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 a nd 920 nm on cell vitality and cellular reproduction has been studied. A to tal 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 sh orter pulses. The damage behavior was found to follow approximately a P-2/t au dependence (P, mean power), indicating that cell destruction is likely b ased on a two-photon excitation process rather than a one- or a three-photo n event. Therefore, femtosecond as well as picosecond pulses provide approx imately the same relative optical window for safe two-photon fluorescence m icroscopy. (C) 1999 Optical Society of America.