High-frequency demodulation of multiphoton fluorescence in long-wavelength hyper-Rayleigh scattering.

Suppression of the multiphoton fluorescence contribution to the hyper-Rayleigh (second-order nonlinear optical) scattering signal was recently achieved by intrinsic demodulation of the fluorescence at high amplitude-modulation (AM) frequencies [Olbrechts et al., Rev. Sci. Instrum. 69, 2233 (1998)]. These high AM frequencies were obtained from the high harmonic content in the Fourier spectrum of a repetitive train of femtosecond pulses from a Ti:sapphire laser emitting at 800 nm. We have used a femtosecond parametric oscillator to shift the fundamental wavelength to 1.3 mum . By further improving 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 fluorophore. The results also indicate that shifting the fundamental wavelength to the near infrared only is not a general solution to the multiphoton fluorescence problem in hyper-Rayleigh scattering.