Lian Nedelchev

Light propagation through photoinduced chiral structures in azobenzene-containing polymers

We investigate light propagation through azobenzene-containing polymers with photoinduced chiral structures. The structures have large pitch but the Mauguin condition is not fulfilled. The eigenmodes are shown to be elliptical and their ellipticity is determined by the ellipticity e0 of the exciting light. In amorphous azopolymers, light induces a macroscopic chiral structure comprising the whole illuminated region. The pitch depends on the value of e0: no chirality is induced if e0 = 1 (circular polarization). In liquid-crystalline azopolymers circularly polarized light induces the formation of many microscopic spirals, which makes the material equivalent to the classical optically active media.

Increase of photoinduced birefringence in a new type of anisotropic nanocomposite: azopolymer doped with ZnO nanoparticles

We report a significant increase of about 50% of the photoinduced birefringence in nanocomposite films of azopolymers doped with ZnO nanoparticles compared with samples made from nondoped azopolymers. This increase is most pronounced at small concentrations of the nanoparticles of 0.5% and for the amorphous polymers used in our study. We observe also an improvement of the response time of more than 25% in some of the polymers, which allows for faster and more effective polarization optical recording.

Anisotropic hybrid organic/inorganic (azopolymer/SiO2 NP) materials with enhanced photoinduced birefringence.

Hybrid materials based on combination of polymers and inorganic nanoparticles (NP) attracted considerable attention in the last decade due to their advantageous electrical, optical, or mechanical properties. Recently, we reported a significant improvement of the photoresponse by doping azopolymers with ZnO NP. To study the influence of the composition of the dopant, in our present work we have synthesized anisotropic organic/inorganic nanocomposite materials by incorporating 5-15 nm sized SiO2 NP in a side-chain azopolymer. As a result we observe an enhancement of the photoinduced birefringence in these composite materials with about 20% compared to the nondoped sample. Additionally, we discuss possible mechanisms leading to this enhancement related with the scattering caused by the NP and the increased mobility of the azochromophores in the vicinity of NP.

Holographic multiplexing using photoinduced anisotropy and surface relief in azopolymer films

The properties of gratings in an azobenzene polymer inscribed with different combinations of the polarizations of the recording beams are investigated and the optimal conditions for polarization multiplexing are found. Two holograms are stored with the same spatial frequency and the same polarization of the reference beam, varying the polarization of the subject beam. Simultaneous and separate read-out of the reconstructed images is demonstrated.

Photoinduced processes and holographic storage in stilbene and stilbenecarboxaldehyde in a polymethylmethacrylate matrix

We report a spectrophotometric investigation of the photoprocesses in PMMA films containing stilbene or stilbenecarboxaldehyde. UV light induces trans–cis isomerization of the stilbene-type molecules accompanied by considerable changes in the absorption spectra. In the stilbenecarboxaldehyde/PMMA films these changes are anisotropic. The photoanisotropy strongly depends on the wavelength of the exciting light. We also report, for the first time to our knowledge, holographic storage in these materials. The recorded holographic gratings are stable for months.

Photoinduced variation of the Stokes parameters of light passing through thin films of azopolymer-based hybrid organic/inorganic materials

Hybrid materials synthesized by combining polymers and inorganic nanoparticles (NPs) have received considerable attention recently due to their advantageous electrical, optical, or mechanical properties. Here we present a polarimetric study of thin films prepared from photoanisotropic hybrid materials: azopolymer (poly [1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with different concentrations of ZnO NPs with sizes below 50 nm. To obtain full information about the polarization response of these thin films, we determined the kinetics of the Stokes parameters of light passing through the films on illumination by linearly polarized light within the absorption band of the material, namely 473 nm. Using the Stokes parameters, the birefringence values were calculated as a function of time and compared for the films with different NP concentrations.

Synthesis, spectroscopic and TD-DFT quantum mechanical study of azo-azomethine dyes. A laser induced trans-cis-trans photoisomerization cycle

This paper describes the synthesis, spectroscopic characterization and quantum mechanical calculations of three azo-azomethine dyes. The dyes were synthesized via condensation reaction between 4-(dimethylamino)benzaldehyde and three different 4-aminobenzene azo dyes. Quantum chemical calculations on the optimized molecular geometry and electron densities of the trans (E) and cis (Z) isomers and their vibrational frequencies have been computed by using DFT/B3LYP density-functional theory with 6-311++G(d,p) basis set in vacuo. The thermodynamic parameters such as total electronic energy E (RB3LYP), enthalpy H298 (sum of electronic and thermal enthalpies), free Gibbs energy G298 (sum of electronic and thermal free Gibbs energies) and dipole moment μ were computed for trans (E) and cis (Z) isomers in order to estimate the ΔEtrans→cis, Δμtrans→cis,ΔHtrans→cis, ΔGtrans→cis and ΔStrans→cis values. After molecular geometry optimization the electronic spectra have been obtained by TD-DFT calculations at same basis set and correlated with the spectra of vapour deposited nanosized films of the dyes. The NBO analysis was performed in order to understand the intramolecular charge transfer and energy of resonance stabilization. Solvatochromism was investigated by UV-VIS spectroscopy in five different organic solvents with increasing polarity. The dynamic photoisomerization experiments have been performed in DMF by pump lasers λ=355nm (mostly E→Z) and λ=491nm (mostly Z→E) in spectral region 300nm - 800nm at equal concentrations and times of illumination in order to investigate the photodynamical trans-cis-trans properties of the CHN and NN chromophore groups of the dyes.

Birefringence induced in azopolymer (PAZO) films with different thickness

In this article we present a study of the photoinduced birefringence (Δn) in films of a water soluble azopolymer: poly[1- [4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). Varying the concentration of the azopolymer in the solution, films with wide range of thicknesses are obtained – from 50 to 2500 nm. The film thickness is determined with a Talystep precision profilometer. Birefringence is measured using a polarimetric setup with a recording laser at 473 nm and probe He-Ne laser at 633 nm. As shown experimentally, the maximal photoinduced birefringence (Δnmax) does not depend on the thickness and is of the order of 0.07 for all of the investigated samples. The recording time however considerably increases for films thicker than 500 nm.

Birefringence improvement in azopolymer doped with MFI zeolite nanoparticles

Abstract Hybrid organic/inorganic materials based on combination of polymers and inorganic nanoparticles (NP) attract considerable attention due to their advantageous electrical, optical, or mechanical properties. Recently it was reported that doping photopolymers with nanoparticles allows to achieve near 100% net diffraction efficiency in case of conventional holographic recording. Thus, we have synthesized novel organic/inorganic composite materials by incorporating MFI (Mordenite Framework Inverted) type zeolite nanoparticles in an amorphous side-chain azopolymer. A considerable improvement of the photoresponse in thin films of these composite materials has been observed compared to the non-doped samples - nearly 25% increase of the saturated value of the birefringence.Moreover the photoinduced birefringence is stable in time which allows these materials to be used as media for diffractive optical elements with high efficiency and unique polarization properties.

Development of Nanostructured Materials with CBRN Agents Sensing Properties

Azobenzene containing polymers are a class of optical materials extensively investigated in the past decade. Due to their possibility to undergo reversible trans-cis-trans photoisomerization cycles and through their attractive photophysical properties, the azopolymers could be used as storage medium in optical devices, as optical switches and sensors, as responsive surfaces for biomedical applications, in polarization holography, and in photonics. The aim of this paper is to summarize the information available in the scientific literature about the possibility to use these materials not only for optical application but also as sensing platforms for the detection of biological agents listed as category A biological weapons. Furthermore, this review attempts to give the readers insights into the difficulties concerning the detection of biological agents within environmental samples as well as to outline the progress and the weaknesses in the development of sensors based on nanostructured materials to help to combat terrorisms.