Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast ele...Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the former two are usually employed to study photoionization processes, the latter is typically used to investigate tunneling ionization. In this review, we briefly overview these timing techniques towards an attosecond temporal resolution of ionization processes in atoms and molecules under intense laser fields. In particular, we review the backpropagation method, which is a novel hybrid quantum-classical approach towards the full characterization of tunneling ionization dynamics. Continued advances in the interrogation techniques promise to pave the pathway towards the exploration of ever faster dynamical processes on an ever shorter timescale.展开更多
We investigate the polarization properties of harmonics from the cyclic H_(3)^(2+) molecular ions in tailored bichromatic counter-rotating circularly polarized(BCCP)fields by solving the time-dependent Schrödinge...We investigate the polarization properties of harmonics from the cyclic H_(3)^(2+) molecular ions in tailored bichromatic counter-rotating circularly polarized(BCCP)fields by solving the time-dependent Schrödinger equation.The allowed harmonics and their helicities are associated with the symmetry compatibility of the field-target systems,and large intensity difference between adjacent harmonics with opposite helicities appears in a wide spectral range when the BCCP field is at certain rotation angles.We try to explain the intensity difference by using a recombination model based on the quantum-orbit theory and by analyzing the ionization pathways.Moreover,to synthesize attosecond pulse trains with tunable polarization,the intensity difference is manipulated by introducing a seed XUV field,and by changing the relative amplitude ratio as well as the helicity of BCCP fields.展开更多
Molecular-frame photoelectron momentum distributions(MF-PMDs) of an H_(2)^(+) molecule ion in the presence of a pair of counter-rotating circularly polarized attosecond extreme ultraviolet laser pulses is studied by n...Molecular-frame photoelectron momentum distributions(MF-PMDs) of an H_(2)^(+) molecule ion in the presence of a pair of counter-rotating circularly polarized attosecond extreme ultraviolet laser pulses is studied by numerically solving the two-dimensional time-dependent Schrodinger equation within the frozen-nuclei approximation. At small time delay, our simulations show that the electron vortex structure is sensitive to the time delay and relative phase between the counterrotating pulses when they are partially overlapped. By adjusting time delay and relative phase, we have the ability to manipulate the MF-PMDs and the appearance of spiral arms. We further show that the internuclear distance can affect the spiral vortices due to its different transition cross sections in the parallel and perpendicular geometries. The lowest-order perturbation theory is employed to interpret these phenomena qualitatively. It is concluded that the internuclear distancedependent transition cross sections and the confinement effect in diatomic molecules are responsible for the variation of vortex structures in the MF-PMDs.展开更多
The intrinsic chirp of high-order harmonic generation is an important factor limiting the production of ultrashort attosecond pulses.Based on numerically solving the time-dependent Schrodinger equation,the generation ...The intrinsic chirp of high-order harmonic generation is an important factor limiting the production of ultrashort attosecond pulses.Based on numerically solving the time-dependent Schrodinger equation,the generation process of highorder harmonic from the He atom under the action of orthogonal two-color combined pulse of fundamental frequency and higher intensity second harmonic fields is studied.In this paper,we propose to achieve quasi-chirp-free isolated attosecond pulses by superimposing a higher second-harmonic field on the orthogonal direction of the fundamental frequency field.It is found that the high-energy part of its harmonic emission exhibits small chirp characteristics,which can be used to synthesize isolated attosecond pulses.Through the analysis of the wave packets evolution and the classical motion trajectories of the electron,it is demonstrated that the quasi-chirp-free harmonic can be attributed to the simultaneous return of electrons ionized at different times to the parent particle.The influence of the relative phase of the two pulses on the harmonics is further analyzed,and it is observed that this phenomenon is sensitive to the relative phase,but it can still generate isolated attosecond pulses within a certain phase.展开更多
We utilized a set of fused silica thin plates to broaden the spectrum of 1kHz,30 fs Ti:sapphire amplified laser pulses to an octave.Following the compression by chirped mirror pairs,the generated few-cycle pulses were...We utilized a set of fused silica thin plates to broaden the spectrum of 1kHz,30 fs Ti:sapphire amplified laser pulses to an octave.Following the compression by chirped mirror pairs,the generated few-cycle pulses were focused onto an argon filled gas cell.We detected high order harmonics corresponding to a train of 209 as pulses,characterized by the reconstruction of attosecond beating by interference of two-photon transition(RABITT)technique.Compared with the conventional attosecond pulse trains,the broad harmonics in such pulse trains cover more energy range,so it is more efficient in studying some typical cases,such as resonances,with frequency resolved RABITT.As the solid thin plates can support high power supercontinuum generation,it is feasible to tailor the spectrum to have different central wavelength and spectral width,which will make the RABITT source work in different applications.展开更多
We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fie...We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive-to negativevalue around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.展开更多
We investigate how the intensity and duration of an attosecond pulse generated from high-order harmonic generation are affected by the pressure and thickness of the gas jet by taking into account the macroscopic propa...We investigate how the intensity and duration of an attosecond pulse generated from high-order harmonic generation are affected by the pressure and thickness of the gas jet by taking into account the macroscopic propagation of both fundamental and harmonic fields. Our simulations show that, limited by the propagation effects, especially the absorption of harmonics, the intensity of an attosecond pulse cannot be improved by just independently increasing the gas pressure or the medium length. On the other hand, due to good phase-matching conditions, the duration of a generated attosecond pulse can be improved by changing the gas pressure.展开更多
Three decades ago,a highly nonlinear nonpertubative phenomenon,now well-known as the high harmonic generation(HHG),was discovered when intense laser irradiates gaseous atoms.As the HHG produces broadband coherent radi...Three decades ago,a highly nonlinear nonpertubative phenomenon,now well-known as the high harmonic generation(HHG),was discovered when intense laser irradiates gaseous atoms.As the HHG produces broadband coherent radiation,it becomes the most promising source to obtain attosecond pulses.The door to the attosecond science was opened ever since.In this review,we will revisit the incredible adventure to the attoworld.Firstly,the progress of attosecond pulse generation is outlined.Then,we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions,utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.展开更多
As a crucial parameter for a few-cycle laser pulse, the carrier envelope phase(CEP) substantially determines the laser waveform. We propose a method to directly describe the CEP of an isolated attosecond pulse(IAP) by...As a crucial parameter for a few-cycle laser pulse, the carrier envelope phase(CEP) substantially determines the laser waveform. We propose a method to directly describe the CEP of an isolated attosecond pulse(IAP) by the vortex-shaped momentum pattern, which is generated from the tunneling ionization of a hydrogen atom by a pair of time-delayed, oppositely and circularly polarized IAP-IR pulses. Superior to the angular streaking method that characterizes the CEP in terms of only one streak, our method describes the CEP of an IAP by the features of multiple streaks in the vortex pattern. The proposed method may open the possibility of capturing sub-cycle extreme ultraviolet dynamics.展开更多
High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase(CEP)of few-cycle laser pulse.We find that sho...High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase(CEP)of few-cycle laser pulse.We find that short pulse duration will cause the negative chirp for the high harmonics.When the laser pulse is shortened to a few cycles,the influence of the laser CEP on the chirp of the harmonics will also become more prominent.展开更多
To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phen...To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phenomena and processes and raises a series of questions of concepts, theories, and methods. Recent studies show that the Coulomb potential can cause the ionization time lag(about 100 attoseconds) between instants of the field maximum and the ionization-rate maximum. This lag can be understood as the response time of the electronic wave function to the strong-field-induced ionization event. It has a profound influence on the subsequent ultrafast dynamics of the ionized electron and can significantly change the time–frequency properties of electron trajectory(an important theoretical tool for attosecond measurement). Here, the research progress of response time and its implications on attosecond measurement are briefly introduced.展开更多
We investigate the role of core potential in high ionization potential systems on high harmonic generation (HHG) spectra and obtain attosecond pulses. In our scheme, we use a standard soft core potential to model high...We investigate the role of core potential in high ionization potential systems on high harmonic generation (HHG) spectra and obtain attosecond pulses. In our scheme, we use a standard soft core potential to model high ionization potential systems and irradiated these systems with fixed laser parameters. We observe the role of these systems on all the three steps involved in HHG process including ionization, propagation and recombination. In our study, the results illustrate that for high ionization potential systems, the HHG process is more sensitive to the ionization probability compared to the recombination amplitude. We also observe that due to the stronger core potential, small oscillations of the electrons during the propagation do not contribute to the HHG spectrum, which implies the dominance of only long quantum paths in the HHG spectrum. Our results, for attosecond pulse generation, show that long quantum path electrons are responsible for the supercontinuum region near the cutoff, which is suitable for the extraction of a single attosecond pulse in this region.展开更多
We investigate the ellipticity of the high-order harmonic generation from the oriented H_(2)^(+)exposed to a linearly polarized laser field by numerically solving the two-dimensional time-dependent Schrodinger equatio...We investigate the ellipticity of the high-order harmonic generation from the oriented H_(2)^(+)exposed to a linearly polarized laser field by numerically solving the two-dimensional time-dependent Schrodinger equation(2 D TDSE).Numerical simulations show that the harmonic ellipticity is remarkably sensitive to the alignment angle.The harmonic spectrum is highly elliptically polarized at a specific alignment angleθ=30°,which is insensitive to the variation of the laser parameters.The position of the harmonic intensity minima indicates the high ellipticity,which can be attributed to the two-center interference effect.The high ellipticity can be explained by the phase difference of the harmonics.This result facilitates the synthesis of a highly elliptical isolated attosecond pulse with duration down to 65 as,which can be served as a powerful tool to explore the ultrafast dynamics of molecules and study chiral light-matter interaction.展开更多
We theoretically investigate the effects of different electronic states as the initial state on the vortex patterns in photoelectron momentum distributions(PMDs)from numerical solutions of the two-dimensional(2D)time-...We theoretically investigate the effects of different electronic states as the initial state on the vortex patterns in photoelectron momentum distributions(PMDs)from numerical solutions of the two-dimensional(2D)time-dependent Schrodinger equation(TDSE)of He^(+)with a pair of counter-rotating circularly polarized attosecond pulses.It is found that the number of spiral arms in vortex patterns is equal to the number of the absorbed photons when the initial state is the ground state.However,the number of spiral arms in vortex patterns is always two more than the number of the absorbed photons when the initial state is the excited state.This sensitivity is attributed to the initial electron density distribution.In addition,we have demonstrated the PMDs for different initial electronic states with the same wavelengths and analyzed their corresponding physical mechanisms.It is illustrated that the method presented can be employed to effectively control the distribution of the electron vortices.展开更多
The attosecond ionization dynamics of atoms has attracted extensive attention in these days.However,the role of the initial state is not clearly understood.To address this question,we perform simulations on the neon a...The attosecond ionization dynamics of atoms has attracted extensive attention in these days.However,the role of the initial state is not clearly understood.To address this question,we perform simulations on the neon atom and its model atom with different initial states by numerically solving the corresponding two-dimensional time-dependent Schrodinger¨equations.We theoretically investigate atomic photoelectron momentum distributions(PMDs)by a pair of elliptically polarized attosecond laser pulses.We find that the PMD is sensitive not only to the ellipticities of the pulses,the relative helicity,and time delay of the pulses,but also to the symmetry of the initial electronic states.Results are analyzed by the first-order time-dependent perturbation theory(TDPT)and offer a new tool for detecting the rotation direction of the ring currents.展开更多
Isolated attosecond pulse generation in argon is theoretically investigated for different gas pressures and medium lengths.The output of attosecond pulse is effectively enhanced by using a longer gas medium with optim...Isolated attosecond pulse generation in argon is theoretically investigated for different gas pressures and medium lengths.The output of attosecond pulse is effectively enhanced by using a longer gas medium with optimized pressure.The peak intensity of the attosecond pulse by using 6 mm gas medium is doubled compared with that of 1-3 mm gas cell,which is usually used in the experiment.Our simulation shows that the distortion of the driving laser waveform and the absorption are the main factors that limit the output of the attosecond pulse for the long gas medium.Optimized generation condition could be found by balancing the medium length and pressure.展开更多
We theoretically investigate high-order harmonic generation in a two-color multi-cycle inhomogeneous field combined with a 27 th harmonic pulse. By considering a bowtie-shaped gold nanostructure, the spatiotemporal pr...We theoretically investigate high-order harmonic generation in a two-color multi-cycle inhomogeneous field combined with a 27 th harmonic pulse. By considering a bowtie-shaped gold nanostructure, the spatiotemporal profiles of enhanced plasmonic fields are obtained by solving the Maxwell equation using finite-domain time-difference method. Based on quantum-mechanical and classical models, the effect of 27 th harmonic pulse, temporal profile of enhanced plasmonic field and inhomogeneity on supercontinuum generation are analyzed and discussed. As a result, broadband supercontinuum can be generated from our approach with optimized gap size of nanostructure. Moreover, these results are not sensitively dependent on the relative phase in the two-color field.展开更多
This paper theoretically investigates the high-order harmonic generation cutoff extension using intense few-cycle linearly chirped laser pulses. It shows that the cutoff of the harmonic can be extended remarkably by o...This paper theoretically investigates the high-order harmonic generation cutoff extension using intense few-cycle linearly chirped laser pulses. It shows that the cutoff of the harmonic can be extended remarkably by optimising the chirping parameters. The time-frequency characteristics of high-order harmonics with different chirping parameters are analysed by means of wavelet transform of the dipole acceleration. It also gives out the classical three-step model pictures of electron. By superposing a properly selected range of the harmonic spectrum, it obtains an isolated 65as pulse.展开更多
The attosecond laser station(ALS) at the Synergetic Extreme Condition User Facility(SECUF) is a sophisticated and user-friendly platform for the investigation of the electron dynamics in atoms, molecules, and condense...The attosecond laser station(ALS) at the Synergetic Extreme Condition User Facility(SECUF) is a sophisticated and user-friendly platform for the investigation of the electron dynamics in atoms, molecules, and condensed matter on timescales ranging from tens of femtoseconds to tens of attoseconds. Short and tunable coherent extreme-ultraviolet(XUV)light sources based on high-order harmonic generation in atomic gases are being developed to drive a variety of endstations for inspecting and controlling ultrafast electron dynamics in real time. The combination of such light sources and end-stations offers a route to investigate fundamental physical processes in atoms, molecules, and condensed matter. The ALS consists of four beamlines, each containing a light source designed specifically for application experiments that will be performed in its own end-station. The first beamline will produce broadband XUV light for attosecond photoelectron spectroscopy and attosecond transient absorption spectroscopy. It is also capable of performing attosecond streaking to characterize isolated attosecond pulses and will allow studies on the electron dynamics in atoms, moleculars, and condensed matter. The second XUV beamline will produce narrowband femtosecond XUV pulses for time-resolved and angle-resolved photoelectron spectroscopy, to study the electronic dynamics on the timescale of fundamental correlations and interactions in solids, especially in superconductors and topological insulators. The third beamline will produce broadband XUV pulses for attosecond coincidence spectroscopy in a cold-target recoil-ion momentum spectrometer, to study the ultrafast dynamics and reactions in atomic and molecular systems. The last beamline produces broadband attosecond XUV pulses designed for time-resolved photoemission electron microscopy, to study the ultrafast dynamics of plasmons in nanostructures and the surfaces of solid materials with high temporal and spatial resolutions simultaneously. The main object of the ALS is to provide domestic and international scientists with unique tools to study fundamental processes in physics, chemistry,biology, and material sciences with ultrafast temporal resolutions on the atomic scale.展开更多
We investigate the ionization dynamics of atoms by chirped attosecond pulses using the strong field approximation method.The pulse parameters are carefully chosen in the regime where the strong field approximation met...We investigate the ionization dynamics of atoms by chirped attosecond pulses using the strong field approximation method.The pulse parameters are carefully chosen in the regime where the strong field approximation method is valid.We analyse the effects of the chirp of attosecond pulses on the energy distributions and the corresponding left-right asymmetry of the ionized electrons.For a single chirped attosecond pulse,the ionized electrons can be redistributed and the left-right asymmetry shows oscillations because of the introduction of the chirp.For time-delayed double attosecond pulses at different intensities with the weaker one chirped,exchanging the order of the two pulses shows a relative shift of the energy spectra,which can be explained by the different effective time delays of different frequency components because of the chirp.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant Nos.92150105,11834004,12227807,and 12241407)the Science and Technology Commission of Shanghai Municipality (Grant No.21ZR1420100)。
文摘Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the former two are usually employed to study photoionization processes, the latter is typically used to investigate tunneling ionization. In this review, we briefly overview these timing techniques towards an attosecond temporal resolution of ionization processes in atoms and molecules under intense laser fields. In particular, we review the backpropagation method, which is a novel hybrid quantum-classical approach towards the full characterization of tunneling ionization dynamics. Continued advances in the interrogation techniques promise to pave the pathway towards the exploration of ever faster dynamical processes on an ever shorter timescale.
基金supported by the National Natural Science Foundation of China(Grant No.91950117)the Fundamental Research Funds for the Central Universities.
文摘We investigate the polarization properties of harmonics from the cyclic H_(3)^(2+) molecular ions in tailored bichromatic counter-rotating circularly polarized(BCCP)fields by solving the time-dependent Schrödinger equation.The allowed harmonics and their helicities are associated with the symmetry compatibility of the field-target systems,and large intensity difference between adjacent harmonics with opposite helicities appears in a wide spectral range when the BCCP field is at certain rotation angles.We try to explain the intensity difference by using a recombination model based on the quantum-orbit theory and by analyzing the ionization pathways.Moreover,to synthesize attosecond pulse trains with tunable polarization,the intensity difference is manipulated by introducing a seed XUV field,and by changing the relative amplitude ratio as well as the helicity of BCCP fields.
基金Project supported by the Natural Science Foundation of Jilin Province,China (Grant No.20220101016JC)the National Key Research and Development Program of China (Grant No.2022YFE0134200)+1 种基金the National Natural Science Foundation of China (Grant Nos.12174147,91850114,and 11774131)the Open Research Fund of State Key Laboratory of Transient Optics and Photonics。
文摘Molecular-frame photoelectron momentum distributions(MF-PMDs) of an H_(2)^(+) molecule ion in the presence of a pair of counter-rotating circularly polarized attosecond extreme ultraviolet laser pulses is studied by numerically solving the two-dimensional time-dependent Schrodinger equation within the frozen-nuclei approximation. At small time delay, our simulations show that the electron vortex structure is sensitive to the time delay and relative phase between the counterrotating pulses when they are partially overlapped. By adjusting time delay and relative phase, we have the ability to manipulate the MF-PMDs and the appearance of spiral arms. We further show that the internuclear distance can affect the spiral vortices due to its different transition cross sections in the parallel and perpendicular geometries. The lowest-order perturbation theory is employed to interpret these phenomena qualitatively. It is concluded that the internuclear distancedependent transition cross sections and the confinement effect in diatomic molecules are responsible for the variation of vortex structures in the MF-PMDs.
基金Project supported by the National Key Research and Development Program of China(Grant No.2019YFA0307700)the National Natural Science Foundation of China(Grant Nos.12074145,11627807,and 11975012)+2 种基金the Research Foundation for Basic Research of Jilin Province,China(Grant No.20220101003JC)the Fundamental Research Funds for the Central Universities of China(Grant No.30916011207)the Outstanding Youth Project of Taizhou University(Grant No.2019JQ002)。
文摘The intrinsic chirp of high-order harmonic generation is an important factor limiting the production of ultrashort attosecond pulses.Based on numerically solving the time-dependent Schrodinger equation,the generation process of highorder harmonic from the He atom under the action of orthogonal two-color combined pulse of fundamental frequency and higher intensity second harmonic fields is studied.In this paper,we propose to achieve quasi-chirp-free isolated attosecond pulses by superimposing a higher second-harmonic field on the orthogonal direction of the fundamental frequency field.It is found that the high-energy part of its harmonic emission exhibits small chirp characteristics,which can be used to synthesize isolated attosecond pulses.Through the analysis of the wave packets evolution and the classical motion trajectories of the electron,it is demonstrated that the quasi-chirp-free harmonic can be attributed to the simultaneous return of electrons ionized at different times to the parent particle.The influence of the relative phase of the two pulses on the harmonics is further analyzed,and it is observed that this phenomenon is sensitive to the relative phase,but it can still generate isolated attosecond pulses within a certain phase.
基金Project supported by the National Key R&D Program of China(Grant No.2017YFB0405202)the Major Program of the National Natural Science Foundation of China(Grant No.61690221)+1 种基金the Key Program of the National Natural Science Foundation of China(Grant No.11434016)the National Natural Science Foundation of China(Grant Nos.11574384,11674386,and 11774277)
文摘We utilized a set of fused silica thin plates to broaden the spectrum of 1kHz,30 fs Ti:sapphire amplified laser pulses to an octave.Following the compression by chirped mirror pairs,the generated few-cycle pulses were focused onto an argon filled gas cell.We detected high order harmonics corresponding to a train of 209 as pulses,characterized by the reconstruction of attosecond beating by interference of two-photon transition(RABITT)technique.Compared with the conventional attosecond pulse trains,the broad harmonics in such pulse trains cover more energy range,so it is more efficient in studying some typical cases,such as resonances,with frequency resolved RABITT.As the solid thin plates can support high power supercontinuum generation,it is feasible to tailor the spectrum to have different central wavelength and spectral width,which will make the RABITT source work in different applications.
基金Project supported by the National Natural Science Foundation of China(Grant No.61575077)the Natural Science Foundation of Jilin Province of China(Grant No.20180101225JC)+1 种基金the China Postdoctoral Science Foundation(Grants Nos.2018M641766 and 2019T120232)the Graduate Innovation Fund of Jilin University,China(Grant No.101832018C105)
文摘We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive-to negativevalue around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11264036 and 11064013)
文摘We investigate how the intensity and duration of an attosecond pulse generated from high-order harmonic generation are affected by the pressure and thickness of the gas jet by taking into account the macroscopic propagation of both fundamental and harmonic fields. Our simulations show that, limited by the propagation effects, especially the absorption of harmonics, the intensity of an attosecond pulse cannot be improved by just independently increasing the gas pressure or the medium length. On the other hand, due to good phase-matching conditions, the duration of a generated attosecond pulse can be improved by changing the gas pressure.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12021004 and 91950202)
文摘Three decades ago,a highly nonlinear nonpertubative phenomenon,now well-known as the high harmonic generation(HHG),was discovered when intense laser irradiates gaseous atoms.As the HHG produces broadband coherent radiation,it becomes the most promising source to obtain attosecond pulses.The door to the attosecond science was opened ever since.In this review,we will revisit the incredible adventure to the attoworld.Firstly,the progress of attosecond pulse generation is outlined.Then,we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions,utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11674243 and 11674242
文摘As a crucial parameter for a few-cycle laser pulse, the carrier envelope phase(CEP) substantially determines the laser waveform. We propose a method to directly describe the CEP of an isolated attosecond pulse(IAP) by the vortex-shaped momentum pattern, which is generated from the tunneling ionization of a hydrogen atom by a pair of time-delayed, oppositely and circularly polarized IAP-IR pulses. Superior to the angular streaking method that characterizes the CEP in terms of only one streak, our method describes the CEP of an IAP by the features of multiple streaks in the vortex pattern. The proposed method may open the possibility of capturing sub-cycle extreme ultraviolet dynamics.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.91950203,11874374,and 61690223)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB16).
文摘High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase(CEP)of few-cycle laser pulse.We find that short pulse duration will cause the negative chirp for the high harmonics.When the laser pulse is shortened to a few cycles,the influence of the laser CEP on the chirp of the harmonics will also become more prominent.
基金Project supported by the National Natural Science Foundation of China(Grant No.91750111)the National Key Research and Development Program of China(Grant No.2018YFB0504400)。
文摘To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phenomena and processes and raises a series of questions of concepts, theories, and methods. Recent studies show that the Coulomb potential can cause the ionization time lag(about 100 attoseconds) between instants of the field maximum and the ionization-rate maximum. This lag can be understood as the response time of the electronic wave function to the strong-field-induced ionization event. It has a profound influence on the subsequent ultrafast dynamics of the ionized electron and can significantly change the time–frequency properties of electron trajectory(an important theoretical tool for attosecond measurement). Here, the research progress of response time and its implications on attosecond measurement are briefly introduced.
文摘We investigate the role of core potential in high ionization potential systems on high harmonic generation (HHG) spectra and obtain attosecond pulses. In our scheme, we use a standard soft core potential to model high ionization potential systems and irradiated these systems with fixed laser parameters. We observe the role of these systems on all the three steps involved in HHG process including ionization, propagation and recombination. In our study, the results illustrate that for high ionization potential systems, the HHG process is more sensitive to the ionization probability compared to the recombination amplitude. We also observe that due to the stronger core potential, small oscillations of the electrons during the propagation do not contribute to the HHG spectrum, which implies the dominance of only long quantum paths in the HHG spectrum. Our results, for attosecond pulse generation, show that long quantum path electrons are responsible for the supercontinuum region near the cutoff, which is suitable for the extraction of a single attosecond pulse in this region.
基金supported by the National Natural Science Foundation of China(Grants Nos.12074142 and 11904122)。
文摘We investigate the ellipticity of the high-order harmonic generation from the oriented H_(2)^(+)exposed to a linearly polarized laser field by numerically solving the two-dimensional time-dependent Schrodinger equation(2 D TDSE).Numerical simulations show that the harmonic ellipticity is remarkably sensitive to the alignment angle.The harmonic spectrum is highly elliptically polarized at a specific alignment angleθ=30°,which is insensitive to the variation of the laser parameters.The position of the harmonic intensity minima indicates the high ellipticity,which can be attributed to the two-center interference effect.The high ellipticity can be explained by the phase difference of the harmonics.This result facilitates the synthesis of a highly elliptical isolated attosecond pulse with duration down to 65 as,which can be served as a powerful tool to explore the ultrafast dynamics of molecules and study chiral light-matter interaction.
基金Project supported by the National Natural Science Foundation of China(Grant No.12074142)the Natural Science Foundation of Jilin Province of China(Grant No.20180101225JC)the Graduate Innovation Fund of Jilin University,China(Grant No.101832020CX337)。
文摘We theoretically investigate the effects of different electronic states as the initial state on the vortex patterns in photoelectron momentum distributions(PMDs)from numerical solutions of the two-dimensional(2D)time-dependent Schrodinger equation(TDSE)of He^(+)with a pair of counter-rotating circularly polarized attosecond pulses.It is found that the number of spiral arms in vortex patterns is equal to the number of the absorbed photons when the initial state is the ground state.However,the number of spiral arms in vortex patterns is always two more than the number of the absorbed photons when the initial state is the excited state.This sensitivity is attributed to the initial electron density distribution.In addition,we have demonstrated the PMDs for different initial electronic states with the same wavelengths and analyzed their corresponding physical mechanisms.It is illustrated that the method presented can be employed to effectively control the distribution of the electron vortices.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11404204 and 11974229)the Natural Science Foundation for Young Scientists of Shanxi Province,China(Grant No.201901D211404)+1 种基金the Scientific and Technological Innovation Program of Higher Education Institutions in Shanxi Province,China(Grant No.2019L0468)the Project of Excellent Course of Shanxi Normal University,China(Grant No.2017YZKC-35).
文摘The attosecond ionization dynamics of atoms has attracted extensive attention in these days.However,the role of the initial state is not clearly understood.To address this question,we perform simulations on the neon atom and its model atom with different initial states by numerically solving the corresponding two-dimensional time-dependent Schrodinger¨equations.We theoretically investigate atomic photoelectron momentum distributions(PMDs)by a pair of elliptically polarized attosecond laser pulses.We find that the PMD is sensitive not only to the ellipticities of the pulses,the relative helicity,and time delay of the pulses,but also to the symmetry of the initial electronic states.Results are analyzed by the first-order time-dependent perturbation theory(TDPT)and offer a new tool for detecting the rotation direction of the ring currents.
基金Project supported by the National Key R&D Program of China(Grant No.2018YFB1107200)the National Natural Science Foundation of China(Grant Nos.11974416 and 91850209)。
文摘Isolated attosecond pulse generation in argon is theoretically investigated for different gas pressures and medium lengths.The output of attosecond pulse is effectively enhanced by using a longer gas medium with optimized pressure.The peak intensity of the attosecond pulse by using 6 mm gas medium is doubled compared with that of 1-3 mm gas cell,which is usually used in the experiment.Our simulation shows that the distortion of the driving laser waveform and the absorption are the main factors that limit the output of the attosecond pulse for the long gas medium.Optimized generation condition could be found by balancing the medium length and pressure.
基金supported by the National Natural Science Foundation of China(Grants Nos.11404153,11175076,11135002,and 11405077)the Fundamental Research Funds for the Central Universities of China(Grant Nos.lzujbky-2014-10,lzujbky-2014-13,and lzujbky-2014-14)
文摘We theoretically investigate high-order harmonic generation in a two-color multi-cycle inhomogeneous field combined with a 27 th harmonic pulse. By considering a bowtie-shaped gold nanostructure, the spatiotemporal profiles of enhanced plasmonic fields are obtained by solving the Maxwell equation using finite-domain time-difference method. Based on quantum-mechanical and classical models, the effect of 27 th harmonic pulse, temporal profile of enhanced plasmonic field and inhomogeneity on supercontinuum generation are analyzed and discussed. As a result, broadband supercontinuum can be generated from our approach with optimized gap size of nanostructure. Moreover, these results are not sensitively dependent on the relative phase in the two-color field.
基金supported by the National Natural Science Foundation of China (Grant No.10974068)
文摘This paper theoretically investigates the high-order harmonic generation cutoff extension using intense few-cycle linearly chirped laser pulses. It shows that the cutoff of the harmonic can be extended remarkably by optimising the chirping parameters. The time-frequency characteristics of high-order harmonics with different chirping parameters are analysed by means of wavelet transform of the dipole acceleration. It also gives out the classical three-step model pictures of electron. By superposing a properly selected range of the harmonic spectrum, it obtains an isolated 65as pulse.
基金Project supported by the National Key R&D Program of China(Grant Nos.2018YFB1107200,2017YFC0110301,and 2017YFB0405202)Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB0703030)the National Natural Science Foundation of China(Grant Nos.11474002,11674386,61575219,and 61690221)
文摘The attosecond laser station(ALS) at the Synergetic Extreme Condition User Facility(SECUF) is a sophisticated and user-friendly platform for the investigation of the electron dynamics in atoms, molecules, and condensed matter on timescales ranging from tens of femtoseconds to tens of attoseconds. Short and tunable coherent extreme-ultraviolet(XUV)light sources based on high-order harmonic generation in atomic gases are being developed to drive a variety of endstations for inspecting and controlling ultrafast electron dynamics in real time. The combination of such light sources and end-stations offers a route to investigate fundamental physical processes in atoms, molecules, and condensed matter. The ALS consists of four beamlines, each containing a light source designed specifically for application experiments that will be performed in its own end-station. The first beamline will produce broadband XUV light for attosecond photoelectron spectroscopy and attosecond transient absorption spectroscopy. It is also capable of performing attosecond streaking to characterize isolated attosecond pulses and will allow studies on the electron dynamics in atoms, moleculars, and condensed matter. The second XUV beamline will produce narrowband femtosecond XUV pulses for time-resolved and angle-resolved photoelectron spectroscopy, to study the electronic dynamics on the timescale of fundamental correlations and interactions in solids, especially in superconductors and topological insulators. The third beamline will produce broadband XUV pulses for attosecond coincidence spectroscopy in a cold-target recoil-ion momentum spectrometer, to study the ultrafast dynamics and reactions in atomic and molecular systems. The last beamline produces broadband attosecond XUV pulses designed for time-resolved photoemission electron microscopy, to study the ultrafast dynamics of plasmons in nanostructures and the surfaces of solid materials with high temporal and spatial resolutions simultaneously. The main object of the ALS is to provide domestic and international scientists with unique tools to study fundamental processes in physics, chemistry,biology, and material sciences with ultrafast temporal resolutions on the atomic scale.
基金Project supported by the National Natural Science Foundation of China (Grant Nos 10704003 and 10821062)the National Basic Research Program of China (Grant No 2006CB806007)
文摘We investigate the ionization dynamics of atoms by chirped attosecond pulses using the strong field approximation method.The pulse parameters are carefully chosen in the regime where the strong field approximation method is valid.We analyse the effects of the chirp of attosecond pulses on the energy distributions and the corresponding left-right asymmetry of the ionized electrons.For a single chirped attosecond pulse,the ionized electrons can be redistributed and the left-right asymmetry shows oscillations because of the introduction of the chirp.For time-delayed double attosecond pulses at different intensities with the weaker one chirped,exchanging the order of the two pulses shows a relative shift of the energy spectra,which can be explained by the different effective time delays of different frequency components because of the chirp.
