Coherent terahertz radiation generation by a flattened Gaussian laser beam at a plasma–vacuum interface

Abstract

We investigate a way for generating strong terahertz (THz) radiation source at a plasma–vacuum interface from laser interactions with a plasma slab. We utilize the special focusing properties of a flattened Gaussian laser beam to generate high-field THz radiation. The laser exerts a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons. The coupling of the oscillatory velocity to the sharp density gradient (at plasma–vacuum interface) generates plasma currents. When the transverse field originated in plasma region crosses the plasma–vacuum interface, electromagnetic radiation at THz frequency is emitted. Flattened Gaussian beams, having the same energy as Gaussian ones, evacuate electrons from a larger area of the plasma, generating stronger plasma current and thus relatively stronger THz fields. Employing a flattened Gaussian laser beam with $\mathrm{P}=3$ (where P is the order of the Gaussian shape) yields stronger THz radiation with relatively higher electric field of about 1 MV/cm with a peak frequency of 18 THz. Compared to the case of an ordinary Gaussian beam, THz field strength is substantially enhanced for the flattened Gaussian beam case.