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Ultrafast Optics|13 Article(s)
Streaking Time Delay and the Oscillation Amplitude of the Momentum Shift
Mengfei XIE, and Weichao JIANG
In usual attosecond streaking schemes, an Extreme Ultraviolet (XUV) pulse of a few hundred attoseconds serving as a pump and a phase-controlled few-cycle Infrared (IR) pulse as a probe. XUV photon can excite the bound electron in atoms to continuum states, resulting in ionization. The ionized electrons can then be accelerated, decelerated, or deflected by the external IR laser field, generating attosecond-level temporal resolution. In the presence of a polarized IR laser pulse, ionization of a bound electron is achieved by absorbing an XUV photon, the energy of the photoelectron ejected forward along the laser polarization is affected by the attosecond streaking of the external IR pulse and depends on the phase of the IR pulse at the moment of ionization. By analyzing attosecond streaking spectra, the photoemission delay can be determined from the final kinetic energy oscillation associated with the relative delay between XUV and IR fields. Both the streaking time delay and amplitude of the energy oscillation depend on the coupling of the potential and the detected IR field. The determination of streaking time delay is a complex task that involves taking into account a variety of effects, such as the Eisenbud-Wigner-Smith (EWS) time delay,Coulomb-Laser Coupling (CLC), electron correlation and dipole-laser coupling. In the context of streaking time delay for ground-state hydrogenic atoms, it is commonly acknowledged that two factors account for this delay. The first is the EWS delay, resulting from the potential's short-range behavior. The second is the CLC delay, due to the joint influence of the IR pulse and the long-range Coulomb potential. Attosecond streaking provides unprecedented insights into the dynamics of time-resolved photoelectron emission in atoms. Moreover, in addition to conventional linearly polarized fields, researchers have recently combined bicircular fields with streaking. This approach enables ionization-time retrieval with remarkable few-attosecond precision. While exploring the streaking dynamics, most of the aforementioned reports have focused mainly on the streaking time delay and have not discussed the oscillation amplitude of the momentum shift of the electron, which defines the strength of the IR field if measured from the spectra, explicitly. However, to truly understand the dynamics, we need to consider the oscillation amplitudeκin addition to the streaking time delay. For low kinetic energies of photoelectrons, κ is greater than 1, while for high kinetic energies, κ approaches the SFA limit of 1. The duration of XUV pulses also has a significant impact on the amplitude of momentum displacement oscillations.In this work, by solving the three-dimensional time-dependent Schr?dinger equations for helium atoms and establishing a numerical analytical model for Weak Field Approximation (WFA), the streaking time delay and oscillation amplitude of the momentum shift were studied. Our findings show that altering the energy of XUV photons and the wavelength of infrared light has a significant impact on both the streaking time delay and the amplitude of momentum shift. Additionally, although the streaking time delay is not sensitive to changes in the duration of the XUV pulse, the oscillation amplitude does changes. The WFA model effectively explains this phenomenon observed in TDSE calculations by accounting for the fact that photoelectrons can be ionized not only at the peak of the XUV pulse but also during times when the XUV field is not negligible. Through averaging over the initial ionization times, we have obtained an analytical estimation for the XUV duration dependence of the κ oscillation amplitude. This equation accurately describes the reduction in the κ amplitude observed in TDSE calculations with increasing XUV pulse duration. The Coulomb effect has a significant impact on the streaking method. Therefore, we used two different model potentials to calculate helium atoms and investigated the influence of different initial electron emission positions on the streaking time delay and oscillation amplitude of the momentum shift. It was found that both the streaking time delay and the oscillation amplitude of the momentum shift are greatly influenced by the choice of initial ionization position. Ultimately, it was discovered that only when the initial ionization position for helium atoms is selected near 0.87 a.u., can we achieve a good agreement between TDSE results and the WFA results. The WFA model, by simplifying the electron's trajectory into a classical mechanics problem, transforms the abstract quantum dynamical process into a series of intuitively understandable electron motions and energy changes, providing an intuitive framework to comprehend the electron's behavior in both XUV and IR fields. In usual attosecond streaking schemes, an Extreme Ultraviolet (XUV) pulse of a few hundred attoseconds serving as a pump and a phase-controlled few-cycle Infrared (IR) pulse as a probe. XUV photon can excite the bound electron in atoms to continuum states, resulting in ionization. The ionized electrons can then be accelerated, decelerated, or deflected by the external IR laser field, generating attosecond-level temporal resolution. In the presence of a polarized IR laser pulse, ionization of a bound electron is achieved by absorbing an XUV photon, the energy of the photoelectron ejected forward along the laser polarization is affected by the attosecond streaking of the external IR pulse and depends on the phase of the IR pulse at the moment of ionization. By analyzing attosecond streaking spectra, the photoemission delay can be determined from the final kinetic energy oscillation associated with the relative delay between XUV and IR fields. Both the streaking time delay and amplitude of the energy oscillation depend on the coupling of the potential and the detected IR field. The determination of streaking time delay is a complex task that involves taking into account a variety of effects, such as the Eisenbud-Wigner-Smith (EWS) time delay,Coulomb-Laser Coupling (CLC), electron correlation and dipole-laser coupling. In the context of streaking time delay for ground-state hydrogenic atoms, it is commonly acknowledged that two factors account for this delay. The first is the EWS delay, resulting from the potential's short-range behavior. The second is the CLC delay, due to the joint influence of the IR pulse and the long-range Coulomb potential. Attosecond streaking provides unprecedented insights into the dynamics of time-resolved photoelectron emission in atoms. Moreover, in addition to conventional linearly polarized fields, researchers have recently combined bicircular fields with streaking. This approach enables ionization-time retrieval with remarkable few-attosecond precision. While exploring the streaking dynamics, most of the aforementioned reports have focused mainly on the streaking time delay and have not discussed the oscillation amplitude of the momentum shift of the electron, which defines the strength of the IR field if measured from the spectra, explicitly. However, to truly understand the dynamics, we need to consider the oscillation amplitudeκin addition to the streaking time delay. For low kinetic energies of photoelectrons, κ is greater than 1, while for high kinetic energies, κ approaches the SFA limit of 1. The duration of XUV pulses also has a significant impact on the amplitude of momentum displacement oscillations.In this work, by solving the three-dimensional time-dependent Schr?dinger equations for helium atoms and establishing a numerical analytical model for Weak Field Approximation (WFA), the streaking time delay and oscillation amplitude of the momentum shift were studied. Our findings show that altering the energy of XUV photons and the wavelength of infrared light has a significant impact on both the streaking time delay and the amplitude of momentum shift. Additionally, although the streaking time delay is not sensitive to changes in the duration of the XUV pulse, the oscillation amplitude does changes. The WFA model effectively explains this phenomenon observed in TDSE calculations by accounting for the fact that photoelectrons can be ionized not only at the peak of the XUV pulse but also during times when the XUV field is not negligible. Through averaging over the initial ionization times, we have obtained an analytical estimation for the XUV duration dependence of the κ oscillation amplitude. This equation accurately describes the reduction in the κ amplitude observed in TDSE calculations with increasing XUV pulse duration. The Coulomb effect has a significant impact on the streaking method. Therefore, we used two different model potentials to calculate helium atoms and investigated the influence of different initial electron emission positions on the streaking time delay and oscillation amplitude of the momentum shift. It was found that both the streaking time delay and the oscillation amplitude of the momentum shift are greatly influenced by the choice of initial ionization position. Ultimately, it was discovered that only when the initial ionization position for helium atoms is selected near 0.87 a.u., can we achieve a good agreement between TDSE results and the WFA results. The WFA model, by simplifying the electron's trajectory into a classical mechanics problem, transforms the abstract quantum dynamical process into a series of intuitively understandable electron motions and energy changes, providing an intuitive framework to comprehend the electron's behavior in both XUV and IR fields.
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0132001 (2025)
Study on the Control System of Picosecond High-voltage Pulse of Framing Camera
Haoming SHUAI, Yanli BAI, and Xingguo QIN
The Microchannel Plate (MCP) and Pulse-dilation Framing Cameras (PDFC) are ultra-fast diagnostic devices,which used to study the spatio-temporal evolution of plasma in the inertial confined fusion. Generally,the control system includes the industrial control system of the streak camera and the group control system of the framing camera. Although both can realize the power on and off,the selection of the DC voltage gear and the monitoring of relevant environmental parameters,they still can not realize the dynamic control of Picosecond High-voltage Pulse (PHVP). Therefore,designing an output performance control system for PHVP generators is not only facilitates an in-depth analysis of the relationship between PHVP and the Temporal Resolution(TR) performance of Framing Camera (FC),but also addressing the lack of dynamic control in the industrial control systems of ultra-fast diagnostic equipment. In order to achieve the control of dynamic performance pulse output,an analysis of the impact of PHVP on the TR of the FC is conducted. Firstly,STM32 master control technology and relays are utilized to design a multi-channel control circuit system. A PHVP generator is designed using the avalanche transistors and the LC high-pass filters,and the two are integrated to achieve control over the PHVP output performance. Subsequently,the photoelectron multiplication model within the MCP channel is established,and the calculation methods for the MCP and PDFC are analyzed. Finally,the PHVP is applied to the study of the TR of the FC,and the influence of dynamic performance of the PHVP on TR is analyzed. In the aspect of study results,the working principle and properties of single chip microcomputer and relay are studied and analyzed. Meanwhile,by combining these two,a multi-channel gating control system is designed . The PHVP generator integrated with the multi-channel gating control system is designed using the avalanche transistors and the LC high-pass filters,it is enabled to produce eight types of V-shaped PHVPs with varying performance characteristics,which the range of the amplitude,the full width at half maximum and the rising slope are -1.212~-1.887 kV,245~327 ps and 3.61~10.01 V/ps,respectively. The Dynamic Multiplication Model of Photoelectrons (DMMP) of MCP is established by Monte Carlo method and the eight V-shaped PHVPs are loaded onto the DMMP. Through the calculation and statistics of the electron gain,the normalized time-gain curve on the MCP is obtained,the impact of the PHVP on the temporal resolution of the MCP framing camera is analyzed. The range of the TR is 95.23~121.79 ps. According to the working principle of PDFC,a calculation method for determining the TR of PDFC is derived. Furthermore,the rising edge of the V-shaped PHVP is utilized for pulse dilation,with consideration of factors such as the TR of the MCP,cathode potential,and drift distance,to investigate the influence of the PHVPs on the TR of PDFC,and the fluctuation range is 1.5~31.4 ps. The multi-channel gating control system is designed by the STM32 microcontroller and relays,the PHVP circuit is constructed using the avalanche triodes and the LC high-pass filters. Meanwhile,by combining these two systems,the multi-channel-controlled of the PHVP generator is built,and the different performance PHVP are applied to the analysis of the temporal resolution of framing camera. The research results are indicated that,the PHVP generator integrated with the multi-channel gating control system can output eight types of V-shaped PHVP. When these pulses are applied to the DMMP of the MCP channel,the TR of the MCP framing camera is obtained. By further integrating the pulses with the cathode voltage and drift distance,and applying them to the PDFC,the TR of the PDFC is obtained. This study provides a new reference direction for improving the functionality of framing camera control systems,especially in terms of dynamic control. It also facilitates an in-depth analysis of the relationship between PHVP and the TR of framing camera. Additionally,it offers a reference method for integrating automatic control technology in the field of ultrafast diagnosis and high-power pulses. In future work,we will continue to conduct in-depth research on the dynamic control system of framing camera,meanwhile,explore new devices and ideas for optimizing picosecond pulse circuits. The Microchannel Plate (MCP) and Pulse-dilation Framing Cameras (PDFC) are ultra-fast diagnostic devices,which used to study the spatio-temporal evolution of plasma in the inertial confined fusion. Generally,the control system includes the industrial control system of the streak camera and the group control system of the framing camera. Although both can realize the power on and off,the selection of the DC voltage gear and the monitoring of relevant environmental parameters,they still can not realize the dynamic control of Picosecond High-voltage Pulse (PHVP). Therefore,designing an output performance control system for PHVP generators is not only facilitates an in-depth analysis of the relationship between PHVP and the Temporal Resolution(TR) performance of Framing Camera (FC),but also addressing the lack of dynamic control in the industrial control systems of ultra-fast diagnostic equipment. In order to achieve the control of dynamic performance pulse output,an analysis of the impact of PHVP on the TR of the FC is conducted. Firstly,STM32 master control technology and relays are utilized to design a multi-channel control circuit system. A PHVP generator is designed using the avalanche transistors and the LC high-pass filters,and the two are integrated to achieve control over the PHVP output performance. Subsequently,the photoelectron multiplication model within the MCP channel is established,and the calculation methods for the MCP and PDFC are analyzed. Finally,the PHVP is applied to the study of the TR of the FC,and the influence of dynamic performance of the PHVP on TR is analyzed. In the aspect of study results,the working principle and properties of single chip microcomputer and relay are studied and analyzed. Meanwhile,by combining these two,a multi-channel gating control system is designed . The PHVP generator integrated with the multi-channel gating control system is designed using the avalanche transistors and the LC high-pass filters,it is enabled to produce eight types of V-shaped PHVPs with varying performance characteristics,which the range of the amplitude,the full width at half maximum and the rising slope are -1.212~-1.887 kV,245~327 ps and 3.61~10.01 V/ps,respectively. The Dynamic Multiplication Model of Photoelectrons (DMMP) of MCP is established by Monte Carlo method and the eight V-shaped PHVPs are loaded onto the DMMP. Through the calculation and statistics of the electron gain,the normalized time-gain curve on the MCP is obtained,the impact of the PHVP on the temporal resolution of the MCP framing camera is analyzed. The range of the TR is 95.23~121.79 ps. According to the working principle of PDFC,a calculation method for determining the TR of PDFC is derived. Furthermore,the rising edge of the V-shaped PHVP is utilized for pulse dilation,with consideration of factors such as the TR of the MCP,cathode potential,and drift distance,to investigate the influence of the PHVPs on the TR of PDFC,and the fluctuation range is 1.5~31.4 ps. The multi-channel gating control system is designed by the STM32 microcontroller and relays,the PHVP circuit is constructed using the avalanche triodes and the LC high-pass filters. Meanwhile,by combining these two systems,the multi-channel-controlled of the PHVP generator is built,and the different performance PHVP are applied to the analysis of the temporal resolution of framing camera. The research results are indicated that,the PHVP generator integrated with the multi-channel gating control system can output eight types of V-shaped PHVP. When these pulses are applied to the DMMP of the MCP channel,the TR of the MCP framing camera is obtained. By further integrating the pulses with the cathode voltage and drift distance,and applying them to the PDFC,the TR of the PDFC is obtained. This study provides a new reference direction for improving the functionality of framing camera control systems,especially in terms of dynamic control. It also facilitates an in-depth analysis of the relationship between PHVP and the TR of framing camera. Additionally,it offers a reference method for integrating automatic control technology in the field of ultrafast diagnosis and high-power pulses. In future work,we will continue to conduct in-depth research on the dynamic control system of framing camera,meanwhile,explore new devices and ideas for optimizing picosecond pulse circuits.
Acta Photonica Sinica
- Publication Date: Dec. 25, 2024
- Vol. 53, Issue 12, 1232002 (2024)
Spatial Resolution of Compressed Ultrafast Photography System
Lijuan XIANG, Fangding YAO, Zeng YE, Jinyuan LIU, and Houzhi CAI
Ultrafast optical imaging technology can acquire image signals of ultrafast scenes in the picosecond or even femtosecond scale. As one of the major devices for ultrafast imaging,streak camera with picosecond-scale time resolution and one-dimensional spatial resolution is commonly used for ultrafast diagnosis of Inertial Confinement Fusion (ICF). Combining streak camera with compressed sensing theory,compressed ultrafast photography has been proposed,which is capable of two-dimensional imaging of ultrafast dynamic scenes at 100 billion frames per second. It can achieve up to hundreds of frames per acquisition without specialized modulated illumination,which has a performance far beyond traditional imaging technologies.The main components of compressed ultrafast photography system include streak camera,Digital Micromirror Device (DMD),and optical path system. The DMD is employed to encode the two-dimensional image signal,whereafter the encoded signal passes through the photocathode,scanning electric field and fluorescent screen of the streak camera successively,and finally arrives at the CCD. Deflected by the scanning electric field in the perpendicular direction,the encoded images of different moments shift and overlap on the fluorescent screen,forming the sampled signal captured by the CCD. Utilizing the reconstruction algorithm,frames of two-dimensional image signals can be reconstructed from the sampled signal. In this paper,a Generalized Alternating Projection (GAP) algorithm based on the PnP framework is used to reconstruct the original images. Based on the image prior of gradient sparsity,the Total Variation (TV) denoise operator is combined with the GAP algorithm for image denoising and meanwhile remaining image detail. The algorithm decomposes the solving operation of the reconstructed images into two sub-processes of alternating projection,and continuously approaches the solution that satisfies all the constraints in the alternating iteration operation.In order to analyze the spatial resolution and imaging quality of the compressed ultrafast photography system,this paper carries out imaging simulation experiments based on MATLAB software and measures the spatial resolution of the system. A mathematical model of image signal acquisition is established according to the working principle of compressed ultrafast photography,with which the sampled signals are obtained. The GAP-TV reconstruction algorithm is employed to iteratively solve and reconstruct the original images from the sampled signal. The simulation results show that the reconstructed images can achieve a dynamic spatial resolution of 10 lp/mm under the frame number of 8. Due to the temporal compressed sampling,the dynamic spatial resolution decreases by 5 lp/mm compared to static imaging.The factors affecting the imaging quality and spatial resolution are researched and analyzed,and the imaging simulation is performed using different DMD coding sampling rates. Higher image quality can be obtained using coding sampling rates between 30% and 50%,and highest spatial resolution of 10 lp/mm can be reached with the coding sampling rates from 40% to 50% when simulating imaging with 8 frames of images. Both large frame number and high coding sampling rate can cause signal aliasing,and thereby lead to the decrease of image quality. Therefore,in order to further analyze the relationship between coding sampling rate and imaging quality,imaging quality simulations were performed with data sets of 16,24 and 32 frames,respectively. It is demonstrated that when the system images dynamic scenes of more frames,better imaging quality can be obtained with lower coding sampling rates due to the reduction of signal aliasing. In order to acquire the optimal coding sampling rate for the highest imaging quality,sets of image data are used for simulation,and the optimal coding sampling rate fitted curve is plotted based on the simulation results. Comparative tests verify that the coding sampling rates from the fitted curve result in better imaging quality,which helps the system to achieve the best spatial resolution. Ultrafast optical imaging technology can acquire image signals of ultrafast scenes in the picosecond or even femtosecond scale. As one of the major devices for ultrafast imaging,streak camera with picosecond-scale time resolution and one-dimensional spatial resolution is commonly used for ultrafast diagnosis of Inertial Confinement Fusion (ICF). Combining streak camera with compressed sensing theory,compressed ultrafast photography has been proposed,which is capable of two-dimensional imaging of ultrafast dynamic scenes at 100 billion frames per second. It can achieve up to hundreds of frames per acquisition without specialized modulated illumination,which has a performance far beyond traditional imaging technologies.The main components of compressed ultrafast photography system include streak camera,Digital Micromirror Device (DMD),and optical path system. The DMD is employed to encode the two-dimensional image signal,whereafter the encoded signal passes through the photocathode,scanning electric field and fluorescent screen of the streak camera successively,and finally arrives at the CCD. Deflected by the scanning electric field in the perpendicular direction,the encoded images of different moments shift and overlap on the fluorescent screen,forming the sampled signal captured by the CCD. Utilizing the reconstruction algorithm,frames of two-dimensional image signals can be reconstructed from the sampled signal. In this paper,a Generalized Alternating Projection (GAP) algorithm based on the PnP framework is used to reconstruct the original images. Based on the image prior of gradient sparsity,the Total Variation (TV) denoise operator is combined with the GAP algorithm for image denoising and meanwhile remaining image detail. The algorithm decomposes the solving operation of the reconstructed images into two sub-processes of alternating projection,and continuously approaches the solution that satisfies all the constraints in the alternating iteration operation.In order to analyze the spatial resolution and imaging quality of the compressed ultrafast photography system,this paper carries out imaging simulation experiments based on MATLAB software and measures the spatial resolution of the system. A mathematical model of image signal acquisition is established according to the working principle of compressed ultrafast photography,with which the sampled signals are obtained. The GAP-TV reconstruction algorithm is employed to iteratively solve and reconstruct the original images from the sampled signal. The simulation results show that the reconstructed images can achieve a dynamic spatial resolution of 10 lp/mm under the frame number of 8. Due to the temporal compressed sampling,the dynamic spatial resolution decreases by 5 lp/mm compared to static imaging.The factors affecting the imaging quality and spatial resolution are researched and analyzed,and the imaging simulation is performed using different DMD coding sampling rates. Higher image quality can be obtained using coding sampling rates between 30% and 50%,and highest spatial resolution of 10 lp/mm can be reached with the coding sampling rates from 40% to 50% when simulating imaging with 8 frames of images. Both large frame number and high coding sampling rate can cause signal aliasing,and thereby lead to the decrease of image quality. Therefore,in order to further analyze the relationship between coding sampling rate and imaging quality,imaging quality simulations were performed with data sets of 16,24 and 32 frames,respectively. It is demonstrated that when the system images dynamic scenes of more frames,better imaging quality can be obtained with lower coding sampling rates due to the reduction of signal aliasing. In order to acquire the optimal coding sampling rate for the highest imaging quality,sets of image data are used for simulation,and the optimal coding sampling rate fitted curve is plotted based on the simulation results. Comparative tests verify that the coding sampling rates from the fitted curve result in better imaging quality,which helps the system to achieve the best spatial resolution.
Acta Photonica Sinica
- Publication Date: Dec. 25, 2024
- Vol. 53, Issue 12, 1232001 (2024)
A Laser Autofocus Method Based on Light Field Distribution and Image Processing Algorithm
Shanming XI, Qisong LI, Jincheng JIANG, Wuqing HONG, and Yi LIU
Femtosecond laser direct writing technology exhibits a variety of significant advantages compared to traditional micromachining technologies, such as the high processing resolution beyond the optical diffraction limit, a wide range of processing materials, and “real” three-dimensional structures fabrication. Due to these unique advantages, it is typically used for fabricating the micro-optical devices, biomedical devices, miniaturized electromechanical systems, etc. In aspect of the fabrication resolution, it usually depends on the use of the objective lens with the high numerical aperture in the femtosecond laser fabrication setup. Caused by the small depth of field of the high numerical aperture, it is easy to produce the out of focus effect, which leads to the error of the fabricated microstructures. Thus, it is vital and significant for quickly and accurately locating the laser focus on the surface of the sample during the micro- and nanostructures fabrication process.To solve the problem of quantitatively achieving the auto-focus function in femtosecond laser three-dimensional fabrication, this paper proposes a new type of automatic focusing method that it includes the computed light field distribution and the image processing algorithms. First of all, in the situation of tightly focusing of the high numerical aperture objective lens, the laser light field distribution function in the multilayer materials after passing the objective lens are derived according to the Richards-Wolf vector diffraction theory. Furthermore, the light field distribution results at the different positions of the medium can be solved by the numerical simulation of the laser light field distribution function. In the meantime, the laser beam diameter as a function of the position in the multilayer mediums can be also theoretically obtained. Secondly, a series of the laser beam images are captured in the interval of 2 μm by moving the laser focus from top to bottom of the multilayer mediums. And the laser beam images are used for the image processing to obtain the laser beam diameter, that specific image processing algorithms are adopted here, including region of interest, open and closed operations, maximum interclass difference method, corrosion and expansion algorithm, Canny edge extraction processing algorithm processing. Next, the laser beam images after the image processing are further processed with the least squares circle fitting method to obtain the laser beam diameter at different positions in the multilayer medium and the curve of the laser beam diameter varying with different positions are also obtained. Finally, the mathematical model of the laser beam diameter and out-of-focus amount is obtained by comparing with the simulated and experimental results. The correction proportion is calculated based on the mathematical model, which is regarded as the amendatory curve. The amendatory model is used to realize the autofocus of the femtosecond laser fabrication system, which is achieved by capturing the real-time laser beam image and comparing with the curve model, then adjusting the actual position to the laser focal position. Finally, an automatic focusing method is successfully obtained based on the calculation of optical field distribution and image processing algorithm. Besides, the ultrashort pulse duration of femtosecond laser means that it has a range width of spectrums. In our experiments, the femtosecond laser from the spectrum-physics company with the central wavelength of ~800 nm and the width of spectrum of ~60 nm is used. Furthermore, the laser light field distribution is studies by considering the influence of different wavelengths, and the ratio coefficient after considering dispersion are theoretically calculated in the paper. The simulation results indicates that with the spectral width increasing, the simulation data are closer to the experimental data, that is, the calculated error ratio coefficient shows an increasing, then a decreasing trend.This kind of autofocus method can obtain the high accuracy out-of-focus amount in real-time during femtosecond laser fabrication process, thereby it can finely tune the focus position after comparing with the curve model. This method provides a solution for the microstructure fabrication of the non-planar and curved substrates and can promote the application field of femtosecond laser fabrication technology. Femtosecond laser direct writing technology exhibits a variety of significant advantages compared to traditional micromachining technologies, such as the high processing resolution beyond the optical diffraction limit, a wide range of processing materials, and “real” three-dimensional structures fabrication. Due to these unique advantages, it is typically used for fabricating the micro-optical devices, biomedical devices, miniaturized electromechanical systems, etc. In aspect of the fabrication resolution, it usually depends on the use of the objective lens with the high numerical aperture in the femtosecond laser fabrication setup. Caused by the small depth of field of the high numerical aperture, it is easy to produce the out of focus effect, which leads to the error of the fabricated microstructures. Thus, it is vital and significant for quickly and accurately locating the laser focus on the surface of the sample during the micro- and nanostructures fabrication process.To solve the problem of quantitatively achieving the auto-focus function in femtosecond laser three-dimensional fabrication, this paper proposes a new type of automatic focusing method that it includes the computed light field distribution and the image processing algorithms. First of all, in the situation of tightly focusing of the high numerical aperture objective lens, the laser light field distribution function in the multilayer materials after passing the objective lens are derived according to the Richards-Wolf vector diffraction theory. Furthermore, the light field distribution results at the different positions of the medium can be solved by the numerical simulation of the laser light field distribution function. In the meantime, the laser beam diameter as a function of the position in the multilayer mediums can be also theoretically obtained. Secondly, a series of the laser beam images are captured in the interval of 2 μm by moving the laser focus from top to bottom of the multilayer mediums. And the laser beam images are used for the image processing to obtain the laser beam diameter, that specific image processing algorithms are adopted here, including region of interest, open and closed operations, maximum interclass difference method, corrosion and expansion algorithm, Canny edge extraction processing algorithm processing. Next, the laser beam images after the image processing are further processed with the least squares circle fitting method to obtain the laser beam diameter at different positions in the multilayer medium and the curve of the laser beam diameter varying with different positions are also obtained. Finally, the mathematical model of the laser beam diameter and out-of-focus amount is obtained by comparing with the simulated and experimental results. The correction proportion is calculated based on the mathematical model, which is regarded as the amendatory curve. The amendatory model is used to realize the autofocus of the femtosecond laser fabrication system, which is achieved by capturing the real-time laser beam image and comparing with the curve model, then adjusting the actual position to the laser focal position. Finally, an automatic focusing method is successfully obtained based on the calculation of optical field distribution and image processing algorithm. Besides, the ultrashort pulse duration of femtosecond laser means that it has a range width of spectrums. In our experiments, the femtosecond laser from the spectrum-physics company with the central wavelength of ~800 nm and the width of spectrum of ~60 nm is used. Furthermore, the laser light field distribution is studies by considering the influence of different wavelengths, and the ratio coefficient after considering dispersion are theoretically calculated in the paper. The simulation results indicates that with the spectral width increasing, the simulation data are closer to the experimental data, that is, the calculated error ratio coefficient shows an increasing, then a decreasing trend.This kind of autofocus method can obtain the high accuracy out-of-focus amount in real-time during femtosecond laser fabrication process, thereby it can finely tune the focus position after comparing with the curve model. This method provides a solution for the microstructure fabrication of the non-planar and curved substrates and can promote the application field of femtosecond laser fabrication technology.
Acta Photonica Sinica
- Publication Date: Nov. 25, 2024
- Vol. 53, Issue 11, 1132002 (2024)
Development of Compressed Ultrafast Photography System for Streak Tube
Houzhi CAI, Zhuoming DU, Fangding YAO, Junkai LIU... and Lijuan XIANG|Show fewer author(s)
Ultrafast diagnosis technologies are primarily employed to acquire images and information from ultrafast phenomena in the fields of physics, optics, and materials at nanosecond even femtosecond scale. Compressed Ultrafast Photography (CUP) is a major breakthrough in the field of ultrafast diagnosis, which is one of the cutting-edge technologies to acquire information efficiently in ultrafast dynamic events.According to Nyquist Theorem, the sampling frequency must be at least twice the frequency of the original signal to ensure that the sampled signal can accurately reconstruct the original signal. But in the condition where the Compressed Sensing (CS) theory is applicable, when the signal satisfies sparsity, the original signal can be reconstructed with high quality in conjunction with a priori knowledge, even when the sampling rate is significantly lower than that required by Nyquist Theorem. The streak camera is one of the major devices for ultrafast diagnosis with ultra-high temporal resolution which is at picosecond scale. The streak camera converts the optical signal into a photoelectron beam through the photocathode. Whereafter, the streak camera deflects the photoelectron beam with a time-varying deflecting electric field, and the imaging position of photoelectron beam changes over time. Therefore, the streak camera is able to realize time-resolved images of the photoelectron beam with high temporal resolution. In order to avoid overlapping of images of different moments, a slit is added to limit the spatial range of the optical signal, thus the streak camera is only able to realize one-dimensional imaging in general. However, since CUP combining CS theory with streak camera, it has both high temporal resolution and two-dimensional spatial-resolved capability. Consequently, CUP is able to realize time-resolved two-dimensional spatial diagnosis of ultrafast events with one single shot.In this paper, a CUP system is designed and presented. Two-dimensional ultrafast diagnosis experiment is simulated, and the experimental platform of CUP system is built to diagnose the intensity evolution process of the picosecond laser pulse in both spatial and time dimensions. The CUP system is mainly composed of streak camera, DMD, optical path system and synchronous modulation module. The system first uses DMD to encode the original ultrafast image I(x, y, t). Then the encoded image transmits to the streak camera through fully opened slit. After being deflected by electric field, images of different moments are overlapped, and finally the overlapped integrated image is sampled by the CCD. In order to reconstruct the image sampled by the CCD, it is necessary to reverse solve I (x, y, t). However, this is an underdetermined problem, which can not be solved by direct operation. According to CS theory, this underdetermined problem can be transformed into an unconstrained optimization problem by regularization method based on the principle of Total Variation (TV) and the prior knowledge of the DMD coding matrix. Once CS theory was applied, each frame of the original images of different moments can be reconstructed. According to the working principle and mathematical model of the CUP system, the sampling and reconstruction processes are simulated. The peak signal-to-noise ratio of the reconstructed images is better than 45 dB, and the structural similarity is better than 99%. The intensity evolution process of the picosecond laser pulse is diagnosed by the experimental platform of CUP system. To measure the accuracy rate of the image reconstruction of the CUP system, the intensity evolution process of the picosecond laser spot is measured by the streak camera in one-dimensional spatial imaging. The accuracy rate of the reconstructed signal images of the system is 96.06%, demonstrating its ability to accurately diagnose ultrafast dynamic events. Ultrafast diagnosis technologies are primarily employed to acquire images and information from ultrafast phenomena in the fields of physics, optics, and materials at nanosecond even femtosecond scale. Compressed Ultrafast Photography (CUP) is a major breakthrough in the field of ultrafast diagnosis, which is one of the cutting-edge technologies to acquire information efficiently in ultrafast dynamic events.According to Nyquist Theorem, the sampling frequency must be at least twice the frequency of the original signal to ensure that the sampled signal can accurately reconstruct the original signal. But in the condition where the Compressed Sensing (CS) theory is applicable, when the signal satisfies sparsity, the original signal can be reconstructed with high quality in conjunction with a priori knowledge, even when the sampling rate is significantly lower than that required by Nyquist Theorem. The streak camera is one of the major devices for ultrafast diagnosis with ultra-high temporal resolution which is at picosecond scale. The streak camera converts the optical signal into a photoelectron beam through the photocathode. Whereafter, the streak camera deflects the photoelectron beam with a time-varying deflecting electric field, and the imaging position of photoelectron beam changes over time. Therefore, the streak camera is able to realize time-resolved images of the photoelectron beam with high temporal resolution. In order to avoid overlapping of images of different moments, a slit is added to limit the spatial range of the optical signal, thus the streak camera is only able to realize one-dimensional imaging in general. However, since CUP combining CS theory with streak camera, it has both high temporal resolution and two-dimensional spatial-resolved capability. Consequently, CUP is able to realize time-resolved two-dimensional spatial diagnosis of ultrafast events with one single shot.In this paper, a CUP system is designed and presented. Two-dimensional ultrafast diagnosis experiment is simulated, and the experimental platform of CUP system is built to diagnose the intensity evolution process of the picosecond laser pulse in both spatial and time dimensions. The CUP system is mainly composed of streak camera, DMD, optical path system and synchronous modulation module. The system first uses DMD to encode the original ultrafast image I(x, y, t). Then the encoded image transmits to the streak camera through fully opened slit. After being deflected by electric field, images of different moments are overlapped, and finally the overlapped integrated image is sampled by the CCD. In order to reconstruct the image sampled by the CCD, it is necessary to reverse solve I (x, y, t). However, this is an underdetermined problem, which can not be solved by direct operation. According to CS theory, this underdetermined problem can be transformed into an unconstrained optimization problem by regularization method based on the principle of Total Variation (TV) and the prior knowledge of the DMD coding matrix. Once CS theory was applied, each frame of the original images of different moments can be reconstructed. According to the working principle and mathematical model of the CUP system, the sampling and reconstruction processes are simulated. The peak signal-to-noise ratio of the reconstructed images is better than 45 dB, and the structural similarity is better than 99%. The intensity evolution process of the picosecond laser pulse is diagnosed by the experimental platform of CUP system. To measure the accuracy rate of the image reconstruction of the CUP system, the intensity evolution process of the picosecond laser spot is measured by the streak camera in one-dimensional spatial imaging. The accuracy rate of the reconstructed signal images of the system is 96.06%, demonstrating its ability to accurately diagnose ultrafast dynamic events.
Acta Photonica Sinica
- Publication Date: Nov. 25, 2024
- Vol. 53, Issue 11, 1132001 (2024)
Research on Driving Technology of Wide Microstrip Amplitude Division Imaging Based on Pulse Power Synthesis Technology
Shiduo WEI, Yongsheng GOU, Yang YANG, Penghui FENG... and Yihao YANG|Show fewer author(s)
When a pulse current with a rise time of about 100 ns and an amplitude of tens of MA is applied to a wire array or jet load, the load will rapidly ionize and form a plasma. Due to the Lorentzian force, these plasms will rapidly implode towards the axis and eventually stagnate in the center, forming a high temperature and high density plasma and further emitting strong X-rays, a process known as Z-pinch. Z-pinch has been widely used in High Energy Density (HED) physics research for decades, including radiation source development, radiation actuation science, dynamic material properties, Magneto-inertial Fusion (MIF) and Inertial Confinement Fusion (ICF). In order to explore the structure, properties and motion laws of matter in the ultra-small space and ultra-fast time scale, the research and measurement techniques of ultra-fast phenomena represented by the variometer framing camera technology have become the main tools in use.X-ray framing cameras are widely used for two-dimensional plasma imaging in the Z-pinch process. This type of frame camera requires selective pulses to excite the Microchannel Plate (MCP). Because the width of the pulse is very narrow, only a microstrip region has voltage at a time, and photoelectrons generated by the X-ray image formed through a pinhole in the region at the input surface of the MCP will be gained and be imaged to the screen on the screen. The exposure time of each image is determined by the half-width of the selected pulse and the characteristics of the framing tube. The MCP with different equivalent impedances will realize the framing camera imaging with different frames. The width and length of the transmission microstrip line of the ultra-wide frame traveling-wave selective framing camera are up to 20 mm and 95 mm, and the equivalent impedance is about 6 Ω. To actuate the beamsplitter, gating pulses with electric field peaks of more than 3 kV, pulse durations on the order of nanoseconds or hundreds of picoseconds, and spectral widths of tens to thousands of megahertz is required. In this paper, the power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. However, limited by the characteristics of the transistor device itself, the pulse source whose amplitude is higher than 5 kV and the front edge is better than 100 ps and the jitter is better than 20 ps is close to the technical limit of electronics. To obtain higher power gate pulse it is necessary to adopt multichannel pulse power synthesis technology.In this paper, a power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. The large bandwidth of the multi-section impedance converter is used to improve the working bandwidth of the power coupling, so as to meet the pulse coupling of different spectrum. The simulation software is used to design the power coupling circuit with the working frequency band of 300 MHz~3 GHz, and the loss generated in the system is optimized to achieve high efficiency coupling. Combined with the high-voltage narrow pulse output and synchronization control circuit of the preceding stage, the high-voltage pulse with peak voltage exceeding 3.2 kV is synthesized by using eight single-channel pulses with peak voltage of about 1.3 kV and pulse width of about 3.5 ns, pulse leading edge of about 600 ps. The pulse width was within 3 ns and the pulse leading edge was within 600 ps. In the pulse spectrum range of 300 MHz to 3 GHz, the two-channel synthesis efficiency is 83.5%, 88% at a specific frequency, and the eight-channel synthesis efficiency is 58%, up to 68% at a specific frequency.Finally, the coupled high-voltage pulse is input into the 20 mm microstrip amplitude-divider. The transmission line of the microchannel plate inside is 20 mm wide and 95 mm long, and the equivalent impedance is 6 Ω. The output pulse amplitude is 1.433 kV, the pulse width is 3.63 ns, and the pulse front is 747.3 ps, which fully conforms to the design requirement that the output voltage of the tube must exceed 800 V. At present, the coupling technique can generate driving pulses for use. In the future, the coupled pulses can be shaped by adjusting the delay of the eight pulses. At present, the high voltage driven pulse source based on this technology has been applied to I-MCP1.0 framing camera and can be used to explore the high energy density physics research with Z-pinch as the core. When a pulse current with a rise time of about 100 ns and an amplitude of tens of MA is applied to a wire array or jet load, the load will rapidly ionize and form a plasma. Due to the Lorentzian force, these plasms will rapidly implode towards the axis and eventually stagnate in the center, forming a high temperature and high density plasma and further emitting strong X-rays, a process known as Z-pinch. Z-pinch has been widely used in High Energy Density (HED) physics research for decades, including radiation source development, radiation actuation science, dynamic material properties, Magneto-inertial Fusion (MIF) and Inertial Confinement Fusion (ICF). In order to explore the structure, properties and motion laws of matter in the ultra-small space and ultra-fast time scale, the research and measurement techniques of ultra-fast phenomena represented by the variometer framing camera technology have become the main tools in use.X-ray framing cameras are widely used for two-dimensional plasma imaging in the Z-pinch process. This type of frame camera requires selective pulses to excite the Microchannel Plate (MCP). Because the width of the pulse is very narrow, only a microstrip region has voltage at a time, and photoelectrons generated by the X-ray image formed through a pinhole in the region at the input surface of the MCP will be gained and be imaged to the screen on the screen. The exposure time of each image is determined by the half-width of the selected pulse and the characteristics of the framing tube. The MCP with different equivalent impedances will realize the framing camera imaging with different frames. The width and length of the transmission microstrip line of the ultra-wide frame traveling-wave selective framing camera are up to 20 mm and 95 mm, and the equivalent impedance is about 6 Ω. To actuate the beamsplitter, gating pulses with electric field peaks of more than 3 kV, pulse durations on the order of nanoseconds or hundreds of picoseconds, and spectral widths of tens to thousands of megahertz is required. In this paper, the power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. However, limited by the characteristics of the transistor device itself, the pulse source whose amplitude is higher than 5 kV and the front edge is better than 100 ps and the jitter is better than 20 ps is close to the technical limit of electronics. To obtain higher power gate pulse it is necessary to adopt multichannel pulse power synthesis technology.In this paper, a power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. The large bandwidth of the multi-section impedance converter is used to improve the working bandwidth of the power coupling, so as to meet the pulse coupling of different spectrum. The simulation software is used to design the power coupling circuit with the working frequency band of 300 MHz~3 GHz, and the loss generated in the system is optimized to achieve high efficiency coupling. Combined with the high-voltage narrow pulse output and synchronization control circuit of the preceding stage, the high-voltage pulse with peak voltage exceeding 3.2 kV is synthesized by using eight single-channel pulses with peak voltage of about 1.3 kV and pulse width of about 3.5 ns, pulse leading edge of about 600 ps. The pulse width was within 3 ns and the pulse leading edge was within 600 ps. In the pulse spectrum range of 300 MHz to 3 GHz, the two-channel synthesis efficiency is 83.5%, 88% at a specific frequency, and the eight-channel synthesis efficiency is 58%, up to 68% at a specific frequency.Finally, the coupled high-voltage pulse is input into the 20 mm microstrip amplitude-divider. The transmission line of the microchannel plate inside is 20 mm wide and 95 mm long, and the equivalent impedance is 6 Ω. The output pulse amplitude is 1.433 kV, the pulse width is 3.63 ns, and the pulse front is 747.3 ps, which fully conforms to the design requirement that the output voltage of the tube must exceed 800 V. At present, the coupling technique can generate driving pulses for use. In the future, the coupled pulses can be shaped by adjusting the delay of the eight pulses. At present, the high voltage driven pulse source based on this technology has been applied to I-MCP1.0 framing camera and can be used to explore the high energy density physics research with Z-pinch as the core.
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0932002 (2023)
Generation of Femtosecond Magnetic Pulses by Circularly Polarized Vortex Laser-driven Plasma
Han WEN, Peng XU, Liangwen PI, and Yuxi FU
Research on pulsed magnetic fields dates back to the early 20th century. Nowadays, ultra-short pulsed magnetic fields are being utilized to better understand ultrafast physical microprocesses, such as domain motion and spin-orbit interaction, with time scales ranging from microseconds to femtoseconds. In particular, femtosecond magnetic field pulses are of great significance for studying ultrafast magnetization, ultrafast demagnetization, ultrafast magnetic storage, and spin ultrafast dynamics. However, traditional pulsed magnetic fields are limited by the performance of the pulse power supply and the mechanical strength of the coil and cannot achieve higher pulsed magnetic field strengths. Additionally, the pulse length of the magnetic pulse generated by the pulse power supply is at the millisecond level, which makes it unsuitable for studying faster magnetic dynamics processes. Fortunately, recent studies have shown that when ultra-short pulse lasers interact with plasmas, hot electrons are produced on the surface of the plasma target. These hot electrons are then excited and pass through the target material, producing strong charge separation on the back surface of the target material. Under the action of the laser, these excited electrons are accelerated, generating strong electromagnetic radiation. Consequently, using ultra-short pulse lasers to drive electron flows is currently the most promising method for generating femtosecond magnetic field pulses. Thus, the goal of this paper is to use a three-dimensional model to simulate the interaction between the driving optical field and the plasma target. This simulation will help to study the physical processes involved, such as the propagation of the optical field, the movement of free electrons, vortex currents, and pulse magnetic field generation. By optimizing the relevant parameters, this research aims to generate femtosecond magnetic field pulses.In this paper, we employ the Particle-In-Cell (PIC) method as our simulation approach. This method utilizes the Vlasov-Maxwell equation set to accurately describe the self-consistent dynamics in plasma simulation. The electrons in the plasma are subject to the Lorenz force, which generates new current density as they move. This equation effectively corrects the electric and magnetic fields through the charge density and current density. The driving light described is a circularly polarized vortex beam, with a wavelength of 800 nm and an optical field intensity of approximately 1016 to 1021 W/cm2. The pulse width of the beam is roughly 10 fs. The plasma density ranges from 1018 to 1020 cm-3, and is confined within a cubic space with a side length of 30 λ0. During the simulation process, we only consider refractive index changes due to electron density and do not account for non-linear effects. Additionally, we assume that the ions are stationary and that the initial velocity and temperature of the plasma are both 0.During theoretical simulation, a proportionality gradient between momentum potential and the strength of the light field is created due to the lowest intensity of the vortex beam at its center. This gradient then forms a potential well, preventing electrons from escaping outward and producing a structured electron beam with a femtosecond duration. In addition, particles acquire angular momentum in their radial motion within the laser field, generating a vortex current. This in turn produces a pulsed magnetic field based on the current magnetic effect.The simulation results indicate that when circularly polarized vortex beams, with light field intensities of the order of 1016 to 1021 W/cm2, interact with plasma densities ranging from 1018 to 1020 cm-3, they can generate ultra-short magnetic pulses with peak intensities of 0.5~50 tesla and pulse time widths of about 10 fs. The effects of driving laser intensity and plasma density on these magnetic pulses are discussed through a simulated system calculation. The results show that the pulsed magnetic field intensity is proportional to the square root of both laser intensity and plasma density. Increasing electron density and laser intensity may facilitate the generation of ultra-short strong magnetic fields, providing numerical references for the production of femtosecond magnetic pulses in experiments.We expect that the simulation results above will facilitate the introduction of ultra-strong, ultra-short magnetic pulses into the femtosecond ultrafast realm, thereby supporting the advancement of research on ultrafast magnetic and spin dynamics, electronic motion and spin microprocessing control, ultrafast spin-electron magnetic storage applications, and magnetic switching. Research on pulsed magnetic fields dates back to the early 20th century. Nowadays, ultra-short pulsed magnetic fields are being utilized to better understand ultrafast physical microprocesses, such as domain motion and spin-orbit interaction, with time scales ranging from microseconds to femtoseconds. In particular, femtosecond magnetic field pulses are of great significance for studying ultrafast magnetization, ultrafast demagnetization, ultrafast magnetic storage, and spin ultrafast dynamics. However, traditional pulsed magnetic fields are limited by the performance of the pulse power supply and the mechanical strength of the coil and cannot achieve higher pulsed magnetic field strengths. Additionally, the pulse length of the magnetic pulse generated by the pulse power supply is at the millisecond level, which makes it unsuitable for studying faster magnetic dynamics processes. Fortunately, recent studies have shown that when ultra-short pulse lasers interact with plasmas, hot electrons are produced on the surface of the plasma target. These hot electrons are then excited and pass through the target material, producing strong charge separation on the back surface of the target material. Under the action of the laser, these excited electrons are accelerated, generating strong electromagnetic radiation. Consequently, using ultra-short pulse lasers to drive electron flows is currently the most promising method for generating femtosecond magnetic field pulses. Thus, the goal of this paper is to use a three-dimensional model to simulate the interaction between the driving optical field and the plasma target. This simulation will help to study the physical processes involved, such as the propagation of the optical field, the movement of free electrons, vortex currents, and pulse magnetic field generation. By optimizing the relevant parameters, this research aims to generate femtosecond magnetic field pulses.In this paper, we employ the Particle-In-Cell (PIC) method as our simulation approach. This method utilizes the Vlasov-Maxwell equation set to accurately describe the self-consistent dynamics in plasma simulation. The electrons in the plasma are subject to the Lorenz force, which generates new current density as they move. This equation effectively corrects the electric and magnetic fields through the charge density and current density. The driving light described is a circularly polarized vortex beam, with a wavelength of 800 nm and an optical field intensity of approximately 1016 to 1021 W/cm2. The pulse width of the beam is roughly 10 fs. The plasma density ranges from 1018 to 1020 cm-3, and is confined within a cubic space with a side length of 30 λ0. During the simulation process, we only consider refractive index changes due to electron density and do not account for non-linear effects. Additionally, we assume that the ions are stationary and that the initial velocity and temperature of the plasma are both 0.During theoretical simulation, a proportionality gradient between momentum potential and the strength of the light field is created due to the lowest intensity of the vortex beam at its center. This gradient then forms a potential well, preventing electrons from escaping outward and producing a structured electron beam with a femtosecond duration. In addition, particles acquire angular momentum in their radial motion within the laser field, generating a vortex current. This in turn produces a pulsed magnetic field based on the current magnetic effect.The simulation results indicate that when circularly polarized vortex beams, with light field intensities of the order of 1016 to 1021 W/cm2, interact with plasma densities ranging from 1018 to 1020 cm-3, they can generate ultra-short magnetic pulses with peak intensities of 0.5~50 tesla and pulse time widths of about 10 fs. The effects of driving laser intensity and plasma density on these magnetic pulses are discussed through a simulated system calculation. The results show that the pulsed magnetic field intensity is proportional to the square root of both laser intensity and plasma density. Increasing electron density and laser intensity may facilitate the generation of ultra-short strong magnetic fields, providing numerical references for the production of femtosecond magnetic pulses in experiments.We expect that the simulation results above will facilitate the introduction of ultra-strong, ultra-short magnetic pulses into the femtosecond ultrafast realm, thereby supporting the advancement of research on ultrafast magnetic and spin dynamics, electronic motion and spin microprocessing control, ultrafast spin-electron magnetic storage applications, and magnetic switching.
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0932001 (2023)
Excitation and Ionization Dynamics of Atomic Rydberg States in Strong Laser Field（Invited）
Yong ZHAO, and Yueming ZHOU
When atoms are exposed to strong femtosecond laser fields, there is a high probability that the electrons in atoms can be excited to the Rydberg states with very high principal quantum numbers and large orbitals, in addition to processes such as multiphoton ionization and tunneling ionization. This highly excited state of the atom is very stable in the ultrashort strong laser pulse and is closely related to many other phenomena in the strong field, such as neutral particle acceleration, multiphoton Rabi oscillations, near-threshold harmonic radiation. It has become one of the research hotspots in the field of strong-field ultrafast physics in the last decade. Among these studies, the mechanism of the generation of Rydberg atoms in strong laser fields, the modulation of Rydberg states by lasers, and the strong-field ionization process and stability of Rydberg states are the main issues of interest.This review provided an overview of the generation mechanisms of the Rydberg states driven by strong laser fields. There are two mechanisms for strong field Rydberg state excitation. One is multiphoton resonance excitation and the other is Frustrated Tunneling Ionization (FTI) excitation. The dependence of the yield of the Rydberg state atoms on the laser ellipticity is usually considered to be an important basis for determining whether the Rydberg state atom generation mechanism is multiphoton resonance excitation or frustrated tunneling ionization. However, this judgment is not reliable. As shown in this review, for multiphoton resonance excitation, the yield of Rydberg state atoms also decreases rapidly with increasing laser ellipticity. In contrast, in the nonadiabatic tunneling ionization region, the yield of Rydberg state atoms in FTI does not always decrease with increasing laser ellipticity, instead, it shows an abnormal increase in the yield. There is no clear boundary between these two mechanisms.This review focused on a variety of interference phenomena during the Rydberg-state excitation of atoms driven by strong laser fields, which are manifested as oscillations of the Rydberg state atomic yield with laser intensity. These interference phenomena are classified into three categories according to the magnitude of the as-Stark shift corresponding to the laser intensity interval of the oscillation period. The first category is the interference with an oscillation period of one photon energy. In multiphoton resonance excitation images, this interference can be understood as channel closing. In tunneling images, it can be understood as coherent recapture, wherein the electrons tunneling ionize at different laser cycles, are recaptured, and interfere, giving rising to the laser-intensity-dependent oscillation of the excitation yield. The second category is the interference with oscillation periods much larger than one photon energy, which usually appears in the long wavelength region. This type of oscillation is explained as the interference of the tunneling electron recaptured at different returns. The third category is the interference with oscillation period much smaller than one photon energy, due to the interference of the excitation during the rising and falling edges of the laser pulse, i.e., the dynamic interference. These interference phenomena provide the dynamic information of the strong-field excitation process of the Rydberg-state atom.In this review, the ionization processes of excited atoms in strong laser fields are also presented, in particular, the circular dichroism of ionization of the Rydberg atoms driven by circularly polarized laser fields. Because of this circular dichroism, the Rydberg state can be considered as the simplest chiral system, which is important for one to explore the chirality of molecules. In addition, the circular dichroism of Rydberg state atoms is also important for the preparing and detecting high-purity single ring current states at ultrafast time scales and the generation of spin-polarized electron pulses. When atoms are exposed to strong femtosecond laser fields, there is a high probability that the electrons in atoms can be excited to the Rydberg states with very high principal quantum numbers and large orbitals, in addition to processes such as multiphoton ionization and tunneling ionization. This highly excited state of the atom is very stable in the ultrashort strong laser pulse and is closely related to many other phenomena in the strong field, such as neutral particle acceleration, multiphoton Rabi oscillations, near-threshold harmonic radiation. It has become one of the research hotspots in the field of strong-field ultrafast physics in the last decade. Among these studies, the mechanism of the generation of Rydberg atoms in strong laser fields, the modulation of Rydberg states by lasers, and the strong-field ionization process and stability of Rydberg states are the main issues of interest.This review provided an overview of the generation mechanisms of the Rydberg states driven by strong laser fields. There are two mechanisms for strong field Rydberg state excitation. One is multiphoton resonance excitation and the other is Frustrated Tunneling Ionization (FTI) excitation. The dependence of the yield of the Rydberg state atoms on the laser ellipticity is usually considered to be an important basis for determining whether the Rydberg state atom generation mechanism is multiphoton resonance excitation or frustrated tunneling ionization. However, this judgment is not reliable. As shown in this review, for multiphoton resonance excitation, the yield of Rydberg state atoms also decreases rapidly with increasing laser ellipticity. In contrast, in the nonadiabatic tunneling ionization region, the yield of Rydberg state atoms in FTI does not always decrease with increasing laser ellipticity, instead, it shows an abnormal increase in the yield. There is no clear boundary between these two mechanisms.This review focused on a variety of interference phenomena during the Rydberg-state excitation of atoms driven by strong laser fields, which are manifested as oscillations of the Rydberg state atomic yield with laser intensity. These interference phenomena are classified into three categories according to the magnitude of the as-Stark shift corresponding to the laser intensity interval of the oscillation period. The first category is the interference with an oscillation period of one photon energy. In multiphoton resonance excitation images, this interference can be understood as channel closing. In tunneling images, it can be understood as coherent recapture, wherein the electrons tunneling ionize at different laser cycles, are recaptured, and interfere, giving rising to the laser-intensity-dependent oscillation of the excitation yield. The second category is the interference with oscillation periods much larger than one photon energy, which usually appears in the long wavelength region. This type of oscillation is explained as the interference of the tunneling electron recaptured at different returns. The third category is the interference with oscillation period much smaller than one photon energy, due to the interference of the excitation during the rising and falling edges of the laser pulse, i.e., the dynamic interference. These interference phenomena provide the dynamic information of the strong-field excitation process of the Rydberg-state atom.In this review, the ionization processes of excited atoms in strong laser fields are also presented, in particular, the circular dichroism of ionization of the Rydberg atoms driven by circularly polarized laser fields. Because of this circular dichroism, the Rydberg state can be considered as the simplest chiral system, which is important for one to explore the chirality of molecules. In addition, the circular dichroism of Rydberg state atoms is also important for the preparing and detecting high-purity single ring current states at ultrafast time scales and the generation of spin-polarized electron pulses.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2023
- Vol. 52, Issue 7, 0732001 (2023)
High-gain Ultra-small Streak Camera and Its Integrated Control System
Yuchi ZHANG, Jinshou TIAN, Yanhua XUE, Zhibing LI... and Wei ZHAO|Show fewer author(s)
As a diagnostic instrument with ultra-high temporal and spatial resolution and spectral resolution, the streak camera is widely used in basic research fields such as physics, life sciences, and materials science, as well as in national strategic fields such as detonation physics, lidar, and inertial confinement fusion. Aiming at the requirements of airborne lidar for miniaturized, high-sensitivity, high-gain, and high spatiotemporal resolution streak camera, a high-brightness-gain compact streak camera and its new integrated control system are developed.Compared with the general picosecond visible light streak camera, the volume and weight of the camera are reduced by more than 2/3. The selected streak camera adopts the theoretical simulation research of cathode semiconductor and the method of optimizing the process to greatly improve the sensitivity of the cathode. Using a slit acceleration grid improves the photoelectron transmittance, enhances the photoelectron energy to give the fluorescent screen higher luminous efficiency, and optimizes the cathode process to improve the brightness gain. The streak image tube has the characteristics of high sensitivity, large detection field, high brightness gain, and high temporal and spatial resolution.Starting from the principle and control requirements, combined with the theoretical analysis of the defects of the active control system, a new type of high-integration control system is developed for the camera, which fully eliminates the low integration, poor reliability and compatibility of the previous version. defect. The hardware of the new control system adopts the design method of modularization and function reuse, and the PCB adopts the multi-layer board design. Compared with the current version, the degree of integration is increased by 2.36 times to achieve multi-device compatibility. The bottom layer of the system hardware is divided into main control module, power supply module, A/D module, D/A module, digital I/O and extended scan switching module: the main control module takes STM32F107VCT6 as the core and is responsible for the information between each module and the host computer Interaction; the power supply module is divided into a high-voltage power supply part and a low-voltage power supply part, which provide corresponding voltages for the stripe tube and each element of the circuit; the A/D module takes ADS1256 as the core, adds anti-static protection and digital-analog isolation to entirely eliminate noise interference, and uses SPI The protocol communicates with the host computer; the D/A module takes DAC8534 as the core to control the output of analog devices such as MCP and high-voltage power supply; the digital I/O and expansion scan switching module use the microcontroller GPIO as the control, and the 24 pins programmable interface supports function multiplexing. The PC-side visualization system realizes human-computer interaction and has functions such as camera control, instant feedback of collected images and data, and operation logs. The interface is concise and optimized, which greatly enhances the operability and maintainability of the camera.Finally, the streak tube static test system is used to test the parameters of the streak image tube: the cathode integral sensitivity is 268 μA/lm, the brightness gain is 20.1, and the time resolution is 36 ps; femtosecond laser, F-P etalon, DG645 delayer, oscilloscope, etc. built a dynamic test system for streak camera, and tested the static/dynamic spatial resolution, time resolution, control system function, etc. of the whole machine. The static spatial resolution is higher than 26 lp/mm, the full-screen scanning time is 600 ps, and the functions of control, monitoring and information exchange of the control system are normal. The developed streak camera works well in the laser radar and Inertial Confinement Fusion (ICF) picosecond laser targeting experiments. As a diagnostic instrument with ultra-high temporal and spatial resolution and spectral resolution, the streak camera is widely used in basic research fields such as physics, life sciences, and materials science, as well as in national strategic fields such as detonation physics, lidar, and inertial confinement fusion. Aiming at the requirements of airborne lidar for miniaturized, high-sensitivity, high-gain, and high spatiotemporal resolution streak camera, a high-brightness-gain compact streak camera and its new integrated control system are developed.Compared with the general picosecond visible light streak camera, the volume and weight of the camera are reduced by more than 2/3. The selected streak camera adopts the theoretical simulation research of cathode semiconductor and the method of optimizing the process to greatly improve the sensitivity of the cathode. Using a slit acceleration grid improves the photoelectron transmittance, enhances the photoelectron energy to give the fluorescent screen higher luminous efficiency, and optimizes the cathode process to improve the brightness gain. The streak image tube has the characteristics of high sensitivity, large detection field, high brightness gain, and high temporal and spatial resolution.Starting from the principle and control requirements, combined with the theoretical analysis of the defects of the active control system, a new type of high-integration control system is developed for the camera, which fully eliminates the low integration, poor reliability and compatibility of the previous version. defect. The hardware of the new control system adopts the design method of modularization and function reuse, and the PCB adopts the multi-layer board design. Compared with the current version, the degree of integration is increased by 2.36 times to achieve multi-device compatibility. The bottom layer of the system hardware is divided into main control module, power supply module, A/D module, D/A module, digital I/O and extended scan switching module: the main control module takes STM32F107VCT6 as the core and is responsible for the information between each module and the host computer Interaction; the power supply module is divided into a high-voltage power supply part and a low-voltage power supply part, which provide corresponding voltages for the stripe tube and each element of the circuit; the A/D module takes ADS1256 as the core, adds anti-static protection and digital-analog isolation to entirely eliminate noise interference, and uses SPI The protocol communicates with the host computer; the D/A module takes DAC8534 as the core to control the output of analog devices such as MCP and high-voltage power supply; the digital I/O and expansion scan switching module use the microcontroller GPIO as the control, and the 24 pins programmable interface supports function multiplexing. The PC-side visualization system realizes human-computer interaction and has functions such as camera control, instant feedback of collected images and data, and operation logs. The interface is concise and optimized, which greatly enhances the operability and maintainability of the camera.Finally, the streak tube static test system is used to test the parameters of the streak image tube: the cathode integral sensitivity is 268 μA/lm, the brightness gain is 20.1, and the time resolution is 36 ps; femtosecond laser, F-P etalon, DG645 delayer, oscilloscope, etc. built a dynamic test system for streak camera, and tested the static/dynamic spatial resolution, time resolution, control system function, etc. of the whole machine. The static spatial resolution is higher than 26 lp/mm, the full-screen scanning time is 600 ps, and the functions of control, monitoring and information exchange of the control system are normal. The developed streak camera works well in the laser radar and Inertial Confinement Fusion (ICF) picosecond laser targeting experiments.
Acta Photonica Sinica
- Publication Date: Oct. 25, 2022
- Vol. 51, Issue 10, 1032003 (2022)
Optimal Control of Isolated Attosecond Pulse Generation in an Ar Crystal（Invited）
Suna PANG, and Feng WANG
The ultrafast motion of electrons in atoms, molecules, and condensed matter can generally involve attosecond timescales. Attosecond light pulse can provide unusual functionalities for probing, initiating, driving, and controlling the ultrafast electronic dynamics with unprecedented high temporal and spatial resolutions simultaneously. The progress of attosecond science is closely linked to the improvement of attosecond light sources in terms of shorter and more intense attosecond pulses. Indeed, following its first synthesis and characterization, with the tendency towards reducing the pulse durations and increasing the pulse intensities, attosecond light pulses have and will continue to open up new venues for studying both fundamental and applied sciences, enabling a number of exciting possibilities.Over the last decade, people have conducted a lot of explorations on new methods of generating single attosecond pulses both experimentally and theoretically. In principle, an isolated attosecond light pulse can be generated via HHG originating from coherent electron motion in atoms, molecules, clusters and bulk crystals exposed to intense few-cycle femtosecond laser pulses. Theoretically, HHG in the atomic case can be well understood in the framework of a semi-classical model consisting of three steps. First, an electron is ionized into the continuum by tunneling through the potential barrier formed by the atomic Coulomb field and the driving laser field. Then, the ionized electron gains energy while being accelerated by the driving laser field. Finally, the electron recombines to the parent ion with an energy release in the form of harmonic photons. The generated harmonic radiation that occurs on successive half-cycles of the driving laser is coherent, leading to the emission of odd harmonics. Ultrashort attosecond pulse can be obtained only when the low-harmonic orders are filtered out. In the last two decades, almost all advances in isolated attosecond laser sources were based on HHG from atoms exposed to intense driving laser pulses. The main problem of isolated attosecond pulse generated by HHG in atoms is its weak intensity and low generation efficiency. To increase the strength of isolated pulses, laser-crystal interaction may be an alternative method worth investigating because in bulk crystals the existence of multiple ionization and recombination sites, the high density and periodic structure makes for richer dynamics allowing the possibility of higher conversion efficiency. At present, it is safe to say that while HHG in atomic gases has been explored extensively, much less has been done for bulk crystals. Interestingly enough, NDABASHIMIYE G et al. reported a direct comparison of HHG in the solid and gas phases of Ar. They found that the HHG spectra of the noble gas solids exhibit multiple platforms, well beyond the atomic limits of the corresponding gas phase harmonics measured under similar conditions, implying that shorter attosecond pulses could be realized in solids. What is most interesting to us is that the dependence of HHG on the laser polarization direction with respect to the Ar crystal, which are currently little studied. We theoretically investigated optimal control of isolated attosecond pulse generation in an Ar crystal irradiated by few-cycle femtosecond pulse, employing quantum time-dependent density-functional theory method. We explored systematically the effect of different laser polarization directions on isolated attosecond pulses generation, showing that the laser polarization direction with respect to the crystal is a sensitive control parameter for producing isolated attosecond pulses. The results indicate that for an Ar crystal, the intensity of isolated attosecond pulses is maximal at an optimal laser polarization direction with respect to the crystal, demonstrating about 11-fold intensity enhancement compared with that generated in an Ar atom under the same driving laser pulses. Our results also suggest opportunities for future investigations for the optimal control of isolated attosecond pulse generation in bulk crystal solids. The ultrafast motion of electrons in atoms, molecules, and condensed matter can generally involve attosecond timescales. Attosecond light pulse can provide unusual functionalities for probing, initiating, driving, and controlling the ultrafast electronic dynamics with unprecedented high temporal and spatial resolutions simultaneously. The progress of attosecond science is closely linked to the improvement of attosecond light sources in terms of shorter and more intense attosecond pulses. Indeed, following its first synthesis and characterization, with the tendency towards reducing the pulse durations and increasing the pulse intensities, attosecond light pulses have and will continue to open up new venues for studying both fundamental and applied sciences, enabling a number of exciting possibilities.Over the last decade, people have conducted a lot of explorations on new methods of generating single attosecond pulses both experimentally and theoretically. In principle, an isolated attosecond light pulse can be generated via HHG originating from coherent electron motion in atoms, molecules, clusters and bulk crystals exposed to intense few-cycle femtosecond laser pulses. Theoretically, HHG in the atomic case can be well understood in the framework of a semi-classical model consisting of three steps. First, an electron is ionized into the continuum by tunneling through the potential barrier formed by the atomic Coulomb field and the driving laser field. Then, the ionized electron gains energy while being accelerated by the driving laser field. Finally, the electron recombines to the parent ion with an energy release in the form of harmonic photons. The generated harmonic radiation that occurs on successive half-cycles of the driving laser is coherent, leading to the emission of odd harmonics. Ultrashort attosecond pulse can be obtained only when the low-harmonic orders are filtered out. In the last two decades, almost all advances in isolated attosecond laser sources were based on HHG from atoms exposed to intense driving laser pulses. The main problem of isolated attosecond pulse generated by HHG in atoms is its weak intensity and low generation efficiency. To increase the strength of isolated pulses, laser-crystal interaction may be an alternative method worth investigating because in bulk crystals the existence of multiple ionization and recombination sites, the high density and periodic structure makes for richer dynamics allowing the possibility of higher conversion efficiency. At present, it is safe to say that while HHG in atomic gases has been explored extensively, much less has been done for bulk crystals. Interestingly enough, NDABASHIMIYE G et al. reported a direct comparison of HHG in the solid and gas phases of Ar. They found that the HHG spectra of the noble gas solids exhibit multiple platforms, well beyond the atomic limits of the corresponding gas phase harmonics measured under similar conditions, implying that shorter attosecond pulses could be realized in solids. What is most interesting to us is that the dependence of HHG on the laser polarization direction with respect to the Ar crystal, which are currently little studied. We theoretically investigated optimal control of isolated attosecond pulse generation in an Ar crystal irradiated by few-cycle femtosecond pulse, employing quantum time-dependent density-functional theory method. We explored systematically the effect of different laser polarization directions on isolated attosecond pulses generation, showing that the laser polarization direction with respect to the crystal is a sensitive control parameter for producing isolated attosecond pulses. The results indicate that for an Ar crystal, the intensity of isolated attosecond pulses is maximal at an optimal laser polarization direction with respect to the crystal, demonstrating about 11-fold intensity enhancement compared with that generated in an Ar atom under the same driving laser pulses. Our results also suggest opportunities for future investigations for the optimal control of isolated attosecond pulse generation in bulk crystal solids.
Acta Photonica Sinica
- Publication Date: Oct. 25, 2022
- Vol. 51, Issue 10, 1032002 (2022)
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