L. Komitov

Device physics of the soft‐mode electro‐optic effect

The soft‐mode ferroelectric liquid‐crystal (SMFLC) effects utilize the electroclinic response of chiral orthogonal smectic phases (A*,B*,E*) in bookshelf geometry. The SMFLC cell may be considered a retardation plate with a field controllable optic axis, with a submicrosecond response time, and a wide continuous dynamic range. A great variety of electro‐optic components and devices can be constructed based on the SMFLC effect, many of them adding novel possibilities and challenges for optical design. We discuss in the present paper different combinations of SMFLC cells with retarders and polarizers, giving optical components capable of light and color modulation. Particularly, multiple electroclinic cells in a special constellation have the potentiality to give nearly achromatic full‐modulation light valves, as well as electrically controlled high‐speed color filters, which can be used for simple and compact TV cameras. Reflective single‐cell electroclinic devices are suitable for integration with semicon...

Linear electro‐optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy

The linear electro‐optic effect in short‐pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro‐optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well‐controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero tr...

Alignment of cholesteric liquid crystals using periodic anchoring

Previous work has shown that when short-pitch cholesteric liquid crystals are aligned in cells with the helical axis in the plane of the substrate in a well-defined direction, then application of an electric field across the cells may result in microsecond in-plane switching due to the flexoelectric effect. Moreover, the response to the electric field is linear and temperature independent. Hence this device geometry has great potential since fast, temperature-independent switching with analogue gray scale may be realized. Up to now, however, the alignment of the helix in the plane of the substrate achieved, for example, by rubbed polyimide layers has been rather unstable, since it relaxes to a Grandjean texture in a time scale determined by the cell thickness. This time is generally of the order of minutes. Here we report a method for the attainment of stable anchoring of the unidirectional lying helix texture using periodic anchoring conditions.

Light-induced anchoring transition in a 4,4'-disubstituted azobenzene nematic liquid crystal

The behaviour of a liquid crystalline azobenzene material possessing a nematic phase was studied under UV illumination. It was found that a uniformly aligned planar sample of the azobenzene undergoes a reversible orientation transition to homeotropic; this can be interpreted as an anchoring transition caused by the photo-isomerization of the azobenzene moiety in the molecular structure of the nematogen. A simple model taking into account the changes in the molecular shape, as well as the magnitude of the molecular net dipole moment, and the changes in the surface density of the cis-isomers with UV exposure time is proposed to explain the light-induced anchoring transition in the azobenzene nematic.

Sign Reversal of the Linear Electro-Optic Effect in the Chiral Nematic Phase

The chiral nematic phase aligned with its helical axis along one preferred direction, being parallel to the confining substrates, exhibits a linear electro‐optic effect when an electric field is applied normally to the substrates. The effect is considered to be of flexoelectric origin. A sign reversal of the effect is found when the helix of the chiral nematic phase changes its handedness, thus revealing the relationship between the sign of the linear electro‐optic effect and the helix handedness. Here, we present and discuss a simple model of this relationship.

Rubbed polyimide films studied by scanning force microscopy

The surfaces of rubbed polyimide films for aligning liquid crystal have been studied by atomic force microscopy. The unrubbed films consisted of randomly distributed polyimide clusters of different sizes. On the rubbed surface, however, the clusters are aligned in long chains along the rubbing direction. The cluster chains were separated by about 100 nm for small rubbing strength. For higher strength the cluster chains coalesced into wider ones.

Volume-stabilized ULH structure for the flexoelectro-optic effect and the phase-shift effect in cholesterics

Flexoelectric coupling gives rise to a linear electro-optic response in cholesterics (flexoelectrooptic effect) with a uniformly lying helix (ULH) structure and this electro-optic effect is strongly reliant on the homogeneity and quality of the texture. The ULH structure, unfortunately, is complicated in itself and may be perturbed by factors such as dielectric coupling, surface/liquid crystal interactions and phase transitions, and often there is a tendency for relaxation into the Grandjean texture (standing helix structure) with time. Hence, in order to exploit the flexoelectro-optic effect in cholesterics any instability of the ULH structure must be ruled out. We have overcome these problems by incorporating a polymer network by means of photopolymerization of a reactive monomer added to the cholesteric. The volume stabilized ULH structure still exhibits the flexoelectro-optic effect, it is stable and it is also retained after heating to the isotropic phase and going back to the cholesteric phase. In a...

Electrically commanded surfaces for nematic liquid crystal displays

Electrically commanded surfaces (ECS) is a liquid crystal display concept whereby the switching of the alignment layer, which is driven by an electric field applied across the layer, is further transferred to the bulk liquid crystal material via elastic forces. This work presents the electro-optic response of a sandwich cell with alignment layer made of siloxane-based ferroelectric liquid crystal polymer, representing the ECS. The bulk liquid crystal material of choice was an in-house nematic mixture comprising fluorinated liquid crystalline compounds with negative dielectric anisotropy (Delta-epsilon < 0). We report a distinct linear electro-optic response, arising from the field-induced in-plane switching of the nematic which in turn is mediated by the ECS.

The flexoelectro-optic effect in cholesterics

The flexoelectro-optic effect in short-pitch cholesterics [1] is analysed in terms of applied electric field strengths and material parameters such as the two flexoelectric coefficients es and eb and the three elastic constants. Starting from the free energy density of the uniformly lying-helix (ULH) configuration, including the flexoelectric polarization term, the equation describing the field-induced tilt angle of the bulk optic axis is derived. It is convenient to introduce the flexoelectric “anisotropy” Deltae as the difference between the splay and bend flexoelectric coefficients, hence defined by Deltae es eb. Our results then show that Deltae is the essential material parameter controlling the sign and magnitude of the electrically induced tilt. In the region of linear approximation, the tilt is proportional to Deltae and to the electric field E, and inversely proportional to the helical wave vector k, as well as the effective elastic constant (K11 K33). The individual values of the elastic constan...

Linear electrooptic effects in the chiral nematic phase

Abstract A linear electro-optic effect is described in the unwound (infinite or sufficiently long pitch) state of the chiral nematic phase N, and attributed to the electroclinic effect. It is similar to the effect already known in the smectic A phase. In particular, it is characterized by the same speed and its induced tilt is a linear function of the applied field. We have, so far, been able to induce tilts of up to several degrees in this mode. An interpretation of the effect is given, based on the symmetry of the N phase in combination with the aligning property of the surfaces. A similar electro-optic effect of exceptionally large amplitude but slower and much more complex has been found in cylindrical N domains near the clearing point. An induced angle of deflection well over 45 degrees, corresponding to switching angles in excess of 90 degrees can be observed in this case.