Short-pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity

TLDR

The study investigates harmonic generation from ultrashort laser pulses interacting with overdense plasma surfaces at relativistic intensities for various incidence angles and polarizations. The authors employ fully relativistic 1‑D PIC simulations and a Fourier‑component model based on relativistic cold‑plasma equations to describe surface oscillations and reproduce the harmonic spectra. Harmonic yield rises with laser intensity and decreases with plasma density, and the simulations reveal that surface oscillations driven by the laser field and ponderomotive force generate harmonics via relativistic reflection, producing polarization‑dependent selection rules for odd and even orders.

Abstract

The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ2=1017–1019 W cm−2 μm2 for normal and oblique incidence and different polarizations. Fully relativistic one-dimensional particle-in-cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.