Influence of Starch-Granule Diameter on Lasing Thresholds of the Molecular Rotor Thioflavin T in a Dual-Cavity Fabry–Perot/Whispering Gallery Mode

A dual-cavity lasing platform is reported in which thioflavin T (ThT), a rotor-sensitive molecular probe, is employed to map molecular-crowding effects within starch granules via coupled Fabry-Perot (FP) and whispering gallery mode (WGM) resonances. In this architecture, global standing-wave feedback is furnished by a planar FP cavity, while size-tunable WGMs are supported by ThT-coated starch granules. Granules were sorted into five diameter classes (<20, 20-30, 30-40, 40-60, and >60 μm), and lasing thresholds alongside fluorescence lifetimes were determined. Lasing was observed only when a WGM eigenfrequency overlapped spectrally with an FP longitudinal mode, rendering the threshold exquisitely sensitive to granule size, local viscosity, and chromophore packing. The lowest lasing threshold (approximately 11 μJ) was exhibited by the 20-30 μm granules, a "sweet-spot" attributed to optimal optical confinement and maximal steric hindrance of ThT rotation. Increased radiative losses were encountered in smaller granules, whereas larger particles (>60 μm) afforded excess rotational freedom in ThT that inhibited stimulated emission. Time-resolved fluorescence measurements corroborated these trends, with lifetimes reaching τ_avg = 0.57 ns in the optimal size regime. Granule diameter is thus established as a precise knob for tuning lasing behavior in crowded systems, with implications for the development of starch-WGM-based photonic sensing in biomolecular processes.