Effect of High-Index Nanoparticles on the Emission Properties of a Dye-Doped Blue Phase Liquid Crystal

Abstract

This study demonstrates a significant advancement in enhancing photoluminescence (PL) intensity by the inclusion of high-index dielectric nanoparticles into a dye-doped blue phase liquid crystal (BPLC). The blue phase I (BPI), the cubic subphase of the blue phase exhibited by highly chiral liquid crystals, is inherently an incomplete photonic band gap (PBG) system due to the low refractive index contrast Δn (<0.1). The Δn is given by the difference between the refractive indices corresponding to the double twisted cylinders and disclination lines that constitute the blue phases. Being an incomplete PBG system, BPI exhibits a narrow PBG width (2 to 3%), affecting the extent of enhancement in PL intensity in the dye-doped BPLC. An elegant pathway of confining highrefractive index selenium nanoparticles (n = 3.07 at optical frequencies) inside the core of disclination lines effectively increases the Δn (1.4) and thus the PBG width (6%), driving BPI toward a complete PBG. This, in turn, enhances the PL intensity by a factor of 3 when compared to the pristine BPLC, as demonstrated by finite element method simulations and reflection spectroscopy experiments. The simulations reveal that the efficient confinement of electromagnetic waves by the high-index nanoparticles leads to increased density of states (DOS) at the band edges, and hence, the enhanced emission. However, the emission gets suppressed at PBG as the DOS is nearly zero at the band gap. The in situ modulation in PL intensity is experimentally established utilizing thermally induced PBG tunability. Simulations also suggest that the enhancement in PL intensity strongly depends on the orientation and position of the dipole (dye) within the photonic cavity. Thus, incorporating high-index nanoparticles in the dyedoped BPLC enhances PL intensity, rendering the system more conducive for various optoelectronic applications such as lightemitting diodes and light-emissive displays.