Guided-Mode Resonance-Assisted Photon Avalanche Emission\nfrom Tm<sup>3+</sup>-Doped NaYF<sub>4</sub> Upconverting Nanoparticles:\nImplications for Biosensing Applications

Photon\navalanche (PA) upconverting nanoparticles (UCNPs) have attracted\ngreat interest because they can exhibit giant changes in upconversion\nluminescence (UCL) intensity through small perturbations of excitation\nlight. The discovery of PA UCNPs opens up their applications in super-resolution\nimaging, microlasers and optical and environmental sensing. These\napplications rely heavily on low PA threshold and high nonlinear order\nPA UCNPs. So far, the PA threshold of PA UCNPs can only be reduced\nthrough material modification. In this work, a strategy is proposed\nto reduce the required excitation intensity of the excitation light\nsource to generate PA UCL from PA UCNPs through the guided-mode resonance\n(GMR) effect of an optimal resonant waveguide grating (RWG) structure.\nBefore using the RWG structure, Tm<sup>3+</sup>-doped NaYF<sub>4</sub> (NaYF<sub>4</sub>:Tm<sup>3+</sup>) core PA UCNPs deposited on a\nglass substrate required an excitation intensity of ∼7.1 kW/cm<sup>2</sup> from the excitation light source to produce PA UCL and with\na no<i>n</i>linear order (<i>n</i>) ∼ 25.9.\nIn comparison, when core PA UCNPs were coated on the surface of the\nwater-covered RWG structure and excited under the GMR condition, only\nan excitation intensity 2.72 kW/cm<sup>2</sup> from the excitation\nlight source was needed to induce PA UCL with nonli<i>n</i>ear order of <i>n</i> ∼ 39.5. This is because a\nstrongly enhanced local electric field is formed on the surface of\nthe RWG structure, which enhances the interaction between the excitation\nlight and core PA UCNPs, thereby significantly reducing the excitation\nintensity of the excitation light source required to generate PA UCL\nfrom core PA UCNPs. The RWG structure can powerfully enhance the PA\nUCL performance of PA UCNPs. The combination of the RWG structure\nand PA UCNPs can be developed into ultrasensitive sandwich-type immunosensors\nfor biosensing applications.