Konstantinos Iliopoulos

Second- and third-order nonlinearities of novel push-pull azobenzene polymers.

In this work, the second- and third-order nonlinear optical response of spin-deposited thin films of three different push-pull side chain azobenzene polymers is investigated by the second- and third-harmonic Maker fringes techniques using 30 ps laser pulses at a fundamental wavelength of 1064 nm. Measurements were carried out before and after aligning the chromophores by corona poling of the films, while different polarization configurations have been utilized. Strong dependence of the response upon the structure of the systems has been found, which is related to the different charge transfer within the molecules. The reported findings are compared with already published results.

Reversible Two-Photon Optical Data Storage in Coumarin-Based Copolymers

A functionalized polymer film allowing for a complete and straightforward second-harmonic generation (SHG)-assisted high-contrast writing-reading-erasing-writing sequence is proposed. The whole process is supported by the reversible photoinduced dimerization of a coumarin chromophore and enables efficient optical data storage that can be detected only by SHG imaging.

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis((4-(4-pyridyl)ethynyl)bicyclo[2.2.2]oct-1-yl)buta-1,3-diyne, 1, contains two 1,4-bis(ethynyl)bicyclo[2.2.2]octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic C2/c centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70-300 K) proton spin-lattice relaxation, (1)H T1(-1), at two different (1)H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol(-1), to the two (1)H spin-lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of 1 should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis(3-((trimethylsilyl)ethynyl)bicyclo[1.1.1]pent-1-yl)buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response.

Multifunctional Bi2ZnOB2O6 single crystals for second and third order nonlinear optical applications

Bi2ZnOB2O6 nonlinear optical single crystals were grown by means of the Kyropoulos method from stoichiometric melt. The second and third harmonic generation (SHG/THG) of Bi2ZnOB2O6 crystals were investigated by the SHG/THG Maker fringes technique. Moreover, SHG microscopy studies were carried out providing two-dimensional SHG images as a function of the incident laser polarization. The high nonlinear optical efficiency combined with the possibility to grow high quality crystals make Bi2ZnOB2O6 an excellent candidate for photonic applications.

Physical origin of the third order nonlinear optical response of orthogonal pyrrolo-tetrathiafulvalene derivatives

We present the third order nonlinear optical study of tetrathiafulvalene-based molecular corners. Degenerate four-wave mixing measurements have been done to provide information about the magnitude and the origin of the nonlinearity, while the nonlinear absorption has been separately measured by performing “open aperture” Z-scan measurements. The response has been found to be strongly dependent upon the structure of the molecular corners.

Effect of the counter cation on the third order nonlinearity in anionic Au dithiolene complexes

In this work, we present the third order nonlinear optical investigation of two gold complexes, which differ by the nature of the counter cations. The impact of the different design in the architecture through a set of hydrogen bonds in the case of Au-Mel of the systems on the nonlinearity has been studied by means of the Z-scan setup under 532 nm, 30 ps laser excitation, allowing for the determination of the nonlinear absorption and refraction of the samples. Significant modification of the nonlinear optical response between the two metal complexes has been found suggesting a clear effect of the counter cation.

Photophysics and transient nonlinear optical response of donor–[60]fullerene hybrids

The photophysical properties of some recently synthesized donor–[60]fullerene hybrids, porphyrinyl– and ferrocenyl–[60]fullerene dyads, are studied by means of absorption, fluorescence and femtosecond transient absorption spectroscopy. The formation of charge transfer states is experimentally confirmed, while it is shown that an efficient photoinduced intra-molecular electron-transfer process takes place between the donors (i.e.porphyrin, ferrocene) and the acceptor (i.e.[60]fullerene). In addition, the transient nonlinear optical response of these [60]fullerene dyads is investigated under visible nanosecond laser excitation. Both dyads were found to exhibit significantly enhanced second hyperpolarizability (γ) compared to that of unfunctionalized [60]fullerene, while the refractive part of their nonlinearity dominated their nonlinear optical response in contrast to what has been observed for pristine fullerene and other functionalized fullerenes. Moreover, their optical limiting action is investigated both experimentally and by means of a five-level model. The results are discussed and compared with other literature reports.

Enhancement of linear and nonlinear optical properties of deoxyribonucleic acid-silica thin films doped with rhodamine

In this work, we present the linear and nonlinear optical properties of DNA as functional material, incorporated into a silica material matrix with rhodamine organic dye. We observed that even low concentration of DNA affects the aggregate behavior of the dyes in silica films. The samples with DNA showed higher transmittance and fluorescence efficiency. Moreover, the presence of DNA has been found to significantly enhance the nonlinear optical response of the systems. In this way, we prove that silica materials can provide suitable matrices for hybridization with functional molecules and can be utilized as active optical waveguide materials with enhanced nonlinear optical properties.

Pyrazoline derivatives with a tailored third order nonlinear optical response

In the present work the third order nonlinear optical response of a series of pyrazoline derivatives has been experimentally investigated. All of the compounds have been prepared as doped poly(methyl methacrylate) thin polymeric films. For the needs of this study the third harmonic generation Maker fringes technique has been employed by using 30 ps laser pulse duration and 1064 nm excitation wavelength. A variety of push–pull groups of pyrazoline-based derivatives have been studied in order to relate the structural properties with the optical nonlinearity. More specifically, the molecules under investigation have in several cases different electro accepting groups, which are located in various positions of the molecular structure providing two pairs of structural isomers. The experimental results demonstrate a high dependence of the optical nonlinearity due to the functionalization with the different moieties on the pyrazoline derivatives. This fact promotes this class of materials as promising candidates for photonics and optoelectronics applications, where the control of the molecular structure as a means of tailoring the optical nonlinearities is crucial.

Ultrafast third order nonlinearities of organic solvents

The third order nonlinear optical parameters of a variety of organic solvents are determined under picosecond and femtosecond laser irradiation. Z-scan measurements have been carried out at 35 ps, 532 nm and at 40 fs, 800 nm, while Optical Kerr Effect studies were performed at 35 ps, 532 nm. The target of the present work is to provide a reference of the nonlinear optical response of various widely used organic solvents under the same experimental conditions and render feasible the direct comparison between their nonlinearities.