Picosecond laser-induced breakdown at 5321 and 3547 Å: Observation of frequency-dependent behavior

Laser-induced dielectric breakdown at 5321 Å has been investigated in six materials: K${\mathrm{H}}_2$P${\mathrm{O}}_4$, fused Si${\mathrm{O}}_2$, NaCl, Ca${\mathrm{F}}_2$, NaF, and LiF. The laser pulses were nominally 21 psec in duration and were obtained by the frequency doubling of single 1.064-μm pulses from a mode-locked YAlG:Nd laser system. The frequency doubling was carried out in a temperature phase-matched Cs${\mathrm{H}}_2$As${\mathrm{O}}_4$ crystal, resulting in spatially smooth, reproducible, diffraction-limited, 5321-Å pulses. Strict attention is given to the complications introduced by self-focusing. The new thresholds for breakdown at 5321 Å are compared with thresholds measured previously at 1.064 μm with the same laser system. This comparison, as a function of material band gap ${\mathcal{E}}_G$, charts experimentally the transition of the character of bulk laser-induced breakdown as it becomes strongly frequency dependent. A frequency-dependent decrease in the threshold of ∼30% is observed for K${\mathrm{H}}_2$P${\mathrm{O}}_4$, for which three 5321-Å photons exceed ${\mathcal{E}}_G$ in energy. Frequency-dependent threshold increases of up to 44%, varying smoothly with ${\mathcal{E}}_G$ are observed for the other materials. At 3547 Å five materials were studied: K${\mathrm{H}}_2$P${\mathrm{O}}_4$, Si${\mathrm{O}}_2$, Ca${\mathrm{F}}_2$, NaF, and LiF. The uv picosecond pulses were obtained by mixing 1.064-μm and 5321-Å pulses in a K${\mathrm{H}}_2$P${\mathrm{O}}_4$ crystal. In spite of uncertainties introduced by walk-off distortion and self-focusing, upper and lower bounds on the breakdown threshold at 3547 Å in these materials are obtained. These results qualitatively confirm and extend the behavior found in the 5321-Å study. These observations are discussed with regard to models for the intrinsic processes involved in the breakdown, which are avalanche and multiphoton ionization.