Analysis of plasmon resonance and surface-enhanced Raman scattering on periodic silver structures

Surface plasmon excitation and surface-enhanced Raman scattering (SERS) are investigated for periodic grating-type substrates such as binary silver gratings and silver gratings on silica. Electromagnetic near fields are calculated by an efficient implementation of a Rayleigh-expansion technique for rectangular-groove gratings. Far-field signals of Raman-active molecules adsorbed at the grating surface are determined by application of the Lorentz reciprocity theorem. SERS enhancement factors are considered for different types of gratings and for structures with different dimensions with respect to both the intensity and angular width of the emitted Stokes light. Thus, consideration of plasmon resonance widths leads to optimum structures for periodic SERS substrates if realistic experimental configurations involving a lens for detection are taken into account. For binary silver gratings, optimum grating depths of more than 80 nm are proposed for SERS measurements in a realistic experimental configuration, whereas maximum SERS signals are emitted into a single direction at shallow gratings with depths between 10 nm and 20 nm. Furthermore, silica gratings with isolated silver layers are superior to binary silver gratings. Due to both the large intensity and angular width of the emitted signals, SERS enhancement factors are additionally increased on such structures.