The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

Abstract

The generation of terahertz (THz) emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses composed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.

Power dependence of terahertz carrier frequency in a plasma-based two-color generation process

“…Since it was first predicted in 1993, terahertz (THz) emission generated by a laser focused in the air has aroused a considerable amount of interest in various fields, [1,2] including material characterization, nondestructive evaluation, and biomedical and pharmaceutical testing. [3][4][5][6][7] An ultrashort laser fundamental (ω) and second harmonic (2ω) pulses are cofocused in the air to ionize the air molecules, and thus generate a THz wave through four-wave mixing. [8][9][10][11][12] In this model, a highly nonlinear and phase-sensitive asymmetric laser field can be generated, in which the bound gas-phase electrons can acquire a nonzero drift velocity and produce a free electron current with simultaneous emission of THz radiation in the far-field.…”