Strong-field ionization with two-color circularly polarized laser fields

Abstract

Strong-field ionization provides fundamental insight into light-matter interactions, encoding the structure of atoms and molecules on the sub-Ångström and sub-femtosecond scales. In this Letter, we explore an important new regime: strong-field ionization by two-color circularly polarized laser fields. In contrast to all past work using linearly polarized drivers, we probe electron trajectories that are driven in a 2D plane, thus separating the tunneling angle from the rescattering angle. This allows us to make several new findings. First, we observe a single-lobed electron distribution for co-rotating fields, and a three-lobed distribution for counter-rotating fields, providing the first experimental validation of the theoretical model explaining the generation of circularly polarized high harmonic light. Second, we discover that there is significant electron-ion rescattering using counter-rotating fields, but not with co-rotating fields. Finally, we show that the rescattered electrons are well separated from the directly-ionized electrons, in striking contrast to similar low-energy structures seen with linearly polarized fields. These findings help overcome the long-standing problem of how to decouple the tunneling and rescattering steps in strong-field ionization, which will enable new dynamic probes of atomic and molecular structure.

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].…”

“…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].…”

Tomographic Reconstruction of Circularly Polarized High Harmonic Fields: 3D Attosecond Metrology

“…This was achieved by delaying the circularly polarized w driving field by three-eighths of its wavelength (292 nm) relative to the counterrotating 2w field. As a result, the orientation of the combined driving field (with threefold symmetry) (22), and hence the circularly polarized HHG field, is rotated by 90°. Although this rotation has a negligible influence on the HHG spectrum and flux, it has a significant impact on the photoelectron interferogram and the corresponding harmonic phases, allowing us to reconstruct the temporal profile of the ŷ-pol HHG field (see the Supplementary Materials).…”

“…Theory predicts that for counter-rotating w and 2w laser fields, the circular harmonics are generated as a superposition of three bursts of linearly polarized EUV light per optical cycle in the time domain, where the polarization of each burst is rotated by 120°from its predecessor (17,19). Experimentally, the electron trajectories have been shown to predominantly move in a two-dimensional (2D) plane (17,22). However, to date, no direct measurement of the temporal characteristics of circularly polarized HHG exists that could be used to inform and validate advanced theory and to harness the enormous potential of extreme nonlinear optics to generate arbitrary spectral, temporal, and polarization-shaped light fields spanning the EUV and soft x-ray regions.…”