High-frequency demodulation of multiphoton fluorescence in long-wavelengthhyper-Rayleigh scattering

Suppression of the multiphoton fluorescence contribution to the hyper-Rayle igh (second-order nonlinear optical) scattering signal was recently achieve d by intrinsic demodulation of the fluorescence at high amplitude-modulatio n (AM) frequencies [Olbrechts et al., Rev. Sci. Instrum. 69, 2233 (1998)]. These high AM frequencies were obtained from the high harmonic content in t he Fourier spectrum of a repetitive train of femtosecond pulses from a Ti:s apphire laser emitting at 800 nm. We have used a femtosecond parametric osc illator to shift the fundamental wavelength to 1.3 mu m. BY further improvi ng the detection electronics, we can now obtain fluorescence suppression at AM frequencies up to 600 MHz. Fluorescence-free hyperpolarizability values were obtained for fluorescent dipolar compounds as well as for an ionic fl uorophore. The results also indicate that shifting the fundamental waveleng th to the near infrared only is not a general solution to the multiphoton f luorescence problem in hyper-Rayleigh scattering. (C) 1999 Optical Society of America.