Probing Fabry–Perot Interference in Self-Assembled Excitonic Microcrystals with Subgap Light Emission

Abstract

Establishing a fundamental understanding of the range of optical effects possible in self-assembled excitonic materials remains crucial to the utility of these materials in future optoelectronic and photonic technologies. In this study, we report the observation and subsequent analysis of Fabry–Perot interference intrinsic to two types of self-assembled excitonic crystalline materials: the hybrid organic–inorganic perovskite-like quantum well superlattice structure of hexyl ammonium lead iodide and the charge-transfer cocrystal of anthracene and pyromellitic dianhydride. The observation of Fabry–Perot interference stems from strong suboptical gap photoluminescence (PL) from both materials in a spectral region of very low material absorption. We characterize this subgap PL in each material to propose permanent defect trap excitons, stabilize their energy, and cause subgap light emission in both materials. We use a model developed to explain interference in the photoluminescence of vacuum-deposited thin films to estimate the thickness, mid-bandgap index of refraction, and surface roughness of our samples. These results indicate that self-assembled excitonic materials may have use in applications such as laser amplifiers, frequency discriminators, and single photon emitters.