Controlling electron-ion rescattering in two-color circularly polarized femtosecond laser fields

Abstract

High harmonic generation driven by two-color counter-rotating circularly polarized laser fields was recently demonstrated experimentally as a breakthrough source of bright, coherent, circularly polarized beams in the extreme ultraviolet and soft X-ray regions. However, the conditions for optimizing the single-atom yield are significantly more complex than for linearly polarized driving lasers and are not fully understood. Here we present a comprehensive study of strong-field ionization-the complementary process to high-harmonic generation-driven by two-color circularly polarized fields. We uncover the conditions that lead to enhanced electron-ion rescattering, which should correspond to the highest single-atom harmonic flux. Using a velocity map imaging photoelectron spectrometer and tomographic reconstruction techniques, we record three-dimensional photoelectron distributions resulting from the strong-field ionization of argon atoms across a broad range of driving laser intensity ratios. In combination with analytical predictions and advanced numerical simulations, we show that "hard" electron-ion rescattering is optimized when the second harmonic field has an intensity approximately four times higher than that of the fundamental driving field. We also investigate electron-ion rescattering with co-rotating fields, and find that rescattering is significantly suppressed when compared with counter-rotating fields.

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

“…However, drifts in the relative phase delay on the few-fs timescale are unavoidable as the data was collected over many hours. Fortunately, since a change in the phase difference between the fundamental and second harmonic simply rotates the resulting electric field waveform [28,33,36], this experiment is not sensitive to slight phase drifts.…”

“…This effect is not seen in the co-rotating case because the angular momentum of the photons precludes resonance-excitation to most states. Note that the presence of low-energy electrons for the counter-rotating field (b) and their absence in the co-rotating field (c) is due to both the shape of the fields as well as the increased role of electron-ion rescattering in counter-rotating fields [33,36].…”

“…/ ! ratios for electron energies > 0 eV is due to the different final drift energies of the fields [36]. The number of atoms in the ensemble is 2.5x10 5 .…”

“…phase matched) source of circularly polarized EUV [23][24][25][26][27][28][29] and soft X-ray [30] beams with sufficient flux for applications in magnetic spectroscopies of materials [28,30]. In the case of SFI, bicircular fields allow electrons to be driven in 2D trajectories prior to rescattering from the parent ion [31][32][33][34][35][36][37][38]. Interestingly, the shape of the bicircular waveform -and therefore the physics of the HHG and SFI processes -can be modified by changing the relative wavelengths, intensities, and ellipticities of the two driving fields [29][30][31]36].…”

“…In the case of SFI, bicircular fields allow electrons to be driven in 2D trajectories prior to rescattering from the parent ion [31][32][33][34][35][36][37][38]. Interestingly, the shape of the bicircular waveform -and therefore the physics of the HHG and SFI processes -can be modified by changing the relative wavelengths, intensities, and ellipticities of the two driving fields [29][30][31]36]. Recent studies have explored the acceleration and rescattering/recombination steps of SFI and HHG driven by a bicircular field [30,36,[39][40][41].…”