intensity.

[0027] By controlling the motor in the mechanical delay line through the host computer to drive the sixth and seventh ultrafast mirrors, the optical path difference between the probe light and the pump light is changed, the terahertz electric field intensity at each moment is detected, and the complete terahertz time-domain spectral waveform is plotted.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects:

[0029] (1) The present invention has good spectral testing performance. The designed strong-field terahertz radiation source has advantages such as high average output terahertz power, high repetition frequency, large single-pulse energy, high energy conversion efficiency, large radiation bandwidth, controllable polarization, and stability and reliability. Compared with traditional methods, it can greatly improve the signal-to-noise ratio, detection speed and efficiency of terahertz time-domain spectra. The present invention has high terahertz electromagnetic intensity. Compared with traditional weak-field terahertz time-domain spectroscopic systems, it can better excite and study the interaction between strong terahertz electric fields and matter, with more powerful testing functions and a wider range of testable samples. The present invention has high system integration. The system occupies a small area and can simultaneously obtain the time-domain spectral signals of transmission and 45-degree reflection of the sample under test, without the need to switch systems or conduct multiple tests, thus improving the testing efficiency of the sample.

[0030] (2) The terahertz radiation source technology used in this invention is mature. The growth, processing, and coating processes of the lithium niobate bulk crystals used are mature and low-cost, enabling mass production; the technical principle of the tilted wavefront method is clear. This invention has a low cost. The industrial-grade laser used as the pump source significantly reduces the procurement cost compared to traditional Ti:sapphire laser amplifiers; moreover, its performance is more stable, its environmental requirements are lower, and its maintenance costs are also significantly reduced.

[0031] (3) The principle of this invention is simple, and its reliability and stability are high. All optical components used are market standard products, and the back-end computer control and data processing programs are implemented based on mature programming languages ​​and are rigorously packaged. The system maintenance cost is low for both software and hardware. This invention can not only meet the relevant needs of scientific research, but also be used in non-destructive testing, industrial thickness measurement and other application scenarios. It is particularly suitable for the transmission and reflection time-domain spectroscopy testing of thicker samples, and its applicable application range is wide. Attached Figure Description

[0032] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] Figure 1 This is a schematic diagram of the integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system provided by the present invention.

[0034] Figure 2 This is a schematic diagram of the terahertz time-domain spectral signals of transmission, reflection, and transmissivity measured using a high-resistivity silicon wafer as a model sample, as provided by the present invention.

[0035] Reference numerals: 1-Concave lens, 2-First convex lens, 3-Chopper, 4-First ultrafast mirror, 5-Second ultrafast mirror, 6-Transmission diffraction grating, 7-First gold mirror, 8-Second gold mirror, 9-Second convex lens, 10-Third convex lens, 11-Lithium niobate crystal, 12-First off-axis parabolic mirror, 13-First ITO flat glass, 14-Second off-axis parabolic mirror, 15-Electric rotary stage, 16-Third off-axis parabolic mirror, 17-Fourth off-axis parabolic mirror, 18-Switch silicon wafer, 19-Second ITO flat glass, 20- 21-Third ultrafast mirror, 22-Fifth ultrafast mirror, 23-Mechanical delay line, 24-Sixth ultrafast mirror, 25-Seventh ultrafast mirror, 26-Eighth ultrafast mirror, 27-Ninth ultrafast mirror, 28-Fifth off-axis parabolic mirror, 29-Zinc telluride electro-optic crystal, 30-Fourth convex lens, 31-Tenth ultrafast mirror, 32-1 / 4 wave plate, 33-Eleventh ultrafast mirror, 34-Fifth convex lens, 35-Wollaston prism, 36-First photodetector, 37-Second photodetector, 40-Pump light, 50-Detector light. Detailed Implementation

[0036] 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.

[0037] This invention provides an integrated high-repetition-rate, high-field terahertz time-domain spectroscopy system that combines transmission and reflection. It can simultaneously test the transmission and 45-degree reflection of a sample in a strong-field terahertz time-domain spectral signal. It utilizes a high-power, high-repetition-rate lithium niobate strong-field terahertz radiation source, and features adjustable terahertz power, single-pulse energy, repetition frequency, and focusing electric field intensity for sample irradiation.

[0038] To illustrate the effectiveness of the method proposed in this invention, the following detailed description of the above technical solution is provided through a specific embodiment, such as... Figure 1 As shown, an integrated transmission-reflection high repetition rate strong field terahertz time-domain spectroscopic system is disclosed, including a femtosecond laser source, a laser spot beam expanding module, a terahertz strong source generation module, a terahertz transmission-reflection focusing and collimation module, a terahertz detection module, and a mechanical delay module.

[0039] The femtosecond laser output from the femtosecond laser source is expanded by the laser spot beam expanding module and then injected into the terahertz strong source generating module to generate strong field terahertz radiation. The strong field terahertz radiation propagates to the terahertz transmission and reflection focusing collimation module, is focused on the sample, and interacts with the sample.

[0040] The mechanical delay module guides the probe light and focuses the probe light and the terahertz radiation after interacting with the sample. The terahertz detection module processes the terahertz time-domain waveform.

[0041] The femtosecond laser source generates 1030nm pump light 40.

[0042] In some embodiments, the femtosecond laser source is a ytterbium-doped fiber-amplified femtosecond laser capable of outputting a single pulse with a maximum energy of 1 mJ, a repetition rate adjustable from 1 Hz to 50 kHz, a center wavelength of 1030 nm, and a pulse width of 570 fs. This type of industrial-grade laser has the advantages of low cost, high stability, and good environmental adaptability. A portion of the laser output is guided into a matching optical parametric amplifier to generate probe light 50. This probe light, with an output wavelength of 800 nm and a pulse width of less than 70 fs, is used for terahertz detection.

[0043] The laser beam expander module includes a lens group consisting of a concave lens 1 and a first convex lens 2, as well as a precision displacement stage (not shown in the figure).

[0044] In some embodiments, the lens group is a beam-expanding lens group, where concave lens 1 is a biconcave lens with a nominal focal length of -50mm, and first convex lens 2 is a biconvex lens with a nominal focal length of 75mm. The beam-expanding lens group is composed of a biconcave lens with a nominal focal length of -50mm and a biconvex lens with a nominal focal length of 75mm, installed sequentially and at specific intervals. This can expand the pump laser spot size to approximately 1.5 times its original size, reducing the power density of the pump laser below the damage threshold of the lithium niobate crystal without affecting the terahertz radiation generation efficiency. This allows the lithium niobate crystal to support femtosecond laser pumping with a maximum average power of 50W. This design avoids creating a focal point in the air, thus preventing air ionization. The lenses are coated with a 1030nm anti-reflection film, allowing for the generation of strong-field terahertz radiation with greater pump laser power. The two lenses are mounted on one-dimensional manual precision displacement stages in the X and Y directions of the horizontal plane, respectively, to finely adjust the size and directionality of the laser beam emitted from the laser beam expander module according to actual conditions.

[0045] The laser beam expanded by the laser beam expansion module passes sequentially through the chopper 3, the first ultrafast reflector 4, and the second ultrafast reflector 5 before entering the terahertz strong source generation module.

[0046] The terahertz strong source generation module includes a transmission diffraction grating 6, a first gold reflector 7, a second gold reflector 8, a 4-f standard imaging module, a lithium niobate crystal 11, and a manual multidimensional displacement stage (not shown in the figure). The manual multidimensional displacement stage is used to finely adjust the position and geometric relationship of the lithium niobate crystal 11 and the 4-f standard imaging module.

[0047] The expanded laser beam passes sequentially through the transmission diffraction grating 6, the first gold reflector 7, the second gold reflector 8, and the 4-f standard imaging module, and is incident on the lithium niobate crystal 11 to generate strong field terahertz radiation.

[0048] In some embodiments, the transmissive diffraction grating 6 can concentrate the incident pump laser power into -1st order diffraction light with a diffraction efficiency of over 90%, thereby improving the power utilization of the pump laser.

[0049] The 4-f standard imaging module includes a second convex lens 9 and a third convex lens 10. In some embodiments, the second convex lens 9 and the third convex lens 10 are two convex lenses with a specific focal length and coated with a 1030nm anti-reflection film, and the imaging ratio is matched with the scribe line number parameter of the transmission diffraction grating.

[0050] In some embodiments, the lithium niobate crystal 11 is a lithium niobate crystal cut into a triangular prism with a base angle of 63 degrees, used to adapt to the tilted wavefront angle and improve the terahertz generation efficiency; the lithium niobate crystal 11 is coated with a 1030nm antireflection film on the incident surface to improve the pump laser utilization and increase the terahertz output.

[0051] The terahertz strong source generation module can ultimately output a broadband terahertz pulse with a peak frequency of approximately 0.6THz and a spectrum covering 0.1-2.5THz.

[0052] The terahertz transmission-reflection focusing and collimation module includes: a first off-axis parabolic mirror 12, a first ITO flat glass plate 13, a second off-axis parabolic mirror 14, an electric rotary stage 15, a third off-axis parabolic mirror 16, a fourth off-axis parabolic mirror 17, and a silicon combining wafer 18. The terahertz pulse passes sequentially through the first off-axis parabolic mirror 12, the first ITO flat glass plate 13, and the second off-axis parabolic mirror 14, and is finally focused onto the sample on the electric rotary stage 15, interacting with the sample. The terahertz waves transmitted through the sample and reflected by the sample surface are reflected by the third off-axis parabolic mirror 16 and the fourth off-axis parabolic mirror 17, respectively, and then combined by the silicon combining wafer 18 into the final terahertz radiation after interaction with the sample.

[0053] In some embodiments, the first off-axis parabolic mirror 12, the second off-axis parabolic mirror 14, the third off-axis parabolic mirror 16, and the fourth off-axis parabolic mirror 17 of the present invention are used for collimation and focusing of terahertz radiation. Compared with the use of terahertz lenses, this design can minimize terahertz loss and dispersion. All off-axis parabolic mirrors used have identical parameters: an aperture diameter of 2 inches and a focal length of 4 inches. This allows for sufficient collection of terahertz radiation while reducing system complexity by using components with the same parameters.

[0054] In some embodiments, a temporary metal plate may be installed in front of either the third off-axis parabolic reflector 16 or the fourth off-axis parabolic reflector 17 to block the propagation of transmitted / reflected terahertz waves into the terahertz detection system.

[0055] In some embodiments, the electric rotary stage 15 is arranged at the terahertz focal point of the terahertz transmission and reflection focusing collimation module for placing the sample to be tested; the electric rotary stage is equipped with a zero-position switch, which can be used to precisely adjust the rotation angle of the sample.

[0056] In some embodiments, the combining silicon wafer 18 is a double-polished high-resistivity silicon wafer with an aperture diameter of 4 inches. It is capable of combining and propagating the transmitted and reflected terahertz signals of the sample to the terahertz detection module.

[0057] The mechanical delay module includes a mechanical delay line 23. The mechanical delay line 23 includes a sixth ultrafast mirror 24, a seventh ultrafast mirror 25, and a one-dimensional motorized translation stage, which moves the sixth ultrafast mirror 24 and the seventh ultrafast mirror 25 together. The 800nm ​​probe light 50 passes sequentially through the third ultrafast mirror 20, the fourth ultrafast mirror 21, and the fifth ultrafast mirror 22. Then, the mechanical delay line 23 delays the light, focuses the probe light and the terahertz radiation after interacting with the sample, and sends them to the terahertz detection module.

[0058] In some embodiments, the mechanical delay module consists of a one-dimensional motorized translation stage with a stroke of 200 mm and a pair of mirrors mounted thereon, enabling the detection of the complete time-domain waveform of a relatively thick sample. The host computer can execute a control program to control the mechanical delay line to achieve automated scanning without manual operation.

[0059] The terahertz detection module includes: a second ITO flat glass plate 19, a fifth off-axis parabolic reflector 28, and an electro-optic sampling module. The electro-optic sampling module includes: a zinc telluride electro-optic crystal 29, a fourth convex lens 30, a quarter-wave plate 32, a fifth convex lens 34, a Wollaston prism 35, a first photodetector 36, and a second photodetector 37.

[0060] The probe light and the terahertz radiation after interacting with the sample pass through the above equipment in sequence, and then through a differential amplifier and a lock-in amplifier to be output to the host computer. The host computer processes the signal to obtain the terahertz time-domain waveform.

[0061] In some embodiments, the first ITO flat glass 13 and the second ITO flat glass 19 have a resistance of 1-2Ω, an aperture size of 10*10cm, and a terahertz reflectivity of over 99%, which enables efficient reflection of terahertz waves while allowing the probe light to pass through completely.

[0062] The process of light propagation is described in detail below.

[0063] The pump light 40 is expanded to approximately 1.5 times its initial size by a laser beam expander module consisting of a concave lens 1 and a first convex lens 2. A chopper 3 is placed in the pump path, operating at a frequency of 500 Hz and phase-locked with the laser trigger signal to suppress noise and improve the signal-to-noise ratio of electro-optic sampling. Subsequently, the pump light is guided by a first ultrafast mirror 4 and a second ultrafast mirror 5 and incident at a specific angle on a transmission diffraction grating 6. Most of the pump laser power exits from the -1 order and is reflected by a first gold mirror 7 and a second gold mirror 8, passing through a 4-f standard imaging module consisting of a second convex lens 9 and a third convex lens 10, and then incident on a lithium niobate crystal 11. The front surfaces of the grating 6 and the lithium niobate crystal 11 are located on the object plane and image plane of the 4-f standard imaging module, respectively. The pump light interacts with the lithium niobate crystal 11, radiating terahertz waves through optical rectification. The radiated terahertz waves are collected and shaped into a parallel beam by a first off-axis parabolic mirror 12 before exiting. After being reflected by the first ITO flat glass plate 13, the terahertz wave is focused by the second off-axis parabolic mirror 14 onto the center of the electric rotary stage 15, i.e., the sample placement point. The single-pulse energy, repetition frequency, and average power of the generated terahertz radiation can be adjusted by adjusting parameters such as the single-pulse energy and repetition frequency of the femtosecond laser (pump source).

[0064] The terahertz waves transmitted through the sample and reflected from the sample surface are collected by the third off-axis parabolic mirror 16 and the fourth off-axis parabolic mirror 17, respectively, and reshaped into parallel beams. The two terahertz beams are combined at the combining silicon wafer 18 and propagate together. Subsequently, the terahertz waves are reflected by the second ITO flat glass 19 to the fifth off-axis parabolic mirror 28 and focused onto the zinc telluride electro-optic crystal 29.

[0065] The probe light 50 is reflected and guided by the third ultrafast mirror 20, the fourth ultrafast mirror 21, and the fifth ultrafast mirror 22 to the mechanical delay line 23, which consists of the sixth ultrafast mirror 24, the seventh ultrafast mirror 25, and a one-dimensional electric translation stage. The probe light continues to be reflected and guided by the eighth ultrafast mirror 26 and the ninth ultrafast mirror 27, passing through the second ITO flat glass 19 to the fifth off-axis parabolic mirror 28, where it is focused together with the terahertz wave onto the zinc telluride electro-optic crystal 29. The terahertz wave alters the birefringence properties within the zinc telluride electro-optic crystal 29, thereby changing the polarization state of the probe light. This invention utilizes this property to detect terahertz time-domain waveforms.

[0066] The probe light passing through the electro-optic crystal is shaped into a parallel beam by the fourth convex lens 30 (collimating lens), reflected by the tenth ultrafast mirror 31, passes through the quarter-wave plate 32, and is then reflected by the eleventh ultrafast mirror 33. It is then focused by the fifth convex lens (focusing lens with a longer focal length). The focused light is then split into two beams according to polarization state after passing through the Wollaston prism 35. The light intensities are detected by the first photodetector 36 and the second photodetector 37, respectively, both located at the focal point of the fifth convex lens 34. The light intensities obtained by the two probes are sent to a differential amplifier, and the resulting differential signal is further amplified by a lock-in amplifier (phase-locked with the laser trigger signal) to obtain the terahertz electric field intensity. The amplified signal is then output to the host computer.

[0067] The host computer controls the motor in the mechanical delay line 23 to drive the sixth ultrafast reflector 24 and the seventh ultrafast reflector 25 to move together, changing the optical path of the probe light, that is, changing the optical path difference between the probe path and the pump (terahertz) path, and detecting the terahertz electric field intensity at various moments, thereby plotting the complete terahertz time-domain spectral waveform. From the obtained time-domain spectral waveform, the main signal peaks of the transmitted and reflected signals can be easily distinguished. If it is not desired to obtain both transmitted and reflected signals simultaneously, temporary metal plates can be installed in front of the third off-axis parabolic reflector 16 and the fourth off-axis parabolic reflector 17 to block the propagation of transmitted / reflected terahertz waves into the terahertz detection system.

[0068] Figure 2 In the diagram, A represents the transmission and reflection terahertz time-domain spectral waveforms of the silicon wafer. B and C represent the simple transmission and reflection terahertz time-domain spectral waveforms obtained when a metal plate is placed between the third off-axis parabolic mirror 16, the fourth off-axis parabolic mirror 17, and the electric rotary table 15, respectively. These waveforms are used to verify and characterize the transmission and reflection spectral testing capabilities of the system.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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. An integrated high-repetition-rate, high-field terahertz time-domain spectroscopic system with transmission and reflection capabilities, characterized in that: include: Femtosecond laser source, laser beam expansion module, terahertz intensity source generation module, terahertz transmission and reflection focusing and collimation module, terahertz detection module and mechanical delay module; The femtosecond laser source outputs femtosecond laser and probe light. The single pulse energy range of the femtosecond laser is 0-1mJ, and the repetition frequency range is 1Hz-50kHz. The femtosecond laser, after being expanded by the laser beam expansion module, is injected into the terahertz strong source generation module to generate a strong field terahertz pulse. The strong field terahertz pulse propagates to the terahertz transmission and reflection focusing collimation module, is focused on the sample, and interacts with the sample. The interaction with the sample includes transmission through the sample and reflection by the sample surface. The mechanical delay module delays the probe light, and the terahertz detection module focuses the probe light and the terahertz pulse after interacting with the sample and processes them to finally obtain the terahertz time-domain spectral waveform. The femtosecond laser is a laser with a center wavelength of 1030 nm and a pulse width of 570 fs; the probe light is a laser with a wavelength of 800 nm and a pulse width of less than 70 fs. The laser beam expander module includes: A lens group consisting of a concave lens (1) and a first convex lens (2) and a precision displacement stage. The concave lens (1) and the first convex lens (2) are mounted on the precision displacement stage. The size and direction of the laser spot emitted by the laser spot beam expanding module are finely adjusted by the precision displacement stage. The laser beam expander module expands the input laser beam to 1.5 times its original size, forming an expanded laser beam. The terahertz strong source generation module includes: The system comprises a transmission diffraction grating (6), a first gold reflector (7), a second gold reflector (8), a 4-f standard imaging module, a lithium niobate crystal (11), and a manual multi-dimensional displacement stage. The manual multi-dimensional displacement stage is used to adjust the relative positions of the lithium niobate crystal (11) and the 4-f standard imaging module. The 4-f standard imaging module includes a second convex lens (9) and a third convex lens (10). The front surfaces of the transmission diffraction grating (6) and the lithium niobate crystal (11) are located on the object plane and image plane of the 4-f standard imaging module, respectively. The lithium niobate crystal (11) is a lithium niobate crystal cut into a triangular prism with a base angle of 63 degrees to accommodate the tilted wavefront angle. The expanded laser beam passes sequentially through a transmission diffraction grating (6), a first gold reflector (7), a second gold reflector (8), and a 4-f standard imaging module, and is incident on a lithium niobate crystal (11) to generate a strong field terahertz pulse. The peak frequency of the strong-field terahertz pulse is 0.6 THz, and the spectral range is 0.1-2.5 THz. The terahertz transmission-reflection focusing collimation module includes: The system comprises a first off-axis parabolic mirror (12), a first ITO flat glass (13), a second off-axis parabolic mirror (14), an electric rotary table (15), a third off-axis parabolic mirror (16), a fourth off-axis parabolic mirror (17), and a bundled silicon wafer (18). The strong-field terahertz pulse passes sequentially through the first off-axis parabolic mirror (12), the first ITO flat glass (13), and the second off-axis parabolic mirror (14), and is finally focused onto the sample on the electric rotary stage (15). After the terahertz pulse is transmitted through the sample and reflected by the sample surface at 45 degrees, it is reflected by the third off-axis parabolic mirror (16) and the fourth off-axis parabolic mirror (17) and then combined by the combined silicon wafer (18).

2. The integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system according to claim 1, characterized in that, A temporary metal plate can be installed before either the third off-axis parabolic reflector (16) or the fourth off-axis parabolic reflector (17) to block the transmitted or reflected terahertz pulses, respectively.

3. The integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system according to claim 2, characterized in that, The mechanical delay module includes a mechanical delay line (23), which includes a sixth ultrafast mirror (24), a seventh ultrafast mirror (25), and a one-dimensional electric translation stage; the mechanical delay line (23) is used to delay the probe light (50).

4. The integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system according to claim 3, characterized in that, The terahertz detection module includes a second ITO flat glass (19), a fifth off-axis parabolic reflector (28), and an electro-optic sampling module. The electro-optic sampling module includes a zinc telluride electro-optic crystal (29), a fourth convex lens (30), a quarter wave plate (32), a fifth convex lens (34), a Wollaston prism (35), a first photodetector (36), and a second photodetector (37). The probe light (50) passes through the second ITO flat glass (19) to the fifth off-axis parabolic mirror (28). The terahertz pulse after interacting with the sample is reflected by the second ITO flat glass (19) to the fifth off-axis parabolic mirror (28). The probe light (50) and the terahertz pulse are focused together onto the zinc telluride electro-optic crystal (29). After passing through the fourth convex lens (30), the quarter wave plate (32), the fifth convex lens (34), and the Wollaston prism (35) in sequence, the light intensity is detected by the first photodetector (36) and the second photodetector (37).

5. The integrated transmission and reflection high repetition rate strong field terahertz time-domain spectroscopic system according to claim 4, characterized in that, The light intensity detected by the first photoelectric probe (36) and the second photoelectric probe (37) is sent to the differential amplifier, and the resulting differential signal is sent to the lock-in amplifier for further amplification to obtain the terahertz electric field intensity. The motor in the upper computer programmable mechanical delay line (23) drives the sixth ultrafast mirror (24) and the seventh ultrafast mirror (25) to move, changing the optical path difference between the probe light (50) and the pump light (40), detecting the terahertz electric field intensity at each moment, and drawing a complete terahertz time-domain spectral waveform.

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