  Surface-enhanced Raman spectroscopy is a powerful and versatile sensing
method with a detection limit down to the single molecule level. In this
article, we demonstrate how topology optimization (TopOpt) can be used for
designing surface enhanced Raman scattering (SERS) substrates adhering to
realistic fabrication constraints. As an example, we experimentally
demonstrated a SERS enhancement factor of 5*10e4 for the 604 cm-1 Raman line of
rhodamine 6G using metal nanostructures with a critical dimension of 20 nm. We
then show that, by relaxing the fabrication constraints, TopOpt may be used to
design SERS substrates with orders of magnitude larger enhancement factor. The
results validate topology optimization as an effective method for engineering
nanostructures with optimal performance and fabrication tolerance.
