Ag Nanoframes Deposited on Au Films Generate Optical Cavities for Surface-Enhanced Raman Scattering

We proposed a film-coupled nanoframe optical cavity which produces a significant near-field enhancement in the visible and near-infrared region. The resonant characteristics of the cavity with cavity dimensions are analyzed in detail. The maximum electric field enhancement was obtained by matching the cavity size with the excitation wavelength, which is different from the traditional SERS strategy with minimized nanogap. Attractively, resonator modes between nanoframes and Au film exist while the gap varies in a large range, which permits for SERS application on the condition that the sample thickness cannot be accurately controlled. Furthermore, the film-coupled nanoframe is of particular interest because it is easy for light to go through the nanoframe for excitation of cavity modes in a broad spectral range from visible to near-infrared. The strong and homogeneous field enhancement in the cavity makes it possible for Raman detection for biological mixtures.