George A. Domrachev

Novel materials from bis(arene)metal-containing polyacrylonitrile

Novel bis(arene)metal-containing polyacrylonitrile materials have been prepared by the polycyanoethylation reaction between acrylonitrile and (arene) 2 M (M = Cr or V; arene = PhH, C 6 H 4 Et 2 or mesitylene) in the absence of solvent. The resulting star-shaped molecules consist of a central (arene) 2 M species with up to four polyacrylonitrile arms covalently bonded to the arene ligands. The materials are readily soluble and films can be cast from solutions in acetonitrile. The IR and solid state 13 C NMR spectra (or EPR spectrum for the oxidized chromium-containing polymer) are consistent with the presence of a metal-arene bond and confirm the persistence of the sandwich structure. The properties of the thermolysed materials are consistent with the formation of conjugated naphthyridine-type structures. The value of |n 2 | determined by the degenerate four-wave mixing technique at 1064 nm with a 6 ns pulse duration for a solution in conc. H 2 SO 4 (1 g l -1 ) of the chromium-containing polymer pyrolysed at 350 °C was found to be 0.8 × 10 -13 cm 2 W -1 corresponding to |Re X (3) | = 0.4 x 10 -11 esu.

Photorefractive properties of new polymer composites incorporating poly[ethynediyl-arylene-ethynediyl-silylene]s

The photorefractive properties of low glass transition temperature, Tg, composites based on oligomeric poly[ethynediyl-arylene-ethynediyl-silylene]s as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm. The photorefractive origin of the refractive index changes was confirmed by the achievement of a two-beam gain as high as 67 cm−1 under an externally applied electric field (E0 = 16 V µm−1). Refractive index gratings affording two-beam gains up to 51 cm−1 can also be written under zero-field conditions. A possible explanation for this unique property of the materials is that chromophore orientation asymmetry can be induced by the longitudinal intensity gradient.

Thin film Z-scan measurements of the nonlinear response of novel conjugated silicon-ethynylene polymers and metal-containing complexes incorporated into polymeric matrices

The third-order optical nonlinearities of new conjugated poly[(arylene)(ethynylene)silylene]s, and a variety of chromium, neodymium or cobalt complexes incorporated into polymeric matrices as thin sol-gel or polyacrylonitrile films have been determined by using a single beam Z-scan technique. The samples were pumped by a single ultrashort pulse of a mode-locked Nd-phosphate glass laser (wavelength 1054 nm) with a 5ps pulse duration (full width at half- maximum), the repetition rate of the Gaussian beam being low (0.3Hz) ro avoid thermal effects. The spot radius of the focused pulse was ca. 60micrometers , its beam waist being in the sample (intensity up to 4x1013 Wm-2). Calibration was done with chloroform and benzene, the value of N2 for the latter (2x10-12esu) being similar to that previously reported. A small-aperture Z-scan (S=0.03) was used to measure the magnitude and the sign of the nonlinear refractive index, n2. Very high nonlinear refractive indices were found for a film containing (a) a poly[(arylene)(ethynylene)silylene]s with pentacoordinated silicon (c 5 gl-1) in a sol-gel matrix (N2 = 6 x 10-13 cm2W-1), (b) a film containing a poly[(arylene)(ethynylene)silylene] with tetracoordinated silicon (c 0.5 gl-1) and a very small proportion of fullerene-C70 incorporated into an NH2-containing sol-gel matrix (n2 = 5x10-13 cm2W-1, and (c) a thin polyacrylonitrile film of polycyanoethylate bis-arenechromium(I) hydroxide (n2 = -5 x 10-12 cm(superscript 2W-1.

Silicon-containing polymers for non-linear optics

A variety of novel poly[ethynediyl-arylidene-ethynediyl-silylene]s (PEAES), including those containing hypercoordinate silicon atom as well as donor and acceptor groups, has been synthesized. The presence of hypercoordinate silicon favours high values of fast Kerr-type non-linearities, and the incorporation of amide groups gives rise to good optical quality nanocomposites in a silica matrix. The inorganic-organic hybrid sol-gel film incorporating 5–14 mass% of the polymer containing pentacoordinate silicon showed Reχ (3) = −9.5 ×10−11 esu. The predominant rôle of the electron nonlinear mechanism has been demonstrated by using test beams with linear and circular polarizations. The photorefractive properties of composites based on PEAES as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm without preliminary electric poling. The photorefractive origin of the refractive index changes was confirmed by the high two-beam gain under an externally applied electric field. Refractive index gratings affording two-beam gain can also be written under zero field conditions. To explain this unique property of the materials it is suggested that chromophore orientation asymmetry arises either during film formation or is induced by the longitudinal intensity gradient.

Photorefractive effects in novel polymer nanocomposites

We present the results of chemical development and optical investigation of the extraordinarily large photorefractive effect in the new polymer nanocomposites. The composites are composed of poly(ethynylene)arylenesilanes as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N- ethylcarbazole and phenyltrimethoxysilane as plasticizer, and C70 and C60 fullerenes as charge generators. The magnitude of the change in photorefractive index and its origin, and temporal behavior were studied at 633 nm by a variety of nonlinear optical techniques, including nonlinear lens method, four-wave mixing and two-beam coupling. The relaxation time of the photorefractive index changed in a range from a few seconds to tens of minutes at changing beam intensity.

Degenerate four-wave mixing measurements of the nonlinear response of conjugated silicon-ethynylene polymers

The third-order optical nonlinearity of a series of new conjugated silicon-ethynylene polymers, poly(aryleneethynylenesilylene)s, have been studied by using the degenerate four-wave mixing technique. The fast nonlinear optical susceptibilities of the polymers containing various groups were determined for the solutions in chloroform, tetrahydrofuran and toluene. The electronic and nuclear contributions of the (chi) (3 susceptibility and the thermal nonlinearity of the solutions were separated. The (chi) (3 susceptibility of the polymer containing tetracoordinate Ph2Si groups was found to be comparable with those of the best polymers.

New materials for nonlinear optics

A variety of novel materials has been synthesized including poly(ethynediyl-arylene-ethynediyl-silylene)s (PEAES) containing hypercoordinate silicon, and films of metal- containing polyacrylonitrile materials. The (chi) (3) properties in various matrices have been investigated by various techniques such as DFWM and Z-scan. The (chi) (3) values are outstandingly high; e.g. sol-gel films incorporating 5-14 mass percent PEAES show Re(chi) (3) equals 3-9 X 10-11 esu, and films of Cr-containing polyacrylonitrile materials synthesized by the polycyanoethylation reaction between acrylonitrile and Ar2Cr show (chi) (3) equals -2.5 X 10-10 esu.