Experimental investigation of optical breakdown using nanosecond 532- and 1064-nm laser pulses delivered at high numerical aperture

We have conducted time-resolved studies of optical breakdown produced by the irradiation water using 6 ns Nd:YAG laser pulses of 1064 nm and 532 nm wavelength focused at a numerical aperture of NA = 0.9. We determined pulse energy threshold values for plasma formation to be 1.89 μJ and 18.3 μJ for 532 and 1064 nm irradiation, respectively. These energy thresholds correspond to irradiance thresholds of 0.77 × 109 W/mm2 for 532 nm irradiation and 1.87 × 109 W/mm2 for 1064 nm irradiation. For pulse energies 1×, 2×, 5×, and 10× above threshold, we determined the length of the laser-induced plasma, the propagation speed and peak pressures of the emitted shock wave, and the mechanical energy dissipated by subsequent cavitation bubble formation, growth, and collapse. This analysis demonstrates that both the breakdown threshold as well as the conversion efficiency of the incident laser energy into mechanical energy is smaller for irradiation at 532 nm than for 1064 nm. These results are consistent with laser parameters employed for a variety of nanosecond pulsed microirradiation procedures using 1064 nm and 532 nm radiation focused by microscope objectives with large numerical apertures (NA ≳ 0.8). These results suggest that laser-induced breakdown is the primary mechanism that drives a variety of cellular micromanipulation techniques which employ nanosecond visible and near-infrared laser pulses