Sub-wavelength imaging system and method based on laser-induced plasma probe

Abstract

The invention relates to the technical field of electromagnetic wave imaging, and discloses a sub-wavelength imaging system and method based on a laser-induced plasma probe. The imaging system comprises a laser regulation and control module, a high-resistance semiconductor substrate, a microwave near-field scattering module and a data acquisition module, the laser regulation and control module is composed of a laser, a galvanometer and a lens assembly, the laser, the galvanometer, the lens assembly, the high-resistance semiconductor substrate and the microwave near-field scattering module are sequentially arranged, and the microwave near-field scattering module is connected with the data acquisition module. The imaging method comprises the following steps: forming a transient plasma region; setting the number of scanning points of the data acquisition module, and setting a scanning matrix; the galvanometer is controlled to jump the laser spot to the current coordinate point; the data acquisition module acquires a reflection parameter S11 of a current point signal; and performing unified processing on the acquired reflection parameter S11 data of all the scanning points. According to the invention, the non-contact high-resolution imaging of the sub-wavelength scale is realized, and the stability, compatibility and scanning efficiency of the system are improved.

Description

Technical Field

[0001] The present invention relates to a method and system for measuring the complex dielectric constant of a standard core, belonging to the technical field of electromagnetic wave imaging. Background Art

[0002] The present invention aims to solve multiple technical bottlenecks commonly existing in current electromagnetic wave imaging technologies.

[0003] Traditional electromagnetic wave imaging methods mainly rely on far-field imaging systems, whose highest spatial resolution is constrained by the diffraction limit, usually about half of the wavelength λ (0.5λ). Therefore, it is difficult to meet the actual needs of high-precision imaging of the electromagnetic properties of materials at the sub-wavelength scale. Although electromagnetic wave near-field imaging technology can image sub-wavelength targets, it still faces many technical bottlenecks. Traditional near-field imaging methods rely on physical probes, which have problems such as poor sample adaptability, easy loss, complex manufacturing, high cost, and short lifespan. Especially in multi-band application scenarios, the probes need to be frequently replaced, and the frequency band compatibility is poor, severely restricting the versatility and scalability of the system. In addition, traditional technologies mostly use mechanical scanning to obtain near-field information, resulting in a slow imaging process and making it difficult to meet the requirements of real-time dynamic detection. Existing methods usually have difficulty in simultaneously achieving high resolution, wide field of view, and sample adaptability, restricting their popularization in practical applications such as in-situ imaging of biological tissues and defect detection of nanomaterials.

[0004] Electromagnetic wave imaging technology analyzes the interaction between electromagnetic waves and samples to reconstruct the internal or surface structure information, covering frequency bands from low-frequency radio (MHz–GHz), microwave (GHz), millimeter wave (30–300 GHz) to terahertz (0.1–10 THz). Far-field imaging systems usually emit microwave or terahertz waves to perform reflection or transmission measurements on targets, and then reconstruct two-dimensional or three-dimensional images based on the amplitude and phase distributions of the echoes. For example, synthetic aperture radar (SAR) uses an antenna array or a moving platform to simulate a large aperture to achieve high-resolution imaging of terrain or buildings; terahertz imaging has good penetration ability for most non-metallic materials (such as plastics, papers) and is superior to millimeter waves in terms of resolution, so it has been widely studied in drug detection, biological tissue imaging, and high-resolution security inspection. Although these methods have advantages in penetration ability and application scope, the spatial resolution of far-field imaging is restricted by the diffraction limit - the spatial resolution is usually at the centimeter level in the X-band and can be reduced to dozens of micrometers or sub-millimeters in the terahertz band. However, no matter what kind of imaging method based on electromagnetic wave far-field detection, it cannot break through the diffraction limit and is difficult to meet the imaging requirements at the sub-wavelength scale.

[0005] To break through this limitation, starting from the principle of near-field detection of electromagnetic waves, the unspread electromagnetic energy changes during propagation are captured in the area close to the sample, thus achieving imaging with sub-wavelength resolution. In this research direction, there are already various near-field imaging techniques, among which the typical near-field scanning optical microscope (NSOM) and scattering-type scanning near-field optical microscope (s-SNOM) have their own advantages and disadvantages. NSOM usually uses a metal-coated nanopore or nanotip probe to couple a light beam or electromagnetic wave beam into an aperture or tip with a size much smaller than the incident wavelength, and collects signals through the near-field region formed by the extremely small distance between the probe and the sample surface. The resolution can reach dozens of nanometers. However, as the aperture of the metal coating becomes smaller, the energy of the transmitted light decreases, the signal-to-noise ratio drops rapidly, and at the same time, the probe is easily blocked by dust or impurities, the probe life is short and mechanical point-by-point scanning is required, resulting in slow imaging speed. In contrast, s-SNOM uses a metal tip probe prepared by an atomic force microscope (AFM) or a scanning tunneling microscope (STM), and captures nano-scale near-field information by means of the scattering signal between the probe tip and the sample. By vibrating the probe to achieve phase-locked detection and improve the signal-to-noise ratio, s-SNOM can achieve nano-scale resolution, but its system requires high-precision mechanical scanning and complex optical interference configurations, is sensitive to environmental vibrations and temperature changes, and the close contact between the probe and the sample is likely to damage the sample, with a limited service life. This limits its practicability and stability.

[0006] Therefore, how to develop a sub-wavelength imaging method that does not rely on mechanical probes, has fast response capabilities, and at the same time takes into account high resolution has become the core technical problem that urgently needs to be broken through in this field. To solve the above problems, researchers have proposed an imaging method for the electromagnetic parameters of materials at the sub-wavelength scale based on laser-induced plasma probes. Summary of the Invention

[0007] Aiming at the deficiencies of existing electromagnetic wave imaging technologies, the present invention provides a sub-wavelength imaging system based on laser-induced plasma probes with high electromagnetic response sensitivity and high resolution at the same time, and also provides an imaging method for this system.

[0008] The sub-wavelength imaging system based on laser-induced plasma probes of the present invention adopts the following technical solutions.

[0009] This imaging system includes a laser control module, a high-resistance semiconductor substrate, a microwave near-field scattering module, and a data acquisition module. The laser control module consists of a laser, a galvanometer, and a lens assembly. The laser, the galvanometer, the lens assembly, the high-resistance semiconductor substrate, and the microwave near-field scattering module are arranged in sequence. The microwave near-field scattering module is connected to the data acquisition module, and both the laser and the galvanometer are connected to the laser galvanometer control card.

[0010] The lens assembly includes at least one lens, and when there are two or more, they are arranged in sequence.

[0011] Both the data acquisition module and the galvanometer control card are connected to the host computer. The host computer is used to regulate the imaging system, store data, and perform image reconstruction.

[0012] The high-resistance semiconductor substrate is an intrinsic silicon wafer with a dielectric constant of ε r , and its resistivity is not less than 2.3×10 5 Ω cm.

[0013] The thickness of the high-resistance semiconductor substrate d is an integer multiple of half the wavelength of electromagnetic waves in the high-resistance semiconductor substrate to generate self-resonance.

[0014] The microwave near-field scattering module is a microwave component that can provide near-field electromagnetic wave radiation, including TM mode, TE mode, or TEM mode.

[0015] The data acquisition module is an integrated device for microwave signal transmission and reception, including a combination of a vector network analyzer or a signal generator and a spectrum analyzer. The data acquisition module collects the amplitude curve and phase curve of the reflection parameter S 11 , and obtains the resonance frequency from the amplitude curve.

[0016] The laser is a continuous laser, with an excitation wavelength of 400 - 1100 nm, an output power of 1 mW - 50 W, a spot diameter of 10 μm - 10 mm, and a laser power density (i.e., power per unit area) of 10 W / cm² - 10 kW / cm².

[0017] The output parameters of the laser should satisfy the following physical condition constraints: ; where: P is the laser power, in watts, L is the spot diameter, in centimeters, λ is the laser wavelength, in nanometers, α (λ) is the absorption coefficient of the high-resistance semiconductor substrate at the laser wavelength λ , in cm -1 , q is the electric charge, μ is the mobility of photo-generated carriers in the high-resistance semiconductor substrate, τ is the carrier lifetime, h is Planck's constant, c is the speed of light, σ min is the minimum conductivity threshold, in S / m, and its minimum is 10 S / m.

[0018] The method for imaging the electromagnetic parameters of materials at the sub-wavelength scale by the above imaging system includes the following steps.

[0019] (1) Attach the measured medium to one side of the high-resistance semiconductor substrate surface close to the microwave near-field scattering module; (2) Focus the laser through a laser, a galvanometer, and a lens on the high-resistance semiconductor substrate surface. After laser irradiation, according to the principle of the photo-generated plasma probe, generate high-concentration photo-generated carriers at a specified position within time t1 to form a transient plasma region; (3) Calibrate the ports and frequencies of the data acquisition module to ensure accurate measurement of the reflection parameter S 11 (S 11 represents the reflection parameter of the port, reflecting the microwave reflection characteristics at this point. The amplitude and phase provide electromagnetic response information).

[0020] (4) Set the number of scan points of the data acquisition module to be no less than 201 points, and the intermediate frequency bandwidth of the data acquisition module to be not lower than 100 kHz to ensure frequency resolution and signal quality; (5) Set the scan matrix to be m × n , initialize the row and column coordinate counters i = 0, j = 0; (6) Control the galvanometer to jump the laser spot to the current coordinate point ( i , j ); (7) Wait for time t; (8) The data acquisition module acquires the reflection parameter S 11 of the current point signal, and saves the data to obtain the amplitude curve and phase curve of the reflection parameter S 11 , and then extracts the resonant frequency and the corresponding amplitude and phase; (9) Increment the column coordinate: j = j + 1; (10) Determine whether the end of the current row has been reached ( j = n ). If not, jump back to step (6) to continue scanning the current row; if so, execute step (11), increment the row coordinate i = i + 1, set the column coordinate j = 0, and continue scanning the next row; (11) Determine whether all rows have been scanned ( i = m ). If not, return to step (6); if so, all points have been measured; (12) For the reflection parameter S 11The data is uniformly processed; for each scanning point, key feature parameters are extracted from its reflection response, including but not limited to: the amplitude of different scanning points at a specific frequency, the phase value of different scanning points at a specific frequency, or the resonance frequency at different scanning spatial positions; any of the above parameters can be used as the physical feature index of this point for subsequent two-dimensional image reconstruction (pseudo-color map, contour map, grayscale map, etc.).

[0021] The principle of the photo-generated plasma probe in step (2) is as follows: After the laser is focused on the surface of the high-resistance semiconductor substrate, a high-density photo-generated carrier is excited in a local area, forming a transient plasma region with high conductivity. This region remains stable under continuous laser irradiation, and the conductivity is calculated by the following formula: ; where e is the electron charge, n (z) is the concentration of photo-generated carriers, n 0 is the carrier concentration in the high-resistance semiconductor substrate, μ e and μ h are the mobilities of electrons and holes respectively; The plasma region is consistent with the laser spot region. By regulating the conductivity of this region, it is used as a local "virtual probe" to interact with electromagnetic waves, increasing the electromagnetic wave reflectivity in this region, thereby achieving near-field regulation of electromagnetic waves.

[0022] The waiting time t in step (7) is not less than t1 + t2, where t1 is the time for forming the transient plasma region in step (2), and t2 is the minimum sampling time of the data acquisition module; this can ensure stable acquisition of data at each laser excitation point to avoid problems such as missing, repeated, or distorted signal acquisition. The t1 is 5 μs - 50 μs, and the t2 is 100 μs - 1 ms.

[0023] The present invention uses the principle of photo-generated plasma probe and self-resonance sensitization mechanism to ensure that the imaging system has sufficiently high spatial resolution and electromagnetic response sensitivity, and has the following characteristics: First, using the transient plasma region excited by the laser on the surface of the high-resistance semiconductor matrix as a virtual near-field probe, without physical contact, non-contact high-resolution imaging at the sub-wavelength scale is achieved, breaking through the resolution bottleneck of traditional systems limited by the diffraction limit.

[0024] Second, the system structure does not require mechanical probes and scanning devices, avoiding problems such as easy damage of probes, poor frequency band adaptability, and slow imaging speed, significantly improving the stability, compatibility, and scanning efficiency of the system.

[0025] In addition, by utilizing the self-resonance effect formed by the reflection interference between the front and back end surfaces of the high resistance semiconductor substrate (high resistance silicon wafer), the present invention can significantly enhance the S 11 The amplitude of the parameter change can be increased to improve the system's sensitivity to tiny differences in electromagnetic properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the structural principle of the plasma probe imaging system.

[0027] Figure 2 It is a schematic diagram of the imaging of the medium being measured.

[0028] Figure 3 This is a flow chart of the imaging method of the electromagnetic parameters of materials at a subwavelength scale based on a laser-induced plasma probe of the present invention.

[0029] Figure 4 It is a schematic diagram of the scanning matrix of the data acquisition module in the present invention.

[0030] Figure 5 The measured S 11 The relationship between the resonant frequency and amplitude in the parameters.

[0031] Figure 6 The measured S 11 The resonant frequency and phase relationship curve in the parameters.

[0032] Figure 7 When the present invention is applied to high-resolution defect detection, S 11 The relationship between the resonant frequency and amplitude in the parameters.

[0033] Figure 8 When the present invention is applied to high-resolution defect detection, S 11 The resonant frequency and phase relationship curve in the parameters.

[0034] Figure 9 It is a high-resolution imaging image of the dielectric constant mutation area of the sample obtained using the characteristic parameters of the resonance point.

[0035] In the figure: 101. Laser, 102. Galvanometer, 103. Lens, 104. Lens, 105. High-resistance semiconductor substrate, 106. Microwave near-field scattering module, 107. Data acquisition module, 108. Host computer, 109. Laser galvanometer control card; 201. Air medium, 202. Measured medium. DETAILED DESCRIPTION

[0036] The present invention is based on the imaging method of the electromagnetic parameters of materials at the sub-wavelength scale using a laser-induced plasma probe. Figure 1The imaging system of the plasma probe shown mainly includes: a laser control module, a high-resistance semiconductor substrate 105, a microwave near-field scattering module 106, and a data acquisition module 107. The laser control module consists of a laser 101, a galvanometer 102, a laser-galvanometer control card 109, and a lens. The lens includes lens 103 and lens 104. The laser 101, the galvanometer 102, the lens 103, the lens 104, the high-resistance semiconductor substrate 105, and the microwave near-field scattering module 106 are arranged in sequence, and the microwave near-field scattering module 106 is connected to the data acquisition module 107. Both the laser 101 and the galvanometer 102 are connected to the laser-galvanometer control card 109, and both the data acquisition module 107 and the laser-galvanometer control card 109 are connected to the host computer 108. A typical high-resistance semiconductor substrate 105 can be an intrinsic silicon wafer, and the dielectric constant of the silicon wafer is ε r , and the resistivity is not less than 2.3×10 5 Ω cm. The microwave near-field scattering module 106 needs to be a microwave component with a relatively uniform mode and a clear field distribution, capable of providing near-field electromagnetic wave radiation, and needs to be able to stably excite the TM mode, TE mode, or TEM mode in the target frequency band, and can be an open waveguide, various antennas, etc., Figure 1 and a typical one used in it is a circular waveguide. The data acquisition module 107 is a microwave signal transmitting and receiving integrated device, capable of generating microwave excitation signals, collecting microwave reflection signals, and extracting amplitude ratio and phase difference information. A vector network analyzer (VNA) can be used as a typical implementation method, and other microwave measurement device combinations with similar functions can also be used for replacement, such as the combination of a signal generator and a spectrum analyzer. All the collected data is finally uniformly regulated, stored, and image-reconstructed by the host computer 108.

[0037] To ensure that the imaging system has a sufficiently high spatial resolution and electromagnetic response sensitivity, the present invention is based on the following key physical principles and mechanisms: the principle of photo-generated plasma probe and the mechanism of self-resonance sensitization. [[ID=!--]]

[0038] Principle of photo-generated plasma probe: After the laser is focused on the surface of the high-resistance semiconductor substrate 105 (high-resistance silicon wafer), a high-density photo-generated carrier is excited in a local area, forming a transient plasma region with high conductivity. This region remains stable under continuous laser irradiation, and the conductivity can be calculated by the following formula: ; where e is the electron charge, n (z) is the concentration of photo-generated carriers, n 0 is the carrier concentration in the high-resistance semiconductor substrate (intrinsic silicon wafer 105) μ e and μh are the mobility of electrons and holes, respectively.

[0039] The plasma region is consistent with the laser spot region. By regulating the conductivity of this region, it can be used as a local "virtual probe" to interact with electromagnetic waves, thereby improving the reflectivity of electromagnetic waves in this region and achieving near-field control of electromagnetic waves.

[0040] Self-resonance sensitization mechanism: This mechanism utilizes the multi-reflection interference behavior at the interface between the waveguide and the high-resistance semiconductor substrate 105 to form self-resonance under specific structural parameters, thereby significantly enhancing the system's response sensitivity to tiny differences in electromagnetic properties.

[0041] The principle of self-resonance generation: electromagnetic waves propagate from the end face of the microwave near-field scattering module 106 to the measured medium 202 (see Figure 2 ), it will experience two reflections at the front and rear end faces. Part of the wave energy is reflected at the front end face of the measured medium 202, resulting in half-wave loss due to the transition from a sparse wave to a dense wave medium. Meanwhile, the other part of the wave energy penetrates the measured medium 202 and is reflected at the rear end face. In this case, the electromagnetic wave does not experience half-wave loss due to the transition from a dense wave to a sparse wave medium.

[0042] By properly selecting the thickness of the high resistance semiconductor substrate 105, a phase difference of π can be generated between the reflected waves at the front and rear ends, thereby causing destructive interference and stimulating self-resonance. 11 The amplitude will reach a minimum value, resonance will occur, and the resonant frequency f r satisfy: ; in c is the speed of light (3×10 8 m / s), ε r The dielectric constant of the high-resistance semiconductor substrate 105 (here, the intrinsic silicon wafer) is greater than that of the air medium 201 (see Figure 2 ) dielectric constant ε 1 (Satisfy ε 1< ε r， ), d is the thickness of the high resistance semiconductor substrate 105, n is a positive integer greater than 0.

[0043] When the thickness of the high resistance semiconductor substrate 105 d When the wavelength is an integer multiple of half the wavelength, self-resonance will occur.

[0044] To effectively excite a local plasma probe on the surface of a high-resistance semiconductor substrate 105 (such as a silicon wafer), it is necessary to generate a region of photo-generated carriers with sufficient density on its material surface to form a plasma probe structure with significant conductivity. For this purpose, the parameter range of the laser 101 needs to meet the following: The laser 101 is a continuous laser, and the excitation wavelength is within the range of 400 - 1100 nm, which is used to excite photo-generated carriers in the semiconductor material. The output power of the laser 101 is fixed or adjustable, within the range of 1 mW - 50 W, to meet the power density requirements under different plasma excitation conditions. The corresponding spot diameter range is 10 μm - 10 mm, and finally, it is ensured that the laser power density (i.e., power per unit area) is within 10 W / cm² to 10 kW / cm². To ensure that the excited plasma probe has the conductivity required to meet electromagnetic detection requirements, the output parameters of the laser 101 should meet the following physical condition constraints: ; where: P is the laser power, in watts (W), L is the spot diameter, in centimeters (cm), λ is the laser wavelength, in nanometers (nm), α α(λ) is the absorption coefficient of the high-resistance semiconductor substrate 105 at the laser wavelength λ λ, in cm -1 , q is the electric charge, μ is the mobility of photo-generated carriers in the semiconductor material, τ is the carrier lifetime, h is Planck's constant, c is the speed of light, σ min is the minimum conductivity threshold, in S / m, and its minimum value is 10 S / m to ensure that the local plasma has a sufficiently large conductivity, so as to generate a sufficiently strong response signal for the probe to manipulate electromagnetic waves for the data acquisition module to capture.

[0045] To ensure the synchronization, accuracy, and rapidity of signal acquisition, the data acquisition module 107 needs to have a microsecond-level response ability. The formation time of the local plasma is denoted as t1, and the typical value is about 5 μs - 50 μs; while the minimum sampling time of the data acquisition module 107 is denoted as t2, and the typical value is about 100 μs - 1 ms. The laser scanning speed is very fast and can be ignored. Therefore, to ensure the stable acquisition of data at each laser excitation point, before the system jumps to the next scanning position, the waiting time t must be ensured to be not less than t1 + t2 to avoid signal acquisition missing, repetition, or distortion, etc.

[0046] Based on the above imaging system and principle composition, the imaging method of the electromagnetic parameters of materials at the sub-wavelength scale based on the laser-induced plasma probe in the present invention has a process as Figure 3 shown, and specifically includes the following steps.

[0047] (1) Step 301: Construct the Figure 1 plasma probe-based imaging system shown in the figure, and start the imaging test process.

[0048] (2) Step 302: As shown in Figure 2 the figure, attach the measured medium 202 to the surface of the high-resistance semiconductor substrate 105 (intrinsic silicon wafer) near the microwave near-field scattering module 106.

[0049] (3) Step 303: Focus the laser on the surface of the high-resistance semiconductor substrate 105 through the laser 101 and the lens system (galvanometer 102, lens 103, and lens 104). After the laser irradiation, generate high-concentration photo-generated carriers at the specified position within time t1 to form a transient plasma region.

[0050] (4) Step 304: Turn on the data acquisition module 107, perform port calibration and frequency calibration to ensure accurate measurement of the reflection coefficient S 11 S 11 represents the reflection parameter of port 1, reflecting the microwave reflection characteristics at this point. The amplitude and phase provide electromagnetic response information.

[0051] The self-resonance enhancement mechanism is an electromagnetic response enhancement phenomenon that naturally appears in the measurement process of the reflection parameter S 11 During the propagation of the excitation electromagnetic wave to the high-resistance semiconductor substrate 105, it takes effect. Through the reflection interference of the front and rear end faces of the high-resistance semiconductor substrate 105, when the thickness condition is met (the thickness of the high-resistance semiconductor substrate 105

[0052] is an integer multiple of half the wavelength of the electromagnetic wave in the high-resistance semiconductor substrate 105), a self-resonance phenomenon is triggered, so that in the reflection parameter S d collected in step (9), it shows significant resonance characteristics (such as extremely small amplitude points or phase mutation points) in the curve, and is extracted as a high-sensitivity characteristic parameter for image reconstruction in step (13). 11 (5) Step 305: Set the number of scan points of the data acquisition module 107 to be no less than 201 points, and the intermediate frequency bandwidth of the data acquisition module 107 to be no less than 100 kHz to ensure frequency resolution and signal quality.

[0053] (6) Step 306: As shown in

[0054] the figure, set the scan matrix to be Figure 4 × m × n , initialize the row and column coordinate counters i = 0, j = 0.

[0055] (7)Step 307: Control the galvanometer 102 to jump the laser spot to the current coordinate point ( i , j ), as shown in Figure 4 .

[0056] (8)Step 308: Wait time t.

[0057] (9)Step 309: The data acquisition module 107 acquires the reflection signal S of the current point 11 , and saves the data. The resonance frequency and amplitude curve of the measured S 11 parameter is as shown in Figure 5 , and the resonance frequency and phase curve is as shown in Figure 6 .

[0058] (10)Step 310: Column coordinate auto-increment: j = j + 1.

[0059] (11)Step 311: Determine whether the end of the current row is reached ( j = n ). If not, jump back to Step 307 to continue scanning the current row; if so, execute Step 312, row coordinate i [[ID=3,6]] = i + 1, column coordinate j = 0, and continue scanning the next row.

[0060] (12)Step 312: Determine whether all rows have been scanned ( i = m ). If not, return to Step 307; if so, all points have been measured.

[0061] (13)Step 313: Uniformly process the S 11 parameter data of all scanned points collected in Step 309. For each scanned point, key feature parameters can be extracted from its reflection response, including but not limited to: the amplitudes of different scanned points at a specific frequency ( Figure 5 in a 1, a 2, a 3, a 4), the phase values of different scanned points at a specific frequency (b1, b2, b3, b4 in Figure 6 ) or the resonance frequencies at different scanned spatial positions ( Figure 5 in f 1, f 2, f 3, f 4). Any of the above parameters can be used as the physical feature index of this point for subsequent two-dimensional image reconstruction (pseudo-color map, contour map, grayscale map, etc.).

[0062] Step 314: End the imaging process, and the system automatically saves the images and data.

[0063] The above method has been systematically verified through an electromagnetic simulation platform in multiple typical scenarios. The simulation results show that the present invention has excellent spatial resolution, electromagnetic sensitivity, and structural compatibility. A typical application example is given below, which is applied to high-resolution defect detection.

[0064] Using the optically controlled plasma virtual probe imaging system, high-resolution detection of internal defects in dielectric materials such as ceramics and composites has been successfully achieved. A high-resistance silicon wafer (0.3 mm thick) is used as the substrate 105, and the sample is scanned by combining laser-induced plasma (focused spot 20 μm). The results show that in the working frequency band, whether it is the 11 amplitude of different scan points at a specific frequency of the S parameter, the phase value of different scan points at a specific frequency, or the resonant frequency at different scan spatial positions (see Figure 7 and Figure 8 ), it can be effectively used for defect imaging. Among them, the image quality reconstructed based on the characteristic parameters of the resonant point is significantly better than the imaging effect of the non-resonant point. Figure 9 Shows the high-resolution imaging results of the dielectric constant mutation region of the sample obtained using the characteristic parameters of the resonant point.

Claims

1. A sub-wavelength imaging system based on a laser-induced plasma probe, characterized by: It includes a laser control module, a high-resistance semiconductor substrate, a microwave near-field scattering module and a data acquisition module. The laser control module consists of a laser, a galvanometer and a lens assembly. The laser, the galvanometer, the lens assembly, the high-resistance semiconductor substrate and the microwave near-field scattering module are arranged in sequence. The microwave near-field scattering module is connected to the data acquisition module, and the laser and the galvanometer are both connected to the laser galvanometer control card.

2. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The lens assembly includes at least one lens, and two or more lenses are arranged in sequence.

3. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The data acquisition module and the laser galvanometer control card are both connected to the master computer.

4. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The high resistance semiconductor substrate is an intrinsic silicon wafer with a dielectric constant of ε r , the resistivity is not less than 2.3×10 5 Ω cm.

5. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The thickness of the high resistance semiconductor substrate d It is an integer multiple of half the wavelength of the electromagnetic wave in the high resistance semiconductor matrix.

6. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The laser is a continuous laser with an excitation wavelength of 400-1100nm, an output power of 1mW-50W, a spot diameter of 10m-10mm, and a laser power density of 10W / cm²-10kW / cm².

7. The sub-wavelength imaging system based on a laser-induced plasma probe according to claim 1, characterized in that: The output parameters of the laser should meet the following physical constraints: ； in: P is the laser power in watts, L is the spot diameter in centimeters, λ is the laser wavelength in nanometers, α (λ) is the high resistance semiconductor substrate at the laser wavelength λ Absorption coefficient under the condition of , in cm -1 , q is the charge, μ is the mobility of photogenerated carriers in a high-resistance semiconductor substrate, τ is the carrier lifetime, h is Planck's constant, c is the speed of light, σ min It is the minimum conductivity threshold in S / m, and its minimum is 10S / m.

8. An imaging method of a sub-wavelength imaging system based on a laser-induced plasma probe according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) The measured medium is attached to the surface of the high-resistance semiconductor substrate near the microwave near-field scattering module; (2) The laser is focused on the surface of the high-resistance semiconductor substrate through a laser, a galvanometer, and a lens. After laser irradiation, a high-concentration photogenerated carrier is generated at a specified location within a time t1 according to the principle of photogenerated plasma probe, forming a transient plasma region; (3) Perform port calibration and frequency calibration on the data acquisition module to ensure that the reflection parameter S 11 Accurate measurement; (4) Set the number of scanning points of the data acquisition module to be no less than 201 points, and the intermediate frequency bandwidth of the data acquisition module to be no less than 100 kHz to ensure frequency resolution and signal quality; (5) Set the scanning matrix to m × n , initialize the row and column coordinate counters i =0, j =0; (6) Control the galvanometer to jump the laser spot to the current coordinate point ( i , j ); (7) Waiting time t; (8) The data acquisition module collects the reflection parameter S of the current point signal 11 , and save the data to obtain the reflection parameter S 11 The amplitude curve and phase curve of the resonance frequency are obtained, and the corresponding amplitude and phase are extracted. (9) Column coordinates increase automatically: j = j +1; (10) Determine whether the end of the current line has been reached. j = n If not, jump back to step (6) to continue scanning the current row; if yes, execute step (11), the row coordinates i = i +1, column coordinate j =0, continue scanning the next line; (11) Determine whether all row scans are completed, i = m , if not, return to step (6); if so, all points have been measured; (12) The reflection parameters S of all the scanning points collected in step (8) 11 The data is processed uniformly; for each scanning point, key characteristic parameters are extracted from its reflection response, including but not limited to: the amplitude of different scanning points at a specific frequency, the phase value of different scanning points at a specific frequency, or the resonant frequency of different scanning spatial positions; any of the above parameters can be used as a physical characteristic indicator of the point for subsequent two-dimensional image reconstruction.

9. The imaging method of the sub-wavelength imaging system based on the laser-induced plasma probe according to claim 7, characterized in that: The principle of the photogenerated plasma probe in step (2) is: After the laser is focused on the surface of a high-resistance semiconductor substrate, a high-density photogenerated carrier is excited in a local area, forming a transient plasma region with high conductivity. This region remains stable under continuous laser irradiation, and the conductivity is calculated by the following formula: ； in e is the electron charge, n (z) is the photogenerated carrier concentration, n 0 is the carrier concentration in the high resistance semiconductor substrate, μ e and μ h are the mobility of electrons and holes, respectively; The plasma region is consistent with the laser spot region. By regulating the conductivity of this region, it is used as a local "virtual probe" to interact with electromagnetic waves, thereby improving the reflectivity of electromagnetic waves in this region and achieving near-field control of electromagnetic waves.

10. The imaging method of the sub-wavelength imaging system based on the laser-induced plasma probe according to claim 7, characterized in that: The waiting time t in step (7) is not less than t1+t2, t1 is the time for forming the transient plasma region in step (2), and t2 is the minimum sampling time of the data acquisition module.

Images