Propagation of TE or TM waves did not produce significant differences in the number of peaks nor in the peak positions from the SER response of 4MPy over the 500 to 3200 cm -1 spectral range. However, Raman counts were uniformly reduced with TM polarization compared to TE polarization, consistent with the theoretical predictions of Chen et al. , who found that the field enhancement factor for Raman scattering with the TE mode in a PW was larger than that compared with the TM mode in the same PW. Metal overlayers on dielectric waveguides act as "mode strippers", preferentially removing TM polarization from mixed TE/TM polarization waves. In general, a metal overlayer on a dielectric waveguide behaves like a high-loss medium with a complex dielectric function 𝜖(𝜔) = 𝜖(𝜔) 1 -𝑖𝜖(𝜔) 2 over the frequency range  of light , because the inertia of charge carriers inside a metal film dominates above the metal plasma frequency 𝜔 𝑝 . As a result, a metal-clad waveguide has a complex propagation constant 𝑘 = 𝛽 -𝑗𝛼 (𝛼 > 0) along the propagation direction. Selective attenuation of the TM polarization is known to depend strongly on the thickness of a buffer (insulator) layer between the metal and the guiding layer .