Enhancement of photoluminescence from defect states in ZnS random photonic crystal: An effect of electronic and photonic mode coupling

Abstract

This paper reports on the enhanced defect state emission from ZnS in the form of a random photonic crystal (RPC) medium. ZnS photonic crystals with varied randomness are fabricated by colloidal self assembly of ZnS nanospheres (215 ± 10 nm). Reflection and transmission studies reveal mid band gap wavelength at ∼435 nm. The band structure calculated for BCC lattice with reduced packing fraction (53%) is in good agreement with experimental results. The reflection due to the photonic band gap diminishes with increased randomness in the nanosphere arrangement. The features of fluorescence from ZnS are modified in the RPC medium, resulting in suppression at wavelengths in the photonic band gap region and an enhancement at band edge wavelengths of 415 and 468 nm. This enhancement becomes less prominent with increasing randomness in the structure. Interestingly these two modes correspond to the electronic defect states of ZnS. Emission enhancement is shown to be due to the strong coupling of electronic defect states and photonic band edge states which is facilitated by randomly scattering slow Bloch modes in the ZnS RPC. Fabrication of RPCs by colloidal self-assembly with specifically designed degrees of randomness (leading to controllable features of emission) provides scope for the design of low threshold random lasing systems.