Femtosecond laser three-dimensional pumping detection imaging method

Abstract

The invention discloses a femtosecond laser three-dimensional pumping detection imaging method, which belongs to the technical field of ultrafast laser observation, and particularly comprises the following steps of: generating a laser pulse by using an ultrashort pulse laser, splitting the laser pulse by using a beam splitter to generate two beams of pulse light, namely pumping light and detection light, ablating a material by using the pumping light as an excitation source, reflection or transmission imaging is realized at different moments after the probe light passes through the time delay table; a detection angle is freely adjusted in space around a sample to be detected by rotating the light path propagation system, and a detection mode is switched between a reflection mode and a detection mode by rotating the imaging system; a multi-angle two-dimensional transient image is fused into a three-dimensional graph, plasma and a double-temperature model are expanded, the electronic dynamic characteristics of three-dimensional space distribution and the plasma eruption process can be calculated in an ultrafast scale, and three-dimensional information of a femtosecond laser ablation material is reduced. The problem that an existing imaging method cannot achieve three-dimensional imaging of the femtosecond laser processing ultrafast process at any angle in the space is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultrafast laser observation, and in particular relates to a femtosecond laser three-dimensional pump detection imaging method. Background Art Femtosecond laser processing is a complex, nonlinear, and nonequilibrium process. Its interaction with materials involves multi-physical processes such as ionization, phase transitions, and ablation. These processes typically occur on extremely short timescales (femtoseconds and picoseconds) and extremely small spatial scales (nanometers and micrometers). When the material interacts with the femtosecond laser, a high-temperature plasma is generated, followed by the rapid eruption of its expander, and dynamic behaviors such as phase transitions, stress propagation, and microscopic removal occur within the material. The dynamic changes in plasma generation and eruption, microscopic material removal, and phase transitions require imaging technology to monitor these dynamics. Therefore, accurate three-dimensional imaging at ultrafast timescales and extremely small spatial scales is crucial for revealing the mechanisms of femtosecond laser-material interactions and improving processing precision and efficiency.

[0002] Traditional pump-probe technology is an important tool for observing the ultrafast dynamic processes of femtosecond lasers. The observation technology relies on two pulses: the pump light is used to excite the sample, and the probe light with adjustable delay time is then used to record the temporal dynamic response of the sample. However, this method usually relies on a single probe beam focused on a fixed point on the surface or inside the sample. Therefore, it can only obtain temporal dynamic information at a single position within the range of the probe spot. It is difficult to obtain three-dimensional information from multiple perspectives in space, thus limiting the understanding of the complex dynamics of femtosecond laser processing. Other existing ultrafast three-dimensional imaging methods often require a layer-by-layer scanning method or the replacement of a light source with penetrating detection capabilities, such as ultrafast X-ray imaging. However, the layer-by-layer scanning method has a slow imaging speed and cannot capture the rapid dynamic processes occurring during femtosecond laser processing in real time. The penetrating detection method also has a complex equipment structure. For non-repeatable processing processes such as ablation, it is difficult to repeatedly and effectively observe in-situ image information.

[0003] Therefore, it is necessary to propose a new femtosecond laser three-dimensional pump-probe imaging method, which can perform three-dimensional imaging of the ultrafast process of femtosecond laser machining at any angle in space. Summary of the Invention

[0004] In response to the above-mentioned deficiencies in the prior art, the present invention provides a femtosecond laser three-dimensional pump-probe imaging method that solves the problem that the existing imaging methods cannot achieve three-dimensional imaging of the ultrafast process of femtosecond laser processing at any angle in space.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is: a femtosecond laser three-dimensional pump detection imaging method, comprising the following steps: S1: Use ultrashort pulse laser to generate laser pulses, and generate pump light and detection light through the beam splitter; S2: The pump light is focused by the objective lens and irradiated onto the sample surface to stimulate the electronic dynamics of the material and produce phase change. The probe light is adjusted by a one-dimensional delay stage to produce a fixed delay time with the pump light. The wavelength is then changed by a frequency doubling crystal, and after the fundamental frequency light is filtered out by a bandpass filter, it is focused on the surface or inside of the sample through the objective lens. S3: Switch between the reflection imaging mode and the transmission imaging mode by adjusting the imaging system. In the reflection imaging mode, the detection light is reflected from the sample surface to the camera. In the transmission imaging mode, the detection light passes through the sample and is transmitted to the camera. S4: Synchronize the ultrashort pulse laser and the camera through the signal generator, output high-level pulses to make the ultrashort pulse laser output a single beam of pulse light, and make the camera receive the detection light signal and collect background images, transient images and relaxation steady-state images; S5: By adjusting the rotation angle of the optical propagation system, the spatial angle between the imaging system and the sample is changed to achieve multi-angle three-dimensional imaging of the sample; S6: After obtaining multi-angle 2D images through pump-probe, the multi-angle 2D images are aligned, normalized, coordinate-transformed, and fused with 3D point clouds to reconstruct the 3D image; S7: Based on the three-dimensional extension of the plasma model and the two-temperature model, the dynamic evolution of electrons and the energy transfer process are analyzed to restore the three-dimensional ultrafast information of femtosecond laser ablation materials.

[0006] Furthermore, the laser pulse width in S1 ranges from 10 fs to 1 ns, and the laser energy adjustment range ranges from 1 μJ to 1 J.

[0007] Furthermore, the one-dimensional time-delay translation stage in S2 is: two reflectors form a retroreflective system, a one-dimensional electric translation stage is equipped with the retroreflective system to control the moving optical path, and a pair of apertures are used for collimation of different optical paths.

[0008] Furthermore, the imaging system in S3 includes a plano-convex lens, a filter and a camera, and is coupled to the back end of the optical path propagation system.

[0009] Furthermore, the reflective imaging mode in S3 is: adjusting the rotation angle of the imaging system to change the positional relationship between the focusing lens and the camera to make them symmetrical; The transmission imaging mode is: adjusting the rotation angle of the imaging system to change the position between the focusing lens and the camera so that they are parallel.

[0010] Furthermore, the optical path propagation system in S5 is composed of connecting rods coupling the reflector, the half-wave plate and the filter, and the connecting rods are connected by a rotating mechanism.

[0011] Furthermore, the S6 includes the following sub-steps: S61: Align multi-angle 2D images using image registration scikit-image functions; S62: Normalize the multi-angle two-dimensional image using the normalized difference method. The formula is:

[0012] in, is the differential reflectivity, is the collected instantaneous reflectivity, is the original reflectivity of the substrate; S63: Map the position of each pixel in the multi-angle two-dimensional image to a three-dimensional coordinate system to obtain corresponding three-dimensional point cloud data. The formula is:

[0013]

[0014]

[0015] in, 、 and is the transformed three-dimensional coordinate, is the distance from the reflected image pixel to the sample center, is the horizontal angle, The tilt angle for pump detection; S64: The 3D point cloud data from multiple angles are combined into a single global point cloud using a weighted average method. The formula is:

[0016] in, is the single global point cloud after merging, For the The weight of the point, For the The reflectivity of a point, is the total number of data points; S65: Perform three-dimensional image visualization on the fused three-dimensional point cloud data through the matplotlib function to obtain a reconstructed three-dimensional image.

[0017] Furthermore, the three-dimensional topology of the plasma model in S7 includes the electron density spatial distribution and the laser field spatial distribution, and the formula is: The three-dimensional evolution equation of electron density is:

[0018] in, is the electron density, For time, is the impact ionization coefficient, is the laser light source term, for Photon ionization coefficient, is the electron relaxation time; Laser light source The formula under action is:

[0019] in, is the maximum intensity of the laser beam waist, is a natural constant, is the absorption rate of the material for laser, is the spot radius.

[0020] Furthermore, the three-dimensional extension of the two-temperature model in S7 includes the evolution of electron temperature and lattice temperature, and the formula is: The evolution equation of electron temperature is:

[0021] in, is the electron heat capacity, is the electron temperature, is the temperature variable, is the Hamiltonian operator, is the electronic thermal conductivity, is the electron-lattice coupling coefficient, is the lattice temperature, is the deposition of laser energy;

[0022] The evolution equation of the lattice temperature is:

[0023] in, is the lattice heat capacity.

[0024] The beneficial effects of the present invention are: (1) The present invention discloses a femtosecond laser three-dimensional pump detection imaging method, which can solve the information loss in traditional single-view imaging by coupling the device with a multi-view imaging method. It can also verify the accuracy of signals from other viewpoints through multi-angle acquisition, thereby promoting the understanding of the femtosecond laser processing mechanism.

[0025] (2) The three-dimensional optical path propagation system and three-dimensional imaging system proposed in this invention can perform ultrafast imaging with continuous angles around the sample to be observed, capture complex three-dimensional processes such as plasma generation, phase change, and microstructure formation, and fully realize the acquisition of complete three-dimensional information in experiments, providing solid observation technology support for precise control of processing quality and development of new functional materials.

[0026] (3) The present invention discloses a femtosecond laser three-dimensional pump-probe imaging method that combines reflection and transmission observation modes into one system, and has higher accuracy in observing the same process under the same excitation conditions.

[0027] (4) The present invention discloses a femtosecond laser three-dimensional pump detection imaging method, which establishes a three-dimensional coordinate system, corresponds the grayscale value of each pixel of the two-dimensional image to the three-dimensional coordinate system, constructs three-dimensional point cloud data, merges the point cloud data from multiple angles into a global point cloud, and constructs a three-dimensional image model after fusion and weighted averaging. The method can fuse two-dimensional transient images from multiple angles into a three-dimensional image with higher image accuracy.

[0028] (5) The present invention discloses a femtosecond laser three-dimensional pump detection imaging method, which is extended to three dimensions through a two-dimensional plasma and dual-temperature model. The ultrafast three-dimensional image after fusion is analyzed in a theoretical model, and the real three-dimensional information after femtosecond laser processing of materials can be restored. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of a femtosecond laser three-dimensional pump detection imaging method of the present invention.

[0030] Figure 2 Schematic diagram of the laser propagation path at the pump-detection front end in the present invention.

[0031] Figure 3 Schematic diagram of the reflection and transmission pump detection observation principles.

[0032] Figure 4 Schematic diagram of the structure of the three-dimensional imaging device in the present invention.

[0033] Figure 5 This is the image of reflection pump detection with a detection delay of 100 picoseconds.

[0034] Figure 6 This is the image of the transmission pump detection with a detection delay of 100 picoseconds.

[0035] Among them, 1. first connecting rod; 2. first hinge; 3. second connecting rod; 4. second hinge; 5. third connecting rod; 6. focusing objective lens; 7. first camera; 8. fourth connecting rod; 9. third hinge; 10. fifth connecting rod; 11. sample to be tested; 12. rotatable translation stage; 13. first beam splitter; 14. first reflector; 15. second reflector; 16. third reflector; 17. fourth reflector; 18. fifth reflector; 19. sixth reflector; 20. seventh reflector; 21. frequency doubling crystal; 22. eighth reflector; 23. first filter; 24. first plano-convex lens; 25. second beam splitter; 26. third beam splitter; 27. second plano-convex lens; 28. second filter; 29. ​​second camera. DETAILED DESCRIPTION

[0036] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0037] like Figure 1 As shown, a femtosecond laser three-dimensional pump-probe imaging method includes the following steps: S1: Use ultrashort pulse laser to generate laser pulses, and generate pump light and detection light through the beam splitter; S2: The pump light is focused by the objective lens and irradiated onto the sample surface to stimulate the electronic dynamics of the material and produce phase change. The probe light is adjusted by a one-dimensional delay stage to produce a fixed delay time with the pump light. The wavelength is then changed by a frequency doubling crystal, and after the fundamental frequency light is filtered out by a bandpass filter, it is focused on the surface or inside of the sample through the objective lens. S3: Switch between the reflection imaging mode and the transmission imaging mode by adjusting the imaging system. In the reflection imaging mode, the detection light is reflected from the sample surface to the camera. In the transmission imaging mode, the detection light passes through the sample and is transmitted to the camera. S4: Synchronize the ultrashort pulse laser and the camera through the signal generator, output high-level pulses to make the ultrashort pulse laser output a single beam of pulse light, and make the camera receive the detection light signal and collect background images, transient images and relaxation steady-state images; S5: By adjusting the rotation angle of the optical propagation system, the spatial angle between the imaging system and the sample is changed to achieve multi-angle three-dimensional imaging of the sample; S6: After obtaining multi-angle 2D images through pump-probe, the multi-angle 2D images are aligned, normalized, coordinate-transformed, and fused with 3D point clouds to reconstruct the 3D image; S7: Based on the three-dimensional extension of the plasma model and the two-temperature model, the dynamic evolution of electrons and the energy transfer process are analyzed to restore the three-dimensional ultrafast information of femtosecond laser ablation materials.

[0038] The laser pulse width in S1 is in the range of 10 fs to 1 ns, and the laser energy adjustment range is 1 μJ to 1 J.

[0039] The one-dimensional time-delay translation stage in S2 is: two reflectors form a retroreflective system, a one-dimensional electric translation stage is equipped with the retroreflective system to control the moving optical path, and a pair of apertures are used for collimation of different optical paths.

[0040] The imaging system in S3 includes a plano-convex lens, a filter and a camera (i.e. Figure 2 The second plano-convex lens 27, the second filter 28 and the second camera 29 are coupled to the rear end of the optical path propagation system.

[0041] The reflective imaging mode in S3 is as follows: adjusting the rotation angle of the imaging system to change the positional relationship between the focusing lens and the camera to make them symmetrical; The transmission imaging mode is: adjusting the rotation angle of the imaging system to change the position between the focusing lens and the camera so that they are parallel.

[0042] The optical path propagation system in S5 is composed of a connecting rod coupling a reflector, a half-wave plate and a filter, and the connecting rods are connected by a rotating mechanism.

[0043] The S6 includes the following sub-steps: S61: Align multi-angle 2D images using image registration scikit-image functions; S62: Normalize the multi-angle two-dimensional image using the normalized difference method. The formula is:

[0044] in, is the differential reflectivity, is the collected instantaneous reflectivity, is the original reflectivity of the substrate; S63: Map the position of each pixel in the multi-angle two-dimensional image to a three-dimensional coordinate system to obtain corresponding three-dimensional point cloud data. The formula is:

[0045]

[0046]

[0047] in, 、 and is the transformed three-dimensional coordinate, is the distance from the reflected image pixel to the sample center, is the horizontal angle, The tilt angle for pump detection; S64: The 3D point cloud data from multiple angles are combined into a single global point cloud using a weighted average method. The formula is:

[0048] in, is the single global point cloud after merging, For the The weight of the point, For the The reflectivity of a point, is the total number of data points; S65: Perform three-dimensional image visualization on the fused three-dimensional point cloud data through the matplotlib function to obtain a reconstructed three-dimensional image.

[0049] The three-dimensional topology of the plasma model in S7 includes the electron density spatial distribution and the laser field spatial distribution, and the formula is: The three-dimensional evolution equation of electron density is:

[0050] in, is the electron density, For time, is the impact ionization coefficient, is the laser light source term, for Photon ionization coefficient, is the electron relaxation time; represents impact ionization, the second term represents the multiphoton ionization term, the third term represents the relaxation term, which expresses the relaxation process of electron density over time; Considering that the input of femtosecond laser energy needs to be described in three-dimensional space, the laser light source term is The formula under action is:

[0051] in, is the maximum intensity of the laser beam waist, is a natural constant, is the absorption rate of the material for laser, describing the attenuation of the laser in depth, is the spot radius.

[0052] The three-dimensional extension of the two-temperature model in S7 includes the evolution of electron temperature and lattice temperature, and the formula is: The evolution equation of electron temperature is:

[0053] in, For electronic hot melt, is the electron temperature, is the temperature variable, is the Hamiltonian operator, is the electronic thermal conductivity, is the electron-lattice coupling coefficient, is the lattice temperature, is the deposition of laser energy;

[0054] The evolution equation of the lattice temperature is:

[0055] in, is the lattice heat capacity.

[0056] In embodiment 1 of the present invention, Figure 2 The figure shows a schematic diagram of the optical path propagation structure of the pump-detection front end. The femtosecond laser source outputs pulsed laser with a central wavelength of 800nm, a pulse width of 50fs, and a repetition rate of 1kHz. The repetition rate can be adjusted from 1Hz to 1kHz through frequency division. The pulsed light is divided into two pulses with different energy ratios by a beam splitter, with the pump light accounting for 70% and the probe light accounting for 30%. The energy of the pump light can be adjusted by a combination of a linear polarizer and a half-wave plate. The probe light is incident on the delay system, which is composed of four reflectors to form a retroreflection system and is placed on a one-dimensional linear translation stage. By adjusting the propagation path of the probe light in the delay system, the propagation path of the probe light can be made different from that of the pump light, thereby enabling the probe light to measure dynamic information under ultrafast delay. Depending on the movement accuracy of the one-dimensional translation stage, the minimum delay step can reach 10fs, and depending on the length of the one-dimensional translation stage, the maximum delay can reach 5ns. The detection light passes through a frequency doubling crystal to double the fundamental frequency light of 800nm ​​to 400nm, and the fundamental frequency light of 800nm ​​is filtered out by a filter. After being focused by a lens, the detection light is irradiated to the surface of the sample, and the reflected signal returns along the original path to be received by the camera. The filter and lens at the front end of the camera adjust the image size and image quality.

[0057] like Figure 3 The center left image shows a schematic diagram of the reflective imaging principle. The light output by an ultrafast pulsed laser is split by a beamsplitter into a pump beam and a probe beam. The pump beam is used to excite and ablate the sample under test, while the probe beam, after being delayed by a delay stage, is used to detect ultrafast image information at a specific moment. In reflective imaging, the pump beam is focused by the objective lens and incident perpendicularly on the sample surface, while the probe beam is focused by the objective lens and incident obliquely on the sample surface at a specific angle. The reflected light is captured by a camera.

[0058] Reflective imaging devices such as Figure 4As shown, the pump light is focused and incident perpendicularly on the sample surface. The linkage mechanism couples the optical path propagation elements. The detection light passes through the first link 1, the second link 3, and the third link 5 before entering the focusing objective 6 and incident on the sample 11 at a certain angle. The reflected or transmitted light signal is collected by the first camera 7 fixed to the fourth link 8. After differential background subtraction processing, the influence of the background environment and noise is reduced, and high-quality ultrafast image information is obtained. By rotating the second link 3 and the fifth link 10, the angle of the entire detection and imaging system relative to the sample can be changed, achieving an angularly continuous image detection mode. By rotating the third link 5 and the fourth link 8, the imaging mode is reflected when the focusing objective 6 and the first camera 7 are symmetrical.

[0059] like Figure 5 The image is acquired using a reflective method. In this example, the acquisition material is a silicon wafer with dimensions of 10mm×10mm×1mm and a polished surface. The reflected light signal is acquired using the above method, and after background subtraction, a differential reflection image is output. In this example, the time delay is set to 100 picoseconds. The central region is excited by the obliquely incident pump spot, exhibiting an elliptical spot shape. The grayscale value at the center of the ellipse is uniform, and the reflectivity of the black surface decreases. The outer ring contains wavy diffraction fringes. This primarily reflects the phenomenon of liquefaction of the material surface layer and the generation of diffraction rings in the probe light at the ultrafast scale of the femtosecond laser processing process, reflecting the interaction mechanism between the ultrafast laser and the material and the optical characteristics of the ultrafast phase transition.

[0060] In step S6, the 3D model is reconstructed from the 2D image as follows: (1) Reflectance preprocessing: Each reflectance image obtained in the experiment needs to go through two steps: image alignment and normalization. During the acquisition process, jitter of the sample or vibration of the equipment will cause the image to be unable to align with the same excitation area in space. This can be corrected by the image registration scikit-image function. In reflectance images at different angles, the incident light is reflected from the sample surface and enters the camera. The angle causes the collected light intensity to differ. Therefore, the image needs to be normalized to the same standard using the normalized difference method to ensure data consistency.

[0061] (2) Coordinate transformation: Two-dimensional graphics at different angles can be viewed as projections of different viewing angles on the sample surface. By constructing a global three-dimensional coordinate system, two-dimensional data from all angles can be counted. By transforming the pixels of the two-dimensional graphics, the data can be converted into three-dimensional point cloud data.

[0062] (3) 3D point cloud generation and fusion. Each 2D image at each angle corresponds to a set of 3D point clouds. The intensity value of each point in the point cloud is derived from the reflectivity data of the 2D image. The point cloud data from multiple angles are merged into a single global point cloud using a weighted average method to process the intensity values ​​of repeated areas.

[0063] (4) Three-dimensional image reconstruction: The fused three-dimensional point cloud data is used to visualize the three-dimensional image through functions such as matplotlib, and a single three-dimensional image is finally obtained by fusing the two-dimensional transient reflectivity from multiple angles, which truly restores the full three-dimensional information of the dynamic process generated after femtosecond laser processing of materials.

[0064] In step S7, the three-dimensional plasma and dual-temperature model expansion calculation steps are as follows: (1) The plasma model is used to analyze the electronic properties of materials after femtosecond laser excitation, including the evolution of properties such as electron density and temperature. Its three-dimensional extension involves three-dimensional dynamic processes such as the spatial distribution of electron density and the spatial distribution of the laser field.

[0065] (2) The two-temperature model is mainly used to describe the heat transfer process between electron lattices, including laser energy absorption, electron lattice coupling and other processes.

[0066] (3) The Druid model corresponds to electron density and ultrafast reflectivity, and is mainly used to compare the three-dimensional reflectivity image obtained from the pump-probe experiment with the simulated reflectivity image inferred from the electron density. The relationship between reflectivity and refractive index is given by the Fresnel formula:

[0067] in, is the material reflectivity; The complex refractive index and dielectric constant are expressed as follows:

[0068] in, is the complex refractive index of the material, represents the dielectric constant, is the frequency, is the dielectric constant of vacuum, is the imaginary unit, represents the plasma frequency, It represents the electron collision frequency and describes the interaction between electrons and the lattice.

[0069] The plasma frequency can be calculated as:

[0070] in, represents the effective mass of the electron, Represents the electron charge. Therefore, the above formula can correspond to the relationship between reflectivity and electron density, thereby calculating the transient reflectivity value, and corresponding to the 3D image, to obtain a comparison between the experiment and the model, and further describe the 3D dynamic process of femtosecond laser processing.

[0071] In embodiment 2 of the present invention, Figure 2 Figure 1 shows the optical propagation structure of the pump-probe front-end. The femtosecond laser source outputs pulsed laser light with a central wavelength of 800 nm, a pulse width of 50 fs, and a repetition rate of 1 kHz. Frequency division allows for adjustable repetition rates from 1 Hz to 1 kHz. A beam splitter splits the pulsed light into two pulses with different energy ratios: 70% pump light and 30% probe light. The pump light energy is adjusted using a combination of a linear polarizer and a half-wave plate. The probe light is then incident on a time-delay system, which consists of four reflectors forming a retroreflection system and is placed on a one-dimensional linear translation stage. By adjusting the optical path length of the probe light in the time-delay system, the path length of the probe light can be adjusted to differ from that of the pump light, enabling the probe light to measure dynamic information under ultrafast time delays. Depending on the accuracy of the one-dimensional translation stage, the minimum delay step can reach 10 fs, and depending on the length of the one-dimensional translation stage, the maximum delay can reach 5 ns. The probe light passes through a frequency-doubling crystal, doubling the 800 nm fundamental frequency to 400 nm. A filter removes the 800 nm fundamental frequency. A camera and lighting system are installed along the probe light propagation path to observe the location of the pump light excitation, facilitating the determination of the overlap of the subsequent probe light and pump light positions. After the probe light position is determined by reflection, the pump light excites the material, the probe light is focused on the pumped region, and the transmission signal is received by another camera.

[0072] Figure 3 The center right figure illustrates the principle of transmission imaging. Light from an ultrafast pulsed laser is split by a beamsplitter into a pump beam and a probe beam. The pump beam excites and ablates the sample under test, while the probe beam, after being delayed by a delay stage, is used to detect ultrafast image information at a specific moment. In transmission imaging, the probe beam is focused by the objective lens, transmitted through the sample at a specific angle, and captured by a camera.

[0073] The pump light focusing objective used in this example is a 150 mm focal length plano-convex lens, and the probe light focusing objective is a 20X lens with a numerical aperture of 0.5. The acquisition method used in this example involves collecting a background image with the probe light before the pump light is applied, an ultrafast image with a time delay after the pump light is turned on, and a steady-state image collected after a long period of detection after the pump light is applied. The difference between the ultrafast image and the background image yields an ultrafast image with a high signal-to-noise ratio.

[0074] Transmission imaging devices such as Figure 4As shown, the pump light is focused and incident perpendicularly on the sample surface. The linkage mechanism couples the optical path propagation elements. The detection light passes through the first link 1, the second link 3, and the third link 5 before entering the focusing objective 6 and incident on the sample 11 at a certain angle. The reflected or transmitted light signal is collected by the first camera 7 fixed to the fourth link 8. After differential background subtraction processing, the influence of the background environment and noise is reduced, and high-quality ultrafast image information is obtained. By rotating the second link 3 and the fifth link 10, the angle of the entire detection and imaging system relative to the sample can be changed, achieving an angularly continuous image detection mode. By rotating the third link 5 and the fourth link 8, the imaging mode is transmission when the focusing objective 6 and the first camera 7 are parallel.

[0075] Figure 6 The image is acquired in a transmission mode. In this example, the acquisition material is a titanium sheet with dimensions of 10mm×10mm×1mm, and both the surface and sides are polished. The transmitted light signal is acquired using the above method, and after background subtraction, a differential transmission image is output. In this example, the time delay is set to 100 picoseconds. The central area is excited by the incident pump light spot, and the signal returned by the detection light shows a circular outline. In the upper air layer, a diffraction ring is formed at the edge of the outline, and internal propagation can be weakly observed inside the lower material. This mainly reflects the generation and propagation of material plasma at the ultrafast scale of the femtosecond laser processing process, and the generation of shock waves that expand in the air, reflecting the interaction mechanism between ultrafast lasers and materials and the optical characteristics of ultrafast phase transitions.

[0076] The image processing of transmission pump detection and the reconstruction method of the two-dimensional image into a three-dimensional image are consistent with the steps in Example 1. Ultimately, a single three-dimensional image can be obtained by fusing the two-dimensional transient transmittances from multiple angles, which truly restores the three-dimensional ultrafast processes such as plasma generation and propagation, shock wave expansion, etc. after femtosecond laser processing of materials.

[0077] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific descriptions and embodiments. Those skilled in the art can make various other specific variations and combinations based on the technical teachings disclosed in the present invention without departing from the essence of the present invention, and such variations and combinations are still within the scope of protection of the invention.

Claims

1. A femtosecond laser three-dimensional pump-probe imaging method, characterized in that: The following steps are involved: S1: Use ultrashort pulse laser to generate laser pulses, and generate pump light and detection light through the beam splitter; S2: The pump light is focused by the objective lens and irradiated onto the sample surface to stimulate the electronic dynamics of the material and produce phase change. The probe light is adjusted by a one-dimensional delay stage to produce a fixed delay time with the pump light. The wavelength is then changed by a frequency doubling crystal, and after the fundamental frequency light is filtered out by a bandpass filter, it is focused on the surface or inside of the sample through the objective lens. S3: Switch between the reflection imaging mode and the transmission imaging mode by adjusting the imaging system. In the reflection imaging mode, the detection light is reflected from the sample surface to the camera. In the transmission imaging mode, the detection light passes through the sample and is transmitted to the camera. S4: Synchronize the ultrashort pulse laser and the camera through the signal generator, output high-level pulses to make the ultrashort pulse laser output a single beam of pulse light, and make the camera receive the detection light signal and collect background images, transient images and relaxation steady-state images; S5: By adjusting the rotation angle of the optical propagation system, the spatial angle between the imaging system and the sample is changed to achieve multi-angle three-dimensional imaging of the sample; S6: After obtaining multi-angle 2D images through pump-probe, the multi-angle 2D images are aligned, normalized, coordinate-transformed, and fused with 3D point clouds to reconstruct the 3D image; S7: Based on the three-dimensional extension of the plasma model and the two-temperature model, the dynamic evolution of electrons and the energy transfer process are analyzed to restore the three-dimensional ultrafast information of femtosecond laser ablation materials.

2. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The laser pulse width in S1 is in the range of 10 fs to 1 ns, and the laser energy adjustment range is 1 μJ to 1 J.

3. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: In S2, the one-dimensional time-delay translation stage is: two reflectors form a retroreflective system, a one-dimensional electric translation stage is equipped with the retroreflective system to control the moving optical path, and a pair of apertures are used for collimation of different optical paths.

4. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The imaging system in S3 includes a plano-convex lens, a filter, and a camera, and is coupled to the back end of the optical path propagation system.

5. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The reflective imaging mode in S3 is as follows: adjusting the rotation angle of the imaging system to change the positional relationship between the focusing lens and the camera to make them symmetrical; The transmission imaging mode is: adjusting the rotation angle of the imaging system to change the position between the focusing lens and the camera so that they are parallel.

6. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The optical path propagation system in S5 is composed of a connecting rod coupling a reflector, a half-wave plate and a filter, and the connecting rods are connected by a rotating mechanism.

7. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The S6 includes the following sub-steps: S61: Align multi-angle 2D images using image registration scikit-image functions; S62: Normalize the multi-angle two-dimensional image using the normalized difference method. The formula is: in, is the differential reflectivity, is the collected instantaneous reflectivity, is the original reflectivity of the substrate; S63: Map the position of each pixel in the multi-angle two-dimensional image to a three-dimensional coordinate system to obtain corresponding three-dimensional point cloud data. The formula is: in, 、 and is the transformed three-dimensional coordinate, is the distance from the reflected image pixel to the sample center, is the horizontal angle, The tilt angle for pump detection; S64: The 3D point cloud data from multiple angles are combined into a single global point cloud using a weighted average method. The formula is: in, is the single global point cloud after merging, For the The weight of the point, For the The reflectivity of a point, is the total number of data points; S65: Perform three-dimensional image visualization on the fused three-dimensional point cloud data through the matplotlib function to obtain a reconstructed three-dimensional image.

8. The femtosecond laser three-dimensional pump-probe imaging method according to claim 1, characterized in that: The three-dimensional topology of the plasma model in S7 includes the electron density spatial distribution and the laser field spatial distribution, and the formula is: The three-dimensional evolution equation of electron density is: in, is the electron density, For time, is the impact ionization coefficient, is the laser light source term, for Photon ionization coefficient, is the electron relaxation time; Laser light source The formula under action is: in, is the maximum intensity of the laser beam waist, is a natural constant, is the absorption rate of the material for laser, is the spot radius.

9. The femtosecond laser three-dimensional pump-probe imaging method according to claim 8, characterized in that: The three-dimensional extension of the two-temperature model in S7 includes the evolution of electron temperature and lattice temperature, and the formula is: The evolution equation of electron temperature is: in, For electronic hot melt, is the electron temperature, is the temperature variable, is the Hamiltonian operator, is the electronic thermal conductivity, is the electron-lattice coupling coefficient, is the lattice temperature, is the deposition of laser energy; The evolution equation of the lattice temperature is: in, is the lattice heat capacity.

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