A system and method for coherent detection of terahertz waves based on organic polymers

Abstract

The application discloses a system and method for detecting terahertz waves based on organic polymers, and relates to the field of terahertz detection.The application utilizes the nonlinear interaction between terahertz waves and detection light in an organic polymer film to generate a target signal, and introduces controlled reference light to perform coherent interference, so that the time-domain waveform, amplitude information and phase information of the terahertz electric field are recovered.The application can expand the application range of organic polymer materials in the field of terahertz detection, and provides a new technical approach for constructing a low-cost, high-sensitivity, wide-band and easy-maintenance terahertz coherent detection platform.

Description

Technical Field

[0001] This invention relates to the field of terahertz detection, and more specifically to a system and method for coherently detecting terahertz waves based on organic polymers. Background Technology

[0002] Coherent detection of terahertz waves is a crucial step in terahertz science and technology, serving as a fundamental basis for terahertz time-domain spectroscopy, ultrafast dynamics measurement, non-destructive material characterization, and terahertz imaging. Compared to detection methods that only acquire intensity information, coherent detection can simultaneously recover the amplitude and phase information of the terahertz electric field, thus playing an irreplaceable role in terahertz time-domain waveform reconstruction, spectral analysis, and polarization characteristic measurement. With the development of strong-field terahertz radiation sources and ultrafast laser technology, developing high-sensitivity, wide-bandwidth, and high-stability coherent terahertz detection methods has become an important research direction in this field.

[0003] Currently, coherent detection methods for pulsed terahertz signals mainly include photoconductive antenna sampling, electro-optic crystal sampling, and plasma coherent detection based on gas media. Photoconductive antenna and electro-optic crystal sampling technologies are relatively mature, but the former is easily limited by carrier dynamics, while the latter is easily affected by crystal phonon absorption, dispersion, and phase matching conditions. Therefore, bandwidth limitations and waveform distortion are common problems in ultra-wideband terahertz waveform measurements. While gas-media-based coherent detection can overcome some bandwidth limitations of solid-state crystals, it usually relies on high probe pulse energy, and the weak nonlinear response of gas media further hinders detection sensitivity improvement. To address these issues, researchers have proposed coherent detection schemes based on solid-state nonlinear media. However, these methods often require external bias voltages or complex device designs, making system implementation complex and still potentially limited by factors such as intrinsic material absorption, coherent coupling length, and damage threshold. However, terahertz coherent detection methods based on organic polymer thin films have not yet been reported. The reasons hindering their adoption as a terahertz detection technology may be the following three: (1) Most existing solid-state terahertz coherent detection technologies are based on semiconductor carrier response, crystal linear electro-optic effect or nonlinear response of specific inorganic thin films. The research focus is usually on photoconductive materials, electro-optic crystals and solid thin film devices with special structural designs. However, relatively little attention is paid to low-cost and easy-to-process organic polymer thin film materials. Therefore, such materials are usually not within the priority consideration of traditional terahertz coherent detection media.

[0004] (2) Low-density polyethylene, polyethylene terephthalate, polypropylene and other organic polymer materials mostly exhibit near-isotropic media. Their response in linear electro-optic effect and traditional transient birefringence detection is usually not as significant as that of commonly used electro-optic crystals. Therefore, when researchers develop high-sensitivity coherent detection schemes, they often believe that such materials are difficult to be directly used for the effective extraction of terahertz electric field amplitude and phase information, thus ignoring their potential as broadband detection media.

[0005] (3) In existing broadband terahertz coherent detection schemes, in order to obtain a resolvable coherent signal, it is usually necessary to rely on gas plasma, external bias electric field, micro / nano structures or specific inorganic nonlinear thin film devices. Although organic polymer thin films have advantages such as low cost, easy replacement and good transmittance, they have not been systematically developed for a long time due to the lack of mature signal acquisition methods, especially the lack of a system structure that can coherently interfere the terahertz-induced nonlinear signal with the reference light and simultaneously recover the terahertz amplitude and phase information. Summary of the Invention

[0006] To address the aforementioned shortcomings in existing technologies, this invention provides a system and method for coherent detection of terahertz waves based on organic polymers, which solves the problems of high cost and low sensitivity in existing technologies.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A system for coherently detecting terahertz waves based on organic polymers is provided, comprising: Femtosecond lasers are used to provide femtosecond laser pulses; A beam splitter is used to split a femtosecond laser pulse into a pump beam and a probe beam; the pump beam enters the pump beam path and the probe beam enters the probe beam path. The pump light path is sequentially configured with an optical parametric amplifier, a second reflector, a terahertz wave generating crystal, a terahertz filter, a first off-axis parabolic reflector, a second off-axis parabolic reflector, a first terahertz polarizer, and a second terahertz polarizer; the terahertz wave emitted from the second terahertz polarizer is reflected and focused onto the organic polymer film by the off-axis perforated parabolic reflector; The probe light path is sequentially configured with a time delay device, a half-wave plate, a first reflector, a first polarizer, a first lens, a BBO crystal, a dual-wavelength wave plate, and a second polarizer. The probe light output from the second polarizer and the controlled second harmonic reference light are focused collinearly with the terahertz wave onto the organic polymer film after passing through the central hole of the off-axis perforated parabolic reflector. The controlled second harmonic reference light is generated by the BBO crystal. Organic polymer films are used to generate terahertz-induced second harmonics (THAs) under the combined action of terahertz waves and probe light; wherein the terahertz-induced second harmonics coherently interfere with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field. The information processing module is used to receive the detection signal, acquire the time-domain waveform of the terahertz wave to be measured, and recover the amplitude and phase information of the terahertz wave.

[0008] Furthermore, the femtosecond laser is a Ti:sapphire femtosecond laser amplifier with an output laser center wavelength of 800 nm and a pulse width of 20 fs to 100 fs.

[0009] Furthermore, the optical parametric amplifier is used to perform pump light wavelength conversion, outputting pump light with a center wavelength of 1200nm~2600nm.

[0010] Furthermore, the terahertz wave generating crystal is one or more of the following: DSTMS crystal, DAST crystal, OH1 crystal, and BNA crystal, used to generate terahertz waves.

[0011] Furthermore, the first off-axis parabolic mirror and the second off-axis parabolic mirror are used to reflect and focus terahertz waves; The first and second terahertz polarizers are used to adjust the polarization state and field strength of the terahertz wave. A time delay device is used to adjust the relative timing between the probe light and the terahertz wave; A dual-wavelength waveplate is used to simultaneously modulate the polarization state of the fundamental frequency probe light and the controlled second harmonic reference light; The second polarizer is used to adjust the polarization relationship between the controlled second harmonic reference light and the terahertz-induced second harmonic signal in order to obtain interference components that are linearly related to the terahertz electric field.

[0012] Furthermore, the organic polymer film is an organic polymer film with terahertz transmission characteristics and nonlinear optical response.

[0013] Furthermore, the thickness of the organic polymer film is 5μm~50μm.

[0014] Furthermore, the information processing module includes a second lens, a third polarizer, a bandpass filter, and a photomultiplier tube arranged sequentially in the optical path; wherein the center wavelength of the bandpass filter is 400 nm and the bandwidth range is 380 nm ~ 420 nm, and it is used to filter out the fundamental frequency probe light and background stray light; the photomultiplier tube is used for photoelectric conversion and signal acquisition.

[0015] A method is provided for a system based on coherent detection of terahertz waves using organic polymers, comprising the following steps: Femtosecond laser pulses are provided by a femtosecond laser; The femtosecond laser pulse is split into pump light and probe light using a beam splitter; Controlled second harmonic reference light is generated by incident probe light on a BBO crystal; terahertz waves are generated by incident pump light on a terahertz generating crystal. The terahertz wave emitted from the pump light path is reflected and focused onto the organic polymer film by an off-axis perforated parabolic mirror; the probe light emitted from the probe light path and the controlled second harmonic reference light are collinearly focused onto the organic polymer film after passing through the central hole of the off-axis perforated parabolic mirror; so that a terahertz-induced second harmonic is generated under the combined action of the terahertz wave and the probe light, and the generated terahertz-induced second harmonic coherently interferes with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field; The information processing module receives the detection signal related to the terahertz electric field, which is formed by the coherent interference between the terahertz-induced second harmonic and the controlled second harmonic reference light. Then, the time-domain waveform of the terahertz wave to be measured is obtained and the amplitude and phase information of the terahertz wave are recovered.

[0016] Furthermore, the method also includes the following operations: By adjusting the first terahertz polarizer, the second terahertz polarizer, and the half-wave plate, detection signals under different polarization conditions are obtained, so as to characterize the polarization characteristics of the terahertz wave under test. By adjusting the time delay device, the detection signal under different delay times can be obtained, thereby recovering the time domain waveform of the terahertz wave to be measured and its amplitude and phase information.

[0017] The beneficial effects of this invention are as follows: 1. This invention innovatively applies organic polymer thin films to coherent terahertz wave detection, and combines the principle of terahertz-induced second harmonic radiation and the controlled second harmonic reference light coherent interference mechanism to achieve synchronous recovery of the time-domain waveform, amplitude information, and phase information of the terahertz electric field. While achieving wide bandwidth and high sensitivity detection, it also possesses advantages such as simple system structure, low construction cost, high stability, ease of maintenance, and a wide range of material selection. This provides a new technical approach for the application of organic polymer materials (organic polymer thin films) in terahertz time-domain spectroscopy, ultrafast dynamics research, material characterization, and terahertz imaging.

[0018] 2. By adjusting the time delay device in the probe optical path, the interference signal can be scanned in the time domain, thus recovering the complete time domain waveform of the terahertz wave to be measured, and further achieving accurate characterization of its amplitude and phase.

[0019] 3. The present invention uses low-density polyethylene and other organic polymer films as materials for detecting terahertz waves. Taking advantage of the good terahertz transmission characteristics, available third-order nonlinear response, high damage threshold and easy processing and replacement of such materials, the dependence on detection light energy is effectively reduced while ensuring broadband detection capability.

[0020] 4. Compared with traditional photoconductive antenna sampling and electro-optic crystal sampling, this invention does not rely on semiconductor carrier response or crystal linear electro-optic effect, and can avoid the limitations of phonon absorption, dispersion and phase matching conditions on the detection bandwidth, thereby realizing coherent detection of ultra-wideband terahertz waveforms.

[0021] 5. Compared with coherent detection schemes based on gas media, the present invention requires lower detection light energy and has higher detection sensitivity.

[0022] 6. Compared with coherent detection schemes based on solid bias structures, the present invention does not require an external bias voltage or complex micro / nano structure devices, avoids signal distortion caused by material phonon absorption, reduces system complexity and improves environmental adaptability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the system structure; Figure 2 The time-domain spectral signal obtained by this system under coherent detection with an 800 nm probe light pulse and an energy of 4 μJ, and a terahertz peak electric field of approximately 0.6 MV / cm; Figure 3 The frequency domain spectrum obtained by coherent detection of this system at a terahertz peak electric field of approximately 0.6 MV / cm is shown. Figure 4 This is a graph showing the relationship between the time-domain peak signal detected by this system and the terahertz electric field.

[0024] The components are: 1. Femtosecond laser; 2. Beam splitter; 3. Time delay device; 4. Half-wave plate; 5. First reflector; 6. First polarizer; 7. First lens; 8. BBO crystal; 9. Dual-wavelength waveplate; 10. Second polarizer; 11. Off-axis perforated parabolic reflector; 12. Organic polymer film; 13. Optical parametric amplifier; 14. Second reflector; 15. Terahertz wave generating crystal; 16. Terahertz filter; 17. First off-axis parabolic reflector; 18. Second off-axis parabolic reflector; 19. First terahertz polarizer; 20. Second terahertz polarizer; 21. Second lens; 22. Third polarizer; 23. Bandpass filter; 24. Photomultiplier tube. Detailed Implementation

[0025] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0026] like Figure 1 As shown, the system based on coherent detection of terahertz waves using organic polymers includes a femtosecond laser 1, a beam splitter 2, a time delay device 3, a half-wave plate 4, a first reflector 5, a first polarizer 6, a first lens 7, a BBO crystal 8, a dual-wavelength waveplate 9, a second polarizer 10, an off-axis perforated parabolic reflector 11, an organic polymer thin film 12, an optical parametric amplifier 13, a second reflector 14, a terahertz wave generating crystal 15, a terahertz filter 16, a first off-axis parabolic reflector 17, a second off-axis parabolic reflector 18, a first terahertz polarizer 19, a second terahertz polarizer 20, a second lens 21, a third polarizer 22, a bandpass filter 23, and a photomultiplier tube 24, all arranged in the optical path. The femtosecond laser 1 is preferably a Ti:sapphire femtosecond laser amplifier, used to output a femtosecond laser pulse with a center wavelength of 800 nm, a pulse width of 35 fs, and a repetition frequency of 1 kHz. The femtosecond laser pulse is split into pump light and probe light by a beam splitter 2. The pump light is wavelength-converted by an optical parametric amplifier 13, then its propagation direction is adjusted by a second reflector 14 and it is incident on a terahertz wave generating crystal 15 to generate a terahertz wave. After the terahertz wave is filtered by a terahertz filter 16 to remove stray near-infrared light, it is reflected and focused by a first off-axis parabolic reflector 17 and a second off-axis parabolic reflector 18. In the propagation path, the polarization state and field strength of the terahertz wave are adjusted by a first terahertz polarizer 19 and a second terahertz polarizer 20, and then it is reflected and focused onto an organic polymer thin film 12 by an off-axis perforated parabolic reflector 11.

[0027] After the probe light is adjusted in relative timing with the terahertz wave by the time delay device 3, it sequentially passes through the half-wave plate 4, the first reflector 5, the first polarizer 6, the first lens 7, the BBO crystal 8, the dual-wavelength waveplate 9, and the second polarizer 10. Then, after passing through the central hole of the off-axis perforated parabolic reflector 11, it is collinearly focused onto the organic polymer film 12 with the terahertz wave. The BBO crystal 8 is used to generate a controlled second harmonic reference light in the probe light path, and the dual-wavelength waveplate 9 is used to synchronously control the polarization state of the fundamental frequency probe light and the controlled second harmonic reference light. The organic polymer film 12 is preferably a low-density polyethylene film. Under the combined action of the terahertz wave and the probe light, a terahertz-induced second harmonic signal is generated in the organic polymer film 12. This terahertz-induced second harmonic signal coherently interferes with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field.

[0028] The interferometric optical signal undergoes post-processing by the information processing module. Specifically, the interferometric optical signal is collected by the second lens 21, then passes sequentially through the third polarizer 22 and the bandpass filter 23 to filter out background light and select the target second harmonic signal. Finally, it is converted into photoelectric signal and acquired by the photomultiplier tube 24. During this process, the time-domain waveform of the terahertz wave to be measured can be obtained by continuously adjusting the time delay device 3, and its amplitude and phase information can be further recovered.

[0029] For example, the pump light output by the optical parametric amplifier 13 has a center wavelength of 1550 nm, which can also be adjusted to other wavelengths in the range of 1200 nm to 2600 nm.

[0030] The terahertz wave generating crystal 15 is a DSTMS crystal, which can also be replaced by organic nonlinear crystals such as DAST crystal, OH1 crystal, and BNA crystal.

[0031] The first terahertz polarizer 19 and the second terahertz polarizer 20 mainly achieve the adjustment of the terahertz peak electric field intensity in the range of 0.02~2 MV / cm.

[0032] The first terahertz polarizer 19 and the second terahertz polarizer 20 can also adjust the polarization state of the terahertz wave within the range of 0~180°.

[0033] Low-density polyethylene film can also be replaced by organic polymer films such as polypropylene film and polyethylene terephthalate film, with a thickness between 5μm and 50μm, preferably 10μm.

[0034] The first polarizer 6 is used to adjust the polarization state of the probe light.

[0035] The focal length of the first lens 7 is between 50 mm and 150 mm, and it is used to focus the probe light onto the organic polymer film 12.

[0036] The dual-wavelength waveplate 9 is mainly used to synchronously adjust the polarization state of the 800 nm fundamental frequency probe light and the 400 nm controlled second harmonic reference light.

[0037] The second polarizer 10 is used to further adjust the polarization state of the probe light and the controlled second harmonic reference light to obtain optical field conditions suitable for coherent detection, so that the controlled second harmonic reference light and the terahertz-induced second harmonic signal generated in the organic polymer film 12 satisfy the coherent interference condition.

[0038] The bandpass filter 23 is a bandpass filter with a center wavelength of 400 nm, used to filter out the fundamental frequency probe light and background stray light.

[0039] The photomultiplier tube 24 is mainly used to collect the second harmonic signal after it has been filtered by the bandpass filter 23 and output the corresponding electrical signal.

[0040] Based on the above parameters and device selection, this system can achieve coherent detection of terahertz wave signals in the range of 0.1 THz to 30 THz.

[0041] In this embodiment, a method for coherently detecting terahertz waves based on organic polymers is also proposed, which mainly includes the following steps: S1. A femtosecond laser pulse is provided by femtosecond laser 1; S2. The femtosecond laser pulse is split into pump light and probe light by beam splitter 2; S3. Controlled second harmonic reference light is generated by incidenting probe light on BBO crystal 8; terahertz wave is generated by incident pump light on terahertz generating crystal. S4. The terahertz wave emitted from the pump light path is reflected and focused onto the organic polymer film 12 by the off-axis perforated parabolic mirror 11; the probe light emitted from the probe light path and the controlled second harmonic reference light are collinearly focused onto the organic polymer film 12 after passing through the central hole of the off-axis perforated parabolic mirror 11; so that a terahertz-induced second harmonic is generated under the combined action of the terahertz wave and the probe light, and the generated terahertz-induced second harmonic interferes coherently with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field; S5. The information processing module receives the detection signal related to the terahertz electric field formed by the coherent interference between the terahertz-induced second harmonic and the controlled second harmonic reference light, thereby obtaining the time-domain waveform of the terahertz wave to be measured and recovering the amplitude and phase information of the terahertz wave.

[0042] The following operations are also included in the use of this method: By adjusting the first terahertz polarizer 19, the second terahertz polarizer 20 and the half-wave plate 4, detection signals under different polarization conditions are obtained to characterize the polarization characteristics of the terahertz wave under test. By adjusting the time delay device 3, the detection signal under different delay times is obtained, thereby recovering the time domain waveform of the terahertz wave to be measured and its amplitude and phase information.

[0043] For example, in actual use, the terahertz wave and the 800 nm probe light are focused together on the tested organic polymer film 12. Under the action of a strong field, a 400 nm terahertz-induced second harmonic signal is generated in the tested sample through a four-wave mixing nonlinear process. At the same time, the BBO crystal 8 in the probe light path generates a controlled second harmonic reference light. The reference light and the terahertz-induced second harmonic signal undergo coherent interference, thereby obtaining a probe signal that is linearly related to the terahertz electric field.

[0044] In one embodiment of the present invention, such as Figure 2 As shown, this system obtains the time-domain spectral signal by coherently probing a terahertz wave with a peak electric field of approximately 0.6 MV / cm under the condition that the energy of the 800 nm probe light pulse is 4 μJ. It can be seen that under the above conditions, this system can obtain a clear terahertz time-domain waveform, indicating that the terahertz-induced second harmonic signal generated in the organic polymer film 12 can effectively coherently interfere with the controlled second harmonic reference light, thereby achieving effective recovery of the time-domain information of the terahertz electric field.

[0045] like Figure 3 As shown, the frequency domain spectrum obtained by coherent detection of this system under a terahertz peak electric field of approximately 0.6 MV / cm is presented. It can be seen that... Figure 2 The time-domain spectral signal shown can be obtained by Fourier transform to obtain the corresponding spectral response result, indicating that the system can not only detect terahertz time-domain waveforms, but also characterize their frequency-domain information, thus meeting the requirements of coherent detection of ultra-wideband terahertz waves.

[0046] like Figure 4 The figure shows the relationship between the time-domain peak signal detected by this system and the terahertz electric field. It can be seen that the detected signal exhibits an approximately linear relationship as the terahertz electric field increases, indicating a good correspondence between the signal measured by this system and the terahertz electric field intensity, which can be used for quantitative characterization of the terahertz electric field under test.

[0047] In summary, this invention is the first to apply organic polymer thin films to coherent terahertz wave detection in ultrawideband. By utilizing the coherent interference between the terahertz-induced second harmonic signal in the organic polymer thin film and the controlled second harmonic reference light, the synchronous recovery of the time-domain waveform, amplitude information, and phase information of the terahertz electric field is achieved. Compared with traditional photoconductive antenna sampling, electro-optic crystal sampling, and gas-based coherent detection schemes, this invention does not require an external bias voltage or complex micro / nano structures. It has advantages such as large detection bandwidth, high sensitivity, low requirement for detection light energy, simple system structure, low construction cost, high stability, and easy maintenance. It expands the selection range of solid-state detection media in the field of terahertz coherent detection and provides a new technical approach for the application of organic polymer materials in terahertz time-domain spectroscopy, ultrafast dynamics research, material characterization, and terahertz imaging, demonstrating strong scientific research value and practical application prospects.

[0048] It will be understood by those skilled in the art that the accompanying drawings are merely illustrative diagrams of embodiments of the present invention, and the structures, devices, and connection methods shown in the drawings do not constitute a limitation on the scope of protection of the present invention. Without affecting the technical effects of the present invention, the modules or devices in the illustrated device can be added, deleted, replaced, combined, split, or repositioned according to actual needs.

[0049] Those skilled in the art will also understand that the organic polymer thin film material, terahertz generating crystal, BBO crystal, polarizing device, mirror, lens, filter and detector in the embodiments of the present invention can be replaced with devices having the same or similar functions according to actual application requirements; the optical path structure, parameter range and implementation conditions can also be adjusted accordingly according to the actual measurement object and system requirements without affecting the realization of the technical solution of the present invention.

Claims

1. A system for coherently detecting terahertz waves based on organic polymers, characterized in that, include: Femtosecond lasers are used to provide femtosecond laser pulses; A beam splitter is used to split a femtosecond laser pulse into a pump beam and a probe beam; the pump beam enters the pump beam path and the probe beam enters the probe beam path. The pump light path is sequentially configured with an optical parametric amplifier, a second reflector, a terahertz wave generating crystal, a terahertz filter, a first off-axis parabolic reflector, a second off-axis parabolic reflector, a first terahertz polarizer, and a second terahertz polarizer; the terahertz wave emitted from the second terahertz polarizer is reflected and focused onto the organic polymer film by the off-axis perforated parabolic reflector; The probe light path is sequentially configured with a time delay device, a half-wave plate, a first reflector, a first polarizer, a first lens, a BBO crystal, a dual-wavelength wave plate, and a second polarizer. The probe light output from the second polarizer and the controlled second harmonic reference light are focused collinearly with the terahertz wave onto the organic polymer film after passing through the central hole of the off-axis perforated parabolic reflector. The controlled second harmonic reference light is generated by the BBO crystal. Organic polymer films are used to generate terahertz-induced second harmonics (THAs) under the combined action of terahertz waves and probe light; wherein the terahertz-induced second harmonics coherently interfere with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field. The information processing module is used to receive the detection signal, acquire the time-domain waveform of the terahertz wave to be measured, and recover the amplitude and phase information of the terahertz wave.

2. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, The femtosecond laser is a Ti:sapphire femtosecond laser amplifier with an output laser center wavelength of 800 nm and a pulse width of 20 fs to 100 fs.

3. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, The optical parametric amplifier is used to perform pump light wavelength conversion, and outputs pump light with a center wavelength of 1200nm~2600nm.

4. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, The terahertz wave generating crystal is one or more of the following: DSTMS crystal, DAST crystal, OH1 crystal, and BNA crystal, used to generate terahertz waves.

5. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, The first off-axis parabolic mirror and the second off-axis parabolic mirror are used to reflect and focus terahertz waves; The first and second terahertz polarizers are used to adjust the polarization state and field strength of the terahertz wave. A time delay device is used to adjust the relative timing between the probe light and the terahertz wave; A dual-wavelength waveplate is used to simultaneously modulate the polarization state of the fundamental frequency probe light and the controlled second harmonic reference light; The second polarizer is used to adjust the polarization relationship between the controlled second harmonic reference light and the terahertz-induced second harmonic signal in order to obtain interference components that are linearly related to the terahertz electric field.

6. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, Organic polymer films are organic polymer films with terahertz transmission characteristics and nonlinear optical response.

7. The system for coherent detection of terahertz waves based on organic polymers according to claim 6, characterized in that, The thickness of the organic polymer film is 5μm~50μm.

8. The system for coherent detection of terahertz waves based on organic polymers according to claim 1, characterized in that, The information processing module includes a second lens, a third polarizer, a bandpass filter, and a photomultiplier tube arranged sequentially in the optical path; the bandpass filter has a center wavelength of 400 nm and a bandwidth range of 380 nm to 420 nm, and is used to filter out the fundamental frequency probe light and background stray light; the photomultiplier tube is used for photoelectric conversion and signal acquisition.

9. A method for a system based on coherent detection of terahertz waves using organic polymers as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Femtosecond laser pulses are provided by a femtosecond laser; The femtosecond laser pulse is split into pump light and probe light using a beam splitter; Controlled second harmonic reference light is generated by incident probe light on a BBO crystal; terahertz waves are generated by incident pump light on a terahertz generating crystal. The terahertz wave emitted from the pump light path is reflected and focused onto the organic polymer film by an off-axis perforated parabolic mirror; the probe light emitted from the probe light path and the controlled second harmonic reference light are collinearly focused onto the organic polymer film after passing through the central hole of the off-axis perforated parabolic mirror; so that a terahertz-induced second harmonic is generated under the combined action of the terahertz wave and the probe light, and the generated terahertz-induced second harmonic coherently interferes with the controlled second harmonic reference light, thereby forming a probe signal related to the terahertz electric field; The information processing module receives the detection signal related to the terahertz electric field, which is formed by the coherent interference between the terahertz-induced second harmonic and the controlled second harmonic reference light. Then, the time-domain waveform of the terahertz wave to be measured is obtained and the amplitude and phase information of the terahertz wave are recovered.

10. The method according to claim 9, characterized in that, This also includes the following operations: By adjusting the first terahertz polarizer, the second terahertz polarizer, and the half-wave plate, detection signals under different polarization conditions are obtained, so as to characterize the polarization characteristics of the terahertz wave under test. By adjusting the time delay device, the detection signal under different delay times can be obtained, thereby recovering the time domain waveform of the terahertz wave to be measured and its amplitude and phase information.

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