Polarization and Spatial Mode Structure of Mid-Infrared-Driven Terahertz-to-Microwave Radiation

Abstract

Polarization-and angle-resolved analysis of microwave-to-terahertz (μWv/THz) radiation from gas-phase plasmas induced by ultrashort mid-infrared laser pulses reveals a complex vectorial electrodynamic picture of μWv/THz generation by a diverse manifold of laser-driven plasma currents and provides a powerful tool to distinguish between currents of different spatial symmetries as sources of μWv/THz radiation. In experiments with a single-color mid-IR driver, the spatial mode of μWv/THz radiation is found to retain a remarkably uniform, conical-emission structure across its entire spectral span, with angular dispersion and a spatial mode profile as dictated by Cherenkov-type phase matching. This spatial mode structure is consistent with its polarization properties of μWv/THz radiation, indicating the dominant role of radiation by ponderomotively driven longitudinal currents. In experiments with a two-color laser driver, the spatialpolarization mode structure of μWv/THz radiation is found to change across its spectrum and as a function of the gas pressure, revealing mixed symmetries of underlying electric currents and indicating the shifting roles of transverse and longitudinal currents as mechanisms behind μWv/THz generation.

Traveling-wave antenna model for terahertz radiation from laser-plasma interactions

“…However, studies of two- [59,69] or even multi-color [70][71][72][73][74] laser pumping THz sources, as well as the phased-array plasma filament emitters, might also benefit from the TWA theory. Since it has been discovered that both transverse and longitudinal currents contribute to the THz emission during filamentation [3,53,[75][76][77][78][79][80][81], and the longitudinal currents tend to become dominant within specific ranges of parameters, like gas pressures, pumping wavelengths, and laser intensities [22,82,83]. Therefore, the suggested TWA model is very promising to show its versatility far beyond what has been presented in this work.…”