Amplified Spontaneous Emission and Temperature Sensor Based on TiO2-μS@Cs2AgBiBr6 Heterostructure Whispering Gallery Mode Resonators

Abstract We present highly stable and optically active microresonators fabricated by thermally evaporating Cs2AgBiBr6 double perovskite films onto TiO2 microspheres (TiO2-μS). This TiO2-μS@Cs2AgBiBr6 core–shell heterostructure exhibits efficient whispering gallery mode (WGM) resonance. The Cs2AgBiBr6 coating is uniform, phase-pure, and exceptionally stable under ambient conditions. Strong coupling between the self-trapped excitonic emission (STE) from Cs2AgBiBr6 and the optical modes of TiO2-μS produces strikingly sharp emission peaks atop the broad STE background, establishing these structures as powerful platforms for light manipulation. Amplified spontaneous emission is observed, with a 2× enhancement in emission intensity at an excitation fluence of 4 ± 0.5 kW cm–2. Temperature-dependent WGM studies further reveal sensitive optical thermometry, with a temperature sensitivity of 32.3 pm K–1 from a ∼5 μm microsphere, alongside consistent repeatability across multiple thermal cycles between 98 and 148 K. Together, these results establish TiO2-μS@Cs2AgBiBr6 core–shell heterostructures as a tunable and photostable gain medium and temperature sensor based on lead-free perovskite materials.