Super-resolution upconversion microscopy of praseodymium-doped yttrium aluminum garnet nanoparticles

We demonstrate background-free subdiffraction optical microscopy with upconverting YAG nanoparticles doped with trivalent praseodymium ions ($Pr:\text{YAG}$). The presented microscopy with $Pr:\text{YAG}$ nanoparticles takes advantage of three facts. First, while excited with visible laser light, $Pr:\text{YAG}$ nanoparticles can emit upconverted ultraviolet (UV) radiation, thus allowing for background-free microscopy. Second, the technique based on exploiting donut-shaped laser beam was introduced to obtain subdiffraction-limited optical resolution. All optical resolution of 50 nm limited by the size of the particles was achieved. Third, $Pr:\text{YAG}$ nanoparticles are absolutely photostable. This technique resembles stimulated emission depletion microscopy (STED) though it is significantly different from STED since it involves stimulated absorption rather than stimulated emission.