Heinz Kitzerow

Fluorescent columnar liquid crystalline 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes

The title compounds were recently demonstrated to be useful as luminescent electron transport materials in organic light emitting diodes. Here, we present studies of the fluorescence of the homologues with chain lengths between 2 and 10 carbon atoms. Dilute solutions of the investigated compounds show absorption in the range between 410 and 490 nm and fluorescence between 475 and 555 nm. However, the columnar phase of the pure compounds exhibits fluorescence at considerably larger wavelength (550-650 nm) and gives larger fluorescence lifetimes. This behaviour can be explained by the formation of excimers in the mesophase. The phase transitions to the liquid crystal state are associated with a distinct change of the fluorescence intensity.

Photonic crystal fiber with a dual-frequency addressable liquid crystal: behavior in the visible wavelength range

Wave-guiding in the visible spectral range is investigated for a micro-structured crystal fiber filled with a dual-frequency addressable nematic liquid crystal mixture. The fiber exhibits a solid core surrounded by just 4 rings of cylindrical holes. Control of the liquid crystal alignment by anchoring agents permits relatively low attenuation. Samples with different anchoring conditions at the interface of the silica glass and the liquid crystal show different transmission properties and switching behavior. Polarization dependent and independent fiber optic switching is observed. Due to a dualfrequency addressing scheme, active switching to both states with enhanced and reduced transmission becomes possible for planar anchoring. Even a non-perfect fiber shows reasonable transmission and a variety of interesting effects.

Polycyclic aromatic hydrocarbons obtained by lateral core extension of mesogenic perylenes: absorption and optoelectronic properties.

Bilaterally extended perylenes were synthesized, characterized, and used to create organic light-emitting devices. A detailed investigation of the electronic and optical properties, and a comparison of perylene derivatives and compounds with unilaterally and bilaterally extended aromatic cores, reveal unexpected changes of the absorption spectrum, which are in agreement with simulations based on DFT.

Hysteresis and memory factor of the Kerr effect in blue phases

The performance of a polymer-stabilized blue phase system based on a nematic host with large dielectric anisotropy and a chiral dopant with high helical twisting power is investigated and the influence of the reactive monomer composition on the electro-optic characteristics is studied. Field-induced birefringence with a Kerr coefficient greater than 1 nm V−2 can be achieved in a large temperature range from well below 20 °C to above 55 °C. The disturbing influences of electro-optic hysteresis and memory effects can be reduced by diligent choice of the composition and appropriate electric addressing.

ORGANIC HETEROJUNCTION PHOTOVOLTAIC CELLS MADE OF DISCOTIC, MESOGENIC MATERIALS

We made photovoltaic cells containing two organic layers. The hole-conducting layer consists of a hexa-peri-hexabenzocoronene (HBC) derivative. The electron-conducting component is an alkylester of perylene tetracarboxylic acid. Both compounds form columnar mesophases which can provide 1D pathways for transport. The current/voltage-characteristics of the two-layer devices indicate a photovoltaic effect with an open circuit voltage of Voc ≈ 0.8 V, a short circuit current of Isc ≈ 3.9 μA/cm2, and a filling factor of FF ≤ 31%. The spectral distribution of the photocurrent of different samples indicates that excitations of the perylene derivative or of the HBC derivative both can lead to a photovoltaic effect.

Scanning near-field optical microscopy of cholesteric liquid crystals

We have studied glasslike cholesteric liquid crystals by means of scanning near-field optical microscopy. A periodic modulation of the optical polarization was used in order to achieve an optical contrast due to the birefringence of the liquid crystal. Variation of the cholesteric pitch indicates an optical resolution of ≈200 nm for a wavelength of 633 nm.

Superposition of patterns in cross-linked liquid crystals

Patterns appearing in liquid crystals due to electrohydrodynamic convection can be permanently stored by in situ photopolymerization. After curing, the respective pattern is fixed in a glasslike state which is stable even if the entropy production becomes zero. If a small amount of photoreactive mesogenic monomers is dissolved in a cholesteric liquid crystal, the reaction can result in the formation of either separate polymer walls or a polymer film with a regularly modulated surface. The symmetry and the orientation of the pattern depend essentially on the ratio between the sample thickness and the helical pitch of the cholesteric phase, while the cross section of the polymer walls or the topography of the polymer film depend on the ultraviolet radiation and the concentration of the reactive monomer.

Effect of alignment on a liquid crystal/split-ring resonator metasurface.

A metasurface comprising a two-dimensional array of split-ring resonators with resonance frequencies in the near-infrared region is fabricated and embedded in a uniformly aligned liquid crystal. The change of the dielectric permittivity in proximity to the plasmonic structure by the replacement of air with the liquid crystal results in a decrease in resonance frequencies. The resonance shift can be attributed to the interaction of the evanescent field of the excited resonant plasmon modes with the liquid crystal. This shift in resonance frequency is found to depend on the liquid-crystal alignment and to vary for different modes. Also, the resulting effects of changes in temperature or applied external electric field on the metasurface depend on the liquid-crystal alignment and may differ from mode to mode. These observations indicate that the characteristic frequencies of the resonant split-ring resonator modes may depend on different evanescent field components interacting with the liquid crystal. Consequently, certain design rules should be taken into account for the development of tunable metasurfaces based on liquid crystals.

All-optical tunability of microdisk lasers via photo-adressable polyelectrolyte functionalization

Photoactive materials are highly promising candidates for novel applications as they enable all-optical control of photonic devices. Photochromic molecules exhibit a reversible change of their dielectric function upon irradiation with light of proper wavelength. The trans- and cis-isomers of azobenzene exhibit different absorption properties due to the effect of the configuration on the polarizability of the molecule. Here, we introduce a novel molecular/semiconductor hybrid device which is fully tunable by all-optical means via the integration of a semiconductor microdisk into a photo-adressable polyelectrolyte material. We demonstrate that such polyelectrolyte superlattices can be used to tune semiconductor photonic resonators with high precision and without any significant degeneration of device performance. Moreover, we demonstrate an all-optically tunable laser based on this hybrid concept.

Electroluminescent and Optoelectronic Properties of OLEDs with Bay-Extended, Distorted Perylene Esters as Emitter Materials

Three esters with a perylene, a unilaterally, and a bilaterally extended perylene core, respectively, were used as emitter materials for organic light-emitting diodes. The electroluminescent properties of these devices were studied. Different spectral shifts were found, which can be attributed to the formation of excited dimers (excimers) in the nanofilms of the emitter materials. Thermal treatment of the unilaterally extended derivative resulted in a red-shift of the electroluminescence owing to the formation of a denser nanofilm. The luminance and efficiency of optoelectronic devices employing the extended perylene esters exceed those of devices using an emitter layer comprised of the perylene ester. Different deposition methods, limitations in the deposition process, and the role of hole-transporting materials are compared.