Thermally Stabilizing and Tuning Photonic Liquid Crystal Phases with Nanoparticles

Photonic liquid crystal (LC) phases, such as blue phases (BPs) and the cholesteric phase (ChP), have contributed to a variety of photonic devices due to their three-dimensional and one-dimensional photonic band gaps (PBGs), respectively. These phases exhibit a helical periodic structure, which functions as a PBG that selectively reflects circularly polarized light (CPL). Tuning the PBG by adjusting its pitch length and extending the thermal stability and design applications of these phases have been significant research objectives over the last decade. Recently, a promising approach for thermally stabilizing blue phases and tuning the PBG to fabricate novel devices involve dispersing nanoparticles (NPs) into liquid crystal hosts that exhibit BPs and ChPs. In this study, various nanoparticles, including Al₂O₃, BaTiO₃, C60, and cellulose nanocrystals (CNCs), were dispersed into two liquid crystal hosts, a thermochromic mixture (TLC) and mixtures of cholesteryl nonanoate (CN) with nematic MBBA at different concentrations, to investigate their effects on the thermal stability and tunability of the PBG. The results demonstrate clear evidence of the extension of the BPs' thermal stability and the tuning of their PBG pitch. Notably, BPs exhibited greater thermal stability during cooling than during heating, which is an indication of increased supercooling effects. Furthermore, an increased cooling rate proportionally enhanced the thermal stability of the BPs, likely because of supercooling phenomena. Additionally, although BPs do not require alignment, their selective reflection wass enhanced by introducing planar alignment. Moreover, enhanced tuning of the pitch length was observed when NPs were added to the BPs and ChP compared to the pure LC host. The thermal stability improvements and enhanced tuning of the PBG suggest potential for developing more robust photonic devices, particularly for lasing and smart window applications.