Studying the wavelength dependence of femtosecond optical breakdown in water helps resolving an ongoing controversy on the relative importance of multiphoton, tunneling and avalanche ionization. Measurements of the bubble formation threshold at 50 wavelengths from UV to near-IR revealed a continuous decrease of the irradiance threshold with increasing wavelength. This is indicative for a dominant role of avalanche ionization, which gains strength with wavelength whereas the multiphoton ionization rate decreases.

Fitting data by a model considering breakdown initiation via a solvated electron state yielded an effective Drude electron collision time of 1 fs. Modeling predicts that the threshold continues to decrease up to 1.3 μm but levels out for longer wavelengths. It remains low in the mid IR because wavelength-independent tunneling ionization ensures a constant level of seed electrons for the ionization avalanche even though the influence of multiphoton ionization ceases.

The low breakdown threshold opens promising perspectives for ultrashort-pulsed laser surgery at wavelengths around 1.3 μm and 1.7 μm, which are attractive due to a favorable combination of low scattering and moderate water absorption. The wavelength dependence of the irradiance threshold together with tissue optical data was used to estimate the wavelength dependence of the energy threshold at various cutting depths. For focusing depths up to 200 μm, pulse energies required for surgery are smallest for < 800 nm. However, the energy minimum shifts to wavelengths around 1350 nm for z = 500 μm, and to the region around 1700 nm for z = 1 mm.

Date Published: 21 April 2017
PDF: 1 pages
Proc. SPIE 10094, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XVII, 1009409 (21 April 2017); doi: 10.1117/12.2252149

Published in SPIE Proceedings Vol. 10094:
Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XVII
Alexander Heisterkamp; Peter R. Herman; Michel Meunier; Roberto Osellame, Editor(s)