Fabry–Perot\nResonances in Rb<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> Microcrystals\nfor Optical Sensing Applications

Rhombus-shaped thin Rb<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> microcrystals\nare investigated for their photoluminescence (PL) spectral properties.\nThe spectra show broad self-trapped excitonic emissions with superimposed\nperiodic spectral modulations. Through numerical analysis and an appropriate\ninterference function model, we demonstrate that these spectral modulations\narise from Fabry–Perot (FP) interferences, which are rarely\nobserved in halide perovskites. The microcrystal with thin, large\nflat facets acts as an FP resonating cavity with two parallel mirror\nsurfaces that capture and resonate the incident light throughout the\ncrystal. The FP resonances have been confirmed through 3D full-field\nfinite difference time domain simulations, which reveal variations\nin electric field eigenmodes with different crystal thicknesses and\nincident wavelengths. These self-resonating highly sensitive FP microcrystals\nhave potential applications in various lead-free optoelectronic devices,\nincluding optical sensors, and as absorption enhancers in perovskite\nsolar cells.