Optimizing the near-field and far-field properties of tips in tip-enhanced Raman scattering

The process of tip-enhanced Raman scattering (TERS) depends critically on the morphology near the apex of the tip used in the experiment. Many tip designs have focused on optimization of electromagnetic enhancement in the near-field, which is controlled to a large extent by subtle details at the nanoscale that remain difficult to reproduce in the tip fabrication process. The use of focused ion beams (FIB) permit modification of larger features on the tip in a reproducible manner, yet this approach cannot produce sub-20-nm structures important for optimum near-field enhancement. Nonetheless, FIB milling offers excellent opportunities for improving the far-field radiation properties of the tip-antenna, a feature that has received relatively little attention in the TERS research community. In this work, we use finite-difference time-domain (FDTD) simulations to study both the near-field and far-field radiation efficiency of several tip-antenna systems that can be constructed with FIB techniques in a feasible manner. Starting from blunt etched tips, we find that excellent overall enhancement of the TERS signal can be obtained with pillar-type tips. Furthermore, by applying vertical grooves on the tip's shaft, the overall efficiency can be improved even more, producing TERS signals that are up to 10-fold stronger than signals obtained from an ideal (unmodified) sharp tip of 10-nm radius. The proposed designs constitute a feasible route toward a tip fabrication process that not only yields more reproducible tips but also promises much stronger TERS signals.