  The authors describe the fabrication of buried waveguides in a highly
nonlinear chalcogenide glass, gallium lanthanum sulfide, using focused
femtosecond laser pulses. Through optical characterization of the waveguides,
they have proposed a formation mechanism and provide comparisons to previous
work. Tunneling has been identified as the dominant nonlinear absorption
mechanism in the formation of the waveguides. Single mode guidance at 633 nm
has been demonstrated. The writing parameters for the minimum propagation loss
of 1.47 dB/cm are 0.36 uJ pulse energy and 50 um/s scanning speed.
