Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously ge...Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.展开更多
Multiple broadband Alfvenic chirping modes(CMs),with frequencies in the wide range of f-35-150 kHz and chirping down rapidly,are found in HL-2A neutral beam injection plasmas,and the CMs can even coexist.The frequency...Multiple broadband Alfvenic chirping modes(CMs),with frequencies in the wide range of f-35-150 kHz and chirping down rapidly,are found in HL-2A neutral beam injection plasmas,and the CMs can even coexist.The frequency chirping down process can be completed within-1 ms,and the frequency shift can reach 30-50 kHz.The CMs propagate in ion diamagnetic drift directions poloidally.The toroidal mode number is confirmed to be n=1,2,3 and 4 for the f-35-65,55-90,70-120 and 100-150 kHz CMs,respectively.The CMs are more like to be energetic-particle continuum modes(EPMs),since the modes almost locate on the Alfven continuum.展开更多
Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum a...Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment t = 0 leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.展开更多
The effect of channel-width chirping on near- and far-field intensity patterns of the six supermodes was investigated. The supermode discrimination was evaluated in various channel-chirped index guided laser arrays. T...The effect of channel-width chirping on near- and far-field intensity patterns of the six supermodes was investigated. The supermode discrimination was evaluated in various channel-chirped index guided laser arrays. The results show that the linearly channel-chirped laser array has very good supermode discrimination which is better than that of a uniform laser array, the V channel-chirped laser array has the smallest radiation angle of the fundamental supermode among the calculated arrays, and the asymmetrically V channel-chirped array has a very small radiation angle of the fundamental supermode, which is smaller than that of the uniform array and also allows for very good fundamental supermode discrimination against the higher-order supermodes, which is better than that of the V channel-chirped laser array.展开更多
Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an or...Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density(n_(e))<2×10^(19) m^(-3) in a wide range of toroidal magnetic fields and plasma currents.This type of instability was identified as compressional Alfven waves,driven by electrons,accelerated during a sawtooth crush.It was found that the mode frequency is sweeping in time,according to the Berk-Breizman hole-clump nonlinear chirping model.The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column,when the plasma tends to decay at extremely low density(n_(e))<2×10^(18) m^(-3) and,in fact,is an instability of the runaway electron beam.The exited modes cover the whole observed frequency range and are divided into several(two or three)frequency regions:approximately 0-30 MHz,60-120 MHz and sometimes 30-60 MHz.Reconnection of the branches was also observed.Single chirps are more rapid than for 1 MHz Alfven instability and follow an exponential law.This paper,to our knowledge,is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.展开更多
Numerical method to solve the problem related with the interactive effect of dispersion (both chromatic dispersion and polarization mode dispersion) and nonlinearity on optical pulse transmission is present. Evolution...Numerical method to solve the problem related with the interactive effect of dispersion (both chromatic dispersion and polarization mode dispersion) and nonlinearity on optical pulse transmission is present. Evolutions of pulses with various initial chirping and shape at bit rate of 10 Gb/s are simulated and compared. Gaussian pulse with appropriate prechirping is propitious for high bit rate transmission.展开更多
The influences of laser pulses with negative frequency chirp and different pulse cycles on pair production are studied.By solving the quantum Vlasov equation,the momentum distribution and the number density of the cre...The influences of laser pulses with negative frequency chirp and different pulse cycles on pair production are studied.By solving the quantum Vlasov equation,the momentum distribution and the number density of the created pairs are obtained numerically.It is found that the chirp rate can enhance the pair production rate in both supercycle and subcycle pulses.Moreover,there is an optimal cycle parameter corresponding to the maximum number density under the same chirp rate.The pair number density is sensitive to the cycle parameter when it is less than the optimal one.Compared to the positive frequency chirp,in the case of the negative frequency chirp,the optimal cycle parameter is increased.展开更多
The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longit...The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase（CEP） breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional（2D） particle-in-cell（PIC） simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.展开更多
Nature is amazing!Sometimes you don’t need a weather App to predict rain or shine-you just need nature!Feel the heat:listen to crickets Want to know the outside temperature?Listen to the crickets!Count how many times...Nature is amazing!Sometimes you don’t need a weather App to predict rain or shine-you just need nature!Feel the heat:listen to crickets Want to know the outside temperature?Listen to the crickets!Count how many times a cricket chirps in 15 seconds and add 40.That’s the temperature in Fahrenheit(华氏温度,1℉≈0.56℃).Do it several times and find the average.展开更多
With the first rays of dawn peeking through the rainforest at Shiti Village,Dehong Dai and Jingpo Autonomous Prefecture,Yunnan Province,Xu Xiaolong,a bird-watching guide from the Jingpo ethnic group,led a German bird-...With the first rays of dawn peeking through the rainforest at Shiti Village,Dehong Dai and Jingpo Autonomous Prefecture,Yunnan Province,Xu Xiaolong,a bird-watching guide from the Jingpo ethnic group,led a German bird-watching group into the jungle.His great-grandfather made a living by hunting“weird-looking birds”in the mountains.Today,the birds(Great Hornbill)are attracting tourist dollars from other parts of the world without dodging bullets.In Shiti Village,bird-watchers can enjoy the chirping of Great Hornbills and feel the rumbling in the air as the birds fly past.The village on the Chinese border with Myanmar was formerly poverty-stricken,with an average annual income of barely 2,000 yuan(US$305)per household 10 years ago.Today,the average annual income has grown to 150,000 yuan(US$23,075).It is a shining example of the successful practice of the notion that“lucid waters and lush mountains are invaluable assets.”展开更多
We propose a scheme for dual-species deceleration and trapping of a cold atom–molecule mixture by a frequency chirping stimulated force.We study the stimulated force exerted on Mg F and Rb using optical Bloch equatio...We propose a scheme for dual-species deceleration and trapping of a cold atom–molecule mixture by a frequency chirping stimulated force.We study the stimulated force exerted on Mg F and Rb using optical Bloch equations based on a direct numerical solution for the time-dependent density matrix.We analyze the relationship between the frequency chirping rate and the number of Mg F molecules and Rb atoms.In addition,we study the dynamical process of molecular deceleration and the effect of transverse diffusion.Monte–Carlo simulations show that buffer-gas-cooled Mg F and Rb beams,with initial velocities of 200 m/s and 130 m/s respectively,can be decelerated to less than 10 m/s.This is achieved with laser powers of as low as 357 m W for Mg F and 10 m W for Rb per traveling wave.The rapid deceleration minimizes molecular loss due to transverse diffusion during the deceleration process.The estimated number of molecules that can be trapped in a magneto-optical trap(MOT)is about 9.0×10^(6),which is an order of magnitude larger than the number of Mg F molecules decelerated by the spontaneous radiation force.The results offer a promising starting point for further studies of sympathetic cooling.展开更多
正交时频空间调制(orthogonal time frequency space,OTFS)技术作为一种多载波技术,同样存在峰均功率比较大的问题,高PAPR信号容易进入功率放大器的非线性工作区,导致接收信号的非线性失真,信号经过信道传输后会进一步影响系统的误码率(...正交时频空间调制(orthogonal time frequency space,OTFS)技术作为一种多载波技术,同样存在峰均功率比较大的问题,高PAPR信号容易进入功率放大器的非线性工作区,导致接收信号的非线性失真,信号经过信道传输后会进一步影响系统的误码率(bit error rate,BER)性能.针对该问题,利用chirp信号相位和为1的特点,采用基于chirp信号预编码的PAPR抑制方法,并在此基础上结合选择映射(selective mapping,SLM)法,将多组频域chirp信号与时频域信号点乘,再经过海森堡变换后选取PAPR最小的一组备选信号作为时域信号.仿真结果表明,与原始OTFS信号相比,基于频域chirp信号的OTFS系统的PAPR降低了0.9 dB,与传统SLM方法相比,使用基于频域chirp信号的SLM改进方法的OTFS系统,减少了IFFT次数的同时保证了系统的PAPR抑制性能.展开更多
In this paper, the nonlinear Schr?dinger equation combining quadratic-cubic nonlinearity is considered, which can be represented by an approximate model of relatively dense quasi-one-dimensional Bose-Einstein condensa...In this paper, the nonlinear Schr?dinger equation combining quadratic-cubic nonlinearity is considered, which can be represented by an approximate model of relatively dense quasi-one-dimensional Bose-Einstein condensate. Based on the bifurcation theory, we proved the existence of solitary and periodic solutions. The methods we take are the trial equation method and the complete discrimination system for polynomial method. Therefore, we obtain the exact chirped solutions, which are more abundant in type and quantity than the existing results, so that the equation has more profound physical significance. These two methods are rigorously mathematical derivation and calculations, rather than based on certain conditional assumptions. In addition, we give some specific parameters to graphing the motion of the solutions, which helps to understand the propagation of nonlinear waves in fiber optic systems.展开更多
To capture the nonlinear dynamics and gain evolution in chirped pulse amplification(CPA)systems,the split-step Fourier method and the fourth-order Runge–Kutta method are integrated to iteratively address the generali...To capture the nonlinear dynamics and gain evolution in chirped pulse amplification(CPA)systems,the split-step Fourier method and the fourth-order Runge–Kutta method are integrated to iteratively address the generalized nonlinear Schrödinger equation and the rate equations.However,this approach is burdened by substantial computational demands,resulting in significant time expenditures.In the context of intelligent laser optimization and inverse design,the necessity for numerous simulations further exacerbates this issue,highlighting the need for fast and accurate simulation methodologies.Here,we introduce an end-to-end model augmented with active learning(E2E-AL)with decent generalization through different dedicated embedding methods over various parameters.On an identical computational platform,the artificial intelligence–driven model is 2000 times faster than the conventional simulation method.Benefiting from the active learning strategy,the E2E-AL model achieves decent precision with only two-thirds of the training samples compared with the case without such a strategy.Furthermore,we demonstrate a multi-objective inverse design of the CPA systems enabled by the E2E-AL model.The E2E-AL framework manifests the potential of becoming a standard approach for the rapid and accurate modeling of ultrafast lasers and is readily extended to simulate other complex systems.展开更多
We investigate theoretically the effects of chirped laser pulses on high-order harmonic generation(HHG)from solids.We find that the harmonic spectra display redshifts for the driving laser pulses with negative chirp a...We investigate theoretically the effects of chirped laser pulses on high-order harmonic generation(HHG)from solids.We find that the harmonic spectra display redshifts for the driving laser pulses with negative chirp and blueshifts for those with positive chirp,which is due to the change in the instantaneous frequency of the driving laser for different chirped pulses.The analysis of crystal-momentum-resolved(k-resolved)HHG reveals that the frequency shifts are equal for the harmonics generated by different crystal momentum channels.The frequency shifts in the cutoff region are larger than those in the plateau region.With the increase of the absolute value of the chirp parameters,the frequency shifts of HHG become more significant,leading to the shifts from odd-to even-order harmonics.We also demonstrate that the frequency shifts of harmonic spectra are related to the duration of the chirped laser field,but are insensitive to the laser intensity and dephasing time.展开更多
The existence and stability of the fundamental, multi-peak, and twisted solitons in Kerr nonlinear media with chirped(amplitude-modulated) lattices are reported. We discover that the chirp rate and lattice depth can d...The existence and stability of the fundamental, multi-peak, and twisted solitons in Kerr nonlinear media with chirped(amplitude-modulated) lattices are reported. We discover that the chirp rate and lattice depth can dramatically change the existence domain of solitons, the energy flow of solitons increases with increasing chirp rate or decreasing lattice depth.We also analyze how the chirp rate and lattice depth affect the stability of solitons. The stable domains of fundamental solitons and twisted solitons exhibit a multi-window distribution, while multi-peak solitons are unstable throughout the entire existence domain.展开更多
This research examines the dynamics of a cosh-Gaussian laser pulse travelling through a vacuum and its impact on electron acceleration. We examine the impact of several critical factors, such as laser electric field a...This research examines the dynamics of a cosh-Gaussian laser pulse travelling through a vacuum and its impact on electron acceleration. We examine the impact of several critical factors, such as laser electric field amplitude, decentered parameter, beam waist, and laser chirp parameter, on the energy gain of electrons using coupled momentum equations. Our results indicate that the energy acquisition of electrons escalates with the amplitude of the laser electric field, decentered parameter, and chirp parameter. An appropriate beam waist is essential for attaining energyefficient electron acceleration in a vacuum. Through the optimization of these parameters, we get a maximum electron energy gain of 2.80 Ge V. This study highlights the significance of customized laser pulse attributes in improving electron acceleration and aids in the progression of high-energy particle physics.展开更多
Laser frequency combs,which are composed of a series of equally spaced coherent frequency components,have triggered revolutionary progress in precision spectroscopy and optical metrology.Length/distance is of fundamen...Laser frequency combs,which are composed of a series of equally spaced coherent frequency components,have triggered revolutionary progress in precision spectroscopy and optical metrology.Length/distance is of fundamental importance in both science and technology.We describe a ranging scheme based on chirped pulse interferometry.In contrast to the traditional spectral interferometry,the local oscillator is strongly chirped which is able to meet the measurement pulses at arbitrary distances,and therefore,the dead zones can be removed.The distances can be precisely determined via two measurement steps based on the time-of-flight method and synthetic wavelength interferometry,respectively.To overcome the speed limitation of the optical spectrum analyzer,the spectrograms are stretched and detected by a fast photodetector and oscilloscope and consequently mapped into the time domain in real time.The experimental results indicate that the measurement uncertainty can be well within±2μm,compared with the reference distance meter.The Allan deviation can reach 0.4μm at 4 ns averaging time and 25 nm at 1μs and can achieve 2 nm at 100μs averaging time.We also measured a spinning disk with grooves of different depths to verify the measurement speed,and the results show that the grooves with about 150 m∕s line speed can be clearly captured.Our method provides a unique combination of non-dead zones,ultrafast measurement speed,high precision and accuracy,large ambiguity range,and only one single comb source.This system could offer a powerful solution for field measurements in practical applications in the future.展开更多
基金Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303603,2016YFA0300303,2017YFA0302901 and 2016YFA0300604the National Natural Science Foundation of China under Grant Nos11774408,11574383 and 11774399+1 种基金the External Cooperation Program of Chinese Academy of Sciences under Grant No GJHZ1826the Interdisciplinary Innovation Team of Chinese Academy of Sciences
文摘Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.
基金Supported by the National Key R&D Program of China (Grant Nos.2018YFE0304102,2019YFE03020000 and 2017YFE0301202)the National Natural Science Foundation of China (Grant Nos.11875024,11875021,11835010 and 12005054)the Sichuan Provincial Science and Technology Project (Grant Nos.2020YFSY0047 and 2020JDJQ0070)。
文摘Multiple broadband Alfvenic chirping modes(CMs),with frequencies in the wide range of f-35-150 kHz and chirping down rapidly,are found in HL-2A neutral beam injection plasmas,and the CMs can even coexist.The frequency chirping down process can be completed within-1 ms,and the frequency shift can reach 30-50 kHz.The CMs propagate in ion diamagnetic drift directions poloidally.The toroidal mode number is confirmed to be n=1,2,3 and 4 for the f-35-65,55-90,70-120 and 100-150 kHz CMs,respectively.The CMs are more like to be energetic-particle continuum modes(EPMs),since the modes almost locate on the Alfven continuum.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11475026 and 11175023)
文摘Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment t = 0 leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.
文摘The effect of channel-width chirping on near- and far-field intensity patterns of the six supermodes was investigated. The supermode discrimination was evaluated in various channel-chirped index guided laser arrays. The results show that the linearly channel-chirped laser array has very good supermode discrimination which is better than that of a uniform laser array, the V channel-chirped laser array has the smallest radiation angle of the fundamental supermode among the calculated arrays, and the asymmetrically V channel-chirped array has a very small radiation angle of the fundamental supermode, which is smaller than that of the uniform array and also allows for very good fundamental supermode discrimination against the higher-order supermodes, which is better than that of the V channel-chirped laser array.
文摘Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density(n_(e))<2×10^(19) m^(-3) in a wide range of toroidal magnetic fields and plasma currents.This type of instability was identified as compressional Alfven waves,driven by electrons,accelerated during a sawtooth crush.It was found that the mode frequency is sweeping in time,according to the Berk-Breizman hole-clump nonlinear chirping model.The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column,when the plasma tends to decay at extremely low density(n_(e))<2×10^(18) m^(-3) and,in fact,is an instability of the runaway electron beam.The exited modes cover the whole observed frequency range and are divided into several(two or three)frequency regions:approximately 0-30 MHz,60-120 MHz and sometimes 30-60 MHz.Reconnection of the branches was also observed.Single chirps are more rapid than for 1 MHz Alfven instability and follow an exponential law.This paper,to our knowledge,is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.
文摘Numerical method to solve the problem related with the interactive effect of dispersion (both chromatic dispersion and polarization mode dispersion) and nonlinearity on optical pulse transmission is present. Evolutions of pulses with various initial chirping and shape at bit rate of 10 Gb/s are simulated and compared. Gaussian pulse with appropriate prechirping is propitious for high bit rate transmission.
基金Supported by the National Natural Science Foundation of China under the grant Nos 11175023,11305010 and 11335013partially by the Open Fund of National Laboratory of Science and Technology on Computational Physics(IAPCM).
文摘The influences of laser pulses with negative frequency chirp and different pulse cycles on pair production are studied.By solving the quantum Vlasov equation,the momentum distribution and the number density of the created pairs are obtained numerically.It is found that the chirp rate can enhance the pair production rate in both supercycle and subcycle pulses.Moreover,there is an optimal cycle parameter corresponding to the maximum number density under the same chirp rate.The pair number density is sensitive to the cycle parameter when it is less than the optimal one.Compared to the positive frequency chirp,in the case of the negative frequency chirp,the optimal cycle parameter is increased.
文摘The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase（CEP） breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional（2D） particle-in-cell（PIC） simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.
文摘Nature is amazing!Sometimes you don’t need a weather App to predict rain or shine-you just need nature!Feel the heat:listen to crickets Want to know the outside temperature?Listen to the crickets!Count how many times a cricket chirps in 15 seconds and add 40.That’s the temperature in Fahrenheit(华氏温度,1℉≈0.56℃).Do it several times and find the average.
文摘With the first rays of dawn peeking through the rainforest at Shiti Village,Dehong Dai and Jingpo Autonomous Prefecture,Yunnan Province,Xu Xiaolong,a bird-watching guide from the Jingpo ethnic group,led a German bird-watching group into the jungle.His great-grandfather made a living by hunting“weird-looking birds”in the mountains.Today,the birds(Great Hornbill)are attracting tourist dollars from other parts of the world without dodging bullets.In Shiti Village,bird-watchers can enjoy the chirping of Great Hornbills and feel the rumbling in the air as the birds fly past.The village on the Chinese border with Myanmar was formerly poverty-stricken,with an average annual income of barely 2,000 yuan(US$305)per household 10 years ago.Today,the average annual income has grown to 150,000 yuan(US$23,075).It is a shining example of the successful practice of the notion that“lucid waters and lush mountains are invaluable assets.”
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12174115 and 91836103)。
文摘We propose a scheme for dual-species deceleration and trapping of a cold atom–molecule mixture by a frequency chirping stimulated force.We study the stimulated force exerted on Mg F and Rb using optical Bloch equations based on a direct numerical solution for the time-dependent density matrix.We analyze the relationship between the frequency chirping rate and the number of Mg F molecules and Rb atoms.In addition,we study the dynamical process of molecular deceleration and the effect of transverse diffusion.Monte–Carlo simulations show that buffer-gas-cooled Mg F and Rb beams,with initial velocities of 200 m/s and 130 m/s respectively,can be decelerated to less than 10 m/s.This is achieved with laser powers of as low as 357 m W for Mg F and 10 m W for Rb per traveling wave.The rapid deceleration minimizes molecular loss due to transverse diffusion during the deceleration process.The estimated number of molecules that can be trapped in a magneto-optical trap(MOT)is about 9.0×10^(6),which is an order of magnitude larger than the number of Mg F molecules decelerated by the spontaneous radiation force.The results offer a promising starting point for further studies of sympathetic cooling.
文摘正交时频空间调制(orthogonal time frequency space,OTFS)技术作为一种多载波技术,同样存在峰均功率比较大的问题,高PAPR信号容易进入功率放大器的非线性工作区,导致接收信号的非线性失真,信号经过信道传输后会进一步影响系统的误码率(bit error rate,BER)性能.针对该问题,利用chirp信号相位和为1的特点,采用基于chirp信号预编码的PAPR抑制方法,并在此基础上结合选择映射(selective mapping,SLM)法,将多组频域chirp信号与时频域信号点乘,再经过海森堡变换后选取PAPR最小的一组备选信号作为时域信号.仿真结果表明,与原始OTFS信号相比,基于频域chirp信号的OTFS系统的PAPR降低了0.9 dB,与传统SLM方法相比,使用基于频域chirp信号的SLM改进方法的OTFS系统,减少了IFFT次数的同时保证了系统的PAPR抑制性能.
文摘In this paper, the nonlinear Schr?dinger equation combining quadratic-cubic nonlinearity is considered, which can be represented by an approximate model of relatively dense quasi-one-dimensional Bose-Einstein condensate. Based on the bifurcation theory, we proved the existence of solitary and periodic solutions. The methods we take are the trial equation method and the complete discrimination system for polynomial method. Therefore, we obtain the exact chirped solutions, which are more abundant in type and quantity than the existing results, so that the equation has more profound physical significance. These two methods are rigorously mathematical derivation and calculations, rather than based on certain conditional assumptions. In addition, we give some specific parameters to graphing the motion of the solutions, which helps to understand the propagation of nonlinear waves in fiber optic systems.
基金supported by the National Natural Science Foundation of China(Grant Nos.62227821,62025503,and 62205199).
文摘To capture the nonlinear dynamics and gain evolution in chirped pulse amplification(CPA)systems,the split-step Fourier method and the fourth-order Runge–Kutta method are integrated to iteratively address the generalized nonlinear Schrödinger equation and the rate equations.However,this approach is burdened by substantial computational demands,resulting in significant time expenditures.In the context of intelligent laser optimization and inverse design,the necessity for numerous simulations further exacerbates this issue,highlighting the need for fast and accurate simulation methodologies.Here,we introduce an end-to-end model augmented with active learning(E2E-AL)with decent generalization through different dedicated embedding methods over various parameters.On an identical computational platform,the artificial intelligence–driven model is 2000 times faster than the conventional simulation method.Benefiting from the active learning strategy,the E2E-AL model achieves decent precision with only two-thirds of the training samples compared with the case without such a strategy.Furthermore,we demonstrate a multi-objective inverse design of the CPA systems enabled by the E2E-AL model.The E2E-AL framework manifests the potential of becoming a standard approach for the rapid and accurate modeling of ultrafast lasers and is readily extended to simulate other complex systems.
基金Project supported by the Natural Science Foundation of Jilin Province of China(Grant No.20230101014JC)the National Natural Science Foundation of China(Grant No.12374265)。
文摘We investigate theoretically the effects of chirped laser pulses on high-order harmonic generation(HHG)from solids.We find that the harmonic spectra display redshifts for the driving laser pulses with negative chirp and blueshifts for those with positive chirp,which is due to the change in the instantaneous frequency of the driving laser for different chirped pulses.The analysis of crystal-momentum-resolved(k-resolved)HHG reveals that the frequency shifts are equal for the harmonics generated by different crystal momentum channels.The frequency shifts in the cutoff region are larger than those in the plateau region.With the increase of the absolute value of the chirp parameters,the frequency shifts of HHG become more significant,leading to the shifts from odd-to even-order harmonics.We also demonstrate that the frequency shifts of harmonic spectra are related to the duration of the chirped laser field,but are insensitive to the laser intensity and dephasing time.
基金Project supported by the Science and Technology Project of Hebei Education Department, China (Grant No. ZD2020200)the Innovation Capability Improvement Project of Hebei Province, China (Grant No. 22567605H)。
文摘The existence and stability of the fundamental, multi-peak, and twisted solitons in Kerr nonlinear media with chirped(amplitude-modulated) lattices are reported. We discover that the chirp rate and lattice depth can dramatically change the existence domain of solitons, the energy flow of solitons increases with increasing chirp rate or decreasing lattice depth.We also analyze how the chirp rate and lattice depth affect the stability of solitons. The stable domains of fundamental solitons and twisted solitons exhibit a multi-window distribution, while multi-peak solitons are unstable throughout the entire existence domain.
文摘This research examines the dynamics of a cosh-Gaussian laser pulse travelling through a vacuum and its impact on electron acceleration. We examine the impact of several critical factors, such as laser electric field amplitude, decentered parameter, beam waist, and laser chirp parameter, on the energy gain of electrons using coupled momentum equations. Our results indicate that the energy acquisition of electrons escalates with the amplitude of the laser electric field, decentered parameter, and chirp parameter. An appropriate beam waist is essential for attaining energyefficient electron acceleration in a vacuum. Through the optimization of these parameters, we get a maximum electron energy gain of 2.80 Ge V. This study highlights the significance of customized laser pulse attributes in improving electron acceleration and aids in the progression of high-energy particle physics.
基金supported by the National Key Research and Development Program of China(Grant No.2022YFC2204601)the National Natural Science Foundation of China(Grant Nos.11925503 and 12275093)+1 种基金the Natural Science Foundation of Hubei Province(Grant No.2021CFB019)the State Key Laboratory of Applied Optics(Grant No.SKLAO2022001A10).
文摘Laser frequency combs,which are composed of a series of equally spaced coherent frequency components,have triggered revolutionary progress in precision spectroscopy and optical metrology.Length/distance is of fundamental importance in both science and technology.We describe a ranging scheme based on chirped pulse interferometry.In contrast to the traditional spectral interferometry,the local oscillator is strongly chirped which is able to meet the measurement pulses at arbitrary distances,and therefore,the dead zones can be removed.The distances can be precisely determined via two measurement steps based on the time-of-flight method and synthetic wavelength interferometry,respectively.To overcome the speed limitation of the optical spectrum analyzer,the spectrograms are stretched and detected by a fast photodetector and oscilloscope and consequently mapped into the time domain in real time.The experimental results indicate that the measurement uncertainty can be well within±2μm,compared with the reference distance meter.The Allan deviation can reach 0.4μm at 4 ns averaging time and 25 nm at 1μs and can achieve 2 nm at 100μs averaging time.We also measured a spinning disk with grooves of different depths to verify the measurement speed,and the results show that the grooves with about 150 m∕s line speed can be clearly captured.Our method provides a unique combination of non-dead zones,ultrafast measurement speed,high precision and accuracy,large ambiguity range,and only one single comb source.This system could offer a powerful solution for field measurements in practical applications in the future.
