Direct conversion of THz emission from a periodic electrostatic field by use of a laser-produced relativistic ionization front

We report on the first demonstration of a novel THz source by use of an ultrashort-laser-produced ionization front. THz radiation is generated through the spatiotemporal change in electron plasma density induced by a relativistic ionization front propagating in a periodic electrostatic field in a ZnSe crystal. The central emission frequency is 1.2 THz with a bandwidth of 0.7 THz (FWHM). The central frequency is attributed to production of the electron plasma density of the order of 1015 cm^-3 and is in good agreement with the expected theoretical value.