Ultrafast white-light continuum generation and self-focusing in transparent condensed media

We report an investigation of white-light continuum generation and self-focusing by 140-fs Ti:sapphire laser pulses in extended transparent media. It is found that continuum generation is triggered by self-focusing and that both phenomena depend on the medium’s bandgap. There is a bandgap threshold for continuum generation. Above that threshold the continuum’s width increases with increasing bandgap. Furthermore, the beam’s self-focal diameter is discontinuous across the threshold. To explain the observations a mechanism is proposed that involves multiphoton excitation of electrons into the conduction band at the self-focus; the generated free electrons cause spectral superbroadening and limit the self-focal diameter. The continuum beam’s surprisingly low divergence is then investigated and explained in terms of a Kerr lensing effect.