Radiation resonance emission from steep overcritical plasma profiles illuminated by femtosecond laser pulses.
A radiation resonance effect observed in the reflection spectra from overdense plasmas illuminated by femtosecond laser pulses at normal incidence is reported from particle-in-cell simulations. Harmonic emission at multiple orders of the fundamental is found to exhibit resonance phenomena, with the number of resonances and power emitted depending on the electron plasma density. For relatively low laser intensities the reflected light at the laser frequency shows prominent resonant emission around specific values of the plasma density, mainly at 4, 16, and 36 times critical. For increasing laser intensities, strong harmonic emission around 4 and 16 times critical dominates the reflection spectra. In the case of the third laser harmonic, the emission is found to be resonant about those densities and presents, additionally, a distinctive resonant region around nine times critical. A simple radiation model for the power of the third harmonic was proposed confirming a resonant effect dependent on the electron plasma density. For higher harmonic numbers, weak radiation resonances persist in the emission spectra, with their number increasing with order. The resonance effect reported in this paper is found to occur at densities that approximately satisfy n(e)/n(c)=4n(2), where n(e) and n(c) are the plasma and the critical density, respectively, and n is an integer. For the third harmonic, the second resonance corresponds to n=1.5.
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Instituto Nacional de Investigaciones Nucleares, Apartado Postal 18-1027, México 11801, Distrito Federal, Mexico.