Detection of Surface Waves During Femtosecond Filamentation

TL;DR

This paper investigates the detection of surface waves during femtosecond filamentation, providing experimental evidence that supports the theory of surface plasmon polariton excitation at plasma boundaries, with detailed near-field measurements aligning with models.

Contribution

The study offers the first direct near-field measurements confirming the excitation of surface plasmon polaritons during femtosecond filamentation, validating theoretical predictions.

Findings

Radial field dependence fits Hankel function

Outer length scale matches plasma parameters

Longitudinal drift in frequency maxima observed

Abstract

Ultrashort pulsed lasers (USPL) can produce thin columns of plasma in air via femtosecond filamentation, and these plasmas have been found to generate broadband TeraHertz (THz) and Radio Frequency (RF) radiation. A recent theory argues that the currents driven at the boundary of the plasma excite a Surface Plasmon Polariton (SPP) surface wave (in particular a Sommerfeld-Goubau wave given the cylindrical symmetry), which proceeds to detach from the end of the plasma to become the RF pulse. We have performed near-field measurements of these plasmas with a D-dot probe, and find an excellent agreement with this theory. The radial field dependence is precisely fit by a Hankel function, with an outer length scale in agreement with plasma conductivity and radius, and a measured longitudinal drift in frequency maxima closely matches both SPP simulations and analytic expectations.