Figure 4.

(a) Sketch of the position of the UCNP within the optical trap. Δx sets the horizontal position of the laser focus, referenced to the position on top of the Au PNP. (b) Dependence of the Purcell factor at 580 nm on the vertical direction. Different colors correspond to the vertical scans performed at different positions (different Δx distances) with respect to the Au PNP. (c) Experimentally determined, laser-power-dependent force for optical trapping of a single UCNP on a substrate with PNPs. Solid lines plot linear fits to the experimental data. (d) Brownian motion trajectory of a single optically trapped UCNP within the transverse section of the laser focus (indicated by the black dashed circle). (e) Dependence of the in-plane spatially averaged Purcell effect at 580 nm along the vertical direction. (f) Upconversion luminescence decay curves recorded for an emission wavelength of 650 nm for UCNPs deposited on a glass substrate (gray) and on a glass substrate containing PNPs (orange) under 980 nm excitation. All of the data included were obtained with the PNPs with plasmon resonance at 574 nm.