Parallel photon avalanche nanoparticles for tunable emission and multicolour sub-diffraction microscopy

Abstract

Photon avalanche (PA) can generate upconversion luminescent emission that grows steeply as a function of excitation power, effectively exhibiting a high order of nonlinearity (N) that is attractive for applications ranging from photophysics studies to biophotonics. Besides the limitations in available material systems, PA is typically sustained by a single reservoir level, limiting the ability to modulate the chromaticity of the emission as well as leading to small values of N and large excitation thresholds. Here we report a parallel PA mechanism in holmium (Ho³⁺)-doped nanoparticles for tunable emission at room temperature. The intermediate ⁵I₇ and ⁵I₆ levels of Ho³⁺ serve as dual reservoir levels that create two parallel energy loops. This activates multiple emissive levels and enables red, green and blue PA emission under 965 nm continuous-wave excitation. By rationally engineering transition kinetics through controlling doping concentration and core/shell configuration, we demonstrate multicolour PA with large N values of 17–22 and mild excitation threshold of ~22 kW cm⁻². Moreover, emission can be tailored from almost pure red to intense red, green and blue by modifying the host lattice and introducing additional cross-relaxation pathways by doping with Ce³⁺/Tm³⁺. When using the nanoparticles to label biological cells, we demonstrate multicolour imaging on a single-continuous-wave-beam microscope with lateral spatial resolution of 78 nm and 102 nm in the green–blue and red channel, respectively. These findings open the way for manufacturing nonlinear multicolour fluorophores for versatile optical and biological applications.