Design of isolated lithographic SERS structures with enhanced sensitivity

Surface-enhanced Raman scattering (SERS) on lithographically fabricated structures has been extensively explored. The consensus is that non-resonant single-molecule sensitivity (SMSERS) is not achievable with isolated structures. Difficult to manufacture closely spaced pairs of structures, akin to the closely approaching (< 1 nm separation) spheres identified in colloidal clusters, are thought to be required. This scale is beyond the capability of current lithographic approaches. FDTD modeling is used to explore improvements in the electric field enhancement for isolated lithographic structures. Electric field enhancement factors (β=E_loc/E_in where E_loc is the maximum local field at the edge of the metal and E_in is the incident laser field) from 40 for conventional structures up to 820 are obtained with simple, scalable, and easily manufacturable isolated structures, with overall SERS enhancements ∝β⁴ up to 5x10¹¹. Improvements include: replacing a lossy metal adhesion layer (AL) with a lossless SiO₂ AL; planarizing the AL with a SiO₂ deposition after the lift-off process; using high curvature elliptical structures; thin-film engineering of the substrate to provide constructive interference at the enhancement structure; and metal-insulator-metal structures to introduce a magnetic dipole resonance that allows increased tuning flexibility and broadens the wavelength response. Together, these improvements open a pathway to non-resonant single-molecule SERS on robust, manufacturable, substrates with isolated nanostructures.