FEMTOSECOND KERR ELLIPSOMETRY IN POLYDIACETYLENE SOLUTIONS-2-PHOTON EFFECTS

We describe a pump-probe femtosecond Kerr experiment in polydiacetylene solutions (4BCMU, yellow form). The Kerr ellipsometry technique permits a separate determination of the real and of the imaginary part of anisotropic index changes. The wavelength (620 nm) of the pump pulses lies in the transparency region of polydiacetylene while the probe pulses have a broad white spectrum. The largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum with beta almost-equal-to 4 x 10(-3) cm/MW is observed close to 4 eV. The two-photon excited states have a life time close to 12 ps and they present a broad absorption band in the 2 eV range.