Wavelength dependence of femtosecond laser-induced breakdown in water and implications for laser surgery

Abstract

The wavelength dependence of the threshold for femtosecond optical breakdown in water provides information on the interplay of multiphoton, tunneling, and avalanche ionization and is of interest for parameter selection in laser surgery. We measured the bubble threshold from ultraviolet to near-infrared wavelengths and found a continuous decrease of the irradiance threshold with increasing wavelength $\ensuremath{\lambda}$. Results are compared to the predictions of a numerical model that assumes a band gap of 9.5 eV and considers the existence of a separate initiation channel via excitation of valence band electrons into a solvated state followed by rapid upconversion into the conduction band. Fits to experimental data yield an electron collision time of $\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$ and an estimate for the capacity of the initiation channel. Using that collision time, the breakdown dynamics were explored up to $\ensuremath{\lambda}=2\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The irradiance threshold first continues to decrease but levels out for wavelengths longer than 1.3 $\ensuremath{\mu}\mathrm{m}$. This opens promising perspectives for laser surgery at wavelengths around 1.3 and 1.7 $\ensuremath{\mu}\mathrm{m}$, which are attractive because of their large penetration depth into scattering tissues.