Femtosecond filamentation in chalcogenide glasses limited by two-photon absorption

Filamentation of 800 nm femtosecond laser pulses in the conditions of strong two-photon absorption was first directly observed in As4Ge30S66 chalcogenide glass, this effect being accompanied by increase in the pulse spectrum width from 8.5 to 11 nm and its modulation indicating the pulse temporal splitting. In contrast, there was no filamentation and pulse spectrum widening in stoichiometric As2S3 glass. The nonlinear figure of merit was shown to be as high as 0.5 and only ∼0.1 in glassy As4Ge30S66 and As2S3, respectively.