Abstract
Using the sol-gel technique we have investigated PbS-doped thin films. The resonant nonlinear refractive index of these samples was measured, using two different techniques: nonlinear m-lines and degenerate four-wave mixing (DFWM). The measured values of the nonlinear refractive index are 
and 
for the m-lines and DFWM, respectively. We explain this difference as being due to the different durations of the laser pulses used (7 ns for m-lines and 30 ps for DFWM).