Joachim Stumpe

Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers

Abstract The light-induced modification of the optical properties of photochromic liquid crystalline (side-group) polymers (LCPs) containing azobenzene moieties was studied. Films of such polymers were irradiated with unpolarized and linearly polarized light. Unpolarized irradiation results in a modification of the order parameter, whereas the director orientation remains constant. The light-induced disturbance of the supramolecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline phases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. This reorientation process is caused by an angular-dependent photoselection within the steady state of the photoisomerization of the azobenzene units. The amount and kinetic of the reorientation differ considerably as a function of certain structural features of the polymers, such as the content of photochromic moieties and the lengths of the spacer chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photoselection process may cause a reorientation of the non-photochromic moieties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in optical data storage.

Surface modified ZrO2 and TiO2 nanoparticles embedded in organic photopolymers for highly effective and UV-stable volume holograms

The improved synthesis of large quantities of small (hydrodynamic diameter 6–8 nm) TiO2 and ZrO2 nanoparticles tailored as a high refractive index agent in holographic acrylate photopolymer composites has been developed. The surface of the particles was modified by p-dodecylbenzenesulfonic acid (organic shell weight fraction is about 50–70%) to render them highly compatible with organic monomer mixtures avoiding aggregation. The exposure to the interference pattern provides fast one-step formation of permanent highly efficient volume phase gratings with excellent transparency in the visible range. A high refractive index modulation amplitude n1 of about 0.0165 has been achieved in the composites containing 25 wt% of TiO2 nanoparticles. The level of light scattering in the gratings does not exceed 8–10% compared to the film without nanoparticles. The holographic grating based on acrylates doped with ZrO2 nanoparticles, first employed in this work, exhibited high photostability in contrast to holographic nanocomposites with TiO2 nanoparticles. Possible applications of such photocurable organic–inorganic nanocomposites are holographic diffractive elements for a number of optical and electro-optical applications or devices based on ordered arrays of functional nanoparticles within organic films.

Photoinduced Optical Anisotropy in Amorphous Films of Liquid Crystalline Polymers

Abstract Amorphous, optically isotropic films of photochromic liquid crystalline copolymers have been prepared and irradiated with linearly polarized light. In this way high values of dichroism and birefringence have been induced.

Photo-orientation in amorphous and aligned films of photochromic liquid crystalline polymers

Abstract Optically isotropic films of liquid crystalline and amorphous copolymers containing azobenzene moieties were prepared and irradiated with linearly polarized light, thus generating an orientational order of the photochromic side groups. This process was compared with the photoinduced modification of the optical anisotropy in aligned liquid crystalline polymer films. It was demonstrated that the photo-orientation of the photochromic side groups resulted in a re-orientation of non-photochromic, rod-like side groups by a co-operative mechanism of this process even in the glassy state. Thus high optical anisotropy values were induced in initially isotropic films. The absolute values of the anisotropy and the kinetics of the induction process were investigated as a function of the structural features of the polymers.

Photoinduced Optical Anisotropy in thin Films of Amorphous Photochromic Side Chain Polymers

Abstract High values of optical anisotropy have been induced in glassy films of amorphous copolymers containing azobenzene moieties and rod—like side groups by angular—dependent photoselection. The photoinduced reorientation of the photochromic groups causes a reorientation of the non—photochromic groups due to a co-operative effect.

Electrically Tunable Polymer DFB Laser

IC A IO N In recent years, there is a trend in optics shifting from hard static objects, such as lenses and fi bers, to softer objects based on fl exible polymers or fl uids. Soft matter based optics can be tuned by external mechanical devices, but can also be coupled to electroactive polymer “artifi cial muscles”, for instance to provide for voltage-controlled, monolithically integrated optical elements. Soft electromechanically active optics could enable a whole new paradigm for compact and effi cient optical devices. We demonstrate the compact monolithic integration of a fl exible polymer distributed feedback (DFB) dye laser with an electroactive dielectric elastomer actuator. This combination enables direct voltage control of the DFB grating period through compression, such that an actuation voltage up to 3.25 kV leads to a continuous wavelength shift from 604 nm to 557 nm, a change of 7.8%. The solid glass lens is the fi rst choice for beam shaping. Researchers have shown that lens-shaped water droplets can be manipulated electrically for fast tuning of focal length, [ 1 , 2 ]

Amplified spontaneous emission in polymer?CdSe/ZnS-nanocrystal DFB structures produced by the holographic method

Amplified spontaneous emission (ASE) is demonstrated in volume-distributed feedback (DFB) structures, formed by colloidal CdSe/ZnS nanocrystals and ZrO2 nanoparticles (NPs) in a polymer matrix. Periodic redistribution of the NPs in an organic matrix was carried out by holographic photopolymerization in a specially developed light-sensitive nanocomposite. The composite consists of two acrylate monomers and two types of inorganic NPs. The NPs provide for the formation of two co-phased gratings-a refractive index grating and an optical gain (losses) grating. The core-shell CdSe/ZnS nanocrystals are used as a gain medium, while ZrO2 NPs create the refractive index grating and enhance the distributed feedback. The period of the volume structure provides the feedback for lasing at the wavelength lambda(las) of about 575 nm in the second diffraction order. In contrast to known laser systems based on volume DFB cavities, in which the different components of the formulation provide optical gain and feedback, in our case the inorganic NPs serve as an emitting material and can provide simultaneously for feedback. By pumping of DFB structures by a titanium-sapphire laser (lambda(pump) = 400 nm, pulse duration of 120 fs) normal to the sample plane, the appearance of a sharp stimulated emission along the grating-vector direction is observed. Output intensity of ASE as a function of the pump energy shows a threshold behavior and full width at half-maximum (FWHM) of the ASE spectral band decreases from 33 to 12 nm.

Interdependence of Photoorientation and Thermotropic Self-Organization in Photochromic Liquid Crystalline Polymers

Abstract By irradiation with linearly polarized light optical anisotropy is generated in isotropic films of photochromic LCPs due to a photoorientation process. Generally, the efficiency is much higher in films of polymers with a high content of azobenzene moieties. On the other hand, the photochemically induced orientation of the photochromic side groups affects also the distribution of non-photochromic but mesogenic moieties. Furthermore, in initially isotropic films it acts as initializing force, governing the direction of thermotropic self-organization. The combination of both ordering principles causes a significant amplification of the light-induced optical anisotropy. Investigating several polymers of the same general structure but with a different content of azobenzene side groups an interdependent behaviour was found for the photoorientation of the photochromic and the co-operative orientation of non-photochromic side groups.

Thermal amplification of photoinduced optical anisotropy of p-cyanoazobenzene polymer films monitored by temperature scanning ellipsometry

The in situ observation of thermal alteration of photoinduced optical anisotropy of ultrathin films of polymethacrylates having p-cyanoazobenzene side chains exhibiting amorphous, crystalline, and liquid-crystalline phases was performed by means of temperature scanning ellipsometry. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photoinduced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers, amplified optical anisotropy was maintained even at 300 °C.

Influence of spacer layer on enhancement of nanoplasmon-assisted random lasing

Threshold reduction and emission enhancement are reported for a gold nanoparticle-based waveguided random laser, exploiting the localized surface plasmon resonance excitation. It was experimentally found that a proper thickness of the spacer layer between the gold nanoparticles and the gain layer enhances the random laser performance. It tunes the coupling between the gain polymer and the gold nanoparticles and avoids the quenching of emission in close contact to the gold nanoparticles which is considered as one of the main sources of loss in the current laser system.