Wavelength dependence of nanosecond and femtosecond optical breakdown in water: Experimental investigations

The wavelength dependence of femtosecond and nanosecond optical breakdown in transparent dielectrics is important for laser micromachining, and for laser surgery of cells and tissues. This work experimentally investigates the intensity threshold, I_th(lambda) for pulse durations of ap 265 fs in the range between 330 nm and 1100 nm at 50 individual wavelengths, and ~ 2 ns in the range between 725 nm and 1030 nm at 21 wavelengths. Laser pulses are focused at two numerical apertures (NA = 0.8, and 0.9) through long-distance water-immersion objectives inserted into the wall of a cell filled with deionized and filtered (0.2 mum) water. Breakdown is identified with the occurrence of bubble formation that is detected using the scattering of a continuous probe laser beam adjusted collinear and confocal with the pulsed laser beam. For ns pulses, the bubble formation threshold coincided with the appearance of bright plasma luminescence.