Stephan J. Zilker

Materials Design and Physics of Organic Photorefractive Systems

Photorefractivity is an intriguing phenomenon which, however, was initially perceived as a nuisance: Laser illumination generates large internal electric fields which, in turn, modify the optical properties of the material. Useful applications in holographic systems were soon recognized and photorefractivity was subsequently demonstrated in many different materials. These materials span from inorganic crystals to semiconductors, from organic liquid crystals to polymers. Nevertheless, nearly 25 years passed from the discovery of inorganic photorefractive materials in 1966 to reports of the first organic systems. This article reviews the rapid progress of organic photorefractive materials in the last decade and focuses on the different chemical concepts and pathways for further optimization. Advanced systems are required to move photorefractivity out of research laboratories into the market. Possible applications and their first demonstration will conclude the article.

Surface relief gratings generated by pulsed holography: A simple way to polymer nanostructures without isomerizing side-chains

Photoaddressable polymers based on azobenzene side-chains have recently drawn a lot of industrial attention due to their possible application as data storage media. From the scientific point of view an additional effect, the occurrence of light induced surface relief gratings, is nowadays extensively studied. This article describes the behavior of side-chain and guest–host polymers (with and without azobenzene groups) in holographic experiments with pulse lasers. We observe surface relief gratings also for polymers, which only contain an absorbing, but nonisomerizing chromophore. Relief formation in pulsed holographic experiments is shown to arise from thermal effects, but not from trans–cis isomerization.

Photorefractive triphenylamine-based glass: a multifunctional low molecular weight compound with fast holographic response

A novel organic photorefractive composite system is presented consisting of the amorphous multifunctional low-molar mass compound DR1-DCTA, doped with C 60 as sensitizer and diisooctyl phthalate (DOP) as plasticizer. DR1-DCTA, a carbazole substituted triphenylamine with a covalently attached nonlinear optical azo chromophore unit has been synthesized in a 4 step reaction. The compound forms a stable glass. Its thermal, optical and electrochemical properties are discussed. Holographic experiments (2BC, DFWM) of the photorefractive composite exhibit a fast response time in the range of only 30 ms and a maximum gain coefficient of 90 cm –1 .

Photoinduced orientations of azobenzene chromophores in two distinct holographic diffraction gratings as studied by polarized Raman confocal microspectrometry

Optically isotropic thin films (0.5 μm) of an amorphous side-chain copolymer containing 10% azobenzene moieties and 90% meso-azobenzene groups (K1-10 sample) were irradiated by two interfering orthogonal (±45°) or parallel (p + p) linearly polarized laser beams; permanent holographic diffraction gratings were thus inscribed. From atomic force microscopy (AFM) measurements, relatively weak amplitudes (about 50–60 nm) in the surface relief modulation (SRM) are detected but strong perturbations in the profiles and new substructures are evidenced at half-period positions. Using a confocal microspectrometric technique we have recorded various pre-resonance enhanced polarized Raman spectra from a large grating area and obtained spatially resolved Raman images of the intensity variations. This yields new insight into the photoinduced orientation effects, the angular distributions of the chromophore species, and finally the grating formation mechanisms. Different theoretical equations of the polarized Raman scattering intensities, taking account of the pump polarization directions and the high numerical aperture objective used, are derived. Then, an analysis of the experimental results allows us to extract values of the second 〈P2〉 and fourth 〈P4〉 coefficients in the chromophore orientation functions at several positions of the SRM, in particular at the top and bottom regions, and to calculate the corresponding normalized distribution functions ia information entropy theory. In both gratings, somewhat broader distributions are found in the bottom regions, regions of greater polymer removal. Thus, mass-transport phenomena have perturbed, to a weak extent in the (±45°) first case and to a larger extent in the (p + p) second case, the primary photoinduced orientations. So, two simultaneous and superimposed mechanisms appear to be responsible for the observed complex SRM in the highly birefringent K1-10 copolymer under study and significant second-order contributions are suggested. The grating surface profiles are thus reproduced by fitting the SRM amplitudes at the fundamental and doubled frequency and the corresponding phase constant parameters: a good agreement between the observed (AFM) and simulated profiles is obtained.

Investigations on the grating dynamics in a fast photorefractive guest–host polymer

Abstract Holographic studies of a new guest–host system based on a highly photoconducting fluorene–triarylamine copolymer are presented. The photorefractive grating dynamics are investigated in detail. We propose a new approach to characterize the temporal behavior of these processes. By performing an inverse Laplace transform analysis using the algorithm CONTIN, we were able to identify several processes and to evaluate the according time constants. We compare this method with a conventional procedure to prove its applicability. Both approaches yield almost identical results for the fast time constant which is down to 1 ms for this material at a writing beam intensity of Iwb=1.44 W/cm2.

COMPARISON OF PHOTON ECHO, HOLE BURNING, AND SINGLE MOLECULE SPECTROSCOPY DATA ON LOW-TEMPERATURE DYNAMICS OF ORGANIC AMORPHOUS SOLIDS

A joint analysis of spectroscopic data obtained at liquid–helium temperatures by three line-narrowing techniques, photon echo (PE), persistent hole burning (HB), and single molecule spectroscopy (SMS), is presented. Two polymer systems, polyisobutylene (PIB) and polymethylmethacrylate (PMMA), doped with tetra-tert-butylterrylene (TBT) were studied via PE and HB techniques and the results are compared with literature data [R. Kettner et al., J. Phys. Chem. 98, 6671 (1994); B. Kozankiewicz et al., J. Chem. Phys. 101, 9377 (1994)] obtained by SMS. Both systems behave quite differently. In TBT/PIB a rather strong influence of a dispersion of the dephasing time T2 was found which plays only a minor role in TBT/PMMA. We have also measured the temperature dependence of T2 for both systems in a broad temperature range (0.4–22 K). Using these data we separated the two different contributions to the optical dephasing — due to an interaction with two-level systems and due to coupling with local low-frequency modes. ...

Thermally induced surface relief gratings in azobenzene polymers

Recently, Ramanujam and co-workers described for the first time that surface relief gratings can be inscribed into azobenzene copolymers with a single pair of nanosecond laser pulses. We performed a detailed investigation of the involved processes. Two contributions to the diffraction efficiency are observed, one arising from a surface relief and one from a transient cis–trans grating which decays on millisecond time scales. The stable surface relief, in contrast, is caused by a thermal effect which sets in at a well-defined threshold value of the pulse energy.

Line broadening mechanisms in spectra of organic amorphous solids: photon echo study of terrylene in polyisobutylene at subkelvin temperatures

Abstract We have developed an experimental setup with a 3 He bath cryostat for picosecond photon echo (PE) measurements in a broad range of temperatures. In this Letter, a two-pulse PE investigation of the amorphous polymer polyisobutylene (PIB) doped with tetra-tert-butylterrylene (TBT) in the temperature region 0.4–4 K is presented. The results are compared with single-molecule spectroscopy (SMS) data for the same system. The PE yields a linewidth which is close to the average of the SMS linewidth distribution. We conclude that a dispersion of the dephasing time T 2 contributes significantly to the linewidth distribution as measured by SMS. Furthermore, the temperature dependence of the linewidth is analyzed.

Zinc-tetrabenzoporphyrine-doped poly(methyl methacrylate):?a new photochromic recording medium

A photochromic reaction in Zn-tetrabenzoporphyrine-doped poly(methyl methacrylate) was investigated at room temperature. A one-quantum thermoreversible photoreaction could be initiated in the sample with 633-nm He-Ne laser irradiation. The photoreaction quantum yield depended strongly on the concentration of an additional electron acceptor. The changes of the refractive index were also measured.

Speed enhancement of photorefractive polymers by means of light-induced filling of trapping states

The fastest organic photorefractive materials show response times in the millisecond range. The origin of this limit is not yet fully understood. Charge-carrier generation and transport processes and reorientation of the nonlinear-optical chromophores may play an important role. We characterize a new photorefractive guest–host polymer by cw two-beam coupling, as well as by cw and pulsed four-wave mixing experiments. The latter were used to determine unambiguously the response time of the material. We show that a decrease of the response time by a factor of 1000 can be achieved by illuminating the sample with a strong cw beam during pulsed exposure. A model of the enhancement process backed by dc photoconductivity measurements is presented.