A. I. Plekhanov

Mechanisms of optical limiting in fullerene-doped π-conjugated organic structures demonstrated with polyimide and COANP molecules

Possible mechanisms of optical limiting of visible radiation in conjugated organic structures based on polyimides and substituted pyridines sensitized with C60 and/or C70 fullerenes are studied using the Z-scan method. It is shown that reverse saturable absorption, two-photon absorption, and complex formation between the donor fragment of a π-conjugated molecule and fullerene contribute to the processes of nonlinear absorption. The absorption cross sections for excited states of the charge transfer complexes based on polyimide 6B-C70 and 2-cyclooctylamino-5-nitropyridine-C70 systems are calculated. The data obtained are quantitatively interpreted and compared with those provided by alternative methods.

Polymer films of J-aggregated cyanine dyes and metal clusters for non-linear optical applications

A new medium with non-linear optical properties, comprising J-aggregates of cyanine dyes entrapped in a polymer matrix is reported. Pseudoisocyanine and benzothiazolium dyes with C2H5 and C18H37 chains were synthesized; Langmuir–Blodgett and polymer films of these dyes were prepared. The temperature stability of the J-aggregates in films was investigated by optical spectroscopy. J-Aggregates in polymer films display a higher thermal stability than those in Langmuir–Blodgett films and can be used in non-linear susceptibility experiments. The cubic susceptibility of the films was measured by four-wave mixing and found to be 0.8 × 10–7 cm3 erg–1 for the degenerate case. Upon addition of metal clusters to the J-aggregate–polymer composite, an increase by a factor of >30 in the susceptibility was observed.

Nanometer films of polymethine dyes in optical memory and nonlinear optics

We present a review of experimental investigations in the synthesis and characterization of thin solid films of polymethine dyes in monomer and aggregate forms, which have been performed at the Institute of Organic Chemistry and the Institute of Automation and Electrometry (Siberian Branch of the Russian Academy of Sciences, Novosibirsk) for the past 15 years. The films, obtained by spin-coating technique on glass substrates, were intended for use in optical memory disks of the WORM (write once read many) type and as nonlinear optical media with cubic nonlinearity. The synthesized symmetric thioflavylium dyes with tert-butyl substituents form non-crystallized films of optical quality, which are suited for the creation of recording layers of WORM disks. Thermal, optical, and nonlinear optical properties of thin films based on J-aggregates of pseudoisocyanine derivatives with long alkyl substituents, the films of such dyes doped with cluster anions of higher boron hydrides, and the films of an aggregated thiocarbocyanine dye are considered. Nanometer films of stable J-aggregates of pseudoisocyanine derivatives exhibit the absolute values of the cubic optical nonlinear susceptibility on the order of 10−5 esu at room temperature and 10−4 esu at liquid nitrogen temperature. Thin films of J-aggregates of organic dyes can also be used as saturable absorbers of laser radiation.

Optical Properties of Solid Pseudoisocyanine Films Doped with Cluster Derivatives of Boron Hydrides

The formation and optical properties of J aggregates of pseudoisocyanine iodide in solid films in the presence of cluster anionic derivatives of boron hydrides and carboranes (B10H102-, B12H122-, B20H182-, [NiIV(1,2-B9C2H11)2]0, 1,2-B9C2H12-, [Co(1,2-B9C2H11)2]-, [Ni(1,2-B9C2H11)2]-, B10H8I22-, [Sn(1,2-B9C2H11)]0) are studied. It is shown that the addition of anions B10H102- or B10H8I22- leads to an efficient formation of stable J aggregates. The addition of carborane complex of nickel [NiIV (1,2-B9C2H11)2]0 also leads to the formation of J aggregates, although less stable ones. Carborane complex of tin [SnII(1,2-B9C2H11)]0 facilitates the formation of a monomeric form of the dye. The remaining compounds yield no distinct pattern of formation of a certain monomeric or J-aggregated film structure. With the aid of the semiempirical AM1 method, the charge distributions in the cation of the dye and anionic derivatives of the boron hydrides are calculated. It is supposed that the bipolar distribution of a negative charge in the B10H102- anion facilitates the formation of a J aggregate. By addition of salts of organic cations to a film of pseudoisocyanine-closo-hydrodecaborate (PCG), J aggregates with a narrow width of the J peak are obtained. The thermal decay of J aggregates in these films is studied. On the basis of the data obtained (the presence of an isosbestic point upon thermal decomposition of J aggregates and their reaggregation; the narrowing and increasing of the J absorption peak, as well as increasing of luminescence, upon dilution of a J-aggregated PCG film with organic cations; and the bipolar character of the electrostatic interaction of the B10H102- anion with the dye), it is assumed that the J peak of pseudoisocyanine in the films studied corresponds to the absorption of a dimeric form of the dye.

Dynamics of pulsed recording of holographic diffraction gratings in photopolymer materials

The dynamics of recording holographic diffraction gratings in photopolymer material using 10-ns pulses of Nd:YAG laser radiation (532 nm) was investigated. The formation of permanent and intermediate gratings was observed. Optimization of the photopolymer composition for pulsed recording was performed. The kinetic photopolymerization models taking into account the mechanisms of radical destruction due to recombination and by impurities are verified. Measurement of the recording kinetics at different recording angles showed that diffusion does not affect the hologram formation for times of the same order of magnitude as the recording duration (0.1 s) at a length of the grating half-period. Samples of the photopolymer composition in triethanolamine were investigated and diffusion of oligomeric acrylamide molecules was revealed. The diffusivity was found to be 1.1 × 10−9 cm2/s.

J-aggregates of pseudoisocyanine with long alkyl substituents in thin films: Synthesis, spectral properties, and thermal stability

The formation and properties of J-aggregates in thin solid films of pseudoisocyanines with long N-alkyl groups, obtained by centrifuging from solutions in organic solvents, were studied. It is shown for the first time that nonsymmetric cyanine dyes, containing a C2H5 group at one nitrogen atom and a C10H21, C15H31, or C18H37 group at another nitrogen atom, spontaneously form J-aggregates stable at room temperature and pressing a narrow absorption band with a half-width at half maximum of 200 cm−1. The thermal stability of J-aggregates in thin films of pseudoisocyanines with alkyl substituents decreases in the following order: C2H5-C2H5> C2H5-C6H13>C2H5-C18H37>C2H5-C10H21>C2H5-C15H31. By introducing 1-octadecyl-2-methylquinolinium iodide in the film, it was found that the J-aggregates studied consist of a small number (2–4) of dye molecules.

Temperature dependence and the dispersion of nonlinear optical properties of chromophore-containing polyimide thin films

Detailed investigations of the quadratic nonlinear response of a series of new polyimides with covalently attached chromophore DR13 are performed by the Maker fringes method in the range of fundamental wavelength from 850 to 1450 nm. Polymer films with thickness of 100–400 nm were spin-coated on glass substrates and corona poled. For these materials, the maximum values of the second harmonic generation coefficients d33 are 80–120 pm/V. A red shift of the nonlinear response dispersion with respect to the linear absorption spectrum was observed for the DR13 chromophore. The temperature dependences of linear absorption and nonlinear coefficients d33 for studied structures are observed. It was found that the temperature changes of the absorption spectra lead to appreciable contribution to the value of the nonlinear coefficient d33. The demonstrated high temperature stability (up to 120 °C) of chromophore-containing polyimide thin films makes it possible to eliminate the degradation of their nonlinear optical ...

Optical and Nonlinear-Optical Properties of Pseudoisocyanine J-Aggregates with the Controlled Exciton Absorption Linewidth

Thin films of molecular J-aggregates of pseudoisocyanine with different linewidths of exciton absorption (from 75 to 210 cm–1) were obtained. A change in the absorption linewidth of J-aggregates was achieved by introducing organic salts into a sample. As an alternative method of varying the exciton absorption linewidth, variation in sample temperature was used. For the samples obtained, the third-order nonlinear optical susceptibility χ(3) was measured at temperatures of 80 and 293 K by the Z scan and pump–probe spectroscopy techniques and the dependence of χ(3) on the absorption linewidth W was found to follow a power law. It was revealed that the character of the function χ(3)(W) is almost independent of the method for measuring the absorption linewidth to have the form χ(3) ∼ Wδ with δ ≈ –(1.36–1.58). Based on this finding, the relation of the exciton delocalization length in the aggregate to the absorption linewidth and optical nonlinearity is discussed.

Accurate measurement of the sticking time and sticking probability of Rb atoms on a polydimethylsiloxane coating

We present the results of a systematic study of Knudsen’s flow of Rb atoms in cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound. The purpose of the investigation is to determine the characterization of the coating in terms of the sticking probability and sticking time of Rb on the two types of coating of high and medium viscosities. We report the measurement of the sticking probability of a Rb atom to the coating equal to 4.3 × 10−5, which corresponds to the number of bounces 2.3 × 104 at room temperature. These parameters are the same for the two kinds of PDMS used. We find that at room temperature, the respective sticking times for high-viscosity and medium-viscosity PDMS are 22 ± 3 μs and 49 ± 6 μs. These sticking times are about million times larger than the sticking time derived from the surface Rb atom adsorption energy and temperature of the coating. A tentative explanation of this surprising result is proposed based on the bulk diffusion of the atoms that collide with the surface and penetrate inside the coating. The results can be important in many resonance cell experiments, such as the efficient magnetooptical trapping of rare elements or radioactive isotopes and in experiments on the light-induced drift effect.

Polarization Anisotropy of Single Crystal Opal Films

Single-crystal opal films have been prepared by the moving-meniscus and gravitational sedimentation methods. It has been found using conoscopy and spectroscopic ellipsometry that these photonic crystal opal films possess birefringence so that the optical indicatrix is a three-axial ellipsoid. The indicatrix axis Ng is directed along the crystal growth axis, the axis Nm lies in the film plane and is orthogonal to the axis Ng, and the axis Np coincides with the normal to the (111) crystallographic plane, which corresponds to the triclinic system of the crystal.