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

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 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.