Fundamental and harmonic emission from the rear side of a thin overdense foil irradiated by an intense ultrashort laser pulse

The emission of fundamental and harmonic radiation from the rear side of thin foils in the thickness range 50–460 nm irradiated by intense frequency doubled Ti:sapphire laser pulses of the duration of 150 fs and intensities up to a few 10¹⁸ W∕cm² was investigated. Following up a previous study of the rear side harmonic emission [ Teubner et al. Phys. Rev. Lett. 92 185001 (2004)], we measured the emission efficiencies, polarization properties, and the spectral shapes of the fundamental frequency and the second harmonic. Rear side emission is only observed when the obliquely incident laser light is p-polarized. Particle-in-cell (PIC) simulations indicate that the foils remain strongly overdense during the interaction with the laser pulse and that the rear side emission is caused by energetic electron bunches which are generated at the front side by resonance absorption. They are accelerated into the foil and drive strong plasma oscillations at the fundamental and higher harmonic frequencies.