We have measured the infrared transmission at 3.39 &mgr;m, for bulk Si and Si:Au and for silicon on sapphire during pulsed laser annealing. The transmission drops sharply and remains near zero for approximately 100 ns in all samples. The infrared transmission in bulk Si requires milliseconds to completely recover. We attribute this effect to a thermally induced change in the index of refraction of the silicon and associated tuning of the Fabry–Perot cavity formed by the faces of the Si wafers. Our experiments yield no evidence for an optical gap as expected for a Bose condensed exciton phase, and thus are supportive of the thermal melting model of laser annealing.