Absolute two-photon absorption coefficients at 355 and 266 nm

The absolute two-photon absorption coefficients of uv-transmitting materials have been measured using well-calibrated single picosecond pulses, at the third and fourth harmonic of a mode-locked YAlG:Nd laser system. Two-photon absorption coefficients of the order of ${10}^{-3\mathrm{}}$ cm/MW were measured for alkali halides, and ${10}^{-4\mathrm{}}$ cm/MW for harmonic-generating crystals. In materials with band gap greater than $2\hslash \omega$, no nonlinear absorption could be observed. Calculations based on the Keldysh theory with one adjustable parameter agree quite well with the experimentally observed dispersions in the two-photon absorption coefficients. The effect of multiphoton absorption on the conversion efficiency of harmonic generating crystals and on the breakdown of uv window materials is also discussed.