Measurement of plasmon-mediated two-photon luminescence action cross sections of single gold bipyramids, dumbbells, and hemispherically capped cylindrical nanorods

Recent advances in two-photon excited photoluminescence of plasmonic gold nanorods have greatly expanded their application. Shape-controlling of nanorods can enhance a particular photophysical process to tailor needs, but its application to two-photon luminescence is yet to be fully developed despite its importance in biolabeling. Here, we report direct, independent measurements of two-photon action cross sections (TPACS) of single gold bipyramids, hemispherically capped cylindrical nanorods, and dumbbells. The effect of radius of curvature at the tip on TPACS are measured and compared per same aspect ratio and volume. Bipyramids have shown 33% more TPACS per volume for the aspect ratio range 3.8-4.2, with 0.28 GM/nm3 compared to 0.21 and 0.20 GM/nm3 for nanorods and dumbbells, respectively, at 27% measurement error. This value is not as high as the field strength at the tip, which can be 3-4 times higher for sharp bipyramids. Such moderate increase is attributed to the small surface area of the tip that the field is subjected to which competes with the increase in field. We have used the z-scan technique to measure nonlinear absorption on a single crystalline gold nanosheet as a bulk gold and an analytical field enhancement theory for prolate spheroids to successfully confirm the trend of TPACS/volume with respect to the aspect ratio. It was found that the prolate spheroids do provide a good approximation of TPACS values for these rods. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement and subsequent two-photon luminescence will need to consider the tip surface area that limits the overall efficiency of the process