Pulse-length dependence of cellular response to intense near-infrared laser pulses in multiphoton microscopes

The influence of the pulse length, tau, of ultrashort laser pulses at 780 and 920 nm on cell vitality and cellular reproduction has been studied. A total of 2400 nonlabeled cells were exposed to a highly focused scanning beam from a mode-locked 80-MHz Ti:sapphire laser with 60-mu s pixel dwell time. For the same pulse energy, destructive effects were more pronounced for shorter pulses. The damage behavior was found to follow approximately a P-2/tau dependence (P, mean power), indicating that cell destruction is likely based on a two-photon excitation process rather than a one- or a three-photon event. Therefore, femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy. (C) 1999 Optical Society of America.