Propagation Characteristics of an Ultrashort Laser Pulse with a Carrier Frequency Close to the GaAs Bandgap

Abstract

The penetration into GaAs of an ultrashort laser pulse with a carrier frequency close to the absorption edge has been studied. It is shown that the frequency dispersion of the dielectric permittivity must be taken into account when describing penetration of such a pulse. It has been established that due to a sharp increase in the absorption coefficient with frequency increasing, as the ultrashort laser pulse penetrates into GaAs the maximum of the spectral energy density of the pulse shifts to the low-frequency region. Consequently, the shorter the pulse, the deeper it penetrates into GaAs, since a significant portion of the short pulse energy is concentrated at low frequencies. As the pulse penetrates deeper into the semiconductor, its shape is primarily determined by harmonics with frequencies lower than the frequency corresponding to the bandgap. In the considered frequency range, the group velocity in GaAs at high frequencies is lower than at low frequencies. Because of this, a small peak in the tail of the ultrashort laser pulse is formed, the main contribution to which comes from high-frequency harmonics that have a lower group velocity. The change in both the spectral composition and the shape of the reflected pulse is minor.