THz radiation generation in semiconductor-metal nanostructure exposed to femtosecond laser pulse of focused radiation

Abstract

Generation of terahertz (THz) radiation under the ponderomotive action of a femtosecond laser pulse on a doped semiconductor layer deposited on a metal surface is studied. The spectral composition, radiation pattern, total energy, and shape of the generated THz radiation pulse are found. There is a peak in the emission spectrum at the electron plasma frequency in semiconductors transparent to THz radiation. The contribution to radiation from the plasma resonance region leads to a relative increase in the total energy and manifests itself in magnetic field oscillations at the stage of pulse switching off. Optimal generation conditions are realized when the focal spot size is comparable with the pulse length and skin depth. The stronger the focusing, the wider the THz radiation pattern. With weak focusing, THz radiation is mainly concentrated along the directions close to normal to the semiconductor surface. When an odd number of half-wavelengths of high-frequency radiation is stacked at twice the thickness of the semiconductor, the generation efficiency increases sharply. In particular, in GaAs and GaSb, the efficiency increases by more than two orders of magnitude.