Strong-field approximation in a rotating frame: High-order harmonic emission from p states in bicircular fields

TL;DR

This paper analyzes high-order harmonic generation in bicircular fields using a rotating frame approach, revealing how effective magnetic fields influence emission asymmetries and connecting s-shell asymmetry to recollision dynamics.

Contribution

It introduces a rotating frame analysis to understand asymmetries in harmonic emission from p states in bicircular fields, incorporating effects of non-inertial transformations.

Findings

Enhanced harmonic emission from co-rotating p orbitals due to effective magnetic fields.

Relation between s-shell asymmetry and recollision velocity's imaginary part.

Insights into polarization and asymmetry in high-order harmonic generation.

Abstract

High-order harmonic generation with bicircular fields - the combination of counter-rotating circularly polarized pulses at different frequencies - results in a series of short-wavelength XUV harmonics with alternating circular polarizations, and experiments show that there is an asymmetry in the emission between the two helicities: a slight one in helium, and a larger one in neon and argon, where the emission is carried out by p-shell electrons. Here we analyze this asymmetry by switching to a rotating frame in which the field is linearly polarized; this induces an effective magnetic field which lowers the ionization potential of the $p_+$ orbital that co-rotates with the lower-frequency driver, enhancing its harmonic emission and the overall helicity of the generated harmonics, while also introducing nontrivial effects from the transformation to a non-inertial frame in complex time. In addition, this analysis directly relates the small asymmetry produced by s-shell emission to the imaginary part of the recollision velocity in the standard strong-field-approximation formalism.