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 investigated experimentally using an acousto-optic pulse sharer to modify femtosecond pulses from a Ti:sapphire laser. By using different spectral window shapes, we find that the measured two photon efficiency can vary by a factor of 2 for differently shared spectra with the same full width at half maximum. We find that these effects are described well by a simple model assuming transform-limited pulses. The fact that even small changes in the spectral wings can significantly affect the efficiency of nonlinear processes has implications for biological multiphoton imaging, where it may be desirable to minimize sample exposure to radiation and maximize fluorescence or harmonic efficiency. (C) 1999 Society of Photo0Optical Instrumenttion Engineers. [S1083-3668(99)00303-1].