Surface enhanced Raman spectroscopy (SERS) has been widely used in many fields, including environmental monitoring, food safety, national security, forensic science, and biological research. SERS generally detects the backscattered light. It has been well recognized that SERS utilizes localized surface plasmon resonance (LSPR) of metallic nanostructures to achieve high sensitivity. Raman light backscattered from conventional SERS substrates is highly divergent, resulting in low collection efficiency. The reverse Kretschmann (RK) Raman spectroscopy is a new technique relative to SERS, which is based on evanescent wave coupled emission (EWCE), including surface plasmon coupled emission (SPCE) and waveguide mode coupled emission (WMCE). EWCE is a directional emission in substrate of photons radiated from molecular dipoles located within the penetration depth of evanescent field, which is based on the near-field interaction with the photons. The excitation source used for EWCE can be either a free-space laser beam or an evanescent wave generated under the condition of propagating surface plasmon resonance (SPR) or waveguide mode resonance (WMR). As long as the core layer satisfies the PSPR or WMR condition, EWCE will inevitably take place for light radiated from the surface of core (the core is a metal film for PSPR and an dielectric layer for WMR). The light radiation from the surface of core can be Raman or fluorescence or elastic scattering. However, the EWCE-based directional emission can be observed only in the presence of the RK configuration. In the absence of RK configuration, the light launched in the substrate by EWCE cannot become directional radiation in free space. This part of light power will be dissipated by SPR absorption or surface scattering, consequently leading to severe impairing of the backscattered Raman light available for collection in air. This means that SPCE has the negative influence on the backscattered Raman detection methods, including conventional SERS and tip enhanced Raman spectroscopy (TERS). EWCE can be significantly weakened by roughing the surface of core because a rough surface enables to weak the evanescent field. According to the Fresnel theory, larger the surface roughness, smaller the field enhancement factor. Based on the optical reciprocity theorem, a small field enhancement factor can effectively suppress the coupling of Raman photons into the PSPR mode, consequently increasing the backscattered Raman photons. As a result, a rough core layer is conducive to enhancing the backscattered Raman signal. This is a new mechanism for SERS enhancement, different from the well-recognized electromagnetic enhancement and chemical enhancement.
© 2016 Optical Society of AmericaPDF Article