Terahertz emission from laser-driven gas plasmas: a plasmonic point of view

We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gasplasmas. For two-color femtosecond pump pulses, a plasmonic resonance is shown to broaden the terahertz emission spectra significantly. We identify the resonance as a leaky mode, which contributes to the emission spectra whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. As a direct consequence, such resonances can be controlled by changing the polarization properties of elliptically shaped driving laser pulses. Both experimental results and 3D Maxwell consistent simulations confirm that a significant terahertz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser beam shape as an additional degree of freedom. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement