Conical Intersection versus Avoided Crossing:Geometric Phase Effect in Molecular High-Order Harmonics

Nonadiabatic dynamics around an avoided crossing or a conical intersection play a crucial role in the photoinduced processes of most polyatomic molecules.The present work shows that the topological phase in conical intersection makes the behavior of pump-probe high-order harmonic signals different from the case of avoided crossing.The coherence built up when the system crosses the avoided crossing will lead to the oscillatory behavior of the spectrum,while the geometric phase erodes these oscillations in the case of conical intersection.Additionally,the dynamical blueshift and the splitting of the time-resolved spectrum allow capturing the snapshot dynamics with the sub-femtosecond resolution.

Modification of laser-induced state in atomic attosecond transient absorption by the XUV pulse pair

Attosecond transient absorption(ATA)has been developed as an all-optical technique for probing electron dynamics in matter.Here we present a scheme that can modify the laserinduced state and the corresponding ATA spectrum via excitation by a pair of XUV attosecond pulses and by a time-delayed mid-infrared(MIR)laser probe.Different from the scheme of the electronic excitation by a single XUV attosecond pulse,the application of a pair of XUV pulses provides extra degrees of freedom,such as the time delay and the intensity ratio between two XUV pulses,which make it possible to adjust the pump process,resulting in the modification of the ATA spectrum.We show that by varying the time delay between the two XUV pulses,the population of the dark state and the ATA spectrum of the laser-induced state have periodic modulations.We also demonstrate that the peak of the ATA spectrum of the laser-induced state appears at a fixed time delay between the XUV pair and the MIR laser when the intensity ratio is large,and it changes with the time delay when the intensity ratio is small,which can be related to either one of two peaks in the population of the dark state.

Optimization of temporal gate by two-color chirped lasers for the generation of isolated attosecond pulse in soft X rays

We propose a simple and efficient method to optimize two-color chirped laser pulses by forming a"temporal gate"for the generation of isolated attosecond pulses(IAPs)in soft X rays.We show that the generation process for higher and cutoff harmonics can be effectively limited within the temporal gate,and the harmonic emission interval can be further reduced with the help of phase-matching by only selecting the contribution from shorttrajectory electrons.This two-color gating mechanism is verified by increasing the pulse duration,raising the gas pressure,and extending the target cutoff.Compared to the five-color waveform in Phys.Rev.Lett.102,063003(2009),our waveform can be used to generate the IAP in the long-duration laser pulse while the cutoff energy is higher without the reduction of harmonic yields.Our work provides an alternative temporal gating scheme for the generation of IAPs by simultaneously improving the harmonic conversion efficiency,thus making the attosecond soft X rays an intense and highly time-resolved tabletop light source for future applications.

Strain effect on the orientation-dependent harmonic spectrum of monolayer aluminum nitride

In this study, we theoretically investigate the strain effect on the orientation-dependent high-order harmonic generation(HHG) of monolayer aluminum nitride(Al N) by solving the multiband semiconductor Bloch equations in strong laser fields. Our simulations denote that the efficiency of the orientation-dependent HHG is considerably enhanced when a 15% biaxial tensile strain is applied to Al N, which is attributed to the downshifting energy level of the conduction band. Furthermore, the odd-even feature in the orientation-dependent high harmonic spectra owing to the strain is considerably different when compared with that in the case without strain. The enhanced quantum interference between different energy bands in strained Al N around the Γ-M direction is responsible for the observed odd-even distributions of the orientation-dependent HHG. This study helps to better understand the HHG in solids by tuning their electronic structures.