Laser-cluster interaction with subcycle pulses

The interaction of intense laser light with atomic nanoclusters is studied with a rigid sphere model and three-dimensional particle-in-cell simulations for ultrashort laser pulses of pulse durations tau down to one optical period T and below. In this subcycle regime not all conventional pulse models are applicable and may lead to unphysical absorption of laser energy. For allowed pulse models, we show that for a given laser peak intensity, and cluster, the efficiency of laser absorption increases as t shortens and reaches a maximum value in the subcycle regime. For deuterium clusters, where inner ionization quickly saturates, the absorbed energy and outer ionization reaches a peak at tau approximate to 0.8T. For argon clusters, however, such a peak disappears, and energy absorption continues even for tau < 0.8T due to additional inner ionization.