Axel Hochstetter, Elke Neu-Ruffing

• Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields. Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk. When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries