Reshaping of ultrashort electromagnetic pulses under reflection from a thin solid-plasma film

Abstract

Temporal reshaping of ultrashort (femtosecond) laser pulses became recently an area of growing interest, which is mainly due to its possible applications. Significant pulse distortion was predicted on reflection from GaAs at Brewster's angle [l] and in the geometry of FTR when surface waves are excited [2]. A new phenomenon of a pulse reshaping after reflection from a thin (comparing to the pulse wavelength) solid-plasma (semiconductor, metal) film placed on a perfectly conducting substrate in the presence of plasma resonance was predicted by authors [3] through the direct numerical calculation of the Fourier transform integral. Here we present a complete analysis of the problem based on several methods: frequency- and time-domain approaches, as well as analogous modelling. It is shown that the Gaussian incident pulse is splitted into two reflected pulses amplitudes of which can be controlled by changing parameters of the wave or the film. In the frequency-domain approach the reflected signal (which we calculated numerically) is given in the form of the Fouirier transformation applied to the spectrum of the incident pulse multiplied by the reflection cdefficient. The splitting can be explained by the strong dispersion of the reflection coefficient in the narrow frequency region near plasma frequency of the film. By using the time-domain technique we derived a first-order differential equation for the reflected pulse and its analytical solution is obtained. A simple electronic circuit (consisting of two oscillatory contours) with the transmission coefficient of the same form as the coeficient of reflection from the plasma film was used for experiments on the analogous modelling of the phenomenon. The described phenomenon may be applied for reshaping of submillimeter wave pulses in semiconductor films and of extremely short optical signals (femtosecond laser pulses) in metal films.