A planar optical platform for generating a Raman signal from a foreign object comprises an input region and an output region, for receiving and extracting optical radiation, optically coupled to a plasmonic band structure region. The plasmonic band structure region comprises a layer of a first material, having a first refractive index, patterned with an array of sub-regions of a second material, having a second refractive index, wherein a side-wall of each sub-region is coated with a metallodielectric layer. The array of sub-regions gives rise to a plasmonic band structure and, in use, each sub-region confines a plasmon resonance excited by optical radiation coupled into the plasmonic band structure region, which gives rise to a Raman signal from a foreign object placed proximate the plasmonic band structure region. The platform may be incorporated into a spectroscopic measurement system and is particularly useful for surface-enhanced Raman spectroscopy of analyte molecules.