Elliptically polarized high-order harmonic generation of Ar atom in an intense laser field

High-order harmonic generation(HHG) of Ar atom in an elliptically polarized intense laser field is experimentally investigated in this work.Interestingly,the anomalous ellipticity dependence on the laser ellipticity(ε) in the lower-order harmonics is observed,specifically in the 13rd-order,which displays a maximal harmonic intensity at ε ≈ 0.1,rather than at ε = 0 as expected.This contradicts the general trend of harmonic yield,which typically decreases with the increase of laser ellipticity.In this study,we attribute this phenomenon to the disruption of the symmetry of the wave function by the Coulomb effect,leading to the generation of a harmonic with high ellipticity.This finding provides valuable insights into the behavior of elliptically polarized harmonics and opens up a potential way for exploring new applications in ultrafast spectroscopy and light–matter interactions.

High-order harmonic generation of the cyclo[18]carbon molecule irradiated by circularly polarized laser pulse

High-order harmonic generation of the cyclo[18]carbon(C_(18) ) molecule under few-cycle circularly polarized laser pulse is studied by time-dependent density functional theory. Compared with the harmonic emission of the ring molecule C_(6)H_(6) having similar ionization potential, the C_(18) molecule has higher efficiency and cutoff energy than C_(6)H_(6) with the same laser field parameters. Further researches indicate that the harmonic efficiency and cutoff energy of the C_(18) molecule increase gradually with the increase of the laser intensity of the driving laser or decrease of the wavelength, both are larger than those of the C_(6)H_(6) molecule. Through the analysis of the time-dependent evolution of the electronic wave packets, it is also found that the higher efficiency of harmonic generation can be attributed to the larger spatial scale of the C_(18) molecule,which leads to a greater chance for the ionized electrons from one atom to recombine with others of the parent molecule.Selecting the suitable driving laser pulse, it is demonstrated that high-order harmonic generation in the C_(18) molecule has a wide range of applications in producing circularly polarized isolated attosecond pulse.

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse.The harmonic spectrum shows a clear cutoff with an energy Ip ＋ 2Up.In particular,the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity.These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.

High-order harmonic generation of N2molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N2 molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.

Role of potential on high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser fields

We investigate high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser pulses by numerically solving the time-dependent Schrödinger equation.It is found that the minimum energy position of the harmonic spectrum and the non-integer order optical radiation are greatly discrepant for different atomic potentials.By analyzing the quantum trajectory of the harmonic emission,discrepancies among the harmonic spectra from different potentials can be attributed to the action of the potential on the ionized electrons.In addition,based on the influence of the driving light intensity on the overall intensity and ellipticity of higher order harmonics,the physical conditions for generating a high-intensity circularly polarized harmonic can be obtained.

Controlling the contributions to high-order harmonic generation from different nuclei of N_2 with an orthogonally polarized two-color laser field

The generation of high-order harmonic and the attosecond pulse of the N2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model.We show that the control of contributions to high-order harmonic generation（HHG） from different nuclei is realized by properly selecting the relative phase.When the relative phase is chosen to be φ= 0.4π,the contribution to HHG from one nucleus is much more than that from another.Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 e V to125 e V.The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical threestep model with a finite initial transverse velocity.By superposing several orders of harmonics,an isolated attosecond pulse with a duration of 80 as can be generated.

Decoding the electron dynamics in high-order harmonic generation from asymmetric molecular ions in elliptically polarized laser fields

The high-order harmonic generation from an asymmetric molecular ion is theoretically investigated based on the Born-Oppenheimer model with two-dimensional electron dynamics.It is shown that the harmonic intensity changes periodically in elliptically polarized laser fields.The periodical character is ellipticity-dependent.By establishing the physical image,the periodicity of the harmonic intensity can be ascribed to the contributions of the ground state and the excited state.Furthermore,the electron dynamics from different electronic states can be selected via combining the elliptically polarized laser field with a static electric field.The harmonics dominated either by ground state or excited state are emitted once in an optical cycle in the combined laser field.

Phase-coherence dynamics of frequency-comb emission via high-order harmonic generation in few-cycle pulse trains

Frequency-comb emission via high-order harmonic generation(HHG)provides an alternative method for the coherent vacuum ultraviolet(VUV)and extreme ultraviolet(XUV)radiation at ultrahigh repetition rates.In particular,the temporal and spectral features of the HHG were shown to carry profound insight into frequency-comb emission dynamics.Here we present an ab initio investigation of the temporal and spectral coherence of the frequency comb emitted in HHG of He atom driven by few-cycle pulse trains.We find that the emission of frequency combs features a destructive and constructive coherences caused by the phase interference of HHG,leading to suppression and enhancement of frequency-comb emission.The results reveal intriguing and substantially different nonlinear optical response behaviors for frequency-comb emission via HHG.The dynamical origin of frequency-comb emission is clarified by analyzing the phase coherence in HHG processes in detail.Our results provide fresh insight into the experimental realization of selective enhancement of frequency comb in the VUV–XUV regimes.

High-order harmonic generation in Ar and Ne with a 45fs intense laser field

Experimental results of high-order harmonic generation (HHG) in Ar and Ne gas driven with a 45fs Ti: sapphire laser are presented. The shortest-wavelength harmonic emission corresponding to the 91st order harmonic (8.63nm) is observed in argon. In neon, the harmonics up to order 131 (5.99nm) is also observed. The effects of gas density, laser intensity, free electron and the focusing geometry parameters of the laser beam on the process of harmonic generation are investigated. The direct experimental evidence that an increased electron density causes a degenerated harmonic radiation is obtained.

Resolving multi-orbital effects on high harmonic generation from aligned N2 molecules in linearly and elliptically polarized intense laser fields

We perform an experimental study of the multi-orbital effect on the high-order harmonic generation(HHG) from aligned N_2 molecules in both linearly and elliptically polarized intense laser fields.Measured by a home-built extreme ultraviolet(XUV) flat grating spectrometer with the pump-probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions.The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization.Our results indicate that both the highest occupied molecular orbital(HOMO) as well as the lower one(HOMO-1) contribute to the HHG of N2 molecules, in either linearly or elliptically polarized intense laser field.The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.

Semi-integer harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized laser field

We theoretically investigated the properties of the high-order harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized(BCCP)laser field.The harmonic emission processes have been illustrated by numerically solving the two-dimensional time-dependent Schr¨odinger equation of an atom in intense laser fields.It is found that with the decrease of the right-circularly polarized laser wavelength,the harmonic spectra are gradually splitting and the harmonic orders move towards the higher frequency.Meanwhile,the integer and semi-integer harmonic emission will be generated when the frequency ratios of right-and left-circularly polarized lasers are semi-integer.The emission mechanism of the semi-integer-order harmonics has been investigated by using the rules of photon absorption and emission.

Helicity of harmonic generation and attosecond polarization with bichromatic circularly polarized laser fields

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.

Generation of non-integer high-order harmonics and significant enhancement of harmonic intensity

High-order harmonics from helium atom in the orthogonally two-color(OTC) laser field are investigated by solving the two-dimensional time-dependent Schrodinger equation.Non-integer high-order harmonics are obtained in some ratio of frequencies of two components.Pure odd and even harmonics from atoms could be separated in two components by adjusting the ratio of frequencies in OTC scheme,and the resolution of harmonics is improved at the same time.The physical mechanism is explained by the periodicity of dipole.With the same intensity of the incident laser,the intensity of the high-order harmonics from the OTC field scheme is improved by three orders of magnitude compared to the monochromatic laser field scheme.A theoretical scheme is provided for experimentally achieving improving energy resolution and separation of pure odd and even harmonics in atoms.Also,we provide a means for improving harmonic intensity.

Role of excited states in the elliptically polarized harmonic generation

We theoretically investigate the contribution of the excited state to the ellipticity of the harmonics from H＋ at different orientation angles irradiated by a linearly polarized laser pulse. It is found that the first excited state has a significant influence to the ellipticity of the harmonics, and the contribution of higher excited states to the ellipticity can be neglected. Moreover, the conclusion is not dependent on the laser intensity.

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses（IAP） are sensitive to the carrier-envelope phase（CEP）of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized（TCCRCP） laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.

Single Ultrashort Attosecond Pulse Generation via Combination of Chirped Fundamental Laser and an Ultraviolet Controlling Pulse

We theoretically study the high-order harmonic generation （HHG） from a hydrogen atom in an intense few-cycle chirped fundamental laser in combination with an ultraviolet （uv） controlling pulse. The high-order harmonic spectrum is calculated by solving the time-dependent Schr6dinger equation using the split-operator method. In our calculation, we present the difference of the high-order harmonic spectrum from one-dimensional （1D） model hydrogen atom and three-dimensional （3D） real hydrogen atom. We found that the plateau of the high-order harmonic generation from the 1D ease and 3D case are all extended effectively to Iv -k 35Up due to the presence of the chirped laser pulse and the HHG supercontinuum spectrum is generated by adding an ultraviolet controlling pulse at a proper time, but the efficiency of the HHC for 3D case is more higher at the near cut-off region than the 1D case. Therefore, the generation of the attosecond pulse by synthesizing the harmonics near cut-off region have some slight differences between 1D and 3D simulations. As a real 3D case study, we show that an isolated 18 as pulse with a bandwidth of 232.5 eV is generated directly by optmizing the combination laser fields.