Metallo-dielectric photonic crystals for reproducible surface-enhanced Raman substrates

Abstract

Planar photonic crystals comprised of metals and dielectrics show huge enhancements in the surface-enhanced Raman scattering of attached molecules. Plasmon engineering is key to these properties including reproducibility (std.dev.<9%), beamed output, resonances and orientation. Surface Enhanced Raman Scattering (SERS) is a widely utilised technique to enhance the weak Raman spectrum of molecules, enabling detection and identification of ultralow concentrations of particular species. Discovered exactly 30 years ago,[1] it employs a roughened metal surface in the vicinity of a molecule to act as an antenna that couples laser light in and SERS photons out. However the major block to its application has been the extreme variability of the SERS signals from the substrates used to elicit this effect. Despite many experiments, there is still considerable debate over how the SERS enhancement arises on a metal, and no first-principle calculations can model the observed effect. We show both experimentally and theoretically how localised surface plasmons engineered on 2D photonic crystals (PCs) fabricated from metals and dielectrics can produce SERS enhancements >10 4 . Our PC SERS substrates [2] exhibit extremely good reproducibility and will enable a new generation of applications based on quantification of the adsorbed molecular concentration, and low-cost widely-applicable SERS devices.Our samples are based on 2D photonic crystal waveguide technologies, in which we have previously demonstrated photonic bandgaps [3], slow light [4], superprism operation [5], and tri-refringence [6]. High refractive index dielectric waveguides are sandwiched between silica buffer and cladding (all grown on silicon wafers), and e-beam lithography is used to pattern a variety of photonic crystal designs. Data is presented here on structures with square lattices of holes, of pitch varying from 500nm-6000nm, and hole diameters from 300-4000nm. The aspect ratio of the holes is varied from depth:width of 1:1 to >10:1. The dielectric samples are then gold-or silver-coated by rfsputtering in an optimised process to produce the SERS substrates (Fig.1).