Photoelectric and photorefractive properties of polyvinylcarbazole composites with graphene in the visible spectral range

The photoelectric and photorefractive characteristics of polyvinylcarbazole (PVK) composites with 0.15 wt % graphene at a wavelength of 532 nm have been measured. The dependence of the quantum efficiency of generation of mobile charge carriers as determined from the photocurrent is well approximated by the Onsager equation calculated accurate to E ₀ ³ for the quantum yield of thermalized electron-hole pairs of φ₀ = 1 and their initial separation radius of r ₀ = 10.9 Å. The long-wavelength edge of optical absorption for PVK lies at 365 nm. Thus, the photogeneration of mobile carriers during illumination of the composite at 532 nm is due to photoexcitation of graphene. The measurement of the photorefractive properties has revealed that the two-beam coupling gain coefficient is Γ = 50 cm^?1 at E ₀ = 150 V/μm and equal intensities of incident beams. It has been found that at E ₀ = 83.3 V/μm, the two-beam coupling gain coefficient increases from 8 to 14 cm^?1 as the ratio of incident beam intensities I ₁(0)/I ₂(0) increases from 1 to 2.4.     

Advantages of polarized two-beam second-harmonic generation in precise characterization of thin films

Polarized second-harmonic generation using two fundamental beams, instead of one, offers significant advantages for characterizing nonlinear optical thin films. The technique is more precise and allows the internal consistency of the results to be verified. The superiority of the two-beam arrangement over the traditional single-beam arrangement is demonstrated by determining the susceptibility tensors of Langmuir-Blodgett films. We show that, for a well-understood reference sample, the results obtained using two fundamental beams agree qualitatively with those obtained with a single fundamental beam, but are more precise. In a more complicated situation, however, the single-beam technique appears to work well but yields results that are, in fact, incorrect. The two-beam technique, instead, yields clearly inconsistent results, thereby highlighting systematic errors in the experimental arrangement or in the theoretical model used to interpret the results.     

Simulation of dispersive Fourier interferograms of gases in the visible wave number range

Dispersive two-beam Fourier interferograms of N₂-gas in the visible wave number range are calculated numerically for nonabsorbing as well as absorbing gas and compared with experiment. Some problems of inverse Fourier transformation of dispersive interferograms are also discussed.     

Interferometric studies on polymer structure

A multiple-beam interferometric method is used to study the change of optical orientation function and molecular structure of nylon-6 fibres due to γ-irradiation under vacuum. It was found that γ-radiation causes alignment to the fibre chains in the direction of the fibre axis, this alignment gives an increase in the optical orientation function. A two-beam interferometric method is used to study the changes in optical orientation function on drawn polypropylene fibres. Empirical formulae are suggested to correlate these changes in the optical orientation function with the dose and with the draw ratio. This study aims to show that the multiple-beam and two-beam interferometric methods can be used to study the changes that take place in polymers and fibres by irradiation or drawing.     

Phase grating generation and optical phase conjugation in photorefractive polymer/dissolved liquid crystal composites

The orientational photorefractive properties of photorefractive mesogenic composites have been investigated by means of two-beam coupling and degenerate four-wave mixing. Photorefractive mesogenic composites, consisting of low molar mass liquid crystals, polymer, and photoconductive sensitizer, constitute novel organic materials possessing high performance photoreractivity. The refractive index change was estimated on the basis of the theory of two-beam coupling and four-wave mixing, and large index modulation of over 0.01 was obtained.     

Near-infrared range photorefractive composites based on poly(vinylcarbazole), multiwall carbon nanotubes, and fullerene C60 1

For composites based on poly(vinylcarbazole) and multiwall carbon nanotubes, photorefractive characteristics at 1064 nm have been studied in the presence and absence of fullerene C₆₀. It has been found that the introduction of fullerene C₆₀ provides an increase in the two-beam gain coefficient of a laser beam and a reduction in the time of hologram recording. The preillumination of the fullerene-containing layer with a continuous light from a He-Ne laser (633 nm) leads to an additional increase in the two-beam gain coefficient.     

CdS nanoparticles sensitized large-scale patterned ZnO nanowire arrays for enhanced solar water splitting

Journal of Solid State Electrochemistry - CdS nanoparticle-sensitized patterned ZnO nanowire arrays (NWAs) were designed and synthesized through two-beam laser interference lithography (2BLIL),...     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive-polymer-stabilized ferroelectric liquid crystals (PPS-FLCs) was examined. The polymer/FLC samples exhibited two-beam coupling gain coefficients of about 6∼12 cm^-1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer-stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two-beam coupling signal was reduced significantly in the PPS-FLC samples.     

Continuous-flow single-molecule CE with high detection efficiency

This paper describes the use of two-beam line-confocal detection geometry for measuring the total mobility of individual molecules undergoing continuous-flow CE separation. High-sensitivity single-molecule confocal detection is usually performed with a diffraction limited focal spot (approximately 500 nm in diameter), which necessitates the use of nanometer-sized channels to ensure all molecules flow through the detection volume. To allow for the use of larger channels that are a few micrometers in width, we employed cylindrical optics to define a rectangular illumination area that is diffraction-limited (approximately 500 nm) in width, but a few micrometers in length to match the width of the microchannel. We present detailed studies that compare the performance of this line-confocal detection geometry with the more widely used point-confocal geometry. Overall, we found line-confocal detection to provide the highest combination of signal-to-background ratio and spatial detection efficiency when used with micrometer-sized channels. For example, in a 2 microm wide channel we achieved a 94% overall detection efficiency for single Alexa488 dye molecules when a 2 microm x 0.5 microm illumination area was used, but only 34% detection efficiency with a 0.5 microm-diameter detection spot. To carry out continuous-flow CE, we used two-beam fluorescent cross-correlation spectroscopy where the transit time of each molecule is determined by cross-correlating the fluorescence registered by two spatially offset line-confocal detectors. We successfully separated single molecules of FITC, FITC-tagged glutamate, and FITC-tagged glycine.     

Interreflection errors in Fourier transform spectroscopy: a preliminary appraisal

The presence of systematic ordinate errors in Fourier transform spectroscopy (FTS) is discussed, largely in terms of interreflection effects between the various interfaces of a two-beam interferometer and its adjacent optical components. Seven distinct categories of interreflection are specified and some consequences are noted. Certain types of optical filter are described which can be used diagnostically to distinguish between ordinate errors arising from interreflections and the intrinsic ordinate errors. The latter are those that remain when the interreflection effects have been removed by optical modifications. A strategy for achieving this is suggested. Finally, a theoretical model is being considered as a general way of handling a complicated problem.     

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 Re ⁽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.     

Fabrication of Two-Dimensional Gratings Using Photosensitive Gel Films and Their Characterization

Two-dimensional (2D) surface-relief gratings have been fabricated by the two-beam interference method on photosensitive gel films, which are derived from chemically modified metal-alkoxides, and characterized with respect to surface morphology and antireflection effect. Photosensitive ZrO₂ gel film was deposited on a silica glass substrate using the sol that was prepared from Zr-tetrabutoxide chemically modified with benzoylacetone. The gel film was exposed to two-beam interference fringe (1.0 m period) of He-Cd laser and then rotated by 90° in its own plane, followed by an additional exposure. Leaching of the gel film with ethanol gave lattice or island types of 2D-gratings depending on the dose of laser irradiation. The morphology of the gratings changed with irradiation time of laser, leaching conditions and so on. The formation of 2D-grating of island type on a silica glass substrate substantially reduced the reflection at its surface in a wavelength range of 1.3 to 2.6 m.     

Quantitative transmission electron microscopy analysis of the pressure of helium-filled cracks in implanted silicon.

The pressure of crack-shaped cavities formed in silicon upon implantation with helium and subsequent annealing is quantitatively determined from the measurement of diffraction contrast features visible in transmission electron micrographs taken under well-defined dynamical two-beam conditions. For this purpose, simulated images, based on the elastic displacements associated with a Griffith crack, are matched to experimental micrographs, thus yielding unambiguous quantitative data on the ratio p of the cavity pressure to the silicon matrix shear modulus. Experimental results demonstrate cavity radii of some 10 nm and p values up to 0.22, which may be regarded as sufficiently high for the emission of dislocation loops from the cracks.     

Coupled calculation of vibrational frequencies and intensities: Part IX. Intensities of methane,ethane, propane,and methionine calculated with the MNDO method

The absolute IR and Raman intensities of methane, ethane and propane are calculated using a combination of a normal coordinate analysis with the MNDO and CNDO/II methods. For the IR intensities the agreement between calculated and experimental observed intensities is better for MNDO than for CNDO/II. The results of both methods are similar for the Raman intensities. The change of the absolute intensities with the change of torsion angles can be used to treat conformational problems of the biomolecule methionine.     

Photopatterning in poly-L-lysine thin films using UV-enhanced hydrophilicity

Deep UV lithography on poly-L-lysine thin films was used to generate microarrays with enhanced hydrophilicity. This was manifested as adsorption of ambient humidity from air by areas exposed to UV fluence around 5 J/cm2 and was made visible by phase-contrast microscopy. Kinetics of adsorption was investigated by a novel technique involving fabrication of submicrometer hydrophilicity grating by two-beam UV interferometry. In an aqueous colloidal medium, gold and polystyrene microspheres preferentially attach to areas that are relatively less hydrophilic, i.e., those areas not exposed to UV light. This observation provides a method for fabricating micro- and nanoporous arrays with controlled porosity. The technique is demonstrated with microspheres of sizes between 250 nm and 10 microm.     

Investigation of nylon-6 fibre structure interferometrically

Multiple-beam Fizeau fringes are used to study the changes in optical properties of Nylon-6 γ-irradiated fibres. Changes in fibre structures due to drawing have been studied using two-beam interference. Some structural parameters such as optical orientation function and electric polarizability difference (Δ/3₀) were determined. Δ/3₀ is found to be constant and depends only on the fibre structure. The generalized Lorantz-Loranz equation given by de Vries is used to determine Nylon-6 fibre structural parameters. Comparison between the results and when using Lorantz-Loranz equation are given. Refractive index profiles also can help in fibre investigations.     

Monochromatic two beam multiphoton dissociation of CF2HCl

Infrared multiphoton dissociation induced by monochromatic two-beam excitation has been carried out by controlling the time delay between the two laser beams. The maximum dissociation yield is found to occur at time delays around 5 μs, when the absorbed energy becomes largely stochastized and an equilibrium among the distinct processes of energy relaxation is manifested. It is shown that this behaviour essentially differs from that which occurs in dichromatic multiphoton dissociation.     

Integrated IR and Raman intensities of aliphatic alcohols in the liquid phase

Integrated molar IR intensities of normal aliphatic alchols, methanol to n-heptanol, are computed by integration of the Lorentz local field corrected function V·(ν·ε″) over the wavenumber interval covered by the absorption band. Raman intensities are measured with the divided cell technique and external standards and the corresponding local field corrections are applied before integration. Absolute values for the integrated IR intensities and relative values for the integrated Raman intensities are given of the OH-stretch and CO-stretch vibrations.     

Thermal lensing in a supercritical water medium

A pulsed, two-beam, thermal lensing experiment was performed to determine the concentration of aqueous solutes above the critical point of water. Despite a very significant mirage effect due to thermal gradients in the cell and absorption by water itself, the thermal lensing signal strength for aqueous benzoic acid in supercritical water was found to be linear with concentration in the sub-millimolar range. Although thermal lensing experiments in aqueous media are notoriously insensitive, the sharp density gradient near the critical point considerably improves the signal intensity. In this study a short-pulse pump 266 nm YAG laser and continuous low-power probe Ar ion beam were both focused into a supercritical water cell, giving a lensing signal whose strength could be maximized by changing the overlap of the two beams.