Energy Deposition in Femtosecond Filamentation: Measurements and Applications

Abstract

Femtosecond filamentation is a nonlinear optical propagation regime of high peak power ultrashort laser pulses characterized by an extended and narrow core region of high intensity whose length greatly exceeds the Rayleigh range corresponding to the core diameter. Providing that a threshold power is exceeded, filamentation can occur in all transparent gaseous, liquid and solid media. In air, filamentation has found a variety of uses, including the triggering of electric discharges, spectral broadening and compression of ultrashort laser pulses, coherent supercontinuum generation, filament-induced breakdown spectroscopy, generation of THz radiation, and the generation of air waveguides. Several of these applications depend on the deposition of energy in the atmosphere by the filament. The main channels for this deposition are the plasma generated in the filament core by the intense laser field and the rotational excitation of nitrogen and oxygen molecules. The ultrafast deposition acts as a delta function-like pressure source to drive a hydrodynamic response in the air. This thesis experimentally demonstrates two applications of the filament-driven hydrodynamic response. One application is the ‘air waveguide’, which is shown to either guide a separately injected laser pulse, or act as a remote collection optic for weak optical signals. The other application is the high voltage breakdown of air, where the effect of filament-induced plasmas and hydrodynamic response on the breakdown dynamics is elucidated in detail. In all of these experiments, it is important to understand quantitatively the laser energy absorption; detailed absorption experiments were performed as a function of laser parameters. Finally, as check on simulations of filament propagation and energy deposition, we measured the axially resolved energy deposition of a filament; in the simulations, this profile is quite sensitive to the choice of the nonlinear index of refraction (n2). We found that using our measured values of n2 in the propagation simulations results in an excellent fit to the measured energy deposition profiles.