Parametric decay of electromagnetic waves into two Langmuir oscillations near the quarter critical density is suggested as being attributable to an enhanced electric field and to surface-enhanced Raman scattering in near-field optics. A nanolayer of silver aggregates localizes the wave in the evanescent region to a one-wavelength span and results in a wave-number mismatch as well as in a reduced growth rate. The fastest-growing mode has a growth rate that scales with the square root of the nanolayer's thickness.