Enhancement of upconversion luminescence using photonic nanostructures

Lanthanide-based upconversion materials convert low energy infrared photons into high energy visible photons. These materials are of interest in a myriad of applications such as solar energy harvesting, color displays and photocatalysis. Upconversion nanoparticles (UCNPs) are also of interest in biological applications as bioimaging and therapeutic agents. However, the intrinsic conversion efficiency of UCNPs remains low for most applications. In this review, we survey the recent work done in increasing the upconversion emission by changing the local electric field experienced by the UCNPs using photonic nanostructures. We review both the underlying theory behind this photonic manipulation as well as experimental demonstrations of enhancement. We discuss the recent developments in the more common plasmonic designs as well as the emerging field of dielectric nanostructures. We find that improvements in design and fabrication of these nanostructures in the last few years have led to reported enhancements of over three orders of magnitude. This large enhancement has been achieved in not only nanostructures on films but also in nanostructures that can be dispersed into solution which is especially relevant for biological applications.