Nonlinear Absorption Initiated Avalanche Breakdown in Dielectric ZrO₂

We report direct measurements of nonlinear transmission in dielectric ZrO₂ prior to the observation of catastrophic damage for nsec laser pulses at 532 nm. The tested cubic zirconia (ZrO₂) specimens are stabilized with yttria (Y₂O₃) having concentrations of 9.4%, 12%, 15%, 18%, and 21%. Transmission measurements were performed for a broad range of pulsewidths from 40 psec to 18 nsec (FWHM) at 532 nm. Our results indicate that the nonlinearity prior to breakdown is mainly attributed to the formation of material defects (color centers) by two-photon absorption (2PA) at high irradiance. These induced color centers appear to be long-lived (the order of hours) but can be bleached by repeated irradiation with relatively low irradiance 532 nm pulses. The accumulated effects of color center absorption during the 2PA measurements makes extraction of the two-photon absorption coefficient difficult. However, the smallest value for the 2PA coefficient (∼0.04 cm/GW) was measured in the ZrO₂ sample stabilized with 9.4% Y₂O₃ for the shortest pulsewidth (40 psec) with which the measurements were performed. The observed nonlinear transmission prior to breakdown is consistent with the interpretation that damage in this material is due to 2PA induced avalanche breakdown.