Guided-Mode Resonance-Assisted Photon Avalanche Emission from Tm³⁺-Doped NaYF₄ Upconverting Nanoparticles: Implications for Biosensing Applications

Photon avalanche (PA) upconverting nanoparticles (UCNPs) have attracted great interest because they can exhibit giant changes in upconversion luminescence (UCL) intensity through small perturbations of excitation light. The discovery of PA UCNPs opens up their applications in super-resolution imaging, microlasers and optical and environmental sensing. These applications rely heavily on low PA threshold and high nonlinear order PA UCNPs. So far, the PA threshold of PA UCNPs can only be reduced through material modification. In this work, a strategy is proposed to reduce the required excitation intensity of the excitation light source to generate PA UCL from PA UCNPs through the guided-mode resonance (GMR) effect of an optimal resonant waveguide grating (RWG) structure. Before using the RWG structure, Tm3+-doped NaYF4 (NaYF4:Tm3+) core PA UCNPs deposited on a glass substrate required an excitation intensity of ∼7.1 kW/cm2 from the excitation light source to produce PA UCL and with a nonlinear order (n) ∼ 25.9. In comparison, when core PA UCNPs were coated on the surface of the water-covered RWG structure and excited under the GMR condition, only an excitation intensity 2.72 kW/cm2 from the excitation light source was needed to induce PA UCL with nonlinear order of n ∼ 39.5. This is because a strongly enhanced local electric field is formed on the surface of the RWG structure, which enhances the interaction between the excitation light and core PA UCNPs, thereby significantly reducing the excitation intensity of the excitation light source required to generate PA UCL from core PA UCNPs. The RWG structure can powerfully enhance the PA UCL performance of PA UCNPs. The combination of the RWG structure and PA UCNPs can be developed into ultrasensitive sandwich-type immunosensors for biosensing applications.