Coupling discrete metal nanoparticles to photonic crystal surface resonant modes and application to Raman spectroscopy

Coupling a tightly packed layer of discrete metal nanoparticles to the resonant mode of a photonic crystal surface has been demonstrated as a means for obtaining additional electromagnetic gain for surface-enhanced Raman spectroscopy (SERS), in which electric fields of the photonic crystal can couple to plasmon resonances of the metal nanoparticles. Because metal nanoparticles introduce absorption that quench the photonic crystal resonance, a balance must be achieved between locating the metal nanoparticles too close to the surface while still positioning them within the enhanced evanescent field to maximize coupling to surface plasmons. In this work, we describe a parametric study into the design of a photonic crystal-SERS substrate, comprised of a replica molded photonic crystal slab as the dielectric optical resonator, a SiO₂ "post" layer spacer, and an Ag "cap" metal nanostructure. Using the Raman signal for trans-1,2bis(4pyridyl)ethane, the coupling efficiency was maximized for a SiO₂ "post" layer thickness of 50 nm and a Ag "cap" height of ∼20 nm, providing an additional enhancement factor of 21.4.