Ionization of highly excited hydrogen atoms by a circularly polarized microwave field

Controlling the atomic-orbital-resolved photoionization for neon atoms by counter-rotating circularly polarized attosecond pulses

We theoretically investigate the atomic-orbital-resolved vortex-shaped photoelectron momentum distributions (PMDs) and ionization probabilities by solving the two-dimensional time-dependent Schrödinger equation (2D-TDSE) of neon in a pair of delayed counter-rotating circularly polarized attosecond pulses. We found that the number of spiral arms in vortex patterns is twice the number of absorbed photons when the initial state is the ψ_m=±1 state, which satisfy a change from c_2n+2 to c_2n (n is the number of absorbed photons) rotational symmetry of the vortices if the 2p state is replaced by 2p₊ or 2p_- states. For two- and three-photon ionization, the magnetic quantum number dependence of ionization probabilities is quite weak. Interestingly, single-photon ionization is preferred when the electron and laser field corotate and ionization probabilities of 2p_- is much larger than that of 2p₊ if the proper time delay and wavelength are used. The relative ratio of ionization probabilities between 2p_- and 2p₊ is insensitive to laser peak intensity, which can be controlled by changing the wavelength, time delay, relative phase and amplitude ratio of two attosecond pulses.

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

We introduce bicircular high-harmonic spectroscopy as a new method to probe dynamical symmetries of atoms and molecules and their evolution in time. Our approach is based on combining a circularly polarized femtosecond fundamental field of frequency ω with its counterrotating second harmonic 2ω. We demonstrate the ability of bicircular high-harmonic spectroscopy to characterize the orbital angular momentum symmetry of atomic orbitals. We further show that breaking the threefold rotational symmetry of the generating medium-at the level of either the ensemble or that of a single molecule-results in the emission of the otherwise parity-forbidden frequencies 3qω  (q∈N), which provide a background-free probe of dynamical molecular symmetries.

Generating Isolated Elliptically Polarized Attosecond Pulses Using Bichromatic Counterrotating Circularly Polarized Laser Fields

We theoretically demonstrate the possibility to generate both trains and isolated attosecond pulses with high ellipticity in a practical experimental setup. The scheme uses circularly polarized, counterrotating two-color driving pulses carried at the fundamental and its second harmonic. Using a model Ne atom, we numerically show that highly elliptic attosecond pulses are generated already at the single-atom level. Isolated pulses are produced by using few-cycle drivers with controlled time delay between them.

Strong-field ionization rate depends on the sign of the magnetic quantum number

We report the first experimental observation of the dependence of strong-field ionization rate on the sign of the magnetic quantum number. We measure the strong-field sequential double ionization yield of argon by two time-delayed near-circularly polarized laser pulses. It is found that double-ionization yield is enhanced more than 3 times if two lasers have the opposite helicities. Analysis shows that the single ionization of both the neutral and ion prefer the same sign of the magnetic quantum number. A qualitative and intuitive model is proposed to help understand this phenomenon.