Intense high-harmonic optical vortices generated from a microplasma waveguide irradiated by a circularly polarized laser pulse

Abstract

A scheme for generating intense high-harmonic optical vortices is proposed. It relies on spin-orbit interaction of light when a relativistically strong circularly polarized laser pulse irradiates a microplasma waveguide. The intense laser field drives a strong surface wave at the inner boundary of the waveguide, which leads to high-order-harmonic generation as the laser is traveling inside. For a circularly polarized drive laser, the optical chirality is imprinted to the surface wave, which facilitates conversion of the spin angular momentum of the fundamental light into orbital angular momenta of the harmonics. A ``shaken waveguide'' model is developed showing that the aforementioned phenomenon arises due to a nonlinear plasma response that modifies the electromagnetic mode at high intensities. We further show that the phase velocities of all the harmonic beams are automatically matched to the driving laser, so that the harmonic intensities increase with propagation distance. The efficiency of harmonic production is related to the surface wave breaking effect, which can be significantly enhanced using a tightly focused laser. Our simulation suggests that an overall conversion efficiency of $\ensuremath{\sim}5%$ can be achieved.