Accurate measure of laser irradiance threshold for near-infrared photo-oxidation with a modified confocal microscope

Femtosecond mode-locked lasers are now being used routinely in multiphoton fluorescence and autofluorescence spectroscopy, are just beginning to be used in refractive surgery, and may be used in the future diagnosis of skin cancer. Pulses from these lasers induce non-linear effects in resultant tissue interactions. Using a modified confocal microscope with dispersion compensation and accurate measurements of beam diameter, a very low threshold was measured for photochemical oxidation in cultured cells. The measured threshold showed non-linear photo-oxidation at a peak irradiance and photon-flux density of 8.4x10(8) W cm-2 and 3.4x10(27) photons cm-2 s-1, respectively (90-fs pulse). The impact of these findings is significant to those using ultrashort lasers because they provide a tangible reference point (microscope-independent) for the generation of photo-oxidative stress in laser-exposed tissues, and because they highlight the importance of dispersion compensation in minimizing collateral tissue damage.