Subwavelength imaging system and method based on laser-induced plasma probe

Abstract

The present invention relates to the field of electromagnetic wave imaging technology, and discloses a subwavelength imaging system and method based on a laser-induced plasma probe. The 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 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 arranged in sequence, and the microwave near-field scattering module is connected to the data acquisition module. The imaging method includes forming a transient plasma area; setting the number of scanning points of the data acquisition module and setting the scanning matrix; controlling the galvanometer to jump the laser spot to the current coordinate point; and the data acquisition module collecting the reflection parameter S of the current point signal. 11 ; Reflection parameters S of all scan points collected 11 The present invention realizes non-contact high-resolution imaging at a sub-wavelength scale, and improves system stability, compatibility and scanning efficiency.

Description

Technical Field

[0001] The invention relates to a method and a system for measuring the complex dielectric constant of a standard rock core, and belongs 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 technology.

[0003] Traditional electromagnetic imaging methods primarily rely on far-field imaging systems. Their maximum spatial resolution is constrained by the diffraction limit, typically around half the wavelength (λ) (0.5λ). Therefore, they struggle to meet the practical needs of high-precision imaging of material electromagnetic properties at the subwavelength scale. While near-field electromagnetic imaging technology can image subwavelength targets, it still faces numerous technical bottlenecks. Traditional near-field imaging methods rely on physical probes, which suffer from poor sample adaptability, high wear and tear, complex manufacturing, high cost, and short lifespan. Especially in multi-band applications, frequent probe replacement and poor frequency band compatibility severely limit the system's versatility and scalability. Furthermore, traditional technologies often rely on mechanical scanning to acquire near-field information, resulting in a slow imaging process that is unable to meet the requirements of real-time dynamic detection. Existing methods often struggle to simultaneously achieve high resolution, a wide field of view, and sample adaptability, hindering their widespread adoption in practical applications such as in situ imaging of biological tissues and defect detection in nanomaterials.

[0004] Electromagnetic imaging technology analyzes the interaction of electromagnetic waves with samples to reconstruct internal or surface structure. It covers frequency bands ranging from low-frequency radio (MHz–GHz), microwaves (GHz), millimeter waves (30–300 GHz), and terahertz waves (0.1–10 THz). Far-field imaging systems typically transmit microwave or terahertz waves, measure reflection or transmission from the target, and then reconstruct a two-dimensional or three-dimensional image based on the amplitude and phase distribution of the echoes. For example, synthetic aperture radar (SAR) uses antenna arrays or moving platforms to simulate a large aperture, enabling high-resolution imaging of terrain or buildings. Terahertz imaging, due to its excellent penetration into most non-metallic materials (such as plastics and paper) and superior resolution compared to millimeter waves, has been widely studied in drug testing, tissue imaging, and high-resolution security inspections. Despite these advantages in penetration and application range, the spatial resolution of far-field imaging is constrained by the diffraction limit—typically centimeter-level in the X-band, but can drop to tens of microns or sub-millimeter levels in the terahertz band. However, no matter what imaging method is used based on far-field detection of electromagnetic waves, it cannot break through the diffraction limit and is difficult to meet the imaging needs at the sub-wavelength scale.

[0005] To overcome this limitation, near-field electromagnetic detection is based on the principle of capturing electromagnetic energy variations before they diffuse in the sample, enabling imaging with subwavelength resolution. Several near-field imaging techniques have been developed in this area of ​​research, including the typical near-field scanning optical microscope (NSOM) and scattering scanning near-field optical microscope (s-SNOM), each with its own advantages and disadvantages. NSOM typically uses a metal-coated nanopore or nanotip probe to couple a light or electromagnetic beam into an aperture or tip much smaller than the incident wavelength. Signals are collected in the near-field region formed by the extremely small distance between the probe and the sample surface, achieving resolutions of tens of nanometers. However, as the metal coating aperture decreases, the energy of the transmitted light decreases, rapidly degrading the signal-to-noise ratio. Furthermore, the probe is easily clogged by dust or impurities, resulting in a short probe lifespan and requiring mechanical scanning point by point, resulting in slow imaging speed. In contrast, s-SNOM utilizes a metal-tip probe fabricated using an atomic force microscope (AFM) or scanning tunneling microscope (STM) to capture nanoscale near-field information by leveraging the scattered signals between the probe tip and the sample. s-SNOM can achieve nanometer-level resolution by using a vibrating probe to achieve phase-locked detection and improve the signal-to-noise ratio. However, the system requires high-precision mechanical scanning and complex optical interferometry configurations, is sensitive to environmental vibrations and temperature changes, and the close contact between the probe and the sample can easily damage the sample, resulting in a limited service life. This limits its practicality and stability.

[0006] Therefore, developing a subwavelength imaging method that does not rely on mechanical probes, has fast response capabilities, and maintains high resolution has become a core technical challenge that needs to be overcome in this field. To address this issue, researchers have proposed a method for imaging the electromagnetic parameters of materials at the subwavelength scale based on a laser-induced plasma probe. Summary of the Invention

[0007] In view of the shortcomings of existing electromagnetic wave imaging technology, the present invention provides a sub-wavelength imaging system based on a laser-induced plasma probe with high electromagnetic response sensitivity and high resolution, and also provides an imaging method of the system.

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

[0009] The 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 is composed of a laser, a galvanometer, and a lens assembly. The laser, galvanometer, lens assembly, high-resistance semiconductor substrate, and 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 galvanometer are both connected to a laser galvanometer control card.

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

[0011] The data acquisition module and the laser galvanometer control card are both connected to the host computer, which is used to control the imaging system, store data, and reconstruct images.

[0012] 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.

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

[0014] The microwave near-field scattering module is a microwave element capable of providing near-field electromagnetic wave radiation, including TM mode, TE mode or TEM mode.

[0015] The data acquisition module is an integrated device for transmitting and receiving microwave signals, including a vector network analyzer or a combination of a signal generator and a spectrum analyzer. 11 The amplitude curve and phase curve are obtained, and the resonant frequency is obtained from the amplitude curve.

[0016] 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 (i.e., power per unit area) of 10W / cm²-10kW / cm².

[0017] The output parameters of the laser should meet the following physical constraints:

[0018] ;

[0019] Where: P is the laser power in watts, L is the spot diameter in centimeters, λ is the laser wavelength in nanometers, and α(λ) is the absorption coefficient of the high-resistance semiconductor substrate at the laser wavelength λ in cm -1 , q is the charge, μ is the mobility of photogenerated carriers in the high resistance semiconductor substrate, τ is the carrier lifetime, h is the Planck constant, c is the speed of light, σ min It is the minimum conductivity threshold in S / m, and its minimum is 10S / m.

[0020] The method for imaging the electromagnetic parameters of a material at a sub-wavelength scale using the imaging system includes the following steps.

[0021] (1) The measured medium is attached to the surface of the high-resistance semiconductor substrate on the side close to the microwave near-field scattering module;

[0022] (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 t1 time according to the principle of photogenerated plasma probe, forming a transient plasma region;

[0023] (3) Perform port calibration and frequency calibration on the data acquisition module to ensure that the reflection parameter S 11 Accurate measurement (S 11 Represents the reflection parameters of the port, reflecting the microwave reflection characteristics of the point, and the amplitude and phase provide electromagnetic response information).

[0024] (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;

[0025] (5) Set the scan matrix to m×n and initialize the row and column coordinate counters i=0, j=0;

[0026] (6) Control the galvanometer to jump the laser spot to the current coordinate point (i, j);

[0027] (7) Waiting time t;

[0028] (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.

[0029] (9) Column coordinates increase: j=j+1;

[0030] (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), row coordinate i = i + 1, column coordinate j = 0, and continue scanning the next row.

[0031] (11) Determine whether all row scans (i=m) are completed. If not, return to step (6); if yes, all points have been measured.

[0032] (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 (false color map, contour map, grayscale map, etc.).

[0033] The principle of the photogenerated plasma probe in step (2) is:

[0034] 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:

[0035] ;

[0036] where e is the electron charge, n(z) is the photogenerated carrier concentration, n0 is the carrier concentration in the high resistance semiconductor substrate, and μ e and μ h are the mobility of electrons and holes, respectively;

[0037] 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.

[0038] 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 ensures stable data acquisition at each laser excitation point to avoid missing, repeated or distorted signal acquisition. t1 is 5μs-50μs, and t2 is 100μs-1ms.

[0039] The present invention adopts the principle of photogenerated 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:

[0040] First, the transient plasma region excited by laser on the surface of a high-resistance semiconductor substrate is used as a virtual near-field probe, achieving subwavelength-scale non-contact high-resolution imaging without the need for physical contact, breaking through the resolution bottleneck of traditional systems that are limited by the diffraction limit.

[0041] Secondly, the system structure does not require mechanical probes and scanning devices, which avoids problems such as probe damage, poor frequency band adaptability, and slow imaging speed, and significantly improves the system's stability, compatibility, and scanning efficiency.

[0042] 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

[0043] Figure 1This is a schematic diagram of the structural principle of the plasma probe imaging system.

[0044] Figure 2 It is a schematic diagram of the imaging of the medium being measured.

[0045] 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.

[0046] Figure 4 It is a schematic diagram of the scanning matrix of the data acquisition module in the present invention.

[0047] Figure 5 The measured S 11 The relationship between the resonant frequency and amplitude in the parameters.

[0048] Figure 6 The measured S 11 The resonant frequency and phase relationship curve in the parameters.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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;

[0053] 201. Air medium, 202. Measured medium. DETAILED DESCRIPTION

[0054] 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 is composed of a laser 101, a galvanometer 102, a laser-galvanometer control card 109 and a lens, and the lens includes a lens 103 and a 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. The laser 101 and the galvanometer 102 are both connected to the laser-galvanometer control card 109, and the data acquisition module 107 and the laser-galvanometer control card 109 are both connected to the master 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 , the resistivity is not less than 2.3×10 5 Ω∙cm. The microwave near-field scattering module 106 must be a microwave component with a relatively uniform mode and clear field distribution, capable of providing near-field electromagnetic wave radiation. It must be able to stably excite the TM mode, TE mode, or TEM mode in the target frequency band. It can be an open waveguide, various antennas, etc. Figure 1 Typically, a circular waveguide is used. Data acquisition module 107 is an integrated microwave signal transmitter and receiver capable of generating microwave excitation signals, collecting microwave reflection signals, and extracting amplitude ratio and phase difference information. A vector network analyzer (VNA) is a typical implementation, but other microwave measurement devices with similar functionality can also be used, such as a signal generator and spectrum analyzer. All collected data is ultimately managed, stored, and reconstructed by the host computer 108.

[0055] To ensure that the imaging system has sufficiently high spatial resolution and electromagnetic response sensitivity, the present invention is based on the following key physical principles and mechanisms: the principle of photogenerated plasma probe and the self-resonance sensitization mechanism.

[0056] Principle of photogenerated plasma probe:

[0057] After the laser is focused on the surface of the high-resistance semiconductor substrate 105 (high-resistance silicon wafer), a high-density photogenerated carrier is excited in a localized area, forming a transient plasma region with high conductivity. This region remains stable under continuous laser irradiation, and the conductivity can be calculated using the following formula:

[0058] ;

[0059] Where e is the electron charge, n(z) is the photogenerated carrier concentration, and n0 is the carrier concentration μ in the high resistance semiconductor substrate (intrinsic silicon wafer 105). e and μ h are the mobility of electrons and holes, respectively.

[0060] 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.

[0061] Self-resonance sensitization mechanism:

[0062] 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.

[0063] 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.

[0064] 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 surfaces, thereby causing destructive interference and stimulating self-resonance. 11 The amplitude will reach a minimum value, resonance will occur, and the resonance frequency f r satisfy:

[0065] ;

[0066] Where 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 ) of the dielectric constant ε1 (satisfying ε1<ε r， ), d is the thickness of the high resistance semiconductor substrate 105, and n is a positive integer greater than 0.

[0067] When the thickness d of the high-resistance semiconductor substrate 105 is an integral multiple of half the wavelength, self-resonance occurs.

[0068] In order to achieve effective excitation of the localized plasma probe on the surface of a high-resistance semiconductor substrate 105 (such as a silicon wafer), it is necessary to generate a photogenerated carrier region of sufficient density on the surface of the material to form a plasma probe structure with significant electrical conductivity. To this end, the parameter range of the laser 101 must meet the following requirements: the laser 101 is a continuous laser with an excitation wavelength in the range of 400-1100nm, which is used to excite photogenerated carriers in the semiconductor material. The output power of the laser 101 is fixed or adjustable in the range of 1mW-50W to meet the power density requirements under different plasma excitation conditions. The corresponding spot diameter range is 10m-10mm, ultimately ensuring that the laser power density (i.e., power per unit area) is within the range of 10W / cm² to 10kW / cm². In order to ensure that the excited plasma probe has the conductivity that meets the requirements of electromagnetic detection, the output parameters of the laser 101 should meet the following physical constraints:

[0069] ;

[0070] 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), and α(λ) is the absorption coefficient of the high-resistance semiconductor substrate 105 at the laser wavelength λ in cm -1 , q is the charge, μ is the mobility of photogenerated carriers in semiconductor materials, τ is the carrier lifetime, h is Planck constant, c is the speed of light, σ min The minimum conductivity threshold is expressed in S / m, with a minimum of 10 S / m to ensure that the local plasma has a sufficiently large conductivity to generate a strong enough response signal for the probe to manipulate the electromagnetic wave for capture by the data acquisition module.

[0071] To ensure synchronous, accurate, and rapid signal acquisition, data acquisition module 107 must possess microsecond-level response capabilities. The localized plasma formation time is denoted as t1, with a typical value of approximately 5μs-50μs. The minimum sampling time of data acquisition module 107 is denoted as t2, with a typical value of approximately 100μs-1ms. Laser scanning speeds are so fast that they are negligible. Therefore, to ensure stable data acquisition at each laser excitation point, the system must ensure a waiting time t of at least t1 + t2 before jumping to the next scanning position to avoid missing, duplication, or distortion of signal acquisition.

[0072] Based on the above imaging system and principle, the present invention uses a laser-induced plasma probe to image the electromagnetic parameters of materials at a subwavelength scale. The process is as follows: Figure 3 As shown, the specific steps include the following steps.

[0073] (1) Step 301: Build Figure 1The plasma probe-based imaging system shown starts the imaging test process.

[0074] (2) Step 302: Figure 2 As shown, the medium to be measured 202 is attached to the surface of the high-resistance semiconductor substrate 105 (intrinsic silicon wafer) close to the microwave near-field scattering module 106 .

[0075] (3) Step 303: The laser is focused onto 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 laser irradiation, a high concentration of photogenerated carriers is generated at the specified position within time t1, forming a transient plasma region.

[0076] (4) Step 304: Start the data acquisition module 107, perform port calibration and frequency calibration, and ensure that the reflection coefficient S 11 Accurate measurement. 11 Represents the reflection parameters of port 1, reflecting the microwave reflection characteristics of this point. The amplitude and phase provide electromagnetic response information.

[0077] The self-resonance sensitization mechanism is the reflection parameter S 11 An electromagnetic response enhancement phenomenon that naturally manifests itself during the measurement process.

[0078] The effect occurs during the propagation of the excitation electromagnetic wave to the high-resistance semiconductor substrate 105. Through the reflection interference of the front and rear end surfaces of the high-resistance semiconductor substrate 105, a self-resonance phenomenon is induced under the thickness condition (the thickness d of the high-resistance semiconductor substrate 105 is an integer multiple of half the wavelength of the electromagnetic wave in the high-resistance semiconductor substrate 105), thereby generating the reflection parameter S collected in step (9). 11 The curve shows significant resonance features (such as extremely small amplitude points or phase mutation points) and is extracted as high-sensitivity feature parameters for image reconstruction in step (13).

[0079] (5) Step 305: Set the number of scanning 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.

[0080] (6) Step 306: Figure 4 As shown, the scan matrix is ​​set to m×n, and the row and column coordinate counters i=0, j=0 are initialized.

[0081] (7) Step 307: Control the galvanometer 102 to jump the laser spot to the current coordinate point (i, j), as shown in Figure 4 shown.

[0082] (8) Step 308: Waiting time t.

[0083] (9) Step 309: The data acquisition module 107 acquires the reflection signal S at the current point 11 , and save the data, the measured S 11 The resonant frequency and amplitude curve of the parameters are as follows: Figure 5 As shown, the resonant frequency and phase curves are as follows Figure 6 shown.

[0084] (10) Step 310: Column coordinate increment: j = j + 1.

[0085] (11) Step 311: Determine whether the end of the current row has been reached (j = n). If not, jump back to step 307 to continue scanning the current row. If yes, execute step 312, row coordinate i = i + 1, column coordinate j = 0, and continue scanning the next row.

[0086] (12) Step 312: Determine whether all row scans (i=m) are completed. If not, return to step 307; if so, all points have been measured.

[0087] (13) Step 313: S of all scan points collected in step 309 11 The parameter data is processed uniformly. For each scanning point, key characteristic parameters can be extracted from its reflection response, including but not limited to: the amplitude of different scanning points at a specific frequency ( Figure 5 a1, a2, a3, a4 in), the phase values ​​of different scanning points at a specific frequency ( Figure 6 b1, b2, b3, b4) or the resonant frequencies at different scanning spatial positions ( Figure 5 Any of the above parameters can be used as a physical characteristic indicator of the point and used for subsequent two-dimensional image reconstruction (false color image, contour map, grayscale image, etc.).

[0088] (14) Step 314: The imaging process ends and the system automatically saves the image and data.

[0089] The above method has been systematically verified in multiple typical scenarios using an electromagnetic simulation platform. 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.

[0090] Using a light-controlled plasma virtual probe imaging system, high-resolution detection of internal defects in dielectric materials such as ceramics and composite materials was successfully achieved. A high-resistance silicon wafer (0.3 mm thick) was used as substrate 105, and the sample was scanned using laser-induced plasma (with a focused spot size of 20 μm). The results showed that within the operating frequency band, both S 11The amplitude of the parameter at different scanning points at a specific frequency, the phase value of the parameter at different scanning points at a specific frequency, or the resonant frequency at different scanning spatial positions (see Figure 7 and Figure 8 ), 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 that of the non-resonant point imaging effect. Figure 9 The high-resolution imaging results of the sample's dielectric constant mutation region obtained using the resonance point characteristic parameters are shown.

Claims

1. An imaging method of a sub-wavelength imaging system based on a laser-induced plasma probe, characterized by: The subwavelength imaging system based on the laser-induced plasma probe 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 a laser galvanometer control card. 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, and its thickness d is an integer multiple of half the wavelength of the electromagnetic wave in the high-resistance semiconductor substrate to generate self-resonance; The microwave near-field scattering module is a microwave element that can provide TM mode near-field electromagnetic wave radiation; the data acquisition module is an integrated device for microwave signal transmission and reception, including a vector network analyzer or a combination of a signal source and a spectrum analyzer, which collects reflection parameters S 11 The amplitude curve and phase curve of , and the resonant frequency is obtained from the amplitude curve; The imaging method of the sub-wavelength imaging system based on the laser-induced plasma probe comprises the following steps: (1) The measured medium is attached to the surface of the high-resistance semiconductor substrate on the side close to 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 t1 time 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 scan matrix to m×n and 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: 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), row coordinate i = i + 1, column coordinate j = 0, and continue scanning the next row. (11) Determine whether all row scans are completed, i = m, if not, return to step (6); if yes, 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.

2. The imaging method of the sub-wavelength imaging system based on the 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 imaging method of the sub-wavelength imaging system based on the 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.

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

5. The imaging method of the sub-wavelength imaging system based on the laser-induced plasma probe according to claim 1, 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: ； Where e is the electron charge, n(z) is the photogenerated carrier concentration, n0 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.

6. The imaging method of the sub-wavelength imaging system based on the laser-induced plasma probe according to claim 1, 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