Alex Boeglin

Rewritable optical data storage in azobenzene copolymers

We propose to encode optical information through the localized depoling of polar chromophores in thin films of grafted polymeric materials with a femtosecond near IR laser source. This disorientation is promoted through the photoisomerization of the azo-dye component induced by a twophoton absorption process. We show that the resulting localized loss in second harmonic generation efficiency can be exploited in data storage applications. The low irradiation powers used allow for a recycling by reheating and repoling the films leading to a rewritable system.

BODIPY-bridged push-pull chromophores for nonlinear optical applications.

A set of linear and dissymmetric BODIPY-bridged push-pull dyes are synthesized. The electron-donating substituents are anisole and dialkylanilino groups. The strongly electron-accepting moiety, a 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) group, is obtained by insertion of an electron-rich ethyne into tetracyanoethylene. A nonlinear push-pull system is developed with a donor at the 5-position of the BODIPY core and the acceptor at the 2-position. All dyes are fully characterized and their electrochemical, linear and nonlinear optical properties are discussed. The linear optical properties of dialkylamino compounds show strong solvatochromic behavior and undergo drastic changes upon protonation. The strong push-pull systems are non-fluorescent and the TCBD-BODIPY dyes show diverse photochemistry and electrochemistry, with several reversible reduction waves for the tetracyanobutadiene moiety. The hyperpolarizability μβ of selected compounds is evaluated using the electric-field-induced second-harmonic generation technique. Two of the TCBD-BODIPY dyes show particularly high μβ (1.907 μm) values of 2050 × 10(-48) and 5900 × 10(-48) esu. In addition, one of these dyes shows a high NLO contrast upon protonation-deprotonation of the donor residue.

Narrow-line single-molecule transducer between electronic circuits and surface plasmons

A molecular wire containing an emitting molecular center is controllably suspended between the plasmonic electrodes of a cryogenic scanning tunneling microscope. Passing current through this circuit generates an ultranarrow-line emission at an energy of ≈1.5  eV which is assigned to the fluorescence of the molecular center. Control over the linewidth is obtained by progressively detaching the emitting unit from the surface. The recorded spectra also reveal several vibronic peaks of low intensities that can be viewed as a fingerprint of the emitter. Surface plasmons localized at the tip-sample interface are shown to play a major role in both excitation and emission of the molecular excitons.

Geometrical structure and nonlinear optical response of a zwitterionic push-pull biphenyl compound

Abstract Solvent polarity effects on a disubstituted biphenyl have been investigated through IR, Raman, UV–visible spectroscopies. The results are interpreted in the light of some empirical and density functional electronic structures calculations. The vibronic structure has been determined and the normal modes in the finger print region have been assigned. This study opens the way to the exploitation of vibrational spectroscopy in the elucidation of the complex phenomena occurring when highly dipolar chromophores are embedded in dielectric media for applications in nonlinear optics (NLO).

Polarization state studies in second harmonic generation signals to trace atherosclerosis lesions

We have performed multi-photon image reconstructions as well as polarization state analyses inside an artery wall affected by atherosclerosis to investigate the changes in collagen structure. Mice, either healthy or affected by spontaneous atherosclerosis, have been used for this purpose. A two-photon imaging system has been used to investigate atherosclerotic lesions in the ascending aorta of mice. Second harmonic imaging has been performed alternatively on healthy samples and on affected region. The reconstructed images show that the spatial distribution of the collagen network seems disorganized by the disease. The polarization state studies reveal however that the apparent disorganization of the collagen is related to its spatially diffuse distribution and that the internal structure of the collagen fibers is not affected by the disease. In addition, a theoretical simulation of the second harmonic polarization states shows that they are consistent with the known 3D structure of the collagen network.

Image storage through gray-scale encoding of second harmonic signals in azo-dye copolymers

The authors investigated the optical data and image storage in polymeric materials functionalized with azo-dye molecules. Thin films are initially poled to generate a spatially homogeneous second harmonic signal. The information is locally inscribed through the disorientation of the azo chromophores by successive isomerization cycles induced through the two-photon absorption processes. The resulting micron-scale controlled decrease in second harmonic signal can be exploited to perform optical storage. As a demonstration of the proposed method, they store a picture with an 8bit gray-scale encoding. This image can only be retrieved through second harmonic detection and not by linear optical imaging techniques.

Toward submicrometer optical storage through controlled molecular disorder in azo-dye copolymer films

While information encoding through optically induced orientation of azo dyes in organic matrices is being extensively pursued, we propose the alternative of starting out with poled films and to locally reduce their second-harmonic generation capacity by a focused near-infrared femtosecond laser beam of moderate intensity. Arrays of dots irradiated under varying conditions are subsequently imaged in situ as dark spots on a bright background. The samples are also examined through conventional optical microscopy and through atomic force microscopy. We demonstrate that, of these techniques, second-harmonic imaging performs best in the task of information retrieval.

Random laser action in organic film during the photopolymerization process

We report on transient laser action during the photopolymerization process in organic thin films of acrylate monomers doped with a laser dye. The emission spectrum was monitored over a period of time in the direction orthogonal to the incident laser beam which is kept at a constant intensity during the experiments. The emission spectra display the signature of laser action after a certain amount of polymerization. We have also recorded the intensity of fluorescence as well as of the amplified stimulated emission (ASE) using a photodiode. Our results confirmed that all the emission is guided by an increase of the refractive index resulting from the photopolymerization process. The spatial fluctuations in the density of the material are thought to act as micro-cavities leading to a random laser effect.

UV–vis absorption spectrum of a novel Ru(II) complex intercalated in DNA: [Ru(2,2′-bipy)(dppz)(2,2′-ArPy)]+

AbstractThe synthesis of a new Ru(II) complex is reported. Its absorption spectrum when interacting with DNA in water was calculated at the hybrid quantum mechanics molecular mechanics level of theory and compared with experimental data. The vertical transitions were computed using time-dependent density functional theory in the linear response approximation. The complex and its environment were treated at the quantum mechanical and molecular mechanical levels, respectively. The effects of the environment were investigated in detail and conveniently classified into electrostatic and polarization effects. The latter were modeled using the computationally inexpensive “electronic response of the surroundings” method. It was found that the main features of the experimental spectrum are nicely reproduced by the theoretical calculations. Moreover, analysis of the most intense transitions utilizing the natural transition orbital formalism revealed important insights into their nature and their potential role in the irreversible oxidation of DNA, a phenomenon that could be relevant in the field of cancer therapy. FigureThe Ru(II) complex intercalated into the DNA helix.

External stimulus driven variable-step grating in a nematic elastomer.

We report on the creation of micro-patterns in an oriented nematic elastomer (an artificial muscle material) by photopolymerization of surface aligned nematic liquid crystal monomers. We demonstrate that microscopic techniques are able to create accurate patterns in rubber-like liquid crystal materials. Two approaches, based on one and two-photon excitations respectively, are implemented using a microscope-based setup. Due to its high spatial selectivity, the two-photon excitation mode yields finer patterns. Benefitting from the intrinsic, thermally-induced, contractile properties of the material, gratings with variable steps in response to temperature changes were fabricated.