Interplay of oxygen vacancies and lanthanide emitters enables reversible upconversion switching

Dynamic modulation of upconversion with spatiotemporal spectral features is essential for advanced optical multiplexing in anti-counterfeiting applications. However, achieving reversible dynamic control of upconversion emissions in a simple nanostructure of fixed composition under external stimulation remains challenging. Here we show that orthogonal upconversion luminescence in erbium-doped bismuth oxyhalides can be temporally modulated by exploiting the interplay of oxygen vacancies and erbium activators. Alternating ultraviolet irradiation and water bleaching enables reversible regulation of the upconversion emissions with high switching contrast and excellent fatigue resistance. Experimental and theoretical evidences reveal that the introduction of oxygen vacancies and lanthanide activators as well as their interactions contributes to the photochromism-induced dynamic modulation of orthogonal upconversion luminescence. Inspired by their special luminescent performance, the developed materials facilitate evolutionary upconversion color and intensity displays in both spatial and temporal dimensions, offering a promising route for high-level anti-counterfeiting and high-capacity optical storage.