Multifunctional terahertz pump-terahertz probe time-domain spectroscopy system and switching method

Abstract

The present application belongs to the technical field of spectrum detector, and particularly relates to a multifunctional terahertz pump-terahertz probe time-domain spectrum system and a switching method. The technical problem to be solved is that independent light paths for terahertz pump and optical pump result in complex system. The system can be switched to the following three working modes: terahertz pump-terahertz probe mode: strong-field pump terahertz laser pulses and weak-field probe terahertz laser pulses are guided to a sample to be measured, and the weak-field probe terahertz laser pulses are measured; optical pump-terahertz probe mode: pump near-infrared laser pulses and strong-field probe terahertz laser pulses are guided to the sample to be measured, and the strong-field probe terahertz laser pulses are measured; terahertz time-domain spectrum measurement mode: including strong-field terahertz laser pulse time-domain spectrum mode and weak-field terahertz laser pulse time-domain spectrum mode. One system of the present application can be switched to multiple different working modes, and the spectrum system has simple structure.

Description

Technical Field

[0001] This invention belongs to the field of spectroscopic detector technology, specifically relating to a multifunctional terahertz pump-terahertz detection time-domain spectroscopic system and switching method. Background Technology

[0002] 1. Terahertz time-domain spectroscopy (THz-TDS): Terahertz time-domain spectroscopy is a coherent detection technique based on the pump-probe principle, using femtosecond lasers. It measures the wavelengths generated by ultrashort laser pulses in the terahertz frequency range (typically 0.1–10 THz, corresponding to wavenumbers of 3–300 cm⁻¹). -1 30μm wavelength -3 The complete waveform of the electric field intensity of the electromagnetic radiation pulse (i.e., terahertz pulse) in the time domain as a function of time is obtained to acquire the physical and chemical information of the substance under test in the terahertz frequency band.

[0003] 2. Tilted-pulse-front (TPF) technique: The best current method for generating strong-field terahertz pulses involves using a lithium niobate crystal with significant second-order nonlinear effects to convert near-infrared (800nm ​​wavelength) laser pulses into terahertz pulses via optical rectification. Because the propagation speeds of electromagnetic waves in the terahertz and near-infrared bands differ greatly within the lithium niobate crystal, phase matching must be achieved by tilting the pulse wavefront to maximize terahertz output power.

[0004] 3. Terahertz detection technology based on optical guide antennas: Terahertz detection optical guide antennas (PCAs) typically use low-temperature grown gallium arsenide (LT-GaAs) as the semiconductor substrate. A dipole or bow-tie shaped antenna structure is fabricated on the LT-GaAs surface, with a micrometer-level gap between the two electrodes, called the optical guide gap. When detecting terahertz waves, a gated femtosecond laser is usually used to illuminate the optical guide gap from one end, generating charge carriers within the gap. When a terahertz pulse is incident on the optical guide gap at the other end of the PCA, its instantaneous electric field... The photocurrent acts on the charge carriers generated by the femtosecond laser within the optical guide gap. Driven by the terahertz electric field, these charge carriers accelerate, forming a transient photocurrent. The current intensity is proportional to both the gating pulse intensity and the terahertz pulse intensity; therefore, the transient electric field of the terahertz pulse can be deduced from the readout photocurrent value. .

[0005] 4. Terahertz pump-terahertz detection spectroscopy system: Pump-probe is a highly efficient tool for studying ultrafast physical and chemical processes within materials. Because terahertz photons have low energy (1 THz ≈ 4.1 meV), they do not induce interband transitions in electrons, but can be directly coupled to low-energy collective excitations, quasi-particle dynamics, and lattice degrees of freedom. Therefore, terahertz pump-terahertz probe can be used to study ultrafast nonequilibrium physical processes in cutting-edge materials science, such as strong-field-induced nonequilibrium superconducting states in high-temperature superconductors, terahertz-field-induced Floquet topological phase transitions in topological insulators, and ultrafast responses to magnetoelectric coupling in ferroelectrics and multiferroic materials.

[0006] This document, published in China (CN206038529U), discloses a terahertz pump-terahertz detection time-domain spectroscopy system. It includes a femtosecond laser source, a first beam splitter, and a second beam splitter. The femtosecond laser source generates a femtosecond laser beam, which is then split into a first beam, a second beam, and a third beam. These beams are used for generating the pump terahertz field, generating the detection terahertz field, and driving the electro-optic sampling measurement of the terahertz time-domain pulse, respectively. The femtosecond laser beam transmitted through the first and second beam splitters serves as the first beam, the femtosecond laser beam reflected by the second beam splitter serves as the second beam, and the femtosecond laser beam reflected by the first beam splitter serves as the third beam. The detection terahertz field with arbitrary polarization can be obtained through simple adjustments.

[0007] This document, published in China (CN105841816A), discloses a terahertz time-domain spectroscopy system. In this system, a femtosecond laser emitted by a femtosecond laser is collimated by a first aperture, and then split into pump and probe beams by a beam splitter. The pump beam generates a terahertz pulse via a first optical path assembly; the probe beam generates a linearly polarized probe beam with the same optical path as the pump beam via the same first optical path assembly. The linearly polarized probe beam and the terahertz pulse are then combined by a beam combiner to obtain a beam to be detected carrying terahertz pulse information. Simultaneously, two electro-optic crystals of equal thickness are used in the detection device. Adjusting the angle between the crystal axes of the two electro-optic crystals provides phase compensation for the phase delay of the two components of the probe beam (o-ray and e-ray) passing through the first electro-optic crystal, thereby achieving linear detection of strong terahertz pulses and improving measurement accuracy.

[0008] This document, published in China (CN208187960), discloses a pump-detection system. The system includes an optical platform and, mounted on the platform, a laser, a sample carrier, control equipment, a reflector, a first delay device, a second delay device, a first beam splitter, a second beam splitter, a third beam splitter, a first reflector, a second reflector, a third reflector, a fourth reflector, a first aperture, a second aperture, a third aperture, a fourth aperture, a terahertz emitting device, a terahertz detecting device, a first terahertz reflector, a second terahertz reflector, and an optical power meter, among other optical components. The pump-detection system is assembled using these optical components. This system provides a fast, comprehensive, and diverse method for studying the nonlinear optical properties of samples.

[0009] Current technological problems: Current terahertz-pump-terahertz-probe (TPTP) and optical-pump-terahertz-probe (OPTP) systems are mostly dedicated to specific systems. However, several problems commonly exist in reported multi-purpose systems: 1. Using a beam splitter to combine strong-field pump terahertz laser pulses and weak-field probe terahertz laser pulses weakens the field intensity of the strong-field pump terahertz laser pulse; 2. Using a beam splitter to combine strong-field pump terahertz laser pulses and weak-field probe terahertz laser pulses generates unwanted reflections of the strong-field pump terahertz laser pulse, leading to difficulties in optical path management; 3. Both strong-field pump terahertz laser pulses and weak-field probe terahertz laser pulses reach the detector, causing the strong-field pump terahertz laser pulse to affect the detection results; 4. Using independent optical paths for weak-field probe terahertz laser pulses and pump near-infrared laser pulses complicates the system. Summary of the Invention

[0010] The purpose of this invention is to provide a multifunctional terahertz pump-terahertz probe time-domain spectroscopy system to solve the technical problem of system complexity caused by the use of independent optical paths for terahertz pumping and optical pumping.

[0011] The term “Terahertz time-domain spectroscopy, THz-TDS” used in this article refers to terahertz time-domain spectroscopy.

[0012] The term “tilted-pulse-front” (TPF) used in this article refers to the tilted pulse-front technique.

[0013] The term "optical rectification" used in this article refers to optical rectification technology.

[0014] The term "PCA" used in this article refers to: Terahertz Probe Optical Guide Antenna.

[0015] The term "bow-tie" used in this article refers to a bow-tie optical antenna.

[0016] The term “Terahertz-pump-terahertz-probe” used in this article refers to: terahertz pump-terahertz probe.

[0017] The term “Optical-pump-terahertz-probe, OPTP” used in this article refers to Optical-pump-terahertz-probe, which is an optical pump-terahertz probe.

[0018] The term "LNO" used in this article refers to Lithium Niobate, a type of lithium niobate crystal.

[0019] The term "ZnTe" used in this article refers to zinc telluride.

[0020] To address this, the present invention provides the following technical solution: In a first aspect, the present invention provides a multifunctional terahertz pump-terahertz detection time-domain spectroscopy system, the system comprising: a femtosecond laser, a strong-field terahertz pump optical path or a strong-field terahertz detection optical path, a weak-field terahertz detection optical path or an optical pump optical path, a weak-field terahertz laser pulse measurement gated optical path, and a strong-field terahertz laser pulse measurement gated optical path; the femtosecond laser generated by the femtosecond laser is split into a first laser pulse, a second laser pulse, a third laser pulse, and a fourth laser pulse; The first laser pulse generates a strong field pumped terahertz laser pulse through a strong field terahertz pump optical path, or generates a strong field detected terahertz laser pulse through a strong field terahertz detector optical path; the second laser pulse generates a weak field detected terahertz laser pulse through a weak field terahertz detector optical path, or obtains a pumped near-infrared laser pulse through an optical pump optical path. The third laser pulse is used to measure the strong-field terahertz laser pulse through a strong-field terahertz laser pulse measurement gated optical path; the fourth laser pulse is used to measure the weak-field terahertz laser pulse through a weak-field terahertz laser pulse measurement gated optical path. The system can switch to the following three working modes: Terahertz pump-terahertz detection mode: The strong field pump terahertz laser pulse and the weak field detection terahertz laser pulse are guided to the sample to be tested, and the weak field detection terahertz laser pulse is measured. Optical pump-terahertz detection mode: The pump near-infrared laser pulse and the strong field detection terahertz laser pulse are guided to the sample to be tested, and the strong field detection terahertz laser pulse is measured. Terahertz time-domain spectral measurement modes include: a strong-field terahertz laser pulse time-domain spectral mode, in which only the strong-field detection terahertz laser pulse is guided to the sample to be measured; and a weak-field terahertz laser pulse time-domain spectral mode, in which only the weak-field detection terahertz laser pulse is guided to the sample to be measured.

[0021] The first preferred embodiment further includes beam splitter one, beam splitter two, and beam splitter three; the femtosecond laser pulse transmitted sequentially through beam splitter one and beam splitter three is used as the first laser pulse; the femtosecond laser pulse transmitted sequentially through beam splitter one and reflected sequentially through beam splitter three is used as the second laser pulse; The femtosecond laser pulse, after being reflected by beam splitter one and transmitted by beam splitter two in sequence, is used as the third laser pulse; the femtosecond laser pulse, after being reflected by beam splitter one and beam splitter two in sequence, is used as the fourth laser pulse.

[0022] The second preferred option also includes a main gated optical path, in which a telescope group consisting of plano-convex lens three and plano-convex lens four is arranged along the light propagation direction, a delay platform one, and a beam splitter two. After being reflected by beam splitter one, the femtosecond laser sequentially enters the telescope group composed of plano-convex lens three and plano-convex lens four. After passing through the telescope group, the femtosecond laser will pass through delay platform one, and then be split by beam splitter two into a third laser pulse after transmission and a fourth laser pulse after reflection. The third laser pulse enters the strong field terahertz laser pulse measurement gated optical path, and the fourth laser pulse enters the weak field terahertz laser pulse measurement gated optical path.

[0023] The third preferred option is that the focal length of the plano-convex lens three is three times that of the plano-convex lens four.

[0024] The fourth preferred option: The gated optical path for weak field terahertz laser pulse measurement is equipped with a neutral density mirror, a half-wave plate, a polarizer, a shutter, and a light guide antenna along the light propagation direction; The fourth laser pulse, after being reflected by the second beam splitter, passes sequentially through the first neutral density mirror, the first half-wave plate, the first polarizer, the second shutter, and the first optical guide antenna.

[0025] The fifth preferred option: The gated optical path for strong field terahertz laser pulse measurement is equipped with two half-wave plates, two polarizers, two neutral density filters, one shutter, and two optical guide antennas along the light propagation direction; The third laser pulse, after being transmitted through beam splitter II, passes sequentially through half-wave plate II, polarizer II, neutral density mirror II, and optical guide antenna II.

[0026] The sixth preferred option is that the lengths of the gated optical path for weak-field terahertz laser pulse measurement and the gated optical path for strong-field terahertz laser pulse measurement are equal.

[0027] The seventh preferred option is as follows: The strong field terahertz pump optical path is provided with a strong field terahertz generation unit, an off-axis parabolic mirror, a terahertz filter, an off-axis parabolic mirror, a plane mirror, and an off-axis parabolic mirror with a through hole along the light propagation direction. The strong-field terahertz generation unit employs tilted pulse wavefront technology. The strong-field terahertz generation unit includes a third delay platform, a first chopper, a diffraction grating, a first cylindrical lens, a second cylindrical lens, a fourth half-wave plate, and an LNO crystal. The first laser pulse first enters the third delay platform, then passes through the first chopper. The first laser pulse emitted from the first chopper enters the diffraction grating, and then enters the first and second cylindrical lenses. After passing through the fourth half-wave plate, it enters the LNO crystal, where a strong-field terahertz laser pulse is generated. The strong-field terahertz laser pulse emitted from the LNO crystal enters the off-axis parabolic mirror 1, terahertz filter 1, and off-axis parabolic mirror 2 for amplification and filtering. The strong-field terahertz laser pulse emitted from off-axis parabolic mirror 2 is reflected by a plane mirror and then acts on the off-axis parabolic mirror 1 with a through hole. Finally, the strong-field terahertz laser pulse is focused onto the sample under test as a strong-field pumped terahertz laser pulse.

[0028] The eighth preferred option: the focal length of the off-axis parabolic mirror one is 0.5 inches, and the focal length of the off-axis parabolic mirror two is 4 inches.

[0029] The ninth preferred option: the strong field terahertz detection optical path shares the optical devices in the strong field terahertz pump optical path along the light propagation direction in front of the sample to be tested, and an off-axis parabolic mirror with a through hole, a TPX terahertz convex lens, and a terahertz filter are set behind the sample to be tested. After the strong-field terahertz laser pulse is transmitted through the sample under test, it is converted into parallel light by the off-axis parabolic mirror with through hole. After being focused by the TPX terahertz convex lens and filtered by the terahertz filter, it finally enters the optical guide antenna as the strong-field detection terahertz laser pulse.

[0030] The tenth preferred option: The weak field terahertz detection optical path is provided with a half-wave plate three, a polarizer three, a shutter three, a chopper two, a delay platform two, a ZnTe crystal, an off-axis parabolic mirror three and an off-axis parabolic mirror four, an off-axis parabolic mirror one with a through hole and an off-axis parabolic mirror two with a through hole along the light propagation direction. The second laser pulse first passes through half-wave plate three and polarizer three, then reaches shutter three. The second laser pulse will pass through chopper two and enter delay platform two, and then irradiate the ZnTe crystal, generating a weak-field terahertz laser pulse. After passing through the ZnTe crystal, it passes through off-axis parabolic mirror three and off-axis parabolic mirror four, reducing the diameter of the weak-field terahertz laser pulse so that it can pass through the through hole in the middle of off-axis parabolic mirror one with a through hole. After interacting with the sample, it passes through the through hole in the middle of off-axis parabolic mirror two with a through hole, and arrives at optical guide antenna one as a weak-field detection terahertz laser pulse.

[0031] Eleventh preferred option: the focal length of off-axis parabolic mirror three is 4 inches, and the focal length of off-axis parabolic mirror four is 0.5 inches.

[0032] The twelfth preferred option: The optical device in front of the sample under test in the weak field terahertz detection optical path is shared in the optical pump optical path, and does not contain ZnTe crystal.

[0033] Secondly, the present invention provides a switching method for a multifunctional terahertz pump-terahertz detector time-domain spectroscopy system, employing the aforementioned system, the method comprising: The frequency of the laser pulse transmitted from the femtosecond laser is controlled by chopper one, and this frequency is used as a reference frequency input to the lock-in amplifier. A strong field pump terahertz laser pulse is generated through the strong field terahertz pump optical path. Shutter three is opened, and the ZnTe crystal is installed in place. A weak field detection terahertz laser pulse is generated by chopper two. Shutter two is opened, and the weak field detection terahertz laser pulse entering the optical guide antenna one is measured using optical guide antenna one. The signal read by optical guide antenna one is connected to the signal input terminal of the lock-in amplifier, and the mode is switched to terahertz pump-terahertz detection mode. The frequency of the laser pulse transmitted by the femtosecond laser is controlled by chopper one and used as a reference frequency to input the lock-in amplifier. A strong field terahertz laser pulse is generated through the strong field terahertz detection optical path. Shutter three is opened and the ZnTe crystal is removed. Pump near-infrared laser pulses are obtained by chopper two and shutter one is opened. The strong field terahertz laser pulse entering the optical guide antenna two is measured by optical guide antenna two. The signal read by optical guide antenna two is connected to the signal input terminal of the lock-in amplifier and the optical pump-terahertz detection mode is switched. The frequency of the laser pulse transmitted by the femtosecond laser is controlled by chopper one, and this frequency is used as a reference frequency input to the lock-in amplifier. The strong field terahertz laser pulse is generated by the strong field terahertz detection optical path. Shutter one is opened, and the strong field terahertz laser pulse entering the optical guide antenna two is measured. The signal read by the optical guide antenna two is connected to the signal input terminal of the lock-in amplifier, and the frequency reference signal output by chopper one is connected to the frequency reference signal input terminal of the lock-in amplifier. The mode is switched to the strong field terahertz laser pulse time domain spectrum mode. Open shutter two, control the frequency of the laser pulse transmission of the femtosecond laser through chopper two, and input the frequency as a reference frequency into the lock-in amplifier. Generate a weak-field terahertz laser pulse through the weak-field terahertz detection optical path. Measure the weak-field terahertz laser pulse entering the first optical guide antenna using a pair of optical guide antennas. Connect the signal read by the first optical guide antenna to the signal input terminal of the lock-in amplifier, and connect the frequency reference signal output by chopper two to the frequency reference signal input terminal of the lock-in amplifier. Switch to the weak-field terahertz laser pulse time-domain spectral mode.

[0034] The present invention has at least the following beneficial effects: Compared to existing multifunctional pump-probe spectroscopic systems, the advantages of this invention are: 1. The system layout is simpler, and it is easier to switch between different detection modes.

[0035] This invention achieves rapid detection mode switching by using a computer-controlled opto-mechanical device. Simultaneously, the same system can switch between multiple different operating modes, resulting in a simple spectral system structure and solving the technical problem of system complexity caused by the use of independent optical paths for terahertz pumping and optical pumping.

[0036] 2. In the terahertz pump-terahertz detection mode, since the strong-field pump terahertz laser pulse and the weak-field detection terahertz laser pulse are not combined in front of the sample using a beam splitter, the electric field intensity of the strong-field pump terahertz laser pulse reaching the sample is increased. Simultaneously, because the strong-field pump terahertz laser pulse does not reach the optical guide antenna for measuring the weak-field detection terahertz laser pulse, the influence of the strong-field pump terahertz laser pulse on the detection results is avoided.

[0037] 3. In the terahertz pump-terahertz detection mode, because the system has separate measurement gated optical paths for the weak-field terahertz laser pulse and the strong-field pump terahertz laser pulse respectively, it can simultaneously acquire the time-domain images of two laser pulses that vary with the time interval between the weak-field detection terahertz laser pulse and the strong-field pump terahertz laser pulse. This gives the system the following advantages: ① Extracting frequency-resolved dynamics: By performing a Fast Fourier Transform (FFT) on the probe time-domain waveform at each delay time, the transient response (such as transmittance, reflectance, and photoconductivity spectrum) at different terahertz frequencies can be directly obtained. This can reveal the response characteristics of charge carriers at different frequencies, such as distinguishing between free carrier absorption and resonant absorption; ② Separate amplitude and phase information: The complete time-domain waveform can simultaneously extract the pump-induced amplitude change (ΔA) and phase change (Δφ). The phase change is extremely sensitive to the imaginary part of the dielectric constant, making it particularly suitable for analyzing thin samples or weakly excited processes; ③ Eliminate probe light jitter error: Since the probe waveform is fully scanned each time, the energy fluctuation and time jitter of the probe light itself will be averaged out, resulting in more robust data; ④ Discovering unknown spectral features: When studying unknown materials or complex processes (such as photoinduced phase transitions), this "panoramic" measurement can capture unexpected spectral evolution, avoiding missing key information by focusing on only one point in time.

[0038] 4. In the optical pump-terahertz detection mode, the frequency of the optical pump can be changed by adding a nonlinear crystal to the optical pump path, thus achieving pumping with different photon energies. The crystal can be placed after the second delay plateau and before the third off-axis parabolic mirror. For example, adding a BBO crystal at this position can convert the 800nm ​​second laser pulse from the femtosecond laser into a 400nm pump laser pulse. If the laser source has a matching optical parametric amplifier (OPA), the third beam splitter can be omitted, and the continuously frequency-converted laser pulse output from the OPA can be directly introduced into the optical pump path through a mirror to achieve continuously frequency-converted optical pumping.

[0039] 5. In various modes using weak-field detection of terahertz laser pulses, the ZnTe crystal generating the weak-field terahertz laser pulse can be replaced with other terahertz generating crystals according to detection requirements. For example, if a wider terahertz band coverage is required, the ZnTe crystal can be replaced with the organic crystal BNA or DAST. If a narrow-linewidth weak-field terahertz laser pulse is required, the ZnTe crystal can be replaced with a PPLN crystal. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the overall structure of the system of the present invention.

[0041] Figure 2 This is a schematic diagram of the system structure under the terahertz pump-terahertz detection mode of the present invention.

[0042] Figure 3This is a schematic diagram of the system structure in the optical pump-terahertz detection mode of the present invention.

[0043] Figure 4 This is a schematic diagram of the system structure under the strong field terahertz laser pulse time-domain spectral mode of the present invention.

[0044] Figure 5 This is a schematic diagram of the system structure in the weak-field terahertz laser pulse time-domain spectral mode of the present invention.

[0045] Figure 6 This is a schematic diagram of the system structure of the present invention, with an external lock-in amplifier.

[0046] Figure label: 1. Femtosecond laser; 2. Beam splitter one; 3. Beam splitter three; 4. Main gate optical path; 4-1. Plano-convex lens three; 4-2. Plano-convex lens four; 4-3. Delay platform one; 4-4. Beam splitter two; 5. Gated optical path for weak-field terahertz laser pulse measurement; 5-1. Neutral density mirror 1; 5-2. Half-wave plate 1; 5-3. Polarizer 1; 5-4. Shutter 2; 5-5. Optical guide antenna 1; 6. Strong-field terahertz laser pulse measurement gated optical path; 6-1. Half-wave plate II; 6-2. Polarizer II; 6-3. Neutral density filter II; 6-4. Shutter I; 6-5. Optical guide antenna II; 7. Strong-field terahertz pump optical path; 7-1. Delay plateau three; 7-2. Chopper one; 7-3. Diffraction grating; 7-4. Cylindrical lens one; 7-5. Cylindrical lens two; 7-6. Half-wave plate four; 7-7. LNO crystal; 7-8. Off-axis parabolic mirror one; 7-9. Off-axis parabolic mirror two; 7-10. Plane mirror; 7-11. Off-axis parabolic mirror with through hole one; 7-12. Terahertz filter one; 8. Strong-field terahertz detection optical path; 8-1. Off-axis parabolic mirror with through hole II; 8-2. TPX terahertz convex lens; 8-3. Terahertz filter II; 9. Weak-field terahertz detection optical path; 9-1. Half-wave plate III; 9-2. Polarizer III; 9-3. Shutter III; 9-4. Chopper II; 9-5. Delay plate II; 9-6. ZnTe crystal; 9-7. Off-axis parabolic mirror III; 9-8. Off-axis parabolic mirror IV; 10. Optical pump path; 11. Sample; 12. Lock-in amplifier. Detailed Implementation

[0047] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way. The following content is merely an exemplary description of the scope of protection claimed by the present invention, and those skilled in the art can make various changes and modifications to the present invention based on the disclosed content, and such changes should also fall within the scope of protection claimed by the present invention.

[0048] Example 1 like Figure 1 As shown, this invention provides a multifunctional terahertz pump-probe time-domain spectroscopy system that integrates terahertz pump-terahertz detection (TPTP), optical pump-terahertz detection (OPTP), and terahertz time-domain spectroscopy measurement capabilities. The system includes: The main parameters of the output laser pulse of femtosecond laser 1 are as follows: The repetition frequency is 1 kHz, the pulse width is 160 ps, ​​the pulse center wavelength is 800 nm, the single pulse energy is 7 mJ, the output laser spot diameter is about 10 mm, the laser polarization state is linearly polarized, and the polarization direction is horizontal.

[0049] For ease of understanding, this invention uses a bow tie optical antenna from Batop as an example. Its terahertz detection / transmitter end has an integrated silicon array lens, which can focus the terahertz beam into the optical guide gap of the optical antenna, or transmit the terahertz wave generated in the optical guide gap as a parallel beam. Its threshold / pump light receiver end integrates a lens operating in the near-infrared band, which can focus the parallel terahertz threshold / pump beam into the optical guide gap of the optical antenna.

[0050] This multifunctional terahertz pump-probe time-domain spectroscopy system has three operating modes: (1) terahertz pump-terahertz detection mode, (2) optical pump-terahertz detection mode, and (3) terahertz time-domain spectroscopy measurement mode. When switching between the modes, there is no need to adjust the optical path of the system; only a few mechanically controlled components need to be operated and the correct detection PCA needs to be connected to the signal measurement system.

[0051] The system optical path analysis is as follows: Referring to Figures 1-6, after passing through beam splitter 2, 90% of the femtosecond laser from femtosecond laser 1 is transmitted through beam splitter 2, while 10% is reflected by beam splitter 2 into the main gated optical path 4. The transmitted femtosecond laser is then split by beam splitter 3. 80% is used as the first laser pulse and enters the strong-field terahertz pump optical path 7 or the strong-field terahertz probe optical path 8 using tilted pulse front (TPF) technology. 20% is used as the second laser pulse and enters the weak-field terahertz probe optical path 9 or the optical pump optical path 10.

[0052] 1. Main Gated Optical Path 4: The femtosecond laser first enters a telescope group consisting of plano-convex lens 3 4-1 and plano-convex lens 4-2, where the focal length of plano-convex lens 3 4-1 is three times that of plano-convex lens 4-2. The laser spot diameter after passing through this telescope group is reduced to approximately 0.3 mm, and can be focused onto the optical guide gap of the optical guide antenna 5-5 by a lens integrated on the optical guide antenna 5-5. After passing through the telescope group, the femtosecond laser passes through delay platform 4-3, and is then split by beam splitter 4-4. 20% is reflected as the fourth laser pulse and enters the weak-field terahertz laser pulse measurement gated optical path 5, while 80% is transmitted as the third laser pulse and enters the strong-field terahertz laser pulse measurement gated optical path 6.

[0053] 1.1 Weak-Field Terahertz Laser Pulse Measurement Gated Optical Path 5: The fourth laser pulse first passes through neutral density mirror 5-1. This neutral density mirror 5-1 reduces the power of the fourth laser pulse to approximately 56 mW. After passing through neutral density mirror 5-1, the fourth laser pulse enters a combination of half-wave plate 5-2 and polarizer 5-3. The function of this combination is to further reduce the power of the fourth laser pulse to below 10 mW and ensure that the polarization direction of the fourth laser pulse entering the optical guide antenna 5-5 is horizontal. Shutter 5-4 is closed when it is not necessary to measure the weak-field terahertz laser pulse to protect the optical guide antenna 5-5.

[0054] 1.2 Strong Field Terahertz Laser Pulse Measurement Gated Optical Path 6: The third laser pulse first passes through a combination of half-wave plate 6-1 and polarizer 6-2. This combination reduces the power of the third laser pulse to below 70 mW and ensures that the polarization direction of the third laser pulse entering the optical guide antenna 6-5 is horizontal. The third laser pulse then passes through neutral density mirror 6-3. This neutral density mirror 6-3 reduces the power of the third laser pulse to approximately 7 mW. When not measuring the strong field terahertz laser pulse, shutter 6-4 is closed to protect the optical guide antenna 6-5.

[0055] 1.3 Ensure that the lengths of the gated optical path 5 for weak-field terahertz laser pulse measurement and the gated optical path 6 for strong-field terahertz laser pulse measurement are equal, thus ensuring that the total optical path length of the gated optical path is always uniformly denoted as L, regardless of which path is used for measurement. gate .

[0056] 2. Strong-field terahertz pump optical path 7: After beam splitter 3, the first laser pulse first enters delay plateau 7-1, then passes through chopper 7-2, which provides a reference frequency for lock-in amplifier 12 in the time-domain spectral mode of the strong-field terahertz laser pulse. The first laser pulse passing through chopper 7-2 enters diffraction grating 7-3. The linear density of diffraction grating 7-3 is 1800 lines / cm. The first laser pulse is incident at an angle of 35.3°. In this system, using the first-order diffraction of diffraction grating 7-3, the exit angle of the first laser pulse after diffraction is 55.7°. The first laser pulse exiting from diffraction grating 7-3 enters a set of cylindrical lenses 7-4 and 7-5. The function of this set of lenses is to adjust the wavefront of the first laser pulse to the optimal angle to achieve the phase-matching condition for generating a strong-field terahertz laser pulse within LNO crystal 7-7. Half-wave plate 4 (7-6) reverses the polarization direction of the first laser pulse from horizontal to vertical. Afterward, the first laser pulse exits from LNO crystal 7-7, becoming a strong-field terahertz laser pulse, which enters the combination of off-axis parabolic mirror 1 (7-8) and off-axis parabolic mirror 2 (7-9). The purpose is to enlarge the diameter of the strong-field terahertz laser pulse, providing a larger numerical aperture for focusing the off-axis parabolic mirror 1 (7-11) with a through-hole. Terahertz filter 1 (7-12) is positioned between off-axis parabolic mirror 1 (7-8) and off-axis parabolic mirror 2 (7-9).

[0057] Among them, the off-axis parabolic mirror 7-8 has a focal length of 0.5 inches, and the off-axis parabolic mirror 7-9 has a focal length of 4 inches. These mirrors can divert the nearly parallel, high-field terahertz laser pulses output from the LNO crystal 7-7 from... Zoom in Here, V and H represent the vertical and horizontal directions, respectively. The strong-field terahertz laser pulse emitted from the off-axis parabolic mirror 7-9 is reflected by a plane mirror 7-10, and then reflected by the off-axis parabolic mirror 7-11 with a through hole as a strong-field pump terahertz laser pulse, which is then focused onto the sample 11.

[0058] 3. Strong-field terahertz detection optical path 8: This path is consistent with the strong-field terahertz pump optical path 7 before passing through sample 11. After passing through sample 11, the strong-field terahertz laser pulse is converted into parallel light by the off-axis parabolic mirror 8-1 with a through-hole. After being focused by the TPX terahertz convex lens 8-2 and then filtered by the terahertz filter 8-3, it enters the optical guide antenna 6-5 as the strong-field detection terahertz laser pulse. The terahertz filter 8-3 before the optical guide antenna 6-5 ensures that no first laser pulse enters the optical guide antenna 6-5 from this direction. TPX is polymethylpentene.

[0059] 4. Weak-field terahertz detection optical path 9: After being reflected by beam splitter 3, the second laser pulse first passes through half-wave plate 39-1 and polarizer 39-2. The combination of half-wave plate 39-1 and polarizer 39-2 is used to control the power and polarization direction of the passing second laser pulse to ensure it is horizontal. Then, the second laser pulse arrives at shutter 39-3. When pump measurement is required, shutter 39-3 will open. The second laser pulse will pass through chopper 29-4, enter delay platform 29-5, and then illuminate the ZnTe crystal 9-6 cut along the

[110] crystal direction. The weak-field terahertz laser pulse generated by the optical rectification effect will continue to propagate in the direction of propagation of the second laser pulse. ZnTe crystal 9-6 has a polymer backplate that allows more than 90% of the weak-field terahertz laser pulse to pass through while blocking >99.99% of the second laser pulse. Following the ZnTe crystal 9-6 is a telescope assembly consisting of two off-axis parabolic mirrors, 9-7 and 9-8. Its function is to reduce the diameter of the weak-field terahertz laser pulse so that it can pass through the through-hole of the subsequent off-axis parabolic mirror 7-11. The focal length of off-axis parabolic mirror 9-7 is 4 inches, and the focal length of off-axis parabolic mirror 9-8 is 0.5 inches. After passing through the two off-axis parabolic mirrors, the diameter of the weak-field terahertz laser pulse is reduced to approximately 1.25 mm. The weak-field terahertz laser pulse then passes through the through-hole of off-axis parabolic mirror 7-11, interacts with sample 11, and then passes through the through-hole in the middle of off-axis parabolic mirror 8-1, arriving at the optical guide antenna 5-5 as a weak-field detection terahertz laser pulse. The total optical path length of this path is denoted as [insert path length here]. .

[0060] 5. Optical pump path 10: Basically the same as the weak-field terahertz detection path 9, but the ZnTe crystal 9-6 that generates terahertz is removed. The optical pump path 10 ultimately outputs a pumping near-infrared laser pulse to sample 11, and its total optical path is denoted as... .

[0061] Combination Figures 1-6 As shown, the three working modes of this system are described in detail below: 1. Terahertz pump-terahertz detection mode: such as Figure 2 As shown, this mode uses a strong-field terahertz laser as the pump pulse (i.e., a strong-field pump terahertz laser pulse) and a weak-field terahertz laser as the probe pulse (i.e., a weak-field probe terahertz laser pulse).

[0062] 1.1 System Preparation: Open shutter speeds 2 (5-4) and 3 (9-3). Control the angle of half-wave plate 3 (9-1) to ensure the power of the second laser pulse reaching the ZnTe crystal 9-6 is approximately 1W. Define the side of sample 11 facing the off-axis parabolic mirror with through-hole 7-11 as the front side and the opposite side as the back side. Move the sample 11 platform (not shown in the figure, but a 3-DOF computer-controlled platform capable of translation in the x, y, and z directions) closer to the off-axis parabolic mirror with through-hole 7-11. Ensure the focal diameter of the strong-field pumped terahertz laser pulse from LNO crystal 7-7 at the back side of sample 11 is no greater than 1.25mm, corresponding to a strong-field pumped terahertz laser pulse field strength of approximately 700 kV / cm.

[0063] 1.2 Time-Domain Spectrum Acquisition of Weak-Field Detected Terahertz Laser Pulses: Set the frequency of chopper 2 (9-4) to 500 Hz and connect its output frequency reference signal to the frequency reference signal input of lock-in amplifier 12. Without starting chopper 1 (7-2), manually adjust chopper 1 (7-2) so that the first laser pulse is blocked by the blocking sector of chopper 1 (7-2). Connect the signal read by optical guide antenna 1 (5-5) to the signal input of lock-in amplifier 12. At this point, the system is equivalent to a conventional terahertz time-domain spectroscopy system (THz-TDS) using a weak-field detected terahertz laser pulse as the test signal. Following the conventional THz-TDS usage, the time-domain spectrum of the weak-field detected terahertz laser pulse can be obtained by scanning the delay platform 1 (4-3).

[0064] 1.3 Acquisition of the Time-Domain Spectrum of a Strong-Field-Pumped Terahertz Laser Pulse: Open shutter 6-4. Set the frequency of chopper 7-2 to 500 Hz and connect its output frequency reference signal to the frequency reference signal input of lock-in amplifier 12. Without starting chopper 9-4, manually adjust chopper 9-4 so that the laser is blocked by its blocking sector. Connect the signal read by optical guide antenna 6-5 to the signal input of lock-in amplifier 12. At this point, the system is equivalent to a conventional terahertz time-domain spectroscopy system (THz-TDS) using a strong-field-pumped terahertz laser pulse as the signal under test. Following the conventional THz-TDS usage, the time-domain spectrum of the strong-field-pumped terahertz laser pulse can be obtained by scanning the delay platform 4-3.

[0065] 1.4 Terahertz Pump-Terahertz Probe Time-Domain Spectrum Acquisition: Close shutter 6-4. Connect the signal output of optical guide antenna 5-5 to the signal input of lock-in amplifier 12. Based on the time-domain spectrum of the weak-field detected terahertz laser pulse, move delay platform 4-3 to maximize the reading of lock-in amplifier 12. Then scan delay platform 7-1 to obtain the terahertz pump-terahertz probe time-domain spectrum.

[0066] 2. Optical pump-terahertz detection mode: such as Figure 3As shown, this mode uses a strong-field terahertz laser pulse as the probe light (i.e., a strong-field probe terahertz laser pulse) and an 800 nm laser pulse as the pump (i.e., a pump near-infrared laser pulse). Because a strong-field terahertz laser pulse is used to perform the detection, samples 11 with low transmittance can be effectively detected.

[0067] 2.1 System Preparation: Remove ZnTe crystal 9-6. Adjust the angle of half-wave plate 3 9-1 to ensure that the second laser pulse reaching sample 11 meets the target optical pump energy requirements. Control the sample platform so that the center of sample 11 is between the off-axis parabolic mirror 1 7-11 with through-hole and the off-axis parabolic mirror 2 8-1 with through-hole. At this time, the focal size of the strong-field terahertz laser pulse is approximately 700 μm, corresponding to a field strength of approximately 1 MV / cm for a single strong-field detection terahertz laser pulse.

[0068] 2.2 Time-Domain Spectrum Acquisition of Strong-Field Detected Terahertz Laser Pulses: Open shutter 6-4. Set the frequency of chopper 7-2 to 500 Hz and connect its output frequency reference signal to the frequency reference signal input of lock-in amplifier 12. Without starting chopper 9-4, manually adjust chopper 9-4 so that the second laser pulse is blocked by its blocking sector. Connect the signal read by optical guide antenna 6-5 to the signal input of lock-in amplifier 12. At this point, the system is equivalent to a conventional terahertz time-domain spectroscopy system (THz-TDS) using a strong-field detected terahertz laser pulse as the test signal. Following the conventional THz-TDS usage, the time-domain spectrum of the strong-field detected terahertz laser pulse can be acquired by scanning the delay platform 4-3.

[0069] 2.3 Optical Pump-Terahertz Probe Time-Domain Spectral Calibration: Open shutter 3 (9-3) and shutter 1 (6-4). Set the frequency of chopper 2 (9-4) to 500 Hz and connect its output frequency reference signal to the frequency reference signal input of lock-in amplifier 12. Without starting chopper 1 (7-2), manually adjust chopper 1 (7-2) so that the first laser pulse is blocked by its blocking sector. Measure the optical pump-terahertz probe time-domain spectrum of a material with known spectra (such as GaAs) using scanning delay platform 2 (9-5), and calibrate the system by comparing the measured spectrum with the known spectrum.

[0070] 2.4 Replace with target sample 11, and obtain the optical pump-terahertz detection time-domain spectrum of sample 11 by scanning delay platform 37-1 according to the parameters obtained in steps 2.1 to 2.3.

[0071] 3. Terahertz Time-Domain Spectroscopy Measurement Mode: This mode utilizes either a strong-field or weak-field terahertz laser pulse for detection, and is a conventional THz-TDS mode. For samples with low transmittance (such as thin-film metal samples), strong-field THz-TDS can be used. For samples with high transmittance, weak-field THz-TDS can be used.

[0072] 3.1 Temporal Spectral Modes of Strong-Field Terahertz Laser Pulses: such as Figure 4 As shown, shutter 6-4 is opened. The frequency of chopper 7-2 is set to 500 Hz, and its output frequency reference signal is connected to the frequency reference signal input terminal of lock-in amplifier 12. The signal read by optical guide antenna 6-5 is connected to the signal input terminal of lock-in amplifier 12. The time-domain spectrum of the strong field detection terahertz laser pulse of sample 11 is obtained by scanning delay platform 4-3.

[0073] 3.2 Time-domain spectral modes of weak-field terahertz laser pulses: such as Figure 5 As shown, open shutter 2 (5-4). Without activating chopper 1 (7-2), manually adjust chopper 1 (7-2) so that the laser is blocked by its blocking sector. Set the frequency of chopper 2 (9-4) to 500Hz and connect its output frequency reference signal to the frequency reference signal input terminal of lock-in amplifier 12. Connect the signal read by optical guide antenna 1 (5-5) to the signal input terminal of the lock-in amplifier. Obtain the time-domain spectrum of the weak-field detection terahertz laser pulse of sample 11 using the scanning delay platform 1 (4-3).

[0074] Example 2 Combination Figure 6 The system structure shown illustrates the switching method of a multifunctional terahertz pump-terahertz detector time-domain spectroscopy system provided in this embodiment. The system described in Embodiment 1 is used, and the method includes: The frequency of the laser pulse transmitted by the femtosecond laser 1 is controlled by chopper 1-7-2, and this frequency is used as a reference frequency and input to lock-in amplifier 12. A strong field pump terahertz laser pulse is generated through strong field terahertz pump optical path 7. Shutter 3-9-3 is opened to install ZnTe crystal 9-6 in place. A weak field detection terahertz laser pulse is generated by chopper 2-9-4. Shutter 2-5-4 is opened to measure the weak field detection terahertz laser pulse entering the optical guide antenna 1-5-5. The signal read by the optical guide antenna 1-5-5 is connected to the signal input terminal of lock-in amplifier 12, and the mode of terahertz pump-terahertz detection is switched. The frequency of the laser pulse transmitted by the femtosecond laser 1 is controlled by chopper 1 7-2, and this frequency is used as a reference frequency and input to lock-in amplifier 12. The strong field terahertz detection laser pulse is generated by strong field terahertz detection optical path 8, shutter 3 9-3 is opened, and ZnTe crystal 9-6 is removed. Pump near-infrared laser pulse is obtained by chopper 2 9-4, shutter 1 6-4 is opened, and strong field terahertz laser pulse entering the optical guide antenna 2 6-5 is measured by optical guide antenna 2 6-5. The signal read by optical guide antenna 2 6-5 is connected to the signal input terminal of lock-in amplifier 12, and the optical pump-terahertz detection mode is switched. The frequency of the laser pulse transmitted by the femtosecond laser 1 is controlled by the chopper 7-2, and this frequency is used as a reference frequency and input to the lock-in amplifier 12. The strong field terahertz laser pulse is generated by the strong field terahertz detection optical path 8. The shutter 6-4 is opened, and the strong field terahertz laser pulse entering the optical guide antenna 6-5 is measured. The signal read by the optical guide antenna 6-5 is connected to the signal input terminal of the lock-in amplifier 12, and the frequency reference signal output by the chopper 7-2 is connected to the frequency reference signal input terminal of the lock-in amplifier 12. The mode is switched to the strong field terahertz laser pulse time domain spectrum mode. Open shutter 2 5-4, control the frequency of laser pulse transmission of femtosecond laser 1 through chopper 2 9-4, and input the frequency as reference frequency into lock-in amplifier 12. The weak field terahertz detection optical path 9 generates a weak field detection terahertz laser pulse. The weak field detection terahertz laser pulse entering the optical guide antenna 1 5-5 is measured. The signal read by the optical guide antenna 1 5-5 is connected to the signal input terminal of lock-in amplifier 12. The frequency reference signal output by chopper 2 9-4 is connected to the frequency reference signal input terminal of lock-in amplifier 12. Switch to the weak field terahertz laser pulse time domain spectrum mode.

[0075] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A multifunctional terahertz pump-terahertz detector time-domain spectroscopic system, characterized in that, The system includes: a femtosecond laser, a strong-field terahertz pump optical path or a strong-field terahertz detection optical path, a weak-field terahertz detection optical path or an optical pump optical path, a weak-field terahertz laser pulse measurement gated optical path, and a strong-field terahertz laser pulse measurement gated optical path; the femtosecond laser generated by the femtosecond laser is split into a first laser pulse, a second laser pulse, a third laser pulse, and a fourth laser pulse; The first laser pulse generates a strong field pumped terahertz laser pulse through a strong field terahertz pump optical path, or generates a strong field detected terahertz laser pulse through a strong field terahertz detector optical path; the second laser pulse generates a weak field detected terahertz laser pulse through a weak field terahertz detector optical path, or obtains a pumped near-infrared laser pulse through an optical pump optical path. The third laser pulse is used to measure the strong-field terahertz laser pulse through a strong-field terahertz laser pulse measurement gated optical path; the fourth laser pulse is used to measure the weak-field terahertz laser pulse through a weak-field terahertz laser pulse measurement gated optical path. The system can switch to the following three working modes: Terahertz pump-terahertz detection mode: The strong field pump terahertz laser pulse and the weak field detection terahertz laser pulse are guided to the sample to be tested, and the weak field detection terahertz laser pulse is measured. Optical pump-terahertz detection mode: The pump near-infrared laser pulse and the strong field detection terahertz laser pulse are guided to the sample to be tested, and the strong field detection terahertz laser pulse is measured. Terahertz time-domain spectral measurement modes include: a strong-field terahertz laser pulse time-domain spectral mode, in which only the strong-field detection terahertz laser pulse is guided to the sample to be measured; and a weak-field terahertz laser pulse time-domain spectral mode, in which only the weak-field detection terahertz laser pulse is guided to the sample to be measured.

2. The system according to claim 1, characterized in that, It also includes beam splitter one, beam splitter two, and beam splitter three; the femtosecond laser after being transmitted through beam splitter one and beam splitter three in sequence is used as the first laser pulse; the femtosecond laser after being transmitted through beam splitter one and reflected by beam splitter three in sequence is used as the second laser pulse; The femtosecond laser pulse, after being reflected by beam splitter one and transmitted by beam splitter two in sequence, is used as the third laser pulse; the femtosecond laser pulse, after being reflected by beam splitter one and beam splitter two in sequence, is used as the fourth laser pulse.

3. The system according to claim 2, characterized in that, It also includes a main gated optical path, in which a telescope group consisting of plano-convex lens three and plano-convex lens four is arranged along the light propagation direction, a delay platform one, and a beam splitter two; After being reflected by beam splitter one, the femtosecond laser sequentially enters the telescope group composed of plano-convex lens three and plano-convex lens four. After passing through the telescope group, the femtosecond laser will pass through delay platform one, and then be split by beam splitter two into a third laser pulse after transmission and a fourth laser pulse after reflection. The third laser pulse enters the strong field terahertz laser pulse measurement gated optical path, and the fourth laser pulse enters the weak field terahertz laser pulse measurement gated optical path.

4. The system according to claim 3, characterized in that, The focal length of the plano-convex lens three is three times that of the plano-convex lens four.

5. The system according to claim 3, characterized in that, The weak-field terahertz laser pulse measurement gated optical path is equipped with a neutral density mirror, a half-wave plate, a polarizer, a shutter, and a light guide antenna along the light propagation direction. The fourth laser pulse, after being reflected by the second beam splitter, passes sequentially through the first neutral density mirror, the first half-wave plate, the first polarizer, the second shutter, and the first optical guide antenna.

6. The system according to claim 5, characterized in that, The strong field terahertz laser pulse measurement gated optical path is equipped with two half-wave plates, two polarizers, two neutral density mirrors, one shutter, and two optical guide antennas along the light propagation direction. The third laser pulse, after being transmitted through beam splitter II, passes sequentially through half-wave plate II, polarizer II, neutral density mirror II, and optical guide antenna II.

7. The system according to claim 3, characterized in that, The lengths of the gated optical paths for weak-field terahertz laser pulse measurement and strong-field terahertz laser pulse measurement are equal.

8. The system according to claim 6, characterized in that, The strong field terahertz pump optical path is provided with a strong field terahertz generation unit, an off-axis parabolic mirror, a terahertz filter, an off-axis parabolic mirror, a plane mirror, and an off-axis parabolic mirror with a through hole along the light propagation direction. The strong-field terahertz generation unit employs tilted pulse wavefront technology. The strong-field terahertz generation unit includes a third delay platform, a first chopper, a diffraction grating, a first cylindrical lens, a second cylindrical lens, a fourth half-wave plate, and an LNO crystal. The first laser pulse first enters the third delay platform, then passes through the first chopper. The first laser pulse emitted from the first chopper enters the diffraction grating, and then enters the first and second cylindrical lenses. After passing through the fourth half-wave plate, it enters the LNO crystal, where a strong-field terahertz laser pulse is generated. The strong-field terahertz laser pulse emitted from the LNO crystal enters the off-axis parabolic mirror 1, terahertz filter 1, and off-axis parabolic mirror 2 for amplification and filtering. The strong-field terahertz laser pulse emitted from off-axis parabolic mirror 2 is reflected by a plane mirror and then acts on the off-axis parabolic mirror 1 with a through hole. Finally, the strong-field terahertz laser pulse is focused onto the sample under test as a strong-field pumped terahertz laser pulse.

9. The system according to claim 8, characterized in that, The focal length of the first off-axis parabolic mirror is 0.5 inches, and the focal length of the second off-axis parabolic mirror is 4 inches.

10. The system according to claim 8, characterized in that, The strong field terahertz detection optical path shares the same optical devices in the strong field terahertz pump optical path in front of the sample under test along the light propagation direction. Behind the sample under test, there is an off-axis parabolic mirror with a through hole, a TPX terahertz convex lens, and a terahertz filter. After the strong-field terahertz laser pulse is transmitted through the sample under test, it is converted into parallel light by the off-axis parabolic mirror with through hole. After being focused by the TPX terahertz convex lens and filtered by the terahertz filter, it finally enters the optical guide antenna as the strong-field detection terahertz laser pulse.

11. The system according to claim 10, characterized in that, The weak-field terahertz detection optical path is equipped with a half-wave plate three, a polarizer three, a shutter three, a chopper two, a delay platform two, a ZnTe crystal, an off-axis parabolic mirror three and an off-axis parabolic mirror four, an off-axis parabolic mirror one with a through hole and an off-axis parabolic mirror two with a through hole along the light propagation direction. The second laser pulse first passes through half-wave plate three and polarizer three, then reaches shutter three. The second laser pulse will pass through chopper two and enter delay platform two, and then irradiate the ZnTe crystal, generating a weak-field terahertz laser pulse. After passing through the ZnTe crystal, it passes through off-axis parabolic mirror three and off-axis parabolic mirror four, reducing the diameter of the weak-field terahertz laser pulse so that it can pass through the through hole in the middle of off-axis parabolic mirror one with a through hole. After interacting with the sample, it passes through the through hole in the middle of off-axis parabolic mirror two with a through hole, and arrives at optical guide antenna one as a weak-field detection terahertz laser pulse.

12. The system according to claim 11, characterized in that, The focal length of the off-axis parabolic mirror three is 4 inches, and the focal length of the off-axis parabolic mirror four is 0.5 inches.

13. The system according to claim 11, characterized in that, The optical pump path uses the same optical device in front of the sample in the weak-field terahertz probe path, and does not contain ZnTe crystal.

14. A switching method for a multifunctional terahertz pump-terahertz detector time-domain spectroscopy system, characterized in that, The method using the system according to any one of claims 11-13 comprises: The frequency of the laser pulse transmitted from the femtosecond laser is controlled by chopper one, and this frequency is used as a reference frequency input to the lock-in amplifier. A strong field pump terahertz laser pulse is generated through the strong field terahertz pump optical path. Shutter three is opened, and the ZnTe crystal is installed in place. A weak field detection terahertz laser pulse is generated by chopper two. Shutter two is opened, and the weak field detection terahertz laser pulse entering the optical guide antenna one is measured using optical guide antenna one. The signal read by optical guide antenna one is connected to the signal input terminal of the lock-in amplifier, and the mode is switched to terahertz pump-terahertz detection mode. The frequency of the laser pulse transmitted by the femtosecond laser is controlled by chopper one and used as a reference frequency to input the lock-in amplifier. A strong field terahertz laser pulse is generated through the strong field terahertz detection optical path. Shutter three is opened and the ZnTe crystal is removed. Pump near-infrared laser pulses are obtained by chopper two and shutter one is opened. The strong field terahertz laser pulse entering the optical guide antenna two is measured by optical guide antenna two. The signal read by optical guide antenna two is connected to the signal input terminal of the lock-in amplifier and the optical pump-terahertz detection mode is switched. The frequency of the laser pulse transmitted by the femtosecond laser is controlled by chopper one, and this frequency is used as a reference frequency input to the lock-in amplifier. The strong field terahertz laser pulse is generated by the strong field terahertz detection optical path. Shutter one is opened, and the strong field terahertz laser pulse entering the optical guide antenna two is measured. The signal read by the optical guide antenna two is connected to the signal input terminal of the lock-in amplifier, and the frequency reference signal output by chopper one is connected to the frequency reference signal input terminal of the lock-in amplifier. The mode is switched to the strong field terahertz laser pulse time domain spectrum mode. Open shutter two, control the frequency of the laser pulse transmission of the femtosecond laser through chopper two, and input the frequency as a reference frequency into the lock-in amplifier. Generate a weak-field terahertz laser pulse through the weak-field terahertz detection optical path. Measure the weak-field terahertz laser pulse entering the first optical guide antenna using a pair of optical guide antennas. Connect the signal read by the first optical guide antenna to the signal input terminal of the lock-in amplifier, and connect the frequency reference signal output by chopper two to the frequency reference signal input terminal of the lock-in amplifier. Switch to the weak-field terahertz laser pulse time-domain spectral mode.

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