INVESTIGATIONS OF TEMPORAL RESHAPING DURING FILAMENTARY PROPAGATION WITH APPLICATION TO IMPULSIVE RAMAN SPECTROSCOPY

Abstract

Femtosecond laser filamentation in gaseous media is a new source of broadband, ultrashort radiation that has the potential for application to many fields of research. In this dissertation filamentation is studied with a view to understanding the underlying physics governing the formation and propagation dynamics of filamentation, as well as to developing a method for vibrational spectroscopy based on the filament-induced impulsive vibrational excitation of molecules in the filamentation region. In pursuit of a better understanding of the underlying physical processes driving filamentation, the development of a new method for characterizing high intensity ultrashort laser pulses is presented, wherein two laser beams generate a transient grating in a noble gas, causing the pulse undergoing filamentation to diffract from the grating. Measuring the spectrum as a function of time delay between the filament and probe beams generates a spectrogram that can be inverted to recover the spectral and temporal phase and amplitude of the filamentary pulse. This technique enables measurement of the filamentary pulse in its native environment, offering a window into the pulse dynamics as a function of propagation distance. The intrinsic pulse shortening observed during filamentation leads to the impulsive excitation of molecular vibrations, which can be used to understand the dynamics of filamentation as well. Combined measurements of the longitudinally-resolved filament Raman spectrum, power spectrum, and fluorescence intensity confirm the propagation dynamics inferred from pulse measurements and show that filamentation provides a viable route to impulsive vibrational spectroscopy at remote distances from the laser source. The technique is applied to thermometry in air and in flames, and an analytical expression is derived to describe the short-time dynamics of the rovibrational wave-packet dispersion experienced by diatomic molecules in the wave of the filament. It is found that no energy is initially partitioned into the distribution of rovibrational states during the filamentation process. Filament-assisted impulsive stimulated Raman spectroscopy of more complex systems is also performed, showing that filament-assisted vibrational measurements can be used as an analytical tool for gas phase measurements and has potential for use as a method for standoff detection. Finally, a study of the nonlinear optical mechanisms driving the filamentation process is conducted using spectrally-resolved pump-probe measurements of the transient birefringence of air. Comparison to two proposed theories shows that a newly described effect, ionization grating-induced birefringence, is largely responsible for saturation and sign inversion of the birefringence at 400 nm and 800 nm, while the magnitude of contributions described by a competing theory that relies on negative terms in the power series expansion of the bound electron response remain undetermined.