Controlling Magnetic and Electric Nondipole Effects with Synthesized Two Perpendicularly Propagating Laser Fields

Nondipole effects are ubiquitous and crucial in light-matter interaction.However,they are too weak to be directly observed.In strong-field physics,motion of electrons is mainly confined in transverse plane of light fields,which suppresses the significance of nondipole effects.Here,we present a theoretical study on enhancing and controlling the nondipole effect by using the synthesized two perpendicularly propagating laser fields.We calculate the three-dimensional photoelectron momentum distributions of strong-field tunneling ionization of hydrogen atoms using the classical trajectory Monte Carlo model and show that the nondipole effects are noticeably enhanced in such laser fields due to their remarkable influences on the sub-cycle photoelectron dynamics.In particular,we reveal that the magnitudes of the magnetic and electric components of nondipole effects can be separately controlled by modulating the ellipticity and amplitude of driving laser fields.This novel scenario holds promising applications for future studies with ultrafast structured light fields.

Probing the orbital angular momentum of intense vortex pulses with strong-field ionization

With the rapid development of femtosecond lasers,the generation and application of optical vortices have been extended to the regime of intense-light-matter interaction.The characterization of the orbital angular momentum(OAM)of intense vortex pulses is very critical.Here,we propose and demonstrate a novel photoelectron-based scheme that can in situ distinguish the OAM of the focused intense femtosecond optical vortices without the modification of light helical phase.We employ two-color co-rotating intense circular fields in the strong-field photoionization experiment,in which one color light field is a plane wave serving as the probing pulses and the other one is the vortex pulses whose OAM needs to be characterized.We show that by controlling the spatial profile of the probing pulses,the OAM of the vortex pulses can be clearly identified by measuring the corresponding photoelectron momentum distributions or angle-resolved yields.This work provides a novel in situ detection scenario for the light pulse vorticity and has implications for the studies of ultrafast and intense complex light fields with optical OAM.

Strong-field photoionization of intense laser fields by controlling optical singularities

The spatial features of a light field, such as in the form of the optical singularities, provide a new degree of freedom for the application of light fields in different areas of science and technology. However, although the exploration of structured light is growing rapidly, the investigation of strong-field photoionization using such light fields is noticeably lagging behind. Here, we present an experimental study that reveals the signatures of intense, structured light fields with controlled optical singularities in strong-field photoionization. The different types of optical singularities can be identified through photoionization observables,i.e., photoelectron momentum distributions(PMDs). By concurrently shifting the locations of the phase and polarization singularities, the focal electric field features can be designated, and subsequently, the photoionization appearances can be manipulated. In this process, the behaviors of the different intense optical singularities are clearly visualized by the PMDs. This work will advance both the strong-field science and singularity optics.

Probing Molecular Frame Wigner Time Delay and Electron Wavepacket Phase Structure of CO Molecule

The time delay of photoelectron emission serves as a fundamental building block to understand the ultrafast electron emission dynamics in strong-field physics.Here,we study the photoelectron angular streaking of CO molecules by using two-color(400+800nm)corotating circularly polarized fields.By coincidently measuring photoelectrons with the dissociative ions,we present molecular frame photoelectron angular distributions with respect to the instantaneous driving electric field signatures.We develop a semiclassical nonadiabatic molecular quantum-trajectory Monte Carlo(MO-QTMC)model that fully captures the experimental observations and further ab initio simulations.We disentangle the orientation-resolved contribution of the anisotropic ionic potential and the molecular orbital structure on the measured photoelectron angular distributions.Furthermore,by analyzing the photoelectron interference patterns,we extract the sub-Coulomb-barrier phase distribution of the photoelectron wavepacket and reconstruct the orientation-and energy-resolved Wigner time delay in the molecular frame.Holographic angular streaking with bicircular fields can be used for probing polyatomic molecules in the future.