Qianguang Li

Broadband water window supercontinuum generation with a tailored mid-IR pulse in neutral media

We propose a new (to our knowledge) scheme to generate a broadband water window supercontinuum in a neutral rare-gas media. Using a phase-stabilized few-cycle mid-IR pulse tailored by a much weaker 800 nm pulse, a supercontinuum from 265 to 425 eV is observed, and the ionization probability is below 0.1%. The nonionized media enable us to adopt the right phase-matching technique to select the short path of the supercontinuum, and an isolated 80 as pulse with a central wavelength of 3.2 nm is straightforwardly obtained.

Asymmetric molecular gating for supercontinuous high harmonic generation in the plateau

Molecular wave packets provide an alternative route to steer the high harmonic generation (HHG). Here we present a new method for isolated attosecond pulse generation from high harmonics in the plateau with asymmetric molecules. It is shown that the photoionization depends on the molecular structure. Through steering the ionization, HHG is controlled with the asymmetric molecule and supercontinuous high harmonics are produced in the plateau, from which an isolated 95-attosecond pulse is generated. In contrast to the cutoff which declines sharply, the plateau shows almost a constant intensity, indicating a higher yield. Moreover, the plateau covers a very broad bandwidth, thus is preferable to produce an isolated attosecond pulse with a rather short duration.

Dissection of electron correlation in strong-field sequential double ionization using a classical model

Recent experiments on strong-field sequential double ionization (SDI) have reported several observations which are regarded as evidence of electron correlation, querying the validity of the standard independent electron approximation for SDI. Here we theoretically study SDI with a classical ensemble model. The experimental results are well reproduced with this model. Back tracing of the ionization process shows that these results are ascribed to the subcycle ionization dynamics of the two electrons, not the evidences of the electron correlation in SDI. Thus, the previously reported observations are not enough to claim the breakdown of the independent electron approximation in SDI.

Isolated-attosecond-pulse generation from asymmetric molecules with an {omega}+2{omega}/3 multicycle two-color field

The high harmonic generation from asymmetric molecules with an {omega}+2{omega}/3 multicycle bichromatic laser pulse has been investigated. It is shown that the ionization asymmetry in consecutive half optical cycles for asymmetric molecules is further enhanced since the 2{omega}/3 control laser pulse further enhances the amplitude of the ionization peak at the center of the laser pulse. The 2{omega}/3 control laser pulse also significantly enlarges the difference of the photon energies emitted from the ejected electron in the half optical cycle at the central laser pulse and its next half optical cycle. In addition, a broadband supercontinuum is produced in the plateau of the spectrum, from which an isolated 90-as pulse can be directly obtained.

Enhanced dissociation of H2+ into highly excited states by UV pulses

We theoretically study the dissociation of H2+ by UV laser pulses as a function of the photon energy ω of the pulse. Our results show that pronounced enhancements of the dissociation into highly excited electronic states can be achieved at some critical ω. This is found to be attributed to a consecutively resonant excitation mechanism where the population is first transferred to the first excited state by absorbing one photon and then to higher states by absorbing another one or more photons at the same internuclear distance. This study indicates that the strong coupling between the lowest two states of H2+ can significantly affect the dissociation through higher lying states.

Macroscopic effects on the broadband supercontinuum driven by an ω and ω/2 bichromatic laser pulse

The macroscopic effects on the broadband supercontinuum generated in the interaction of an ω and ω/2 bichromatic laser pulse and an atom are investigated with a three-dimensional propagation model. It is shown that a broadband supercontinuum can still be generated when the macroscopic effects are included, and the macroscopic effects can ameliorate the temporal characteristic of the broadband supercontinuum of a single atom. When the broadband supercontinuum propagates through the low-density gas medium, the modulations of the broadband supercontinuum can be weakened or even eliminated by phase matching. Moreover, this unmodulated broadband supercontinuum can steadily last for a relative long distance of propagation and can be directly filtered to generate an isolated sub-100 attosecond pulse.