Abnormal absorption of extremely intense laser pulses in relativistically underdense plasmas

It is generally believed that relativistically underdense plasmas are transparent for intense laser radiation. However, particle-in-cell simulations reveal abnormal laser field absorption above the intensity threshold of about 3 × 10 24 W cm − 2 for the wavelength of 1 μ m. Above the threshold, the further increase in the laser intensity does not lead to an increase in the propagation distance. The simulations take into account emission of hard photons and subsequent pair photoproduction in the laser field. These effects lead to onset of a self-sustained quantum electromagnetic cascade and to formation of dense electron–positron ( e + e −) plasma right inside the laser field. The plasma absorbs the laser field efficiently, which ensures the plasma opacity. The role of a weak longitudinal electron–ion electric field in the cascade growth is discussed.