Phys. Rev. Lett. 116, 205003 (2016)

Intense, high-brightness table-top sources of coherent terahertz (THz) radiation, which remove the need for large and expensive accelerator facilities, are of interest for applications ranging from biomedical imaging to wireless communications. Now, Guo-Qian Liao and colleagues have achieved coherent THz radiation by harnessing the interaction between laser-driven beams of relativistic electrons and a suitable target material. The laser system, based at the Laboratory for Laser Plasmas, Shanghai Jiao Tong University, was used to provide 30-fs-duration p-polarized pulses with an energy and wavelength of 2 J and 800 nm, respectively. Focusing these pulses onto the targets with a peak irradiance of ∼1.5 × 1019 W cm−2 enabled coherent transition radiation when the laser-produced electron beam passed through the rear solid/vacuum interface of the target. Three types of target were investigated: polyethylene (PE), Cu foils, and metal–PE samples with variable thickness of the PE layer. The dependence of the THz radiation on the structural parameters of the target was studied in detail and particle-in-cell simulations supported the experiments. The estimated total THz energy from the rear side of the metal foils is ∼400 μJ per pulse, corresponding to an energy conversion efficiency from the laser into THz radiation of around 2 × 10−4.