Theory of dielectric nanofilms in strong ultrafast optical fields

We theoretically predict that a dielectric nanofilm subjected to a normally incident strong but ultrashort (a few optical oscillations) laser pulse exhibits deeply nonlinear (nonperturbative) optical responses which are essentially reversible and driven by the instantaneous optical field. Among them is a high optical polarization and a significant population of the conduction band, which develop at the peak of the pulse and almost disappear after its end. There is also a correspondingly large increase of the pulse reflectivity. These phenomena are related to Wannier-Stark localization and anticrossings between the Wannier-Stark ladders originating from the valence and conduction bands leading to optical ``softening'' of the dielectric. Theory is developed by solving self-consistently the Maxwell equations and the time-dependent Schr\"odinger equation. The results point out to a fundamental possibility of optical-field effect devices with the bandwidth on the order of optical frequency.