Highly sensitive Surface Enhanced Raman Spectroscopy (SERS) sensors are described in the form of a optical resonator and a metal nanostructure deposited on surface of the optical resonator. In one embodiment the optical resonator is in the form of a photonic crystal, but other optical resonators are contemplated. Examples are described in which the resonant near-fields of a large-area replica molded photonic crystal efficiently couples light from a laser to dielectric-metal “post-cap” nanostructures deposited on the photonic crystal surface by a glancing angle evaporation technique, achieving a high SERS enhancement factor. Other constructions are also contemplated a metal nanostructure formed on a dielectric support deposited on the photonic crystal, including a metallic film deposited over close-packed surface of nanospheres, arrays of metallic nanotriangles, metallic nanorods, metallic nanohelices, arrays of metallic nanospheres, and roughened metal surfaces.