We discuss the director deformation inside a hybrid aligned nematic (HAN) cell, assuming that the surface interaction is of the (w/2) sin2 (ϕ — Φ) kind, and we suppose that the anchoring energy for homeotropic alignment is greater than the planar one. In the absence of an external electric field, we show that a critical thickness dc, below which the nematic liquid crystal (NLC) is undistorted, exists. Furthermore we investigate theoretically dc vs. the reduced electric field (E/Ec, where Ec is a threshold field depending upon the anchoring energies and the intrinsic constants of the NLC). The last analysis shows that there are two critical thicknesses dc(H) and dc(P): for d < dc(H) the sample is homeotropic (HOM), while for d > dc(P) it is planar (PLAN). Finally we point out that if the anchoring energies are the same on the upper and lower plates of the sample, the HAN-configuration is realizable only if E/Ec < 1

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Förster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

Ellipsometric measurements of a nematic drop spreading on a rough surface are reported. The thickness profile of the drop shows two typical lengths belonging to the microscopic and to the mesoscopic scales (a few tens and some hundreds of Å, respectively). No intermediate thicknesses are observed during the spreading. A simple model involving the balance among elastic, anchoring, and spreading energies is used to interpret the results.