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Continuous-wave upconverting nanoparticle microlasers
Fernandez-Bravo, Angel,Yao, Kaiyuan,Barnard, Edward S.,Borys, Nicholas J.,Levy, Elizabeth S.,Tian, Bining,Tajon, Cheryl A.,Moretti, Luca,Altoe, M. Virginia,Aloni, Shaul,Beketayev, Kenes,Scotognella, F Nature Publishing Group UK 2018 Nature nanotechnology Vol.13 No.7
Reducing the size of lasers to microscale dimensions enables new technologies<SUP>1</SUP> that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors<SUP>2</SUP> to live brain tissue<SUP>3</SUP>. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers<SUP>4–7</SUP> that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs)<SUP>8</SUP> as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm<SUP>3+</SUP>-doped UCNPs<SUP>9</SUP> to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm<SUP>−2</SUP>. Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions<SUP>10</SUP>. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.