Beam transformation and permanent modification in fused silica induced by femtosecond filaments

Results of an investigation of lateral beam structure by propagating femtosecond pulses in fused silica are presented. Observed spatial transformations of the beam were closely related to the onset of continuum generation. Further beam propagation was in the form of an intense central spike (filament) and surrounding rings. Increasing power led to beam splitting and the formation of a regular structure of multiple filaments. The filament was responsible for the spontaneous formation of permanent modification inside the material. Experimental results are backed by numerical simulations that include self-guided channels of a permanent change of the refractive index in a bulk medium. (c) 2005 Optical Society of America