Plasma formation in water by picosecond- and nanosecond- Nd:YAG laser pulses: Transmission, scattering, and reflection

We investigated the transmission, scattering and reflection of plasmas produced in water by Nd:YAG laser pulses of 6 ns and 30 ps duration. The transmission measurements comprise a large energy range at a wavelength of 1064 nm and various focusing angles between 1.7° and 22°. This parameter range covers the parameters used for intraocular microsurgery, but also allows to asses the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for ps pulses than for ns pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self- focussing, which occurs at focusing angles below 2, leads to a further increase of transmission. The efficacy of plasma- mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the ns pulses nearly 50% of the laser pulse energy is absorbed already at threshold, whereas it is only 8% with the ps-pulses. The small fractional energy deposition with ps pulses together with a low energy threshold for breakdown can, however, be useful for the generation of very fine tissue effects. Structures beyond the laser focus are 2-6 times more effectively shielded from laser radiation by plasmas generated with ns pulses than by ps plasmas. The transmitted energy at equal normalized energy E/E_th is, nevertheless, always by more than a factor of 8 less than for ps-pulses because of their lower energy threshold for plasma formation.