Apparatus and method for determining the identity of a sequence of 
chemical groups in a 
linear polymer sample, such as the sequence of bases in a 
nucleic acid are disclosed. The apparatus has a substrate having a mirror surface which is formed of a 
plasmon resonant 
metal, a source of a beam of light, and a lens 
assembly composed of one or more 
plasmon resonance particles disposed about an opening defining a detection region. The particles arranged to produce, when the 
light beam is directed onto a sample at the detection region, near-field electromagnetic gap 
modes in a space between the nanolens and a confronting detection region on the 
substrate surface, in a gapbetween the nanolens and substrate having a selected spacing of 40 nm or less. Also included in the apparatus are a 
detector for receiving light emitted by or scattered from the sample at the detection region, and for converting the received light into a gap-mode enhanced Raman spectrum, and a translation mechanism for moving the sample relative to lens 
assembly, through the opening in the lens 
assembly, to position successive 
chemical groups in the sample at the detection region. Apparatus and method for determining the identity of a sequence of 
chemical groups in a 
linear polymer are disclosed. The apparatus has a substrate having a mirror surface which is formed of a 
plasmon resonant 
metal, a source of a beam of light, and a lens assembly composed of one or more plasmon 
resonance particles disposed about an opening defining a detection region. The particles arranged to produce, when the 
light beam is directed onto a sample at the detection region, near-field electromagnetic gap 
modes in a space between the nanolens and a confronting detection region on the 
substrate surface. Also, included are a 
detector for receiving light emitted by or scattered from the sample at the detection region, and for converting the received light into a gap-mode enhanced Raman spectrum.