Abstract
We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gas-plasmas. 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.
Highlights
Terahertz (THz) radiation has become a ubiquitous tool for many applications in science and technology [1,2]
We have shown that plasmonic effects can significantly broaden the terahertz emission spectrum from femtosecond-laser-induced gas plasmas when the corresponding plasma wavelengths are larger than the transverse plasma size
In the framework of a simplified model considering a plasma slab, we identified the plasmonic resonance responsible for the observed spectral features as a leaky mode
Summary
Terahertz (THz) radiation has become a ubiquitous tool for many applications in science and technology [1,2]. A femtosecond (fs) two-color (2C) laser pulse composed of fundamental-harmonic (FH) and second-harmonic (SH) frequency is focused into an initially neutral gas, creating free electrons via tunnel ionization. These electrons are accelerated by the laser electric field and produce a macroscopic current leading to broadband THz emission. A simple analytical model developed in Section 3 allows us to link this resonant broadening to a leaky mode of the ellipsoidal plasma This model shows that the resonance has a strong impact on the spectrum whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength.
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