Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. I. Optical breakdown at threshold and superthreshold irradiance

We investigated plasma formation in distilled water by 30-ps and 6-ns Nd:YAG laser pulses of 1064-nm and 532-nm wavelength for focusing angles between 1.7° and 32°. We determined the optical breakdown thresholds and analyzed the plasma length achieved at superthreshold irradiance, The parameter range investigated covers the parameters used for intraocular laser surgery. The experimental results are compared to theoretical models for the calculation of breakdown thresholds and the description of plasma growth for superthreshold breakdown. We found that at λ=1064 nm the measured thresholds for both pulse durations coincide with the calculated thresholds for the generation of seed electrons by multiphoton ionization. The breakdown process is completed by avalanche ionization. The seed electron density required for breakdown is about 4×10/sup 9/ cm for the 6-ns pulses, and 1.4×10/sup 11/ cm/sup -3/ for the 30-ps pulses. No spot size dependence of the irradiance threshold for breakdown was observed. The average threshold is by a factor of 5.9 higher for 30-ps pulses (I/sub th/=4.5×1011 W/cm²) than for 6-ns pulses (I_th=0.76×10/sup 11/ W/cm/sup 2/). At angles below approximately 2°, the threshold is influenced by self-focusing effects. The breakdown thresholds at 532 nm are slightly lower than at 1064 nm. Here, multiphoton ionization contributes considerably to the generation of free electrons throughout the whole process of plasma formation. Our results for plasma formation at superthreshold energies support a "breakdown wave" mechanism of plasma growth. For picosecond pulses, the breakdown threshold can be considered to be time-invariant, but for nanosecond pulses there is probably a decrease of the threshold during the laser pulse which may be due to UV-radiation emitted from plasma created at the beginning of the pulse.