Shielding effectiveness of femtosecond laser-induced plasmas in ultrapure water

The ability of laser-induced plasmas to shield biological tissue from intense exposure to laser pulse energy has been in question for years. Current ocular surgical techniques rely on cone angles of incident pulses to protect delicate tissue anterior and posterior to the site of intraocular laser surgery. The so-called 'shielding' effect of the evolving plasma has been a proposed mechanism for providing additional protection to the retina. Concern is heightened when the procedures require work deeper int the vitreous cavity. For various pulse durations, we have examined the efficiency of the pulse energy to plasm conversion as well as the shielding effectiveness. Work has been done in both the nanosecond (ns) and picosecond (ps) time regimes, but little is known about breakdown and plasma behavior induced by femtosecond (fs) pulses. A simplistic model of the eye is developed using a quartz cell filled with ultrapure water. To focus the pulses in the cell and to induce optical breakdown, a 17 millimeter (mm) aspheric lens was selected to approximate the 17.1 mm reduced focal length of the human eye. Pulses of 10 ns at 1064 nanometer (nm) and 5 ps at 580 nm are used to compare with previous work. In the femtosecond time regime, 100 fs pulses at 580 nm are used with energies ranging from 0.5 - 120 microjoules (μJ). For all pulse widths, the ratio of the energy exiting the water cell to the input energy is recorded for cases with and without optical breakdown. Comparisons of shielding effectiveness are made between all three time regimes.