A high-repetition-rate strong-field terahertz coupling near-field imaging system

Abstract

The application relates to a high-repetition-frequency strong-field terahertz coupling near-field imaging system, and belongs to the technical field of terahertz time-domain spectroscopy. The system solves the problem of low terahertz source power and single-pulse energy in a near-field probe scanning system in the prior art, and comprises a femtosecond laser light source (1) for providing femtosecond pump laser; a non-polarized beam splitter (3) for splitting the pump laser provided by the femtosecond laser light source (1) into two beams, which are respectively provided to the light path of the pump laser and the light path of detection light, and converting one of the pump lasers into terahertz radiation through the light path of the pump laser; and a sample scanning module comprising a terahertz near-field probe (20) for receiving the terahertz radiation to perform terahertz scanning and imaging on a sample surface.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz time-domain spectroscopy, and particularly relates to a high-repetition-rate strong-field terahertz coupling near-field imaging system. BACKGROUND

[0002] Terahertz waves are electromagnetic waves with a frequency in the range of 0.1-10 THz, between low-energy electronics and high-energy photonics. This unique frequency band endows it with many excellent characteristics. In terms of penetration, terahertz waves have strong penetration for non-polar materials such as paper and plastic, can easily penetrate the wrapping layer and see the internal structure, and play an important role in express security inspection and cultural relic non-destructive testing. For polar molecules, especially water molecules, terahertz waves have sensitive response characteristics. By using this point, auxiliary disease diagnosis can be achieved in the field of biomedicine. Terahertz waves have great application value in many fields such as material science, biomedicine, food safety, non-destructive testing, and security inspection.

[0003] Terahertz imaging technology is one of the most promising development directions in terahertz science and technology. The performance parameters of the imaging device, including signal bandwidth, imaging resolution, signal-to-noise ratio, and imaging speed, directly determine the application range of the terahertz imaging device. Terahertz imaging technology mainly includes continuous wave imaging and pulse imaging. Continuous wave imaging is mainly based on intensity imaging, has the advantages of high radiation power, fast imaging speed, and portable system, but has the disadvantages of less data information and lack of phase information. Although the subsequent development of continuous wave coherent detection can obtain phase information, it still faces the technical bottleneck 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, but its rich information provides important value for multispectral imaging research.

[0004] In the terahertz pulse imaging technology system, different types of technology show different characteristics. Terahertz real-time two-dimensional imaging is based on the linear Pockels effect of electro-optic crystals, and realizes detection and imaging by means of free-space electro-optic sampling technology. Its advantage is fast imaging speed, but it has the limitations of lower signal-to-noise ratio than time-domain spectroscopy, lack of phase information, and inability to perform spectral analysis. Terahertz computed tomography (Terahertz-CT) has a similar principle to X-CT. By collimating terahertz waves to scan the sample and combining with the inverse filtered projection algorithm to reproduce the image, a large amount of information can be obtained to analyze the amplitude and phase of the sample. However, the signal-to-noise ratio is highly dependent on the intensity of the terahertz wave and the absorption rate of the sample, and is limited by the lack of high-power radiation sources.

[0005] As the earliest and most mature pulsed imaging technology, the terahertz time-domain spectroscopy imaging technology synchronously acquires amplitude and phase information through coherent detection, and can be used for intensity analysis and spectral analysis, but has problems of limited spatial resolution by terahertz focusing spot size (diffraction limit) and complex device.

[0006] An existing terahertz imaging method directly detects near-field information by using a terahertz wave generated by a photoconductive antenna to break the diffraction limit, and relies on a terahertz probe to be very close to a sample. The method is limited by low terahertz source power and single pulse energy, resulting in insufficient peak focusing electric field intensity and penetration, and the probe needs to be very close to the sample, which can easily damage the probe. The method has technical bottlenecks in high-energy and high-repetition-frequency terahertz radiation generation.

[0007] An existing lithium niobate terahertz radiation source using a tilted pulse front technology is limited by laser and lithium niobate crystal damage threshold, and it is difficult to generate terahertz radiation with a repetition frequency of more than 1 kHz, resulting in low test speed and efficiency.

[0008] In addition, the excitation light condition of the existing near-field probe is less than 7 μJ / cm2, and the current application is only in a weak-field terahertz system of a megahertz laser (for an 80 MHz excitation light, the spot size is 40 microns, and the excitation light power is about 3.5 mW). For a strong-field terahertz system, a low-frequency high-single-pulse-energy laser is usually selected to generate strong terahertz waves, so that the single pulse energy of the detection excitation light is also much larger than that of the megahertz laser, thereby exceeding the excitation threshold of the near-field probe and causing damage to the near-field probe. Therefore, there is currently no suitable terahertz strong source system for the near-field probe. In other words, the generation of a strong-field terahertz requires a laser with large single pulse energy, and the laser with large single pulse energy can damage the probe when exciting the probe. 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

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

[0011] The application provides a high-repetition-frequency strong-field terahertz coupling near-field imaging system, which can also be used as a terahertz time-domain spectroscopy system. The system can break through the terahertz diffraction limit to obtain a 20-micron imaging spatial resolution, uses a high-power high-repetition-frequency lithium niobate strong-field terahertz radiation source, has stronger penetration capability, and can adjust the terahertz power, single-pulse energy, repetition frequency and focused electric field intensity as required without changing the system.

[0012] The embodiment of the application provides a high-repetition-frequency strong-field terahertz coupling near-field imaging system, which comprises:

[0013] A femtosecond laser source is arranged to provide femtosecond pump laser.

[0014] A non-polarizing beam splitter is arranged to divide the pump laser provided by the femtosecond laser source into two beams, which are respectively provided to the optical path of the pump laser and the optical path of the probe light, and one of the two beams of the pump laser is converted into terahertz radiation through the optical path of the pump laser.

[0015] A sample scanning module is arranged and comprises a terahertz near-field probe, which is used to receive the terahertz radiation to realize scanning and imaging on the surface of a sample.

[0016] The optical path of the pump laser comprises, in sequence:

[0017] A laser spot expansion module is arranged to receive one of the two beams of the pump laser provided after the beam splitting of the non-polarizing beam splitter to perform laser spot expansion.

[0018] A terahertz strong source generation module is arranged to receive the pump laser after the laser spot expansion, generate terahertz radiation, and provide the terahertz radiation to the sample scanning module.

[0019] The other beam of the pump laser is converted into probe light through the optical path of the probe light, and the optical path of the probe light comprises, in sequence:

[0020] An optical parametric amplifier is arranged to receive the other beam of the pump laser provided after the beam splitting of the non-polarizing beam splitter and convert the other beam of the pump laser into probe light.

[0021] A chopper is arranged to suppress the noise of the probe light to improve the signal-to-noise ratio of electro-optic sampling.

[0022] A mechanical delay module is arranged to adjust the optical path of the probe light and provide the probe light to the sample scanning module synchronously with the terahertz radiation to excite the terahertz near-field probe.

[0023] Optionally, the femtosecond laser source is a ytterbium-doped fiber-amplified femtosecond laser, which is arranged to provide pump laser with a center 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 divides the pump laser emitted by the femtosecond laser source into two beams with powers of 95% and 5%, wherein the pump laser with a power of 95% is guided into the optical path of the pump laser, and the pump laser with a power of 5% is guided into the optical path of the probe light.

[0025] Optionally, the optical path of the pump laser further comprises a pump laser optical path adjusting module arranged between the non-polarizing beam splitter and the laser spot beam expander module, which is used to receive the split pump laser and guide it to the laser spot beam expander module downstream of the optical path; the pump laser optical path adjusting module comprises a first ultrafast mirror, a first zero-degree mirror and a second zero-degree mirror.

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

[0027] Optionally, the terahertz strong source generation module outputs broadband terahertz radiation with a peak frequency of 0.6 THz and a spectral coverage range of 0.1-2.5 THz, and comprises 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 the pump laser incident at an angle of 31°, and converges the incident pump laser power to the -1 order diffracted light; the gold mirror group comprises 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 comprises 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 biconvex lenses coated with 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 the image plane of the 4-f standard optical imaging module, respectively.

[0028] Optionally, the sample scanning module further comprises a three-dimensional moving sample holder (21) and a CCD camera microscope; the three-dimensional moving sample holder is used to hold the sample to be detected, and adjusts the position and distance of the sample to be detected in three dimensions, aligns the sample to be detected with the terahertz radiation emitted by the terahertz strong source generation module; the terahertz near-field probe is an 800 nm terahertz probe, which is arranged close to the surface of the sample to be detected on the three-dimensional moving sample holder to implement near-field detection; the CCD camera microscope is arranged at the three-dimensional moving 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 moving 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 mirror;

[0050] 6- second zero-degree mirror;

[0051] 7- concave lens;

[0052] 8- first convex lens;

[0053] 9- second ultrafast mirror;

[0054] 10- transmission diffraction grating;

[0055] 11- first gold mirror;

[0056] 12- second gold mirror;

[0057] 13- second convex lens;

[0058] 14- third convex lens;

[0059] 15- lithium niobate crystal;

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

[0061] 17- ITO flat glass;

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

[0063] 19- CCD camera microscope;

[0064] 20- terahertz near-field probe;

[0065] 21- three-dimensional moving sample holder;

[0066] 22- third ultrafast mirror;

[0067] 23- fourth convex lens;

[0068] 24- fourth ultrafast mirror;

[0069] 25- fifth ultrafast mirror;

[0070] 26- mechanical delay module;

[0071] 27- sixth ultrafast mirror;

[0072] 28- seventh ultrafast mirror;

[0073] 29- eighth 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 source 1 can be a 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, a repetition frequency adjustable between 1 kHz and 50 kHz, a maximum single-pulse energy of 1 mJ, and a pulse width of 570 fs. The single-pulse energy, the repetition frequency, and the pulse width of the ytterbium-doped fiber-amplified femtosecond laser can be adjusted to adjust the single-pulse energy, the repetition frequency, and the average power of the generated terahertz radiation. The single-pulse energy of the ytterbium-doped fiber-amplified femtosecond laser can be set to range between 1% and 100% with a maximum single-pulse energy of 1 mJ, and the repetition frequency can be set to range between 30 kHz and 50 kHz to prevent damage to the terahertz near-field probe 20 caused by excessively low laser repetition frequency.

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

[0084] The structures arranged in the optical path of the pump laser are described below.

[0085] The high-repetition strong-field terahertz-coupled near-field imaging system provided by the first embodiment can further include a pump laser optical path adjustment module arranged between the non-polarizing beam splitter 3 and the laser spot expansion module, for receiving the split pump laser and guiding the pump laser to the laser spot expansion module downstream of the optical path, so as to increase the optical path in a smaller space to match the optical path of the probe light. The pump laser optical path adjustment module can include a first ultrafast mirror 4, a first zero-degree mirror 5, and a second zero-degree mirror 6. Optionally, the first ultrafast mirror 4 can be an ultrafast mirror with a working wavelength of 1030 nm; the first zero-degree mirror 5 and the second zero-degree mirror 6 are used to lengthen the optical path of the split pump laser to match the optical path of the probe light.

[0086] The laser spot expansion module can include an expansion lens group and a precision displacement 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 transmission 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 transmission diffraction grating 10 at an angle of 31°. The transmission 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 combined by two lenticular lenses with specific focal length, and the imaging ratio is matched with the line number parameter of the transmission diffraction grating to realize high-efficiency optical imaging. Specifically, the 4-f standard optical imaging module can 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 lenticular lenses coated with a 1030 nm anti-reflection film, combined with a focal length of 3.7:1, and the imaging ratio is matched with the line number parameter of the transmission diffraction grating to realize high-efficiency optical imaging. Alternatively, the focal length of the second convex lens 13 is 370 mm, the focal length of the third convex lens 14 is 100 mm, and the distance between the second convex lens 13 and the third convex lens 14 is set to 470 mm. The above-mentioned 4-f standard optical imaging module can improve the generation efficiency of terahertz radiation.

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

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

[0096] The off-axis parabolic mirror set collimates and focuses the terahertz radiation emitted from the lithium niobate crystal 15. The off-axis parabolic mirror set can include a first off-axis parabolic mirror 16, an ITO flat glass 17, and a second off-axis parabolic mirror 18, which collimate and focus the generated terahertz radiation, and compared with using terahertz lenses, can minimize terahertz loss and dispersion. Specifically, the first off-axis parabolic mirror 16 receives and collects terahertz, shapes it into a parallel light beam, and emits it to the ITO flat glass 17. After reflection by the ITO flat glass 17, it is incident to the second off-axis parabolic mirror 18, which focuses it to the sample to be detected. The first off-axis parabolic mirror 16 and the second off-axis parabolic mirror 18 can be gold-coated off-axis parabolic mirrors, and can both have the same parameters of an aperture diameter of 2 inches and a focal length of 4 inches. By standardizing the components, the system complexity is reduced, and the change in terahertz polarization caused by the gold-coated off-axis parabolic mirror set is avoided, and compared with using terahertz lenses, the 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, the first embodiment of the application provides a high-repetition-rate strong-field terahertz coupled near-field imaging system, which comprises a femtosecond laser light source 1, an optical parametric amplifier 2, a non-polarizing beam splitter 3, a first ultrafast mirror 4, a first zero-degree mirror 5, a second zero-degree mirror 6, a concave lens 7, a first convex lens 8, a second ultrafast mirror 9, a transmission diffraction grating 10, a first gold mirror 11, a second gold mirror 12, a second convex lens 13, a third convex lens 14, a lithium niobate crystal 15, a first 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, a mechanical delay module 26 composed of 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 working process of the high-repetition-rate strong-field terahertz coupled near-field imaging system of the first embodiment is as follows. The femtosecond pump laser emitted by the femtosecond laser light source 1 passes through the 95:5 non-polarizing beam splitter 3 and is divided into pump laser and probe light. The transmission paths of the pump laser and the probe light are as follows.

[0116] The light path of the pump laser is as follows. The part of the pump laser divided by the non-polarizing beam splitter 3 passes through the ultrafast mirror 4, then passes through the first zero-degree mirror 5 and the second zero-degree mirror 6 to lengthen the optical path and match the optical path of the probe light. Then, it passes through the laser spot expansion module composed of the concave lens 7 and the first convex lens 8, and the spot size is expanded to about 1.5 times the initial size. Then, the expanded pump laser is guided by the ultrafast mirror 9 and then enters the transmission diffraction grating 10. More than 90% of the pump laser power is emitted from the -1 order. After being reflected by the first gold mirror 11 and the second gold mirror 12, it passes through the 4-f standard optical imaging module composed of the second convex lens 13 and the third convex lens 14 in the horizontal direction. Then, it is incident to the incident surface of the lithium niobate crystal 15 at a specific angle. The pump laser interacts with the lithium niobate crystal 15, and terahertz waves are generated from the exit surface of the lithium niobate crystal 15 through the optical rectification mechanism. The emitted terahertz waves are collected by the first off-axis parabolic mirror 16 and shaped into a parallel beam, which is then emitted to the ITO flat glass 17. After being reflected by the ITO flat glass 17, it is focused by the second off-axis parabolic mirror 18 to the three-dimensional moving sample holder 21 of the sample scanning module, i.e., to the sample to be detected on the three-dimensional moving sample holder 21. The single pulse energy, repetition rate, and average power of the generated terahertz radiation can be adjusted by adjusting the single pulse energy, repetition rate, and other parameters of the femtosecond laser (pump source).

[0117] The light path of the probe light is as follows: the part of the pump laser split by the non-polarizing beam splitter 3 is guided into the matched optical parametric amplifier 2 through the ninth ultrafast mirror 31 and the tenth ultrafast mirror 32 to obtain the probe light with a central wavelength of 800 nm and a pulse width of 70 fs; then the probe light 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 further improve the signal-to-noise ratio of the electro-optic sampling; the probe light passing through the chopper 30 continues to be guided to the mechanical delay module 26 composed of the sixth ultrafast mirror 27, the seventh ultrafast mirror 28 and the stepper motor through the reflection of the eighth ultrafast mirror 29; the probe light continues to be guided through the fifth ultrafast mirror 25 and the fourth ultrafast mirror 24, is focused by the fourth convex lens 23, and is irradiated to the excitation position of the terahertz near-field probe 20 through the guidance of the third ultrafast mirror 22. The probe light is coherent with the pump laser, and the probe light is used to excite the terahertz near-field probe 20.

[0118] The sample scanning module simultaneously receives the terahertz radiation and the probe light, and the three-dimensional movable sample holder 21 and the long-working-distance CCD camera microscope 19 are used to accurately control the distance between the sample and the terahertz near-field probe 20. The terahertz near-field probe 20 receives the signal converted from the terahertz radiation scanning of the sample into an output current signal, and the current signal is converted into a voltage signal through a current amplifier. The voltage signal output from the current amplifier can be connected to the input port of a lock-in amplifier, the output port of the lock-in amplifier is connected to a data acquisition card, and the data acquisition card is connected to a computer; the mechanical delay module 26 and the electrically-driven three-dimensional movable sample holder 21 can be connected to the computer, so that the automatic control of the terahertz scanning imaging can be realized.

[0119] The current amplifier can be a current amplifier with a magnification of 10 8 V / A.

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

[0121] Figure 2 The strong-field terahertz at the sample in the terahertz spot diagram generated by the example of the high-repetition-rate strong-field terahertz coupled near-field imaging system according to the first embodiment of the application. Figure 3 The terahertz time-domain spectrum waveform diagram when there is no sample in the embodiment of the high-repetition-rate strong-field terahertz coupled near-field imaging system according to the first embodiment of the application.

[0122] As Figure 2 and Figure 3As shown, the terahertz spot generated by the embodiment is regular circular, the intensity is in accordance with Gaussian distribution, and the focusing diameter is about 1 mm. Figure 3 The terahertz time-domain waveform measured at the sample (but without the sample) at the laser frequency of 50 kHz is used to calculate the peak electric field intensity, 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 by the above embodiment can also be modularly packaged as a whole, realizing a high-integration system with a smaller volume.

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

[0125] It should be understood that the size of the serial number of each step in the above embodiments does not mean the order of execution, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0126] The above description is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A near-field imaging system with high repetition rate and strong field terahertz coupling, characterized in that, include: Femtosecond laser source (1) provides femtosecond pump laser; The non-biased beam splitter (3) splits the pump laser provided by the femtosecond laser source (1) into two beams, which provide the optical path to the pump laser and the optical path to the probe light, respectively. The optical path of the pump laser converts one beam of pump laser into terahertz radiation. The sample scanning module includes a terahertz near-field probe (20) for receiving terahertz radiation to scan and image the sample surface; The pump laser's optical path includes the following components arranged in sequence: The laser beam expander module receives a pump laser beam provided by the non-biased beam splitter (3) after beam splitting and expands the laser beam. 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. In this process, another pump laser beam is converted into a probe beam in the optical path of the probe beam, including the following sequential steps: The optical parametric amplifier (2) receives another pump laser beam provided by the non-biased beam splitter (3) and converts it into probe light; A chopper (30) is used to suppress noise in the probe light to improve the signal-to-noise ratio of electro-optic sampling; The mechanical delay module (26) adjusts the optical path of the probe light and provides it to the sample scanning module in sync with the terahertz radiation to excite the terahertz near-field probe (20). The femtosecond laser source (1) is a ytterbium-doped fiber amplified femtosecond laser. The ytterbium-doped fiber amplified femtosecond laser is set up to provide a pump laser with a center wavelength of 1030nm, a repetition frequency range of 30-50kHz, and a maximum single pulse energy of 1mJ. The unbiased beam splitter (3) is a 95:5 unbiased beam splitter that splits the pump laser emitted by the femtosecond laser source (1) into two beams with 95% power and 5% power. The 95% power pump laser is guided to the optical path of the pump laser, and the 5% power pump laser is guided into the optical path of the probe light.

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

3. The near-field imaging system with high repetition rate and strong field terahertz coupling according to claim 1, characterized in that, The laser beam expander module includes a beam expanding lens group and a precision displacement stage; The beam-expanding lens group expands the spot size of the incident pump laser to 1.5 times the original size. It includes a concave lens (7) and a first convex lens (8) arranged in sequence. The concave lens (7) is a concave lens with a focal length of -50mm, and the first convex lens (8) is a biconvex lens with a focal length of 75mm. The distance between the concave lens (7) and the first convex lens (8) is set to 25mm. The precision displacement stage is used to mount the concave lens (7) and the first convex lens (8) for precise adjustment of their positions.

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

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

6. The near-field imaging system with high repetition rate and strong field terahertz coupling according to claim 2, characterized in that, The optical parametric amplifier (2) converts a received pump laser beam into a 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 the trigger signal of the ytterbium-doped fiber amplified femtosecond laser.

7. The near-field imaging system with high repetition rate and strong field terahertz coupling according to claim 1, characterized in that, The mechanical delay module (26) includes a sixth ultrafast reflector (27), a seventh ultrafast reflector (28), an electric displacement stage, and a stepper motor for controlling the electric displacement stage; Among them, the sixth ultrafast reflector (27) and the seventh ultrafast reflector (28) are placed at 90° to each other on the electric displacement stage; The stepper motor controls the horizontal movement of the sixth ultrafast mirror (27) and the seventh ultrafast mirror (28) by controlling the movement of the electric displacement stage, thereby changing the optical path of the probe light.

8. The near-field imaging system with high repetition rate and strong field terahertz coupling according to claim 1, characterized in that, Between the mechanical delay module (26) and the terahertz near-field probe (20), there are also arranged in sequence: a fifth ultrafast reflector (25), a fourth ultrafast reflector (24), a fourth convex lens (23) and a third ultrafast reflector (22), which are used to guide the probe light emitted from the mechanical delay module (26) to the excitation point of the terahertz near-field probe (20); The fourth convex lens (23) is used to focus the probe light. The focal length of the fourth convex lens (23) is 200mm, and the optical path from the fourth convex lens (23) to the terahertz near-field probe (20) is set to 100mm.

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