Control of photonic bandgaps in chiral liquid crystals for distributed feedback effect

This paper describes two strategies to control the photonic bandgap (PBG) in chiral liquid crystals (CLCs) to generate laser action through the distributed feedback effect. First, we demonstrated the electrical control of supramolecular helical structure and lasing action from the CLCs, which were comprised of an optically active agent and an achiral nematic liquid crystal, doped with a small amount of a fluorescent dye. When the dye-doped CLC cell was optically pumped with linearly polarized laser beam, the laser action took place at the edge(s) of the CLC reflection bands. The threshold pumping energy to induce laser action was dependent on the PBG position of LC hosts. Subsequently, applying voltages to the optically pumped CLC cells over 20 V brought about immediate disappearance of the laser emission due to the supramolecular structural changes of CLCs from planar to focal conic texture. The laser emission could be observed again by reconstruction of the initial planar structure as high voltages over 75 V were switched off. Secondly, the photoinduced tunability of PBG band in CLC and laser oscillation wavelength was explored by using a photoreactive mixture of cholesteryl derivatives. Photoirradiation of the dye-doped CLC cell with 254 nm light brought about continuous changes in the chiral PBG as a result of the predominant photolysis of cholesteryl iodide in the CLC host, leading to the phototunable laser oscillation in a wavelength ranging from 560 to 620 nm. The present report opens promising ways to design novel active mirrorless laser devices by electro- and photo-modulation of the self-organized chiral PBG structures.