Propagation length of guided waves in lossy Si film sandwiched by identical dielectrics

We have measured a propagation length L of TM0 and TE0 guided waves in a lossy Si film surrounded by identical dielectrics 632.8 nm in wavelength. The propagation length LATR(E) estimated from experimental attenuated total reflection (ATR) spectra deviated largely from the propagation length LATR(T) estimated from calculated ATR spectra in the range of small optical thickness of Si film hopt. This discrepancy may arise from an angular beam spread of incident light. We have proposed a method to measure L directly. This method gave us an L for TM0 guided waves of 1.6 mm at hopt = 6.1 nm. This value is in good agreement with LATR(T) when LATR(E)=0.2 mm, under certain conditions.