Ultrafast optical dephasing measurements were performed on thin films of the oxazine dye Nile Blue in poly-[vinyl alcohol] (PVOH). The experimental technique used two beam, time delayed degenerate four wave mixing (DFWM) with incoherent light.\footnote{T. Kobayashi, A. Terasaki, T. Hattori and K. Kurokawa, Appl. Phys. B 47, 107 (1988).} Under the conditions that the sample was at relatively high temperatures (between 200°K and room temperature) and the measurements were made on the blue side of the Nile Blue inhomogeneous distribution, small time delay oscillations in the were observed. A time delay period was measured at 1.96 +/- .07 fs, which corresponds to the central period of the incoherent light source (centered at 575 nm with a period of 1.92 fs). A comparison between the measured oscillation depth and a numerical simulation of the DFWM scattered intensity yields an estimate of a phase relaxation time between 2 and 3 femtoseconds. This technique offers a new method of measuring femtosecond dephasing times with applications to samples of broad homogeneous linewidths, at high temperatures or in highly interacting environments. The depth of the oscillation decreased with lower temperature and completely disappeared below T=100°K. The temperature dependence of this effect is currently under investigation.
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