Model for transmission of ultrastrong laser pulses through thin foil targets

To explore the physical mechanism of the anomalously high transmittivity seen in recent experiments, we present a simple self-consistent model for the transmission of an intense short-pulse laser through a thin foil target. The model is based on the idea that the ultra-intense light pressure of an ultrashort light pulse compresses the electrons in the thin target into a much thinner layer before significant ion motion sets in, such that the the effective target width becomes of the order of or less than the skin depth. For analytical simplicity, the normal incidence of a circularly polarized laser pulse onto a highly overdense thin plasma layer is considered. It is found that the transmittivity can increase rapidly with the laser strength parameter when the latter is above 3. For the circularly polarized light considered, the transmittivity is found to be about 42% for an ion density 50 times critical, laser strength parameter 3, and foil thickness 0.1 times the wavelength. This result agrees well with that from a particle-in-cell simulation.