Environmentally Sensitive Photon Avalanche Upconversion Enabled by Coupling Photon Avalanche Upconversion Nanoparticles to a Resonant Waveguide Grating for Refractive-Index Sensing Applications

Photon avalanche upconversion nanoparticles exhibit extremely high nonlinear responses and threshold-dependent upconversion luminescence, while cyclic olefin copolymer resonant waveguide grating structures transduce small ambient refractive index changes into guided-mode resonance wavelength shifts. By coupling these two elements, we developed a hybrid nanophotonic platform with exceptional environmental sensitivity. Tm 3+ -doped NaYF 4 photon avalanche upconversion nanoparticles were deposited onto a water-covered cyclic olefin copolymer resonant waveguide grating designed to support a guided-mode resonance at 1064 nm, matching the excitation laser wavelength and generating strong local excitation fields that enhance the photon avalanche process and yield intense upconversion luminescence. Small variations in the environmental refractive index shifted the guided-mode resonance wavelength away from the excitation wavelength, markedly reducing the local excitation power density and dramatically altering the photon avalanche upconversion luminescence characteristics. At an excitation power density of 1.18 kW/cm 2, increasing the environmental refractive index ( n ) from 1.33250 to 1.37233 caused an exponential decrease in upconversion luminescence intensity, yielding a remarkably high local relative sensitivity of 9477%/RIU at n = 1.33250. In contrast, under the same environmental refractive index variation range but at a higher excitation power density of 7.5 kW/cm 2, photon avalanche upconversion nanoparticles on a water-covered glass substrate showed only minor luminescent intensity changes, corresponding to a substantially lower average relative sensitivity of 737 %/RIU. These findings demonstrate that coupling photon avalanche upconversion nanoparticles to a resonant waveguide grating structure enables ultrahigh ambient refractive-index sensitivity and highlights the strong potential of this hybrid nanophotonic platform for next-generation ultrasensitive biosensing applications.