Diverging Surface Plasmon Polaritons for Single Upconverting Nanoparticle Imaging

Abstract Upconverting nanoparticles (UCNPs) offer great potential for single‐molecule labeled biosensing because of their background‐free detection and high photostability. However, the weak intensity emitted by one individual UCNP, inherent to its low intrinsic upconversion efficiency, represents a challenge to tackle when imaging single UCNPs. The motivation for this work is to study how diverging surface plasmon polaritons (SPPs) can be used in the imaging of individual UCNPs. The analytical solution to describe the SPP coupling and the diverging mode propagation is derived. Numerical simulations are conducted, and their results are in excellent agreement with the analytical solution. The phenomenon is also studied experimentally by using diverging SPPs to excite UCNPs immobilized on a gold surface and imaging their emissions. The results show that the detection of single UCNPs over a large area is possible. This work offers a novel approach toward single‐molecule digital detection in biosensor applications.