We determined the bubble radius $R_{\max }$ for femtosecond optical breakdown in water at 347, 520, and 1040 nm with an unprecedented accuracy ($\pm 10\mathrm{nm}$). At threshold, $R_{\max }$ was smaller than the diffraction-limited focus radius and ranged from 190 nm to 320 nm. The increase of $R_{\max }$ with laser energy $E_L$ is slowest at 347 nm, providing optimum control of cell surgery. Experimental results agree with a model of bubble formation in heated and thermoelastically stretched liquids. Theory predicts a threshold temperature $T_{\mathrm{th}}\approx 168°\mathrm{C}$. For $T>300°\mathrm{C}$, a phase explosion sets in, and $R_{\max }$ increases rapidly with $E_L$.

• Received 4 September 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.038102