High-repetition-frequency strong-field terahertz coupled near-field imaging system

Abstract

The invention relates to a high-repetition-frequency strong-field terahertz coupled near-field imaging system, which belongs to the technical field of terahertz time-domain spectroscopy, solves the problems of low terahertz source power and low monopulse energy in a near-field probe scanning system in the prior art, and comprises a femtosecond laser light source (1) for providing femtosecond pumping laser; the non-polarized beam splitting sheet (3) divides the pump laser provided by the femtosecond laser light source (1) into two beams, the two beams are respectively provided for a light path of the pump laser and a light path of the probe light, and one beam of pump laser is converted into terahertz radiation through the light path of the pump laser; and the sample scanning module comprises a terahertz near-field probe (20) and is used for receiving terahertz radiation to carry out terahertz scanning and imaging on the surface of the sample.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz time-domain spectroscopy, and in particular to a high-repetition-rate, strong-field terahertz-coupled near-field imaging system. Background Art

[0002] Terahertz waves are electromagnetic waves with a frequency range of 0.1-10 THz, lying between low-energy electronics and high-energy photonics. This unique frequency band endows them with numerous exceptional properties. Terahertz waves have strong penetrability for non-polar materials like paper and plastic, easily piercing through layers of packaging to reveal internal structures. This makes them crucial for express delivery security inspections and non-destructive testing of cultural relics. Terahertz waves are sensitive to polar molecules, particularly water molecules, and this can be leveraged to aid disease diagnosis in the biomedical field. Terahertz waves have significant application value in numerous fields, including materials science, biomedicine, food safety, non-destructive testing, and security inspections.

[0003] Terahertz imaging technology is one of the most promising development directions in terahertz science and technology. The performance parameters of imaging devices, including signal bandwidth, imaging resolution, signal-to-noise ratio, and imaging speed, directly determine the scope of their application. Terahertz imaging technology is mainly divided into two types: continuous wave imaging and pulse imaging. Continuous wave imaging is mainly based on intensity imaging, and has advantages such as high radiation power, fast imaging speed, and system portability. However, it suffers from the disadvantages of small data information and lack of phase information. Although the subsequent development of continuous wave coherent detection can obtain phase information, it faces the technical bottlenecks of narrow bandwidth and high noise. Terahertz pulse imaging obtains time-domain signals through coherent detection, and can simultaneously obtain the amplitude and phase information of the sample through Fourier transform. It has the significant characteristics of wide bandwidth and high signal-to-noise ratio. Although the imaging speed is slow, its rich information content provides important value for multispectral imaging research.

[0004] Within the terahertz pulse imaging technology landscape, different types of technologies exhibit distinct characteristics. Real-time terahertz two-dimensional imaging, based on the linear Pockels effect of electro-optical crystals, utilizes free-space electro-optical sampling technology to achieve detection imaging. While its advantage lies in its fast imaging speed, it suffers from limitations such as a lower signal-to-noise ratio than time-domain spectral imaging, a lack of phase information, and an inability to perform spectral analysis. Terahertz tomography (Terahertz-CT), similar in principle to X-CT, scans a sample with collimated terahertz waves and reconstructs the image using an inverse filtered projection algorithm. This method can capture a wealth of information for analyzing the sample's amplitude and phase, but its signal-to-noise ratio is highly dependent on the terahertz wave intensity and sample absorptivity, limiting the need for high-power radiation sources.

[0005] Terahertz time-domain spectral imaging technology is the earliest and most mature pulse imaging technology. This technology synchronously obtains amplitude and phase information through coherent detection and can be used for intensity analysis and spectrum analysis. However, it has problems such as spatial resolution being limited by the size of the terahertz focused spot (diffraction limit) and the complexity of the device.

[0006] An existing terahertz imaging method generates terahertz waves through a photoconductive antenna. It relies on the terahertz probe to be extremely close to the sample to directly detect near-field information to break the diffraction limit. However, it is limited by the low power of the terahertz source and the low energy of the single pulse, resulting in insufficient peak focused electric field intensity and penetration. The probe needs to be very close to the sample, which can easily damage the probe. There is a technical bottleneck in the generation of high-energy, high-repetition-rate terahertz radiation.

[0007] Existing lithium niobate terahertz radiation sources using tilted pulse front technology are limited by the damage threshold of lasers and lithium niobate crystals, making it difficult to generate terahertz radiation with a repetition rate above 1 kHz, resulting in low test speed and efficiency.

[0008] In addition, the excitation light conditions of existing near-field probes are less than 7μJ / cm2, and are currently only used in weak-field terahertz systems using megahertz-class lasers (for 80MHz excitation light, when the spot size is 40 microns, the excitation light power is about 3.5mW). For strong-field terahertz systems, in order to generate strong terahertz waves, low-frequency, high-single-pulse-energy lasers are usually selected, so that the energy of the separated detection excitation light single pulse is also much greater than that of the megahertz-class laser, thereby exceeding the excitation threshold of the near-field probe and causing damage to the near-field probe. Therefore, there is currently no terahertz strong source system suitable for near-field probes. In other words, the generation of strong-field terahertz requires a laser with large single-pulse energy, and a laser with large single-pulse energy will damage the probe when exciting it. This contradiction makes it difficult to achieve strong-field terahertz radiation and efficient terahertz near-field detection.

[0009] Therefore, there is a need in the art for an improved lithium niobate terahertz radiation near-field imaging system to provide strong-field terahertz radiation and efficient terahertz near-field detection. Summary of the Invention

[0010] In view of the above problems, the present invention provides a high-repetition-rate, strong-field terahertz-coupled near-field imaging system, which solves the problems of low terahertz source power and single pulse energy in existing near-field probe scanning systems.

[0011] The present invention provides a high-repetition-rate, high-field terahertz-coupled near-field imaging system. It can also function as a terahertz time-domain spectroscopy system. It can break the terahertz diffraction limit and achieve an imaging spatial resolution of 20 μm. It utilizes a high-power, high-repetition-rate lithium niobate, high-field terahertz radiation source, resulting in enhanced penetration. Furthermore, the terahertz power, single-pulse energy, repetition rate, and focused electric field strength can be adjusted on demand without modifying the system.

[0012] According to an embodiment of the present invention, a high-repetition-rate strong-field terahertz-coupled near-field imaging system is provided, comprising:

[0013] Femtosecond laser light source, providing femtosecond pump laser;

[0014] The non-polarizing beam splitter splits the pump laser provided by the femtosecond laser light source into two beams, providing them to the optical path of the pump laser and the optical path of the detection light respectively, and converts one beam of pump laser into terahertz radiation through the optical path of the pump laser;

[0015] A sample scanning module, including a terahertz near-field probe, is used to receive terahertz radiation to scan and image the sample surface;

[0016] The optical path of the pump laser includes the following arranged in sequence:

[0017] The laser spot beam expansion module receives a beam of pump laser provided after being split by the non-polarizing beam splitter to expand the laser spot;

[0018] The terahertz strong source generation module receives the pump laser after the laser spot is expanded, generates terahertz radiation, and provides it to the sample scanning module;

[0019] The process of converting another pump laser beam into a probe light in the optical path of the probe light includes the following steps:

[0020] an optical parametric amplifier, receiving another pump laser beam provided after being split by the non-polarizing beam splitter and converting it into a detection light;

[0021] A chopper suppresses the noise of the detection light to improve the signal-to-noise ratio of electro-optical sampling;

[0022] The mechanical delay module adjusts the optical path of the detection light and provides it to the sample scanning module synchronously with the terahertz radiation to excite the terahertz near-field probe.

[0023] Optionally, the femtosecond laser light source is an ytterbium-doped fiber amplified femtosecond laser, which is configured to provide a pump laser with a central wavelength of 1030 nm, a repetition frequency range of 30-50 kHz, and a maximum single pulse energy of 1 mJ.

[0024] Optionally, the non-polarizing beam splitter is a 95:5 non-polarizing beam splitter, which splits the pump laser emitted by the femtosecond laser light source into two beams of 95% power and 5% power, wherein the 95% power pump laser beam is guided into the optical path of the pump laser, and the 5% power pump laser beam is guided into the optical path of the detection light.

[0025] Optionally, the optical path of the pump laser also includes a pump laser optical path adjustment module arranged between the non-polarizing beam splitter and the laser spot beam expansion module, which is used to receive the split pump laser and guide it to the laser spot beam expansion module downstream of the optical path; the pump laser optical path adjustment module includes a first ultrafast reflector, a first zero-degree reflector and a second zero-degree reflector.

[0026] Optionally, the laser spot expansion module includes a beam expansion lens group and a precision translation stage; the beam expansion lens group expands the spot size of the incident pump laser to 1.5 times the original size, including a concave lens and a first convex lens arranged in sequence, the concave lens is a concave lens with a focal length of -50mm, the first convex lens is a double convex lens with a focal length of 75mm, and the distance between the concave lens and the first convex lens is set to 25mm; the precision translation stage is used to install the concave lens and the first convex lens to precisely adjust their positions.

[0027] Optionally, a terahertz strong source generation module outputs broadband terahertz radiation with a peak frequency of 0.6 THz and a spectrum coverage range of 0.1-2.5 THz, including a transmission diffraction grating, a gold mirror group, a 4-f standard imaging module, a lithium niobate crystal and an off-axis parabolic mirror group; the transmission diffraction grating receives a pump laser incident at an angle of 31°, and focuses the incident pump laser power on the -1 order diffraction light; the gold mirror group includes a first gold mirror and a second gold mirror, which converts the pump laser into a horizontal direction and guides it to the 4-f standard optical imaging module; the 4-f standard optical imaging module includes a second convex lens and a third convex lens, the focal length ratio of the second convex lens and the third convex lens is 3.7:1, and both are double convex lenses coated with a 1030 nm anti-reflection film; the incident surfaces of the transmission diffraction grating and the lithium niobate crystal are located at the object plane and image plane of the 4-f standard optical imaging module, respectively.

[0028] Optionally, the sample scanning module further includes a three-dimensional movable sample holder (21) and a CCD camera microscope; the three-dimensional movable sample holder is used to hold the sample to be detected, to perform three-dimensional adjustment on the position and distance of the sample to be detected, and to align the sample to be detected with the terahertz radiation emitted by the terahertz strong source generation module; the terahertz near-field probe is an 800nm ​​terahertz probe, which is arranged at the three-dimensional movable sample holder to be close to the surface of the sample to be detected on the three-dimensional movable sample holder to implement near-field detection; the CCD camera microscope is arranged at the three-dimensional movable sample holder and the terahertz near-field probe, and is used to accurately amplify and display the positional relationship between the sample to be detected on the three-dimensional movable sample holder and the terahertz near-field probe.

[0029] Optionally, the optical parametric amplifier converts a received pump laser into probe light with a center wavelength of 800 nm and a pulse width of 70 fs; the chopper 30 operates at a frequency of 500 Hz and is phase-locked with a trigger signal of the ytterbium-doped fiber amplified femtosecond laser.

[0030] Optionally, the mechanical delay module comprises a sixth ultrafast mirror, a seventh ultrafast mirror, a motorized displacement stage, and a stepper motor for controlling the motorized displacement stage; the sixth ultrafast mirror and the seventh ultrafast mirror are placed at 90° to each other on the motorized displacement stage; the stepper motor controls the simultaneous horizontal movement of the sixth ultrafast mirror and the seventh ultrafast mirror by controlling the movement of the motorized displacement stage, so as to change the optical path of the probe light.

[0031] Optionally, between the mechanical delay module and the terahertz near-field probe, a fifth ultrafast mirror, a fourth ultrafast mirror, a fourth convex lens, and a third ultrafast mirror are sequentially arranged for guiding the probe light emitted from the mechanical delay module to irradiate the excitation position of the terahertz near-field probe; the fourth convex lens is used for focusing the probe light, the focal length of the fourth convex lens is 200 mm, and the optical path from the fourth convex lens to the terahertz near-field probe (20) is 100 mm.

[0032] Compared with the prior art, the high-repetition-rate strong-field terahertz coupling near-field imaging system provided by the embodiment of the present application has at least the following beneficial effects:

[0033] (1) The present application provides improved spectral test effect, and the built terahertz radiation source has the advantages of high average power output, high repetition frequency, large single pulse energy, and stability and reliability. Compared with the traditional method of photoconductive antenna terahertz source, it can provide deeper penetration depth and greatly improve the signal-to-noise ratio of terahertz time-domain spectroscopy.

[0034] (2) The present application generates high-intensity terahertz electromagnetic waves, which can reach 500kV / cm. It can better excite the interaction of strong terahertz electric field and matter, and can perform special design of post-interaction imaging. The test function is more powerful, and the sample range is wider. With deeper penetration depth, the probe can be slightly away from the sample to read the same information, reducing the damage to the probe and the sample.

[0035] (3) By setting the laser spot expansion module and adjusting the parameters of the laser, the energy of the single pulse is reduced, the terahertz radiation with a repetition frequency of more than 1 kHz is generated, the lithium niobate crystal is not damaged, and the test speed and test efficiency are effectively improved.

[0036] (4) By setting the detection lens focal length and attenuation, the detection light spot size of the excitation near-field probe is controlled and the energy of the single pulse is reduced, so that the near-field probe can work normally under the excitation of kilohertz-level laser.

[0037] (5) The present application adopts lithium niobate block crystal as core material, the physical mechanism of tilt wave front method is clear, and the stability and repeatability of terahertz radiation generation process are ensured. The terahertz wave generated by the tilt pulse of lithium niobate has a large increase in single pulse intensity (electric field) and power.

[0038] (6) The present application has low cost. Industrial-grade laser is used instead of traditional titanium-sapphire laser amplifier as pump source, which not only greatly reduces the procurement cost, but also significantly improves the system stability and environmental adaptability, reduces the demand for auxiliary facilities such as temperature control and shockproof, and reduces the whole life cycle maintenance cost by more than 70%.

[0039] (7) The present application has high system integration. The system occupies small area, and far-field and near-field signals can be obtained by controlling the movement of the sample displacement table; the system can be modularized and packaged as a whole to ensure system reliability and scalability, reduce daily maintenance workload by 80%, and shorten fault troubleshooting time to hours. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. The features and advantages of the present application can be more clearly understood by referring to the drawings. The drawings are schematic and should not be understood as any limitation on the present application. Those skilled in the art can obtain other drawings without creative labor on the basis of these drawings.

[0041] Figure 1 A schematic diagram of a high-repetition-rate strong-field terahertz coupled near-field imaging system according to an embodiment of the present application.

[0042] Figure 2 A terahertz spot diagram at a sample of a strong-field terahertz generated by an example of a high-repetition-rate strong-field terahertz coupled near-field imaging system according to an embodiment of the present application.

[0043] Figure 3 A terahertz time-domain spectrum waveform diagram when there is no sample in an embodiment of a high-repetition-rate strong-field terahertz coupled near-field imaging system according to an embodiment of the present application.

[0044] Reference signs:

[0045] 1 - femtosecond laser light source;

[0046] 2 - optical parametric amplifier;

[0047] 3- non-polarizing beam splitter;

[0048] 4-First ultrafast mirror;

[0049] 5-first zero-degree reflector;

[0050] 6- second zero-degree reflector;

[0051] 7-concave lens;

[0052] 8-first convex lens;

[0053] 9-second ultrafast mirror;

[0054] 10-transmission diffraction grating;

[0055] 11-First gold reflector;

[0056] 12- second gold reflector;

[0057] 13- second convex lens;

[0058] 14- third convex lens;

[0059] 15-lithium niobate crystal;

[0060] 16-first off-axis parabolic reflector;

[0061] 17-ITO flat glass;

[0062] 18-second off-axis parabolic reflector;

[0063] 19-CCD camera microscope;

[0064] 20-THz near-field probe;

[0065] 21-3D mobile sample holder;

[0066] 22-third ultrafast mirror;

[0067] 23- fourth convex lens;

[0068] 24-the fourth ultrafast mirror;

[0069] 25-fifth ultrafast mirror;

[0070] 26-Mechanical delay module;

[0071] 27-sixth ultrafast mirror;

[0072] 28-7th ultrafast mirror;

[0073] 29-8th ultrafast mirror;

[0074] 30-chopper;

[0075] 31 - ninth ultrafast mirror;

[0076] 32 - tenth ultrafast mirror. DETAILED DESCRIPTION

[0077] In order to enable a more complete understanding of the above-mentioned objects, features and advantages of the present application, the application will be described in further detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict, if possible.

[0078] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, and therefore the scope of protection of the present application is not limited by the specific embodiments disclosed below. The first, second, etc. mentioned in the following description are not a limitation of the importance and order of the features, but only for the purpose of distinguishing the parts.

[0079] A high-repetition strong-field terahertz coupling near-field imaging system provided by an embodiment of the present application is described in detail below with reference to the accompanying drawings.

[0080] A high-repetition strong-field terahertz coupling near-field imaging system provided by a first embodiment of the present application is described below. The high-repetition strong-field terahertz coupling near-field imaging system provided by the first embodiment divides the pump laser output by the femtosecond laser source 1 into two paths, respectively for generating terahertz radiation and probe light, and provides them to the sample scanning module via the pump laser light path and the probe light path respectively, for comprehensive and complete scanning of the sample to be detected.

[0081] The high-repetition strong-field terahertz coupling near-field imaging system provided by the first embodiment of the present application includes: a femtosecond laser source 1 providing femtosecond pump laser; a non-polarizing beam splitter 3 dividing the femtosecond pump laser provided by the femtosecond laser source 1 into pump laser and probe light, and providing them to the pump laser light path and the probe light path respectively; and further including: a laser spot beam expander module arranged in sequence in the pump laser light path, receiving the pump laser provided after being split by the non-polarizing beam splitter 3 for laser spot beam expansion; a terahertz strong source generation module receiving the pump laser after laser spot beam expansion and generating terahertz radiation; a sample scanning module receiving the terahertz radiation for scanning the sample; an optical parametric amplifier 2 arranged in sequence in the probe light path, receiving the pump laser provided after being split by the non-polarizing beam splitter 3 and converting it into probe light; a chopper 30 suppressing the noise of the probe light to improve the signal-to-noise ratio of electro-optical sampling; a mechanical delay module 26 changing the optical path of the probe light and providing it to the sample scanning module synchronously with the terahertz radiation.

[0082] The femtosecond laser light source 1 can be an ytterbium-doped fiber-amplified femtosecond laser. The ytterbium-doped fiber-amplified femtosecond laser can provide a pump laser with a central wavelength of 1030 nm, an adjustable repetition rate of 1-50 kHz, a maximum single-pulse energy of 1 mJ, and a pulse width of 570 fs. Parameters such as the single-pulse energy, repetition rate, and pulse width of the ytterbium-doped fiber-amplified femtosecond laser can be adjusted to regulate the single-pulse energy, repetition rate, and average power of the generated terahertz radiation. The single-pulse energy of the ytterbium-doped fiber-amplified femtosecond laser can be set within a range of 1-100%, with a maximum single-pulse energy of 1 mJ; the repetition rate range is 30-50 kHz, which prevents excessively low laser repetition rates from damaging the terahertz near-field probe 20.

[0083] The non-polarizing beam splitter 3 can be a 95:5 non-polarizing beam splitter, which is used to split the pump laser emitted by the femtosecond laser light source 1 into two beams of 95% power and 5% power, wherein the 95% power pump laser beam is directed into the optical path of the pump laser, and the 5% power pump laser beam is directed into the optical path of the probe light. After the pump laser is split by the non-polarizing beam splitter 3, it is guided along the optical path of the pump laser to the laser spot beam expansion module, the terahertz strong source generation module, etc., for generating terahertz radiation and providing it to the sample scanning module; the probe light after the non-polarizing beam splitter 3 is guided along the optical path of the probe light to the optical parametric amplifier 2, the chopper 30, the mechanical delay module, etc., and finally provided to the sample scanning module for exciting the terahertz near-field probe 20.

[0084] The following describes the structure provided in the optical path of the pump laser.

[0085] The high repetition rate strong field terahertz coupled near-field imaging system provided in the first embodiment may also include a pump laser optical path adjustment module provided between the non-polarizing beam splitter 3 and the laser spot beam expansion module, which is used to receive the pump laser after beam splitting and guide it to the laser spot beam expansion module downstream of the optical path, thereby increasing the optical path in a smaller space to match the optical path of the detection light. The pump laser optical path adjustment module may include a first ultrafast reflector 4, a first zero-degree reflector 5, and a second zero-degree reflector 6. Optionally, the first ultrafast reflector 4 can be an ultrafast reflector with an operating wavelength of 1030 nm; the first zero-degree reflector 5 and the second zero-degree reflector 6 are used to extend the optical path of the pump laser after beam splitting to match the optical path of the detection light.

[0086] The laser spot beam expansion module may include a beam expansion lens group and a precision translation stage.

[0087] The beam expanding lens group can include a concave lens and a convex lens arranged in sequence and at a specific interval. Specifically, the beam expanding lens group can include a concave lens 7 and a first convex lens 8 arranged in sequence to expand the spot size of the pump laser. Optionally, the concave lens 7 can use a concave lens with a focal length of -50 mm, the first convex lens 8 can use a biconvex lens with a focal length of 75 mm, and the distance between the concave lens 7 and the first convex lens 8 can also be set to 25 mm. The set beam expanding lens group can expand the incident pump laser spot size to about 1.5 times the original size. This design can reduce the power density of the pump laser below the damage threshold of the lithium niobate crystal 15 without affecting the efficiency of terahertz radiation generation, so that the lithium niobate crystal 15 can support femtosecond laser pumping with an average power of up to 50 W, and the pump laser does not generate a focal point in the air, avoiding air ionization.

[0088] The precision displacement stage of the laser spot beam expanding module is used to install the concave lens 7 and the first convex lens 8 and precisely adjust their positions as needed, so that the expansion multiple of the pump laser spot can be adjusted according to actual conditions.

[0089] The high-repetition strong-field terahertz coupling near-field imaging system provided by the first embodiment can further include a second ultrafast mirror 9 arranged between the laser spot beam expanding module and the terahertz strong source generation module, for turning the pump laser emitted from the laser spot beam expanding module and providing it to the subsequent terahertz strong source generation module. The second ultrafast mirror 9 can be an ultrafast mirror with a working wavelength of 1030 nm.

[0090] The terahertz strong source generation module can include a transmissive diffraction grating 10, a gold mirror group, a 4-f standard imaging module, a lithium niobate crystal 15, and an off-axis parabolic mirror group. This terahertz strong source module can output broadband terahertz radiation with a peak frequency of about 0.6 THz and a spectral coverage range of 0.1-2.5 THz.

[0091] The position and angle of the second ultrafast mirror 9 in front are adjusted, and the expanded pump laser is incident to the transmissive diffraction grating 10 at an angle of 31°. The transmissive diffraction grating 10 can converge the incident pump laser power to the -1 order diffracted light, with a diffraction efficiency of more than 90%, significantly improving the power utilization rate of the pump laser.

[0092] The gold mirror group can include a first gold mirror 11 and a second gold mirror 12, for converting the pump laser into a horizontal direction and guiding it to the 4-f standard optical imaging module.

[0093] The 4-f standard optical imaging module can be composed of two double convex lenses with a specific focal length, and its imaging ratio matches the line number parameter of the transmission diffraction grating to achieve efficient optical imaging. Specifically, the 4-f standard optical imaging module may include a second convex lens 13 and a third convex lens 14. The second convex lens 13 and the third convex lens 14 can be double convex lenses coated with a 1030nm anti-reflection film, combined with a focal length of 3.7:1, and its imaging ratio matches the line number parameter of the transmission diffraction grating to achieve efficient optical imaging. Optionally, the focal length of the second convex lens 13 is 370mm, the focal length of the third convex lens 14 is 100mm, and the distance between the second convex lens 13 and the third convex lens 14 is set to 470mm. The above-mentioned 4-f standard optical imaging module can improve the efficiency of terahertz radiation generation.

[0094] The lithium niobate crystal 15 is a block-shaped lithium niobate crystal with a triangular prism structure and a base angle of 63°, which is used to adapt to the tilted pulse front angle and thus improve the terahertz generation efficiency. The incident surface of the lithium niobate crystal 15 can be coated with a 1030nm anti-reflection film to further improve the utilization rate of the pump laser and increase the terahertz output power. The pump laser incident on the incident surface of the lithium niobate crystal 15 at a specific angle interacts with the lithium niobate crystal 15, generating a terahertz wave through the optical rectification mechanism, which is emitted from the exit surface of the lithium niobate crystal 15. By setting a 4-f standard optical imaging module, the pump laser can be incident on the incident surface of the lithium niobate crystal 15 at a 90-degree angle.

[0095] The incident surfaces of the transmission diffraction grating 10 and the lithium niobate crystal 15 are respectively located at the object plane and the image plane of the 4-f standard optical imaging module.

[0096] The off-axis parabolic reflector assembly collimates and focuses the terahertz radiation emitted from the lithium niobate crystal 15. The off-axis parabolic reflector assembly may include a first off-axis parabolic reflector 16, an ITO flat glass 17, and a second off-axis parabolic reflector 18. This collimates and focuses the generated terahertz radiation, minimizing terahertz loss and dispersion compared to using a terahertz lens. Specifically, the first off-axis parabolic reflector 16 receives and collects the terahertz radiation, shaping it into a parallel beam that is emitted to the ITO flat glass 17. After reflection from the ITO flat glass 17, the beam is incident on the second off-axis parabolic reflector 18, which focuses it onto the sample to be tested. The first off-axis parabolic reflector 16 and the second off-axis parabolic reflector 18 can be gold-plated off-axis parabolic reflectors, and can both have the same parameters of an aperture diameter of 2 inches and a focal length of 4 inches. While fully collecting terahertz radiation, the system complexity is reduced through standardized components, and the change of terahertz polarization caused by the gold-plated off-axis parabolic reflector group is avoided. Compared with the use of terahertz lenses, terahertz loss and dispersion can be minimized.

[0097] As described above, the transmission diffraction grating 10, the first gold mirror 11, the second gold mirror 12, the second convex lens 13, the third convex lens 14, the lithium niobate crystal 15, the first off-axis parabolic mirror 16, the ITO flat glass 17, and the second off-axis parabolic mirror 18 together constitute the terahertz strong source generation module.

[0098] The terahertz strong source generation module can further include a manual multi-dimensional displacement table, and each component in the terahertz strong source generation module can be installed on the manual multi-dimensional displacement table, and the position and angle of each component can be adjusted through the manual multi-dimensional displacement table.

[0099] The sample scanning module is arranged downstream of the off-axis parabolic mirror group, and can include a three-dimensional moving sample holder 21, a terahertz near-field probe 20, and a CCD camera microscope 19, for providing a terahertz scanning of a sample surface with adjustable position and distance.

[0100] The three-dimensional moving sample holder 21 is used to hold a sample to be detected, and can adjust the position and distance of the sample in three dimensions, so as to align the sample with the terahertz radiation emitted by the terahertz strong source generation module. The three-dimensional moving sample holder 21 can be electrically controlled. The three-dimensional moving sample holder 21 is arranged in the optical path downstream of the terahertz strong source generation module, and is used to hold a sample to be detected, and the three-dimensional moving sample holder 21 can be precisely controlled to adjust the position and angle of the sample to be detected thereon, so as to perform a complete and comprehensive scanning.

[0101] The terahertz near-field probe 20 can use an 800 nm terahertz probe. The terahertz near-field probe 20 can be arranged close to the three-dimensional moving sample holder 21, so as to implement near-field detection close to the surface of the sample to be detected on the three-dimensional moving sample holder 21, break through the diffraction limit constraint, and greatly improve the spatial resolution of the device to a sub-wavelength level. Optionally, the terahertz near-field probe 20 can be arranged at a distance of about 50 microns from the sample surface, so as to better perform detection. According to the probe technical parameters, the spatial resolution of the 800 nm terahertz probe can reach 20 microns. The 800 nm terahertz probe has two types of measuring transverse electric field and longitudinal electric field, and by replacing the terahertz probe of different types, the imaging function of the electric field in different directions can be realized. For example, the terahertz probe for measuring the transverse electric field can be used to scan the terahertz light field distribution, and the terahertz probe for measuring the longitudinal electric field can be used to scan the surface wave field distribution of the sample.

[0102] The CCD camera microscope 19 is a high-power microscope, which can be a microscope with a long working distance and a high magnification and with a CCD camera recording, so that the distance of the terahertz near-field probe 20 to the sample on the three-dimensional moving sample holder 21 can be clearly displayed and controlled through the CCD camera microscope 19. The CCD camera microscope 19 is arranged close to the three-dimensional moving sample holder 21 and the terahertz near-field probe 20, for accurately magnifying and displaying the positional relationship between the sample on the three-dimensional moving sample holder 21 and the terahertz near-field probe 20. The three-dimensional moving sample holder 21 and the terahertz near-field probe 20 can be adjusted based on the display of the CCD camera microscope 19, so as to control the relative position and distance between the sample and the terahertz near-field probe 20, and achieve a better detection effect.

[0103] The following references are cited Figure 1 The module arranged in the light path of the probe light is described.

[0104] In the pump laser output by the ytterbium-doped fiber amplification femtosecond laser, 5% of the power of the pump laser obtained by the non-polarizing beam splitter 3 is guided into the optical parametric amplifier 2 to obtain probe light with a center wavelength of 800 nm and a pulse width of about 70 fs. The pulse width is controlled by the optical parametric amplifier 2.

[0105] Optionally, a tenth ultrafast mirror 32 and a ninth ultrafast mirror 31 can also be arranged between the non-polarizing beam splitter 3 and the optical parametric amplifier 2, for guiding part of the pump laser split by the non-polarizing beam splitter 3 to the optical parametric amplifier 2.

[0106] In the light path of the probe light, the probe light output by the optical parametric amplifier 2 receiving part of the pump laser split by the non-polarizing beam splitter 3 passes through the chopper 30, the mechanical delay module 26, the fifth ultrafast mirror 25, the fourth ultrafast mirror 24, the fourth convex lens 23, and the third ultrafast mirror 22, and is provided to the terahertz near-field probe 20 of the sample scanning module.

[0107] The chopper 30 can be set to work at a frequency of 500 Hz and be phase-locked with the trigger signal of the laser, for suppressing noise and thereby improving the signal-to-noise ratio of electro-optic sampling.

[0108] Optionally, an eighth ultrafast mirror 29 can also be arranged between the chopper 30 and the mechanical delay module 26, for turning and providing the probe light passing through the chopper 30 to the mechanical delay module 26.

[0109] The mechanical delay module 26 can include a sixth ultrafast mirror 27, a seventh ultrafast mirror 28, an electrically driven displacement stage and a stepper motor. The mechanical delay module 26 can be an electrically driven delay line or a one-dimensional mechanical optical delay line. The sixth ultrafast mirror 27 and the seventh ultrafast mirror 28 are placed at 90° on the electrically driven displacement stage, and the movement of the electrically driven displacement stage controlled by the stepper motor can control the simultaneous horizontal movement of the two mirrors to change the optical path. The waveform of the terahertz wave radiation is measured by fine changes in the optical path to test the terahertz signal.

[0110] The mechanical delay module 26 can further include a control program.

[0111] The fifth ultrafast mirror 25, the fourth ultrafast mirror 24, the fourth convex lens 23 and the third ultrafast mirror 22 guide the probe light emitted from the mechanical delay module 26 to irradiate the excitation position of the terahertz near-field probe 20.

[0112] The fourth convex lens 23 is used to focus the probe light to the excitation position of the terahertz near-field probe 20. By setting the focal length of the fourth convex lens 23, the size of the light spot focused to the excitation position of the terahertz near-field probe 20 can be controlled, so as to control the average flux of the probe light hitting the probe. Optionally, the focal length of the fourth convex lens 23 is 200 mm, and the optical path of the fourth convex lens 23 to the terahertz near-field probe 20 is controlled to be 100 mm. In addition, an attenuation sheet can be added between the fourth convex lens 23 and the third ultrafast mirror 22 according to the actual probe light flux.

[0113] In the first embodiment, the third ultrafast mirror 22, the fourth ultrafast mirror 24, the fifth ultrafast mirror 25, the sixth ultrafast mirror 27, the seventh ultrafast mirror 28, the eighth ultrafast mirror 29, the ninth ultrafast mirror 31 and the tenth ultrafast mirror 32 can be ultrafast mirrors with a working wavelength of 800 nm, which are used to convert and guide the optical path of the probe light split by the non-polarizing beam splitter 3.

[0114] Specifically, as Figure 1As shown, according to the first embodiment of the present invention, a high repetition rate strong field terahertz coupled near-field imaging system is provided, comprising: a femtosecond laser light source 1, an optical parametric amplifier 2, a non-polarizing beam splitter 3, a first ultrafast reflector 4, a first zero-degree reflector 5, a second zero-degree reflector 6, a concave lens 7, a first convex lens 8, a second ultrafast reflector 9, a transmission diffraction grating 10, a first gold reflector 11, a second gold reflector 12, a second convex lens 13, a third convex lens 14, a lithium niobate crystal 15, a first off-center reflector 16, a first convex lens 17, a second convex lens 18, a second convex lens 19, a first convex lens 20, a first convex lens 21, a second convex lens 22, a second convex lens 23, a third convex lens 24, a first convex lens 25, a first convex lens 26, a first convex lens 27, a first convex lens 28, a second convex lens 29, a first convex lens 30, a first convex lens 31, a first convex lens 32, a first convex lens 33, a second convex lens 34, a first convex lens 35, a first convex lens 36, a first convex lens 37, a first convex lens 38, a first convex lens 39, a first convex lens 40, a first convex lens 41, a first convex lens 42, a first convex lens 43, a first convex lens 44, a first convex lens 45, a first convex lens 46, a first convex lens 47, a first convex lens The invention relates to a mechanical delay module 26 consisting of an off-axis parabolic mirror 16, an ITO flat glass 17, a second off-axis parabolic mirror 18, a CCD camera microscope 19, a terahertz near-field probe 20, a three-dimensional moving sample holder 21, a third ultrafast mirror 22, a fourth convex lens 23, a fourth ultrafast mirror 24, a fifth ultrafast mirror 25, a sixth ultrafast mirror 27 and a seventh ultrafast mirror 28, an eighth ultrafast mirror 29, a chopper 30, a ninth ultrafast mirror 31, and a tenth ultrafast mirror 32.

[0115] The high-repetition-rate, high-field terahertz-coupled near-field imaging system of the first embodiment operates as follows. The femtosecond pump laser light emitted by femtosecond laser source 1 passes through a 95:5 non-polarizing beam splitter 3 and is split into pump laser light and probe light. The transmission paths of the pump laser and probe light are as follows.

[0116] The optical path of the pump laser is as follows: the part of the pump laser split by the non-polarizing beam splitter 3 passes through the ultrafast reflector 4 and then passes through the first zero-degree reflector 5 and the second zero-degree reflector 6 to extend the optical path to match the optical path of the detection light; then passes through the laser spot expansion module composed of the concave lens 7 and the first convex lens 8, and its spot size is expanded to about 1.5 times the initial size; then the expanded pump laser is guided by the ultrafast reflector 9 and enters the transmission diffraction grating 10, and the pump laser power with a proportion greater than 90% is emitted from the -1 level, and after being reflected by the first gold reflector 11 and the second gold reflector 12, it passes through the second convex lens 13 and the third convex lens 1 in the horizontal direction. The 4-f standard optical imaging module, consisting of a laser beam and a laser diode, is then incident at a specific angle on the incident surface of a lithium niobate crystal 15. The pump laser interacts with the lithium niobate crystal 15, generating terahertz waves through an optical rectification mechanism. These waves are then transmitted from the exit surface of the lithium niobate crystal 15. The radiated terahertz waves are collected by a first off-axis parabolic reflector 16 and shaped into a parallel beam that is then emitted to an ITO flat glass 17. After being reflected by the ITO flat glass 17 and then reflected by a second off-axis parabolic reflector 18, they are focused by the second off-axis parabolic reflector 18 onto the three-dimensional movable sample holder 21 of the sample scanning module, specifically onto the sample to be inspected on the three-dimensional movable sample holder 21. The single pulse energy, repetition rate, and average power of the generated terahertz radiation can all be adjusted by adjusting parameters such as the single pulse energy and repetition rate of the femtosecond laser (pump source).

[0117] The optical path of the probe light is as follows: after being split by the non-polarizing beam splitter 3, the portion of the pump laser light is guided by the ninth and tenth ultrafast mirrors 31 and 32 into the accompanying optical parametric amplifier 2, generating a probe light with a central wavelength of 800 nm and a pulse width of 70 fs. The probe light then passes through the chopper 30 and is phase-locked with the laser trigger signal (i.e., the frequency of the femtosecond laser source 1) to suppress noise and thereby improve the signal-to-noise ratio of the electro-optical sampling. After passing through the chopper 30, the probe light is further reflected by the eighth ultrafast mirror 29 and guided to the mechanical delay module 26, which consists of the sixth and seventh ultrafast mirrors 27 and 28, and a stepper motor. The probe light then passes through the fifth and fourth ultrafast mirrors 25 and 24, is focused by the fourth convex lens 23, and is then guided by the third ultrafast mirror 22 to the excitation point of the terahertz near-field probe 20. The probe light is coherent with the pump laser and is used to excite the terahertz near-field probe 20.

[0118] The sample scanning module simultaneously receives terahertz radiation and probe light, and utilizes a three-dimensional movable sample holder 21 and a long-working-distance CCD camera microscope 19 to accurately control the distance between the sample and the terahertz near-field probe 20. The terahertz near-field probe 20 converts the received signal from the sample's terahertz radiation scanning into an output current signal, which is then converted into a voltage signal via a current amplifier. The voltage signal output from the current amplifier can be connected to the input port of a lock-in amplifier, and the output port of the lock-in amplifier is connected to a data acquisition card, which is then connected to a computer. The mechanical delay module 26 and the motorized three-dimensional movable sample holder 21 can also be connected to the computer, enabling automated control of terahertz scanning imaging.

[0119] The current amplifier can be amplified by a factor of 10 8 V / A current amplifier.

[0120] The high-repetition-rate, strong-field terahertz-coupled near-field imaging system of the first embodiment, in addition to the above-mentioned terahertz scanning imaging of samples, can also obtain the amplitude and phase information of the terahertz pulse through the sample scanning module, and obtain parameters such as the absorption coefficient and refractive index of the sample by analyzing the time domain and frequency domain waveforms, which can be used for terahertz spectral analysis of the sample.

[0121] Figure 2 The terahertz spot pattern of the strong field terahertz at the sample is generated by an example of the high repetition rate strong field terahertz coupled near-field imaging system provided by the first embodiment of the present invention. Figure 3 This is a waveform diagram of the terahertz time-domain spectrum when there is no sample in an embodiment of the high-repetition-rate strong-field terahertz-coupled near-field imaging system provided by the first embodiment of the present invention.

[0122] like Figure 2 and Figure 3As shown, the terahertz light spot generated by this embodiment is a regular circle, the intensity conforms to the Gaussian distribution, and the focus diameter is about 1 mm. Figure 3 This is the terahertz time domain waveform measured at the sample (but without the sample placed) at a laser frequency of 50 kHz. The peak electric field intensity is calculated based on this waveform, which proves that the terahertz radiation is a strong-field terahertz source.

[0123] Optionally, the high repetition rate, strong field terahertz-coupled near-field imaging system provided in the above embodiment can be packaged in an overall modular manner to achieve a highly integrated system with a smaller volume.

[0124] All of the above optional technical solutions can be combined in any way to form optional embodiments of the present application, and will not be described in detail here.

[0125] It should be understood that the size of the serial numbers of the steps in the above embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0126] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A high repetition rate strong field terahertz coupled near-field imaging system, characterized in that: include: A femtosecond laser light source (1) provides femtosecond pump laser; A non-polarizing beam splitter (3) is used to split the pump laser provided by the femtosecond laser light source (1) into two beams, which are provided to the optical path of the pump laser and the optical path of the detection light respectively, and convert one beam of the pump laser into terahertz radiation through the optical path of the pump laser; A sample scanning module, comprising a terahertz near-field probe (20), configured to receive terahertz radiation to scan and image the sample surface; The optical path of the pump laser includes the following arranged in sequence: A laser spot beam expansion module receives a beam of pump laser provided after being split by the non-polarizing beam splitter (3) to perform laser spot beam expansion; The terahertz strong source generation module receives the pump laser after the laser spot is expanded, generates terahertz radiation, and provides it to the sample scanning module; The process of converting another pump laser beam into a probe light in the optical path of the probe light includes the following steps: An optical parametric amplifier (2) receives another beam of pump laser light provided after being split by the non-polarizing beam splitter (3) and converts the beam into a detection light; a chopper (30) for suppressing noise of the detection light to improve the signal-to-noise ratio of the electro-optical sampling; The mechanical delay module (26) adjusts the optical path of the detection light and provides it to the sample scanning module synchronously with the terahertz radiation, so as to excite the terahertz near-field probe (20).

2. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The femtosecond laser light source (1) is an ytterbium-doped fiber amplified femtosecond laser, which is configured to provide a pump laser with a central wavelength of 1030nm, a repetition frequency range of 30-50kHz, and a maximum single pulse energy of 1mJ.

3. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The non-polarizing beam splitter (3) is a 95:5 non-polarizing beam splitter, which splits the pump laser emitted by the femtosecond laser light source (1) into two beams of 95% power and 5% power, wherein the 95% power pump laser beam is guided to the optical path of the pump laser, and the 5% power pump laser beam is guided to the optical path of the detection light.

4. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The optical path of the pump laser also includes a pump laser optical path adjustment module arranged between the non-polarizing beam splitter (3) and the laser spot beam expansion module, which is used to receive the split pump laser and guide it to the laser spot beam expansion module downstream of the optical path; The pump laser optical path adjustment module comprises a first ultrafast reflector (4), a first zero-degree reflector (5) and a second zero-degree reflector (6).

5. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The laser spot beam expansion module includes a beam expansion lens group and a precision translation stage; The beam expansion lens group expands the spot size of the incident pump laser to 1.5 times the original size, and includes a concave lens (7) and a first convex lens (8) arranged in sequence, the concave lens (7) being a concave lens with a focal length of -50 mm, the first convex lens (8) being a double convex lens with a focal length of 75 mm, and the distance between the concave lens (7) and the first convex lens (8) being 25 mm; The precision displacement stage is used to install the concave lens (7) and the first convex lens (8) so as to precisely adjust their positions.

6. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The terahertz strong source generation module outputs broadband terahertz radiation with a peak frequency of 0.6 THz and a spectrum coverage range of 0.1-2.5 THz, and includes a transmission diffraction grating (10), a gold reflector group, a 4-f standard imaging module, a lithium niobate crystal (15) and an off-axis parabolic reflector group; The transmission diffraction grating (10) receives the pump laser incident at an angle of 31 degrees and focuses the incident pump laser power into the -1st order diffraction light; The gold reflector group includes a first gold reflector (11) and a second gold reflector (12), which converts the pump laser into a horizontal direction and guides it to a 4-f standard optical imaging module; The 4-f standard optical imaging module includes a second convex lens (13) and a third convex lens (14), wherein the focal length ratio of the second convex lens (13) and the third convex lens (14) is 3.7:1, and both are double convex lenses coated with a 1030nm anti-reflection film; The incident surfaces of the transmission diffraction grating (10) and the lithium niobate crystal (15) are respectively located at the object plane and the image plane of a 4-f standard optical imaging module.

7. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The sample scanning module also includes a three-dimensional movable sample holder (21) and a CCD camera microscope (19); The three-dimensional movable sample rack (21) is used to hold the sample to be detected, to adjust the position and distance of the sample to be detected in three dimensions, and to align the sample to be detected with the terahertz radiation emitted by the terahertz strong source generation module; The terahertz near-field probe (20) is an 800 nm terahertz probe, which is arranged at the three-dimensional movable sample holder (21) so as to be close to the surface of the sample to be detected on the three-dimensional movable sample holder (21) to perform near-field detection; A CCD camera microscope (19) is arranged at the three-dimensional movable sample holder (21) and the terahertz near-field probe (20) and is used to accurately magnify and display the positional relationship between the sample to be detected on the three-dimensional movable sample holder (21) and the terahertz near-field probe (20).

8. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 2, characterized in that: The optical parametric amplifier (2) converts a received pump laser beam into a detection light with a central wavelength of 800 nm and a pulse width of 70 fs; The chopper 30 operates at a frequency of 500 Hz and is phase-locked with the trigger signal of the Yb-doped fiber-amplified femtosecond laser.

9. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: The mechanical delay module (26) includes a sixth ultrafast reflection mirror (27), a seventh ultrafast reflection mirror (28), an electric displacement stage, and a stepping motor for controlling the electric displacement stage; wherein the sixth ultrafast reflection mirror (27) and the seventh ultrafast reflection mirror (28) are placed on the electric translation stage at 90 degrees to each other; The stepping motor controls the movement of the electric displacement stage to control the simultaneous horizontal movement of the sixth ultrafast reflection mirror (27) and the seventh ultrafast reflection mirror (28), thereby changing the optical path of the detection light.

10. The high repetition rate strong field terahertz coupled near-field imaging system according to claim 1, characterized in that: A fifth ultrafast reflector (25), a fourth ultrafast reflector (24), a fourth convex lens (23), and a third ultrafast reflector (22) are sequentially arranged between the mechanical delay module (26) and the terahertz near-field probe (20), for guiding the detection light emitted from the mechanical delay module (26) to illuminate the excitation location of the terahertz near-field probe (20); The fourth convex lens (23) is used to focus the detection light, the focal length of the fourth convex lens (23) is 200 mm, and the optical distance from the fourth convex lens (23) to the terahertz near-field probe (20) is set to 100 mm.

Images