Physical characterization of light-plasma filaments in water using time resolved shadowgraphy

Filamentation of intense femtosecond laser pulses has recently attracted interest for its potential in applications such as laser-written waveguides in glasses and eye surgery. The evolution of the laser pulse shape and the population of electrons generated during the laser-matter interaction is clearly a key issue for understanding the temporary and permanent physical-chemical changes induced by light filamentation in the medium, which are particularly relevant for application of light filaments to eye surgery. This paper implements for the first time a time-resolved shadowgraphic method to measure with unprecedented resolution the changes in the complex refractive index induced by a loosely focused 120 fs pulse undergoing filamentation in water. The shadowgraphic method is a sensitive optical technique to characterize small-amplitude refractive index changes, such as those induced by a low density plasma in a transparent material.small-amplitude refractive index changes, such as those induced by a low density plasma in a transparent material. The evolution of the filamentation process has been tracked with a time resolution of only 25 fs over a time span of about 1 ps. Variations of the complex refractive index along two characteristic time scales, namely a "fast" (sub-100 fs) and a "slow" (ps) scale can be distinguished. The fast features are observed within the filamenting pulse and are characterized by a positive variation of the real part of the refractive index (Deltan) and a strong absorption.