Nonlinear refraction in aqueous colloidal gold at 527 nm was studied u
sing the z-scan technique. While a z-scan with a 35 ns laser showed a
large negative lensing, a z-scan with a 4 ps laser showed no measurabl
e refraction. The observed nonlinear refraction is shown to be of ther
mal origin resulting from energy transfer from gold particles to the w
ater molecules.