Nonlinear characterization of PbS nanocrystals embedded in sol-gel planar waveguides.

Thin silica-titania planar waveguides doped with different concentrations of lead sulfide (10-25 mol%) have been prepared by a sol-gel process. It consists of three steps: a) preparation of a colloidal sol of semiconductor particles; b) preparation of an alkoxide solution, precursor of the glass-like matrix; c) mixing of the colloidal sol and the alkoxide solution. Films were deposited on fused silica by dipping and densified for 1h at 300 degrees C in nitrogen. The mean particle size is 3 nm and the optical absorption edge is situated around 1100 nm. The nonlinear properties have been investigated using degenerated four wave mixing (DFWM) and a nonlinear m-line technique. Different Nd:YAG lasers with pico- and nanosecond pulses at 1064 nm have been used. Depending on the PbS concentration we measured a high negative nonlinear refractive index of n(2) = -3 to -9 10(-8) cm(2)/kW for nanosecond pulses (m-lines) and -1 to -10 10(-10) cm(2)/kW for picosecond pulses (m-lines and DFWM). The response time of the nonlinearity is below 30 ps. All observed nonlinear effects are fully reversible and we did not observe any photodarkening. Straight, monomode channel waveguides have been fabricated on theses films.