Effect of pulse shape on the efficiency of multiphoton processes: Implications for biological microscopy

The effects of spectral shape on two photon fluorescence excitation are inv estigated experimentally using an acousto-optic pulse sharer to modify femt osecond pulses from a Ti:sapphire laser. By using different spectral window shapes, we find that the measured two photon efficiency can vary by a fact or of 2 for differently shared spectra with the same full width at half max imum. We find that these effects are described well by a simple model assum ing transform-limited pulses. The fact that even small changes in the spect ral wings can significantly affect the efficiency of nonlinear processes ha s implications for biological multiphoton imaging, where it may be desirabl e to minimize sample exposure to radiation and maximize fluorescence or har monic efficiency. (C) 1999 Society of Photo0Optical Instrumenttion Engineer s. [S1083-3668(99)00303-1].