Electro-optical effects in liquid crystals (LCs) have been widely utilized in many optical components and photonic devices, thanks to the anisotropic media that can be easily manipulated by an electric field to modulate the light. In general, dielectric heating in LC applications is negligible because their orientational dielectric relaxations occur at high frequencies. Here we focus on a dual-frequency LC characterized by its much lower relaxation frequency. The fieldinduced heat strongly affects the LC ordering and optical properties. The electrothermo-optical effect reveals an unusual behavior compared with the well-known electro-optical effect in regular LCs. Based on the electrothermo-optical effect, some applications such as optical modulators or tunable optical shutters are demonstrated.
The texture observation has long been the core technique in liquid crystal (LC)-based bioassays. Its working principle stems from the dark-to-bright texture change induced by the interruption of the initially homeotropic alignment in nematic bulks or from the radial-to-bipolar configuration transition in LC droplets in the presence of biomolecules. One of the drawbacks of this observational scheme, which requires a polarizing optical microscope, is the difficulty in quantitative analysis. In this invited paper, we report on our recent development of alternative optical biosensing techniques based on cholesteric LCs (CLCs) without the use of a polarizer. The increase in structural order in a vertically anchored CLC cell in the quasi-planar state provides a means to allow detection and quantification of the concentration of biomolecules immobilized on the interface between the mesophase and the surfactant DMOAP for LC vertical alignment.
Thermodielectric effect in dual-frequency cholesteric liquid crystals (DFCLCs) is an important issue and has rarely been studied in the past. DFCLC materials have many applications such as fast-switching CLCs, light modulators, and tunable photonic devices. However, DFCLCs characteristically need high operation voltage, which hinders their further development in thin-film-transistor operation. Here we present a lower-voltage switching method based on thermodielectric effect. Dielectric heating effect entails applying an electromagnetic wave to occasion dielectric oscillation heating so to induce the increase in crossover frequency. The subsequent change in dielectric anisotropy of the DFCLC permits the switching, with a lower voltage, from the planar state to the focal conic or homeotropic state. Furthermore, we also demonstrate the local deformation of the CLC helical structure achieved by means of the thermodielectric effect. The wavelength of the deformation-induced defect mode can be tuned upon varying the dielectric heating power. The physics and the calculation of dielectric heating in DFCLCs are described.