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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. Moreover, it provides information about the interplay of the different ionization mechanisms governing breakdown, such as tunnelling and multiphoton ionization, avalanche ionization, and thermal ionization. For nanosecond breakdown, it enables to distinguish between extrinsic initiation of breakdown by impurities and intrinsic breakdown. Initiation of the breakdown process by multiphoton ionization should result in a stepwise increase of the threshold irradiance, Ith, at wavelengths above which one more photon is required to overcome the band gap [1].