Plasmon-enhanced upconversion: engineering enhancement and quenching at nano and macro scales

Photon upconversion (UC) is the sequential absorption of two or more low frequency photons and subsequent emission of light at a higher frequency. Because of a large number of potential applications of this anti-Stokes process, extensive studies of UC have taken place in the last decades. The most crucial challenge in this field is the development of an efficient strategy to enhance the inherently low efficacy of the UC process. Among the various intensively developed approaches, local tailoring of the electromagnetic field with metal nanoparticles (MNPs) to the position of the UC material has been considered as the most promising one. However, distinctive features of photon UC imply the emergence of fluorescence quenching near the MNP. Along with different near-and far-field MNP responses and non-trivial competition of enhancement and quenching of the UC signal in suspension of MNPs on the macroscale, the search of optimal MNP configuration for UC enhancement becomes quite the challenging task to solve. In this paper, we thoroughly analyze these effects with a rigorous and comprehensive theoretical model based on the extended Mie theory for multilayered spheres and the effective medium approach. We provide general guidelines for highly efficient UC enhancement by Ag and Au spherical MNPs.