Ultrafast Magnetic Field Generation in Molecular $$\pi $$-Orbital Resonance by Circularly Polarized Laser Pulses

Optically induced magnetic fields,from the femtosecond nuclear to attosecond electron time scales are shown to be produced by intense ultrashort laser pulses due to highly nonlinear nonperturbative optical response. The light-matter interaction results in coherent electron currents, giving rise to magnetic field generation. Schemes with bichromatic high-frequency co-rotating and counter-rotating circularly polarized UV light pulses are used to produce the spatial and temporal evolution of the generated magnetic field. The one-electron molecular ion H-2(+) as a benchmark model is used to describe the ultrafast photophysics process. Under the condition of molecular resonance excitation, results obtained from ab-inito simulations showa strong dependence on themolecular alignment. In bicircular polarization processes, the interference effects between multiple resonant excitations modulate the evolution of the generated magnetic field, thus leading to pulse relative phase dependence. It is found that the modulation of generated magnetic fields is dependent on the pulse frequency and helicity combination. Molecular resonant excitation induces coherent ring electron currents, resulting in suppression of the phase dependence. Pulse helicity effects illustrate laser induced electron dynamics in bichromatic circular polarization processes. These highly nonlinear phenomena are described by attosecond ionisation and coherent electron current models. The results offer a guiding principle for generating ultrafast magnetic fields and for studying coherent electron dynamics in complex molecular systems.