  We report the experimental observation of the photothermal effect. The
measurements are performed by modulating the laser power absorbed by the
mirrors of two high-finesse Fabry-Perot cavities. The results are very well
described by a recently proposed theoretical model [M. Cerdonio, L. Conti, A.
Heidmann and M. Pinard, Phys. Rev. D 63 (2001) 082003], confirming the
correctness of such calculations. Our observations and quantitative
characterization of the photothermal effect demonstrate its critical importance
for high sensitivity interferometric displacement measurements, as those
necessary for gravitational wave detection.
