The differences and similarities between the flexoelectro-optic effect in chiral nematic liquid crystals in the uniform lying helix geometry and the standing helix geometry have been considered. We present results on the response times measured for a bimesogenic liquid crystal in the uniform lying helix and compare this with the response observed for the standing helix geometry for similar field strengths. For the standing helix geometry an electric field is applied perpendicular to the helix axis by means of parallel gold deposited electrodes which were coated on the surface of one substrate. These electrodes provide a uniform electric field across the sample. From theoretical simulations we find that the response for the two geometries should be identical although, experimentally, some differences are observed. Furthermore, solutions to the governing equations reveal that, unlike the uniform lying helix geometry, the direction of the applied field relative to the alignment of the liquid crystal at the surface boundaries is of importance to the overall response. These results are compared with experimental observations.
In this paper, we review our recent experimental work on coherent and incoherent liquid crystal lasers. For the coherent lasers, results are presented on thin-film photonic band edge lasing using dye-doped low molar mass liquid crystals in the self-organised chiral nematic phase. We show that potentially high Q-factor lasers can be fabricated from these materials by demonstrating that a single mode output with a very narrow linewidth is readily achievable in well-aligned monodomain samples. Moreover, from our investigations we have found that the performance of the laser, i.e. the slope efficiency and the excitation threshold, are dependent upon the physical parameters of the low molar mass liquid crystal. Specifically, the slope efficiency was found to vary from 1% to 12% depending upon the liquid crystalline material employed. Using this information, the important parameters of the host liquid crystal are highlighted. As regards to the functionality, we demonstrate how the wavelength of the laser can be tuned using an in-plane electric field in a direction perpendicular to the helix axis. Finally, for the incoherent lasers, we summarise our findings on random lasers that are fabricated from liquid crystals which exhibit a smectic A* phase.
Liquid crystal variable phase retarders have been incorporated into prototype devices for optical communications system applications, both as endless polarization controllers1,2,3, and as holographic beam steerers4. Nematic liquid crystals allow continuous control of the degree of retardation induced at relatively slow switching speeds, while ferroelectric liquid crystal based devices allow fast (sub millisecond) switching, but only between two bistable states. The flexoelectro-optic effect5,6 in short-pitch chiral nematic liquid crystals allows both fast switching of the optic axis and continuous, electric field dependent control of the degree of rotation of the optic axis.
A novel geometry for the flexoelectro-optic effect is presented here, in which the helical axis of the chiral nematic is perpendicular to the cell walls (grandjean texture) and the electric field is applied in the plane of the cell. This facilitates deflection of the optic axis of the uniaxial negatively birefringent material from lying along the direction of propagation to having some component in the polarization plane of the light. The device is therefore optically neutral at zero field for telecommunications wavelengths (1550nm), and allows a continuously variable degree of phase excursion to be induced, up to 2π/3 radians achieved so far in a 40μm thick cell. The retardation has been shown both to appear, on application of the field, and disappear on removal, at speeds of 100-500 μs. The direction of deflection of the optic axis is also dependent on the direction of the field, allowing the possibility, in a converging electrode "cartwheel cell", of endless rotation of the liquid crystal waveplate at a higher rate than achievable through dielectric coupling to plain nematic materials.