Tunable reflecting/ transparent mirror in a photonic gel

The proposed BP gel switchable mirror device with the corresponding polarizing microscopy textures exhibiting (a) Green reflection with electric field (b) Transparent / dark homeotropic state with further higher field and (c) Red reflection when the field is completely removed

Soft photonic crystals have attracted immense research interest recently due to simple fabrication by bottom-up self-assembly and soft-stimuli responsive photonic band gap-tunability. Blue phase (BP), a unique liquid crystalline phase that exists between the isotropic and cholesteric, can be considered as photonic crystal owing to the cubic lattice formed by its molecular arrangement. As the BPs are soft systems, their lattice and thereby the selective reflection wavelength can be tuned by external stimuli. Electric field, especially is known to provide the widest range of possibilities for wavelength tuning especially due to the field induced lattice distortion/reorientation or electrostriction. Upon gelating the BP system, perfect and reversible tunability of colors (and hence the photonic band gap) between vivid red and green reflecting states is achieved by varying the operating electric field. Interestingly, the gelated BPI additionally gives rise to a homeotropic nematic state which appears dark (light completely blocked) or transparent (light completely passes through) with and without crossed polarizers respectively. This feature of switching between the reflecting and transparent states brings in the possibility to fabricate a tunable mirror device using the BP composite. See ACS Appl. Mater. Interfaces, 2017, 9(45), 39569-39575.

Posted by S. Vimala