Bright single-cycle terahertz source based on gas cells irradiated by two-color laser pulses

Abstract

Abstract We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for a sufficiently small 20μm-size interaction volume the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation emitted in the terahertz frequency domain. Simultaneously, strong quasi-static electric fields of maximal strength Em ≃ 10MV/cm are shown to exist inside the plasma during several hundred femtoseconds after the ionizing two-color laser pulse has gone.