Dynamic sheath formation and sub-THz radiation from laser–metal interactions

The generation of low-frequency radiation from a short pulse (∼100 fs) laser with mJ energy incident on a metal surface is investigated. The electrons within the metal surface absorb energy from the laser pulse, increasing in temperature to a few electron volts and resulting in some at the high-energy tail of the distribution to overcome the work function barrier. Emission of these electrons from the surface contributes to a surface current which sources secondary radiation emission. The Richardson–Dushman emission picture breaks down due to Coulombic effects and a modified emission model is presented. Previous precedence exists for modified thermionic emission models for laser–metal interactions of a similar nature, such as the one presented by Riffe et al. [J. Opt. Soc. Am. B 10, 1424–1435 (1993)] upon which we elaborate. Surface currents generated by such a mechanism are modeled with a particle-in-cell (PIC) simulation together with a Monte Carlo treatment of electron–neutral collisions with air molecules. The modified emission model, together with the PIC model, provides a numerical basis from which the radiated spectra are estimated due to the time-varying currents emitted from the surface of the metal. Experiments and numerical simulations for air pressure between 0.1 and 1 atm show qualitative agreement as to the pressure dependence of the measured signal energy from the secondary radiation, with E measured ∝ P air − α for some α near but less than 1. Quantitative agreement between experiments and simulations is improved by including the effect of an additional contribution to secondary emission energy arising from inverse-bremsstrahlung collisions within a nm-scale vapor layer.