Broadband terahertz radiation from metal targets irradiated by a short pulse laser

Abstract

The generation of low-frequency radiation from sub-picosecond laser pulses incident on metal targets is investigated. The laser field drives time-varying currents in a thin sub-surface layer of the metal, which emits broadband radiation that peaks at terahertz frequencies. We present a one-dimensional electrostatic model for copper appropriate for the interaction of laser pulses at normal incidence combined with a radiation model for an infinitely thin disk. The latter uses as input a single parameter, the temporal dependence of the integrated current density on axis, which is derived from the electrostatic model. The salient characteristics of the emitted radiation, such as power, energy, and spectra, are calculated for laser pulses with various intensities and pulse durations. The radiated energy per unit solid angle peaks at a small angle off the target normal and tapers off at larger angles. Analytical scaling of radiated energy with incident laser energy, in the low frequency limit, is obtained in the form ϵ rad ∼ ϵ laser 3 / 2. For accurate results, it is imperative to use the full expression for the heat capacity of electrons, in both the degenerate and ideal gas limits. Failure to do so may result in inaccuracies for the computed radiated energy, as large as one order of magnitude. A comparison of calculated and measured radiation energy in the 8-12 GHz frequency range indicates a similar trend with laser energy and comparable magnitude (∼1 fJ).