  We report on the characterization of silica aerogel thermal optical
nonlinearity, obtained by z-scan technique. The results show that typical
silica aerogels have nonlinear optical coefficient similar to that of glass
$(\simeq 10^{-12} $m$^2/$W), with negligible optical nonlinear absorption. The
non\-li\-near coefficient can be increased to values in the range of $10^{-10}
$m$^2/$W by embedding an absorbing dye in the aerogel. This value is one order
of magnitude higher than that observed in the pure dye and in typical highly
nonlinear materials like liquid crystals.
