Free-electron absorption of 1064-nm photons is measured photoacoustically in NaCl and ${\mathrm{SiO}}_2$. The electrons are generated with a 266-nm pump pulse by two- or three-photon transitions. For a given pump pulse energy, the photoacoustic signal and the rise in lattice temperature are observed as a function of the energy of the 1064-nm pulse and found to be in agreement with the theory of free-carrier heating. Temperatures approaching the melting point are produced in these materials under prebreakdown conditions. No evidence of electron-impact ionization and ensuing avalanche formation is found up to the intrinsic damage threshold.

• Received 24 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.2711