Michael Büchel

Photochromic command surface induced switching of liquid crystal optical waveguide structures

We report on optical waveguide structures containing a thin liquid crystal (LC) film held between two photochromic command surfaces. The command surfaces consist of three monomolecular layers of a polymer with azobenzene side chains deposited according to the Langmuir–Blodgett–Kuhn technique. When exposed to light of appropriate wavelength, the command surfaces undergo a trans↔cis photoisomerization process that induces a reversible change in the liquid crystalline orientation. Such an orientation change of the LC alters the optical properties of the optical waveguide. We present experiments on the dynamics of the LC orientation process. The transition is shown to be continuous, with the degree of orientation dependent on the ratio of the cis‐trans chromophore concentration ratio in the command surface.

PHOTOINDUCED ALIGNMENT OF AZOBENZENE MOIETIES IN THE SIDE-CHAINS OF POLYGLUTAMATE FILMS

Abstract The ‘trans↔cis’ reversible photoisomerization process tends to align azobenzene derivatives perpendicular to the polarization direction of the pumping beam. It is shown that in the trans→cis optical transition the cis state is aligned perpendicular to the pumping light polarization. This is shown for spin-coated films of ‘hairy-rod’ polyglutamate with azobenzene moieties in the side chains. Photoselection in both trans and cis states is demonstrated using the attenuated total reflection method.

ANISOTROPIC ALIGNMENT OF A NEMATIC LIQUID-CRYSTAL CONTROLLED BY A POLARIZATION-SENSITIVE LANGMUIR-BLODGETT COMMAND LAYER

Abstract We investigate the homeotropic⇔planar switching in the alignment of a nematic liquid crystal (LC) controlled by Langmuir-Blodgett command layers of polymer containing photochromic azobenzene molecules in the side chain. Waveguide spectroscopy is used to probe the changes in the optical properties of the LC induced by the cis ⇹ trans photoisomerization of the azobenzene units. This optical method provides a high sensitivity for studying the orientation of the LC both within and perpendicular to the plane of waveguide cell. The study of the dynamics of the switching induced by polarized uv light shows that initially the LC molecules follow the movement of the azobenzene units but that eventually they align with the dipping direction. An explanation for this effect based on the movement of the azobenzene units themselves is proposed.

Command surface controlled liquid crystalline waveguide structures as optical information storage

We report on optical waveguide structures in which light propagates within a liquid crystal (LC) thin film. In this configuration, the orientation, and therefore the optical properties of the LC waveguide structure are controlled by two photochromic command surfaces, consisting of an ultra thin film of a polymer with azobenzene side chains [K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, Langmuir 4, 1214 (1988)]. When exposed to light of appropriate wavelength, the azobenzene side chains undergo a trans↔cis photoisomerization process inducing a commensurate change in the LC alignment and therefore, in the set of refractive indices of the LC film. Using this effect we could reversibly write information into the LC cell. The size of our test structure was in the range of 50 μm. The stored information was read out by optical waveguide microscopy [W. Hickel and W. Knoll, Appl. Phys. Lett. 57, 1286 (1990)]; the storing times were in the range of several hours.

Command surface induced switching of the optical properties of liquid crystalline thin film structures

Abstract We report on optical waveguide structures with light propagation in a nematic liquid crystalline thin film. The liquid crystal (LC) is sandwiched between two so-called photochromic command layers, consisting of a polymer with azobenzene side chains. When exposed to light of appropriate wavelength, the azobenzene side chains undergo a transcis isomerization process, inducing a homeotropicparallel change in the LC alignment. These orientational changes affect the set of refractive indices of the LC thin film and, hence, can be detected by optical waveguide methods. As a possible application of the command surface effect in optical waveguide structures, we present an optical switching and information storage device.

Command layers with high azimuthal anisotropy: static and dynamic behavior

Abstract We describe the tilted orientational anchoring of nematic liquid crystals on polymeric Langmuir-Blodgett (LB) layers. The LB film exposes mesogenic groups (azobenzenes) to the bulk liquid crystal which allow for interdigitation. The mesogens are attached to the spacer groups at the meta-position of one of the phenyl rings. This leads to an inclined conformation of the mesogens on the molecular scale. The in-plane anisotropy required for alignment layers is provided by the LB dipping process. The pretilt angle of the liquid-crystal director with respect to the surface at room temperature is about the same as the microscopic bonding angle of the mesogens with respect to the surface. It decreases with temperature. On irradiation, the azobenzene molecules undergo trans-cis isomerization which changes the alignment to homogeneous. We compare the kinetics of switching to the kinetics of a similar substance, with less microscopic inclination. In contrast to the latter material, the system with high anisotropy does not have an induction phase in the beginning of the UV-induced anchoring transition.

Organized Photochromic Azo-polymeric Structures: Self-assembled and Langmuir-Blodgett-Kuhn Layers

Abstract Self-assembled monolayers (SAMs) of azo-silane molecules exhibit persistent dichroism upon linearly polarized light irradiation. This dichroism could be both written and erased by irradiation with light of an appropriate wavelength. In addition, the sign of this dichroism can be inverted by choosing the appropriate polarization of the irradiating light, thus showing a ‘smart communication’ between the light polarization and the ultra-thin photochromic layers. Langmuir—Blodgett—Kuhn (LBK) multilayer assemblies of ‘hairy rod’ polyglutamates with stiff main chains and flexible side chains containing photochromic azo units exhibit a highly optically anisotropic structure when the azo molecules are in the trans form, and a nearly optically isotropic structure when the azo molecules are in the cis form. The trans ⇔ cis photoisomerization of the azo molecules switches them between a highly oriented trans configuration and a bent cis configuration, thus turning the anisotropy ‘on’ and ‘off’. In contrast to spun-cast polymer films containing azobenzene units, photoselection under polarized light irradiation does not occur in these LBK structures.

Langmuir-Blodgett Films of Photochromic Polyglutamates: Structures and Photochemically Induced Structural Changes

Abstract Attachment of azobenzene moieties, substituted with alkyl chains, to a polyglutamate backbone give photochromic “hairy rod” polymers, capable of forming monolayers at an air/water interface. These monolayers can be transferred to substrates as Langmuir-Blodgett films. Irradiation and concomitant photoisomerization of the chromophore result in structural changes of the monolayers and the LB films, which give the systems interesting photoresponsive properties.

Optical switching of azobenzene sidechain molecules observed by surface plasmon spectroscopy

The use of surface plasmons is reported for the optical characterization of thin organic films, in particular thin polymer films at a metal dielectric interface. The basic properties of PSP are presented, including excitation and the technique of surface plasmon microscopy, which allows to obtain an image. As an example studies are presented on the trans-cis photoisomerization of azobenzene sidechain polymers in ultra thin films, prepared by the Langmuir-Blodgett-Kuhn technique. The transition from the trans to the cis state induces a change in the refractive index of the film. These changes were detected and their kinetics measured by a surface plasmon spectroscopy technique. A refractive index pattern could be written into the films and the areas of different refractive indices have been detected by surface plasmon microscopy.