The attenuated total reflection (ATR) method in a prism/gap/metal configuration was used in Raman spectroscopy of molecules adsorbed on silver to improve the detection limit. The thickness of the air gap was controlled between λL/2 and 2λL (λL: wavelength of the incident light) with a biaxial suspension. All the scattered light within the SPP cone was collected with a Weierstrass prism and a 1:1 objective. The enhancement factor of the Raman scattering intensity for the use of in-and out-coupling of the SPP was found to be 134 and 70 for carbon and copper-phthalocyanine (CuPc), respectively. The scattered light inside the SPP cone is additionally enhanced by the interference in the air gap. This interference is also useful to enhance the Raman signals of the adsorbates. Nearly the same enhancement factor was observed for the different symmetry species of vibrations in CuPc, because CuPc is randomly oriented with respect to the silver surface normal.
M. Futamata, P. Borthen, J. Thomassen, D. Schumacher, and A. Otto, "Application of an ATR Method in Raman Spectroscopy," Appl. Spectrosc. 48, 252-260 (1994)