Short-Pulse Laser Harmonics From Oscillating Plasma Surfaces Driven At Relativistic Intensity
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 Iλ^2=10^17 - 10^20 Wcm-2μm^2. Fully relativistic one-dimensional particle-in-cell (PIC) simulations are performed with high spectral resolution. Harmonics of order 100 with conversion efficiency η_100=10-7 are obtained at highest intensity. Harmonic emission increases with intensity and when lowering the plasma density. Also ramped density profiles are simulated showing resonant enhancement of low harmonics with a cut-off at the bulk plasma frequency for ramps 0.2-0.5 λ thick and intensities below 10^18 Wcm-2μm^2. The generation of harmonics is interpreted geometrically as reflection from an oscillating steplike surface taking account of retardation. The oscillations are driven at 2ω by the fast component of the ponderomotive force and at ω by direct action of the laser field. The model, based on a cold plasma description, well reproduces the PIC spectra. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- November 1996
- Bibcode:
- 1996APS..DPP..4R08L