NIR-to-NIR Two-Photon Excited CaF<sub>2</sub>:Tm<sup>3+</sup>,Yb<sup>3+</sup> Nanoparticles: Multifunctional Nanoprobes for Highly Penetrating Fluorescence Bio-ImagingNingning Dong, Adolfo Speghini, Francisco Sanz-Rodrı́guez et al.|ACS Nano|2011Cited by 421