Expanding the toolbox for lanthanide-doped upconversion nanocrystals

The ability to convert low-energy quanta into a quantum of higher energy is critical for a variety of applications, including photovoltaics, volumetric display, bioimaging, multiplexing sensing, super-resolution imaging, optogenetics, and potentially many others. Although the processes of second harmonic generation and multiphoton (or two-photon) absorption can be used to generate photon upconversion, lanthanide-doped upconversion nanocrystals have emerged as an attractive alternative for nonlinear upconversion of near-infrared light with pump intensities several orders of magnitude lower than required by conventional nonlinear crystals. Over the past five years, considerable efforts have been made to tune the photoluminescence of upconversion nanocrystals, and significant progress has been achieved. In this review, we focus on manipulation of the wavelength, emission intensity and lifetime of upconversion nanocrystals. Here, we outline the fundamental principle for the upconversion phenomenon, review the current experimental state-of-the-art for controlling photon upconversion in lanthanide-doped nanocrystals and highlight the prospects for multifunctional upconversion nanocrystals currently in development.