Observation of ionization enhancement in two-color circularly polarized laser fields

Abstract

When atoms are irradiated by two-color circularly polarized laser fields the resulting strong field processes are dramatically different than when the same atoms are irradiated by a single-color ultrafast laser. For example, electrons can be driven in complex 2D trajectories before rescattering or circularly polarized high harmonics can be generated, which was once thought impossible. Here, we show that two-color circularly polarized lasers also enable control over the ionization process itself and make a surprising finding: the ionization rate can be enhanced by up to 700% simply by switching the relative helicity of the two-color circularly polarized laser field. This enhancement is experimentally observed in helium, argon and krypton over a wide range of intensity ratios of the twocolor field. We use a combination of advanced quantum and fully classical calculations to explain this ionization enhancement as resulting in part due to the increased density of excited states available for resonance-enhanced ionization in counter-rotating fields compared with co-rotating fields. In the future, this effect could be used to probe the excited state manifold of complex molecules.

Pulse-length effects in strong-field ionization of atoms by co-rotating and counter-rotating bicircular laser pulses

“…These findings are also predicted by calculations of TIYs based on tunneling theory. We have also considered the long pulse limit, for which case the TDSE calculations at specific intensities show larger TIYs for CRBC compared to COBC pulses, in agreement with the recent experiments [22].…”

“…The present study is motivated by recent experiments on strong-field ionization of rare gases [22]. In the experiments, the rare-gas atoms Ar, Kr, and He were ionized by 40 fs BC pulses with various intensity ratios and the 1ω-2ω frequency combination (ω corresponds to 800 nm wavelength).…”

Attoclock with counter-rotating bicircular laser fields

“…There is thus a true need to devise an approach for cleaner investigation of the time structure of ionization in linear polarization. In this work we demonstrate that the ionization step in (almost) linear polarization can be probed using an attoclocklike setup involving bicircular fields [42], which have already been used successfully to investigate various aspects of strong-field ionization [43][44][45][46][47][48][49]. The term bicircular field refers to the superposition of two circularly polarized fields with different wavelength.…”

Compact and flexible harmonic generator and three-color synthesizer for femtosecond coherent control and time-resolved studies

“…While many recent studies use orthogonally polarized [20] or counter-rotating circular polarized ω/2ω fields [21,52], the three-color device introduced here permits tailoring the fields with all three colors and thereby extend such studies to more complex fields. Using such tailored laser fields is also expected to be very beneficial for high-harmonic generation [33].…”