title: Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy.
abstract: Lanthanide-doped glasses and crystals are attractive for laser applications because the metastable energy levels of the trivalent lanthanide ions facilitate the establishment of population inversion and amplified stimulated emission at relatively low pump power. At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be made with precisely controlled phase, dimension and doping level. When excited in the near-infrared, these UCNPs emit stable, bright visible luminescence at a variety of selectable wavelengths, with single-nanoparticle sensitivity, which makes them suitable for advanced luminescence microscopy applications. Here we show that UCNPs doped with high concentrations of thulium ions (Tm3+), excited at a wavelength of 980 nanometres, can readily establish a population inversion on their intermediate metastable3H4level: the reduced inter-emitter distance at high Tm3+doping concentration leads to intense cross-relaxation, inducing a photon-avalanche-like effect that rapidly populates the metastable3H4level, resulting in population inversion relative to the3H6ground level within a single nanoparticle. As a result, illumination by a laser at 808 nanometres, matching the upconversion band of the3H4→3H6transition, can trigger amplified stimulated emission to discharge the3H4intermediate level, so that the upconversion pathway to generate blue luminescence can be optically inhibited. We harness these properties to realize low-power super-resolution stimulated emission depletion (STED) microscopy and achieve nanometre-scale optical resolution (nanoscopy), imaging single UCNPs; the resolution is 28 nanometres, that is, 1/36th of the wavelength. These engineered nanocrystals offer saturation intensity two orders of magnitude lower than those of fluorescent probes currently employed in stimulated emission depletion microscopy, suggesting a new way of alleviating the square-root law that typically limits the resolution that can be practically achieved by such techniques.
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China.|School of Life Sciences and Biotechnology, Shanghai Jiao Tong University, Shanghai 200241, China.
authors.forename: Yujia
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Yiqing
authors.initials: Y
authors.lastname: Lu
authors.affiliation: Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China.
authors.forename: Xusan
authors.initials: X
authors.lastname: Yang
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Xianlin
authors.initials: X
authors.lastname: Zheng
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
authors.forename: Shihui
authors.initials: S
authors.lastname: Wen
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.|Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
authors.forename: Fan
authors.initials: F
authors.lastname: Wang
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Xavier
authors.initials: X
authors.lastname: Vidal
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Jiangbo
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Deming
authors.initials: D
authors.lastname: Liu
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
authors.forename: Zhiguang
authors.initials: Z
authors.lastname: Zhou
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Chenshuo
authors.initials: C
authors.lastname: Ma
authors.affiliation: Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
authors.forename: Jiajia
authors.initials: J
authors.lastname: Zhou
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: James A
authors.initials: JA
authors.lastname: Piper
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China.
authors.forename: Peng
authors.initials: P
authors.lastname: Xi
authors.affiliation: Advanced Cytometry Laboratories, ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), Macquarie University, Sydney, New South Wales 2109, Australia.|Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.|Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
authors.forename: Dayong
authors.initials: D
authors.lastname: Jin