Apparatus and method for examining the identity of chemical groups in a sample are disclosed. The apparatus has a substrate having a plasmon resonant surface on which the sample is supported, a source of a beam of light, and a lens assembly having a tip region and a nanolens composed of one or more plasmon resonance particles (PRPs) on the tip region. The PRPs are arranged to produce near-field electromagnetic gap modes in a space between the nanolens and a confronting detection region on the substrate surface when the gap between the nanolens and substrate is 30 nm or less. A focusing mechanism in the apparatus operates to move the lens assembly toward and away from the substrate surface, with a gap of less than 30 nm, to produce electromagnetic gap modes that enhance the Raman spectroscopy signals produced by the sample in the detection region.