Harald Knobloch

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.

PROBING THE EVANESCENT FIELD OF PROPAGATING PLASMON SURFACE-POLARITONS BY FLUORESCENCE AND RAMAN SPECTROSCOPIES

Flourescence and Raman scattering intensities have been measured for different probe molecules positioned at various defined separation distances from a Ag grating carrying a propagating plasmon surface polariton mode. Thus, the evanescent decay length of such a surface–electromagnetic wave could be mapped out, and was found to agree with theoretical predictions based on a Fresnel formalism.

Dispersive surface plasmon microscopy for the characterization of ultrathin organic films

We report on experiments, applying a novel microscopy method for ultrathin‐film characterization which uses the dispersion properties of plasmon surface polaritons (PSP) as a contrast giving mechanism. In contrast to prior methods, we apply white light instead of a laser for resonant PSP excitation. For a given incident angle and an area of given thickness, a narrow spectral band of the incident light is at resonance for PSP excitation. Therefore, we obtain images with areas of different thickness appearing in different color. As an example, we present microscopic images taken from a thin Cu–phthalocyanine film, evaporated onto solid support through a mask. In addition, we compare the obtained images to theoretical considerations on the contrast mechanism.

On the laser‐wavelength dependence of plasmon surface polariton field‐enhanced Raman spectroscopy

We analyze plasmon surface polariton field‐enhanced Raman spectroscopic data for the CH stretching vibrational range (2800–3000 cm−1) of cadmium arachidate multilayer assemblies, taken as a function of the exciting laser wavelength λL. In addition to the well‐known λ−4 dependence of ordinary (photon) Raman scattering we find a variation of the intensity ratio of the methylene antisymmetric to symmetric stretching band (νa=2885 cm−1 and νs=2850 cm−1, respectively) which can be attributed quantitatively to a specific surface plasmon field property: a longitudinal field component parallel to the propagation direction adds to the dominant perpendicular field with increasing relative strength for decreasing wavelength.

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.

Raman‐imaging and ‐spectroscopy with surface plasmon light

Plasmon surface polariton field‐enhanced Raman scattering is combined with an optical multichannel detection scheme in order to increase the sensitivity for vibrational spectroscopy with Langmuir–Blodgett–Kuhn monolayer and multilayer assemblies of cadmium arachidate. Moreover, the imaging capabilities of the employed (CCD‐) TV camera are used to spatially resolve (in one dimension) the Raman‐spectral information of heterogeneous samples with a lateral materials contrast.

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.

Electro-Optical Waveguide-Spectroscopy and -Microscopy

Abstract This paper deals with the application of some recently developed evanescent wave-optical techniques for the characterization of electro- optically active macromolecular systems. In particular, guided optical wave-spectroscopies and - microscopies are used to determine the linear and nonlinear optical properties.

Metal ion adsorption to Langmuir-Blodgett films monitored by the optical waveguide technique

Abstract We applied optical waveguide methods to monitor the metal ion adsorption to ultrathin organic films. As a metal ion-sensitive material we used a polymer with azacrown ether sidegroups which are highly selective to Cu 2+ cation complexation. Ultrathin films of this material were prepared by applying the Langmuir-Blodgett technique. When exposing such a multilayer structure to an aqueous Cu 2+ ion solution, the effective index of the propagating waveguide mode increases, indicating that the ions are complexed to the molecules of the ultrathin film structure. The prepared layered structure was sensitive to solutions with a concentration down to c ≈ 3 × 10 −7 M; the response time was in the range of τ = 20–30 min.