In this study,an optical setup for generating terahertz(THz)pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time.β-bari...In this study,an optical setup for generating terahertz(THz)pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time.β-barium borate(BBO),α-BBO,and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers.Tilting ofα-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect.The maximum out-put THz pulse energy reached 21μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate.The corresponding conversion efficiency exceeded 0.35%.展开更多
The effects of turbulence intensity and turbulence region on the distribution of femtosecond laser filaments are experimentally elaborated.Through the ultrasonic signals emitted by the filaments,it is observed that in...The effects of turbulence intensity and turbulence region on the distribution of femtosecond laser filaments are experimentally elaborated.Through the ultrasonic signals emitted by the filaments,it is observed that increasing turbulence intensity and an expanding turbulence active region cause an increase in the start position of the filament and a decrease in filament length,which can be well explained by theoretical calculation.It is also observed that the random perturbation of the air refractive index caused by atmospheric turbulence expands the spot size of the filament.Additionally,when the turbulence refractive index structure constant reaches 8.37×10^(-12)m^(-2/3),multiple filaments are formed.Furthermore,the standard deviation of the transverse displacement of filament is found to be proportional to the square root of the turbulent structure constant under the experimental turbulence parameters in this paper.These results contribute to the study of femtosecond laser propagation mechanisms in complex atmospheric turbulence conditions.展开更多
The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results sugges...The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.展开更多
Broadband and energetic terahertz （THz） pulses can be remotely generated in air through filamentation. We review such THz generation and detection in femtosecond Ti-sapphire laser induced remote filaments. New resul...Broadband and energetic terahertz （THz） pulses can be remotely generated in air through filamentation. We review such THz generation and detection in femtosecond Ti-sapphire laser induced remote filaments. New results are presented on the direct relationship between THz generation in a two color filament and induced N2 fluorescence through population trapping during molecular alignment and revival in air. This further supports the new technique of remote THz detection in air through the sensitive measurement of N2 fluorescence.展开更多
We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser’s ellipticity from linear polarization to circular polarization, the onset position o...We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser’s ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.展开更多
Due to the promising applications of femtosecond laser filamentation in remote sensing,great demands exist for diagnosing the spatiotemporal dynamics of filamentation.However,until now,the rapid and accurate diagnosis...Due to the promising applications of femtosecond laser filamentation in remote sensing,great demands exist for diagnosing the spatiotemporal dynamics of filamentation.However,until now,the rapid and accurate diagnosis of a femtosecond laser filament remains a severe challenge.Here,a novel filament diagnosing method is proposed,which can measure the longitudinal spatial distribution of the filament by a single laser shot-induced acoustic pulse.The dependences of the point-like plasma acoustic emission on the detection distance and angle are obtained experimentally.The results indicate that the temporal profile of the acoustic wave is independent of the detection distance and detection angle.Using the measured relation among the acoustic emission and the detection distance and angle,a single measurement of the acoustic emission generated by a single laser pulse can diagnose the spatial distribution of the laser filament through the Wiener filter deconvolution(WFD)algorithm.The results obtained by this method are in good agreement with those of traditional point-by-point acoustic diagnosis methods.These findings provide a new solution and idea for the rapid diagnosis of filament,thereby laying a firm foundation for femtosecond laser filament-based promising applications.展开更多
The characteristics of plasmas play an important role in femtosecond laser filament-based applications.Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air fil...The characteristics of plasmas play an important role in femtosecond laser filament-based applications.Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates.In our experiments,the measured average plasma density of the filament is 1.54×10^(17)cm^(-3)and the temperature of the plasma is about 5100 K under 100 Hz pulse repetition rate.The plasma density decreases to1.43×10^(17)cm^(-3)and the temperature increases to 6230 K as the pulse repetition rate increases to 1000 Hz.The experimental observation agrees with the numerical simulation by solving the nonlinear Schr?dinger equations with repetition rate related“low density hole”correction.展开更多
We applied a spatial high-order finite-difference-time-domain (HO-FDTD) scheme to solve 2D Maxwell’s equations in order to develop a fluid model employed to study the production of terahertz radiation by the filament...We applied a spatial high-order finite-difference-time-domain (HO-FDTD) scheme to solve 2D Maxwell’s equations in order to develop a fluid model employed to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma. We examined the performance of the applied scheme, in this context, we implemented the developed model to study selected phenomena in terahertz radiation production, such as the excitation energy and conversion efficiency of the produced THz radiation, in addition to the influence of the pulse chirping on properties of the produced radiation. The obtained numerical results have clarified that the applied HO-FDTD scheme is precisely accurate to solve Maxwell’s equations and sufficiently valid to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma.展开更多
As intense,ultrashort,kHz-repetition-rate laser systems become commercially available,pulse cumulative effects are critical for laser filament-based applications.In this work,the pulse repetition-rate effect on femtos...As intense,ultrashort,kHz-repetition-rate laser systems become commercially available,pulse cumulative effects are critical for laser filament-based applications.In this work,the pulse repetition-rate effect on femtosecond laser filamentation in air was investigated both numerically and experimentally.The pulse repetition-rate effect has negligible influence at the leading edge of the filament.Clear intensity enhancement from a high-repetition pulse is observed at the peak and tailing edge of the laser filament.As the repetition rate of the laser pulses increases from 100 to 1000 Hz,the length of the filament extends and the intensity inside the filament increases.A physical picture based on the pulse repetition-rate dependent‘low-density hole’effect on filamentation is proposed to explain the obtained results well.展开更多
In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma c...In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma column,is observed. When the shorter focal length lens is used, a high density plasma isgenerated near the geometrical focus and coexists with a weak plasma channel of the filamemt. Under thetightest focusing condition, filamentation is prevented and only a strong plasma volume appears at tehgeometrical focus.展开更多
The temporal evolutions of electron density and plasma diameter of 1 kHz femtosecond laser filament in air are experimentally investigated by utilizing a pump-probe longitudinal diffraction method.A model based on sca...The temporal evolutions of electron density and plasma diameter of 1 kHz femtosecond laser filament in air are experimentally investigated by utilizing a pump-probe longitudinal diffraction method.A model based on scalar diffraction theory is proposed to extract the spatial phase shift of the probe pulse from the diffraction patterns by the laser air plasma channel.The hydrodynamic effect on plasma evolution at 1 kHz filament is included and analyzed.The measured initial peak electron density of~10^(18)cm^(-3) in our experimental conditions decays rapidly by nearly two orders of magnitude within200 ps.Moreover,the plasma channel size rises from 90μm to 120μm as the delay time increases.The experimental observation is in agreement with numerical simulation results by solving the rate equations of the charged particles.展开更多
We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride(BaF2)crystal.It is found that the depolarization of the su...We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride(BaF2)crystal.It is found that the depolarization of the supercontinuum depends strongly on the crystal orientations with respect to the incident laser polarization.At most crystal orientations,the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates.While at 45°,the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies.These peculiar depolarization properties of the supercontinuum can be ascribed to the orientation dependence of the cross-polarized wave(XPW)generation and ionization-induced plasma scattering in the BaF2 crystal.展开更多
We have developed a remote sea salt aerosol fluorescence spectroscopy system integrating a high-power industrial-grade femtosecond laser to enhance detection sensitivity and precision in complex environments.This syst...We have developed a remote sea salt aerosol fluorescence spectroscopy system integrating a high-power industrial-grade femtosecond laser to enhance detection sensitivity and precision in complex environments.This system successfully detects sea salt aerosol particles,achieving a detection limit of 0.015 ng/m3 for neutral Na element(Na I)at 589 nm,with a detection range of 30 m.Our findings demonstrate significant improvements in remote aerosol monitoring,addressing previous challenges in long-range and high-precision sensing with a detection accuracy previously unattainable below 10 ng/m3.展开更多
Laser polarization and its intensity inside a filament core play an important role in filament-based applications.However,polarization dependent clamping intensity inside filaments has been overlooked to interpret the...Laser polarization and its intensity inside a filament core play an important role in filament-based applications.However,polarization dependent clamping intensity inside filaments has been overlooked to interpret the polarization-related filamentation phenomena.Here,we report on experimental and numerical investigations of polarization dependent clamping intensity inside a femtosecond filament in air.By adjusting the initial polarization from linear to circular,the clamping intensity is increased by 1.36 times when using a 30 cm focal length lens for filamentation.The results indicate that clamping intensity inside the filament is sensitive to laser polarization,which has to be considered to fully understand polarizationrelated phenomena.展开更多
基金National Natural Science Foundation of China(12061131010,12074198)Russian Science Foundation(21-49-00023).
文摘In this study,an optical setup for generating terahertz(THz)pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time.β-barium borate(BBO),α-BBO,and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers.Tilting ofα-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect.The maximum out-put THz pulse energy reached 21μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate.The corresponding conversion efficiency exceeded 0.35%.
基金supported by the National Key Research and Development Program of China(No.2018YFB0504400)the Fundamental Research Funds for the Central Universities(No.63223052)。
文摘The effects of turbulence intensity and turbulence region on the distribution of femtosecond laser filaments are experimentally elaborated.Through the ultrasonic signals emitted by the filaments,it is observed that increasing turbulence intensity and an expanding turbulence active region cause an increase in the start position of the filament and a decrease in filament length,which can be well explained by theoretical calculation.It is also observed that the random perturbation of the air refractive index caused by atmospheric turbulence expands the spot size of the filament.Additionally,when the turbulence refractive index structure constant reaches 8.37×10^(-12)m^(-2/3),multiple filaments are formed.Furthermore,the standard deviation of the transverse displacement of filament is found to be proportional to the square root of the turbulent structure constant under the experimental turbulence parameters in this paper.These results contribute to the study of femtosecond laser propagation mechanisms in complex atmospheric turbulence conditions.
基金supported by the National Key R&D Program of China (No.2018YFB0504400)。
文摘The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.
基金supported in part by NSERC,Canada Research Chair,the Canada Foundation for Innovation,the Canadian Institute for Photonics Innovation,and leFonds Québécois pour la Recherche sur la Nature et les Technologies
文摘Broadband and energetic terahertz （THz） pulses can be remotely generated in air through filamentation. We review such THz generation and detection in femtosecond Ti-sapphire laser induced remote filaments. New results are presented on the direct relationship between THz generation in a two color filament and induced N2 fluorescence through population trapping during molecular alignment and revival in air. This further supports the new technique of remote THz detection in air through the sensitive measurement of N2 fluorescence.
基金supported in part by the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB16000000)the Key Project from the Bureau of International Cooperation of the Chinese Academy of Sciences(No.181231KYSB20160045)+1 种基金the 100 Talents Program of the Chinese Academy of Sciencesthe support from Laval University in Canada
文摘We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser’s ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.
基金supported by the National Natural Science Foundation of China(Nos.12074198,12061131010,and 12304381)the Russian Science Foundation(RSF)(No.21-49-00023).
文摘Due to the promising applications of femtosecond laser filamentation in remote sensing,great demands exist for diagnosing the spatiotemporal dynamics of filamentation.However,until now,the rapid and accurate diagnosis of a femtosecond laser filament remains a severe challenge.Here,a novel filament diagnosing method is proposed,which can measure the longitudinal spatial distribution of the filament by a single laser shot-induced acoustic pulse.The dependences of the point-like plasma acoustic emission on the detection distance and angle are obtained experimentally.The results indicate that the temporal profile of the acoustic wave is independent of the detection distance and detection angle.Using the measured relation among the acoustic emission and the detection distance and angle,a single measurement of the acoustic emission generated by a single laser pulse can diagnose the spatial distribution of the laser filament through the Wiener filter deconvolution(WFD)algorithm.The results obtained by this method are in good agreement with those of traditional point-by-point acoustic diagnosis methods.These findings provide a new solution and idea for the rapid diagnosis of filament,thereby laying a firm foundation for femtosecond laser filament-based promising applications.
基金in part supported by the NSAF(No.U2130123)the International Partnership Program of Chinese Academy of Sciences(Nos.181231KYSB20200033 and 181231KYSB20200040)the Shanghai Science and Technology Program(No.21511105000)。
文摘The characteristics of plasmas play an important role in femtosecond laser filament-based applications.Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates.In our experiments,the measured average plasma density of the filament is 1.54×10^(17)cm^(-3)and the temperature of the plasma is about 5100 K under 100 Hz pulse repetition rate.The plasma density decreases to1.43×10^(17)cm^(-3)and the temperature increases to 6230 K as the pulse repetition rate increases to 1000 Hz.The experimental observation agrees with the numerical simulation by solving the nonlinear Schr?dinger equations with repetition rate related“low density hole”correction.
文摘We applied a spatial high-order finite-difference-time-domain (HO-FDTD) scheme to solve 2D Maxwell’s equations in order to develop a fluid model employed to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma. We examined the performance of the applied scheme, in this context, we implemented the developed model to study selected phenomena in terahertz radiation production, such as the excitation energy and conversion efficiency of the produced THz radiation, in addition to the influence of the pulse chirping on properties of the produced radiation. The obtained numerical results have clarified that the applied HO-FDTD scheme is precisely accurate to solve Maxwell’s equations and sufficiently valid to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma.
基金the NSAF(No.U2130123)the International Partnership Program of the Chinese Academy of Sciences(Nos.181231KYSB20200033 and 181231KYSB20200040)the Shanghai Science and Technology Program(No.21511105000).
文摘As intense,ultrashort,kHz-repetition-rate laser systems become commercially available,pulse cumulative effects are critical for laser filament-based applications.In this work,the pulse repetition-rate effect on femtosecond laser filamentation in air was investigated both numerically and experimentally.The pulse repetition-rate effect has negligible influence at the leading edge of the filament.Clear intensity enhancement from a high-repetition pulse is observed at the peak and tailing edge of the laser filament.As the repetition rate of the laser pulses increases from 100 to 1000 Hz,the length of the filament extends and the intensity inside the filament increases.A physical picture based on the pulse repetition-rate dependent‘low-density hole’effect on filamentation is proposed to explain the obtained results well.
文摘In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma column,is observed. When the shorter focal length lens is used, a high density plasma isgenerated near the geometrical focus and coexists with a weak plasma channel of the filamemt. Under thetightest focusing condition, filamentation is prevented and only a strong plasma volume appears at tehgeometrical focus.
基金supported in part by NSAF(No.U2130123)International Partnership Program of Chinese Academy of Sciences(Nos.181231KYSB20200033 and 181231KYSB20200040)Shanghai Science and Technology Program(No.21511105000)。
文摘The temporal evolutions of electron density and plasma diameter of 1 kHz femtosecond laser filament in air are experimentally investigated by utilizing a pump-probe longitudinal diffraction method.A model based on scalar diffraction theory is proposed to extract the spatial phase shift of the probe pulse from the diffraction patterns by the laser air plasma channel.The hydrodynamic effect on plasma evolution at 1 kHz filament is included and analyzed.The measured initial peak electron density of~10^(18)cm^(-3) in our experimental conditions decays rapidly by nearly two orders of magnitude within200 ps.Moreover,the plasma channel size rises from 90μm to 120μm as the delay time increases.The experimental observation is in agreement with numerical simulation results by solving the rate equations of the charged particles.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11527807,11674356,11834015,and 91850121)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB21010400)the Open Fund of the State Key Laboratory of High Field Laser Physics,China
文摘We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride(BaF2)crystal.It is found that the depolarization of the supercontinuum depends strongly on the crystal orientations with respect to the incident laser polarization.At most crystal orientations,the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates.While at 45°,the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies.These peculiar depolarization properties of the supercontinuum can be ascribed to the orientation dependence of the cross-polarized wave(XPW)generation and ionization-induced plasma scattering in the BaF2 crystal.
基金supported by the National Natural Science Foundation of China(Nos.12074198,12061131010,and 12304381).
文摘We have developed a remote sea salt aerosol fluorescence spectroscopy system integrating a high-power industrial-grade femtosecond laser to enhance detection sensitivity and precision in complex environments.This system successfully detects sea salt aerosol particles,achieving a detection limit of 0.015 ng/m3 for neutral Na element(Na I)at 589 nm,with a detection range of 30 m.Our findings demonstrate significant improvements in remote aerosol monitoring,addressing previous challenges in long-range and high-precision sensing with a detection accuracy previously unattainable below 10 ng/m3.
基金supported in part by the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB16010400)the International Partnership Program of Chinese Academy of Sciences(Nos.181231KYSB20160045 and 181231KYSB20200033)the support from the Russian Science Foundation(No.2149-00023)。
文摘Laser polarization and its intensity inside a filament core play an important role in filament-based applications.However,polarization dependent clamping intensity inside filaments has been overlooked to interpret the polarization-related filamentation phenomena.Here,we report on experimental and numerical investigations of polarization dependent clamping intensity inside a femtosecond filament in air.By adjusting the initial polarization from linear to circular,the clamping intensity is increased by 1.36 times when using a 30 cm focal length lens for filamentation.The results indicate that clamping intensity inside the filament is sensitive to laser polarization,which has to be considered to fully understand polarizationrelated phenomena.
