Harmonic Generation with Optical Vortex Beams

Vortex beams carry two types of angular momentum: spin angular momentum (SAM) due to the time-dependence of the light polarization, and orbital angular momentum (OAM) due to the spatial phase variation of their wavefront. When a noble gas is illuminated by an optical vortex beam, odd harmonics of the driven laser frequency are generated. For long and weak laser pulses, the generated harmonics are known to possess a well-defined topological number that scales linearly with the harmonic order. In this work, we consider high-order harmonic generation (HHG) of a helium gas exposed to an optical Bessel vortex beam. The single-atom response is computed through time-dependent calculations, and the collective response of the atomic gas is simulated by the Fraunhofer diffraction formula. We find that the harmonics produced by an intense few-cycle optical vortex beam exhibit a mixture of OAMs, indicating the emergence of high-order processes and the breakdown of the perturbative regime as we increase the pulse intensity. The produced far field exhibits the fingerprint of these non-perturbative processes and lead to a complex light intensity profile and phase variation, which could be observed experimentally. We discuss the possible applications of this approach.