Interaction of One-Dimensional Photonic Crystals and Metal Nanoparticle Arrays and Its Application for Surface-Enhanced Raman Spectroscopy

We introduce a new concept to localize and strongly enhance electromagnetic fields by covering one-dimensional photonic crystals with ordered metal nanoparticles arrays. When designed properly, the combined photonic–plasmonic composite shows a significant interaction of the plasmonic resonance and the photonic band gap. For this purpose we fabricated one-dimensional photonic crystals based on porous silicon by electrochemical etching of silicon in hydrofluoric acid and deposited a silver nanoparticle array on top by nanosphere lithography. The composite structure was designed in such a way that the plasmonic resonance coincides with the photonic band gap, leading to highly confined electromagnetic fields at the interface between both structures. The samples were characterized using spectroscopic ellipsometry and reflectance measurements and were modeled using effective medium theories and finite-element methods. Surface-enhanced Raman spectroscopy measurements of this unique photonic–plasmonic hybrid system show extraordinary enhancement factors that can be explained only by an interaction mechanism. The optical properties of the composite structure are very versatile, providing a promising platform for improved sensing applications and superior substrates for surface-enhanced Raman spectroscopy.