Geometry-optimised plasmon-enhanced diamond NV− scanning probes for high-efficiency quantum sensing

Nitrogen-vacancy (NV−) centers in diamond are a leading platform for room-temperature quantum sensing with nanoscale spatial resolution. In scanning-probe implementations, however, overall performance remains limited by the efficiency of optical excitation and fluorescence collection, which directly constrain readout signal-to-noise ratio, spin contrast, and magnetic-field sensitivity. Here, we present a comparative numerical study of plasmon-enhanced diamond NV− scanning probes in which the optical response is engineered through the tip geometry and local metallic functionalization. We analyze paraboloid, truncated-paraboloid, single-cone, double-cone, and trench-cone tip geometries, considering both bare and Au-coated structures. Using full-vector electromagnetic simulations, we compare the fraction coupled into the collection objective, Tobj, alongside the normalized total radiative decay rate Ttot, the waveguide-coupled transfer Twg, the effective β-factor, and the collection efficiency. Tobj is used as the main practical comparison metric, while the other quantities clarify the underlying optical behavior. Among the investigated designs, the truncated-paraboloid probe offers the best bare-tip balance of local enhancement and useful photon extraction, while thin Au coatings further improve both the effective β-factor and the collection efficiency. In the explored design space, the 50 nm Au-coated truncated-paraboloid probe gives the highest objective-coupled signal, Tobj = 0.92, together with Ttot = 1.17, β = 94%, and a collection efficiency of 79%, establishing a practical route toward improved optical readout and enhanced quantum sensing with scanning NV− probes.