Generation of Huygens' dipoles for any spherical nanoparticle excited by counter-propagating plane waves: study of scattered helicity.

Helicity and directionality control of scattered light by nanoparticles is an important task in different photonic fields. In this paper, we theoretically demonstrate that scattered light of lossy spherical nanoparticles excited by using two counter-propagating dephased plane waves with opposite helicity ±1 and the adequate selection of dephase and intensity shows a well defined helicity and a controllable scattering directivity. Numerical examples of Si nanospheres are studied showing their potential application to directional nanoantennas with a well defined helicity. The proposed method is valid for any type of nanoparticle, not only lossy ones.

An explanation for the non-uniform grating effects during recording of diffraction gratings in photopolymers.

The recent results reported in reference 1 have produced an increased interest in explaining deviations from the ideal behavior of the energetic variation of the diffraction efficiency of holographic gratings. This ideal behavior occurs when uniform gratings are recorded, and the index modulation is proportional to the energetic exposure. As a result, a typical sin(2) curve is obtained reaching a maximum diffraction efficiency and saturation at or below this value. However, linear deviations are experimentally observed when the first maximum on the curve is lower than the second. This effect does not correspond to overmodulation and recently in PVA/acrylamide photopolymers of high thickness it has been explained by the dye concentration in the layer and the resulting molecular weight of the polymer chains generated in the polymerization process. In this work, new insights into these deviations are gained from the analysis of the non-uniform gratings recorded. Therefore, we show that deviations from the linear response can be explained by taking into account the energetic evolution of the index modulation as well as the fringe bending in the grating.

Optical singularities and power flux in the near-field region of planar evanescent-field superlenses.

We rigorously analyze the optical singularities and power flux in the near-field region of the novel superlenses reported in [Science317, 927 (2007)] For this purpose, we derive near-field expressions and a general criterion to classify the optical singularities in the vacuum, which are valid when the (s- or p-polarized) electromagnetic fields are generated by any planar field distribution with Cartesian or azimuthal symmetry. Such general results are particularized to the superlenses [Science317, 927 (2007)], for which we identify a sequence of optical vortices and saddles that arise from evanescent-field interference. While the saddles are always located around the focal region, the vortex locations depend on the source field. The features of the topological connection between vortices and saddles are also discussed.

Theoretical approach to photoinduced inhomogeneous anisotropy in bacteriorhodopsin films.

The aim of this work was to perform a complete study of the dynamic and steady-state photoinduced processes of thick bacteriorhodopsin (bR) films, taking into account all the physical parameters and the coupling of rate equations with the energy transfer equation. The theoretical approach was compared with experimental data, and good concordance was found between both sets of data. The theoretical approach shows that the values of the rate constants for solid bR films are about two or three orders of magnitude lower than those observed in solution. It can also be noted that the temperature change during the experiment had a great influence on the final values of transmittance and, consequently, on the inhomogeneous distribution along the coordinate of light propagation. The study shows that, depending on the intensity and wavelength of the pump beam, we can obtain a very inhomogeneous profile of the population densities, which implies an inhomogeneous profile of the birefringence and dichroism. Therefore, this must be taken into account in the applications described for this system.

Holographic reflection gratings in photopolymerizable solgel materials.

The recording of holographic reflection gratings with a spatial frequency higher than 5400 lines/mm in photopolymerizable solgel materials is experimentally demonstrated. Diffraction efficiencies near 60% and a FWHM of 2.5 nm centered at 531.5 nm are achieved. Moreover, the effect of the energetic exposure is characterized at different recording intensities.

Matrix method for the study of wave propagation in one-dimensional general media.

A matrix method which relates the field and its derivative is presented for the study of wave propagation in any type of one-dimensional media. The transfer matrix is obtained from the canonical solutions of Helmholtz equations at normal incidence. The method is applied to different optical systems like a Fabry-Perot cavity formed by uniform fiber Bragg gratings, periodic dielectric structures and different quasi-periodic structures based on Fibonacci and Thue-Morse sequences of layers with constant and variable refractive index.

Upper limits of dielectric permittivity modulation in bacteriorhodopsin films.

A theoretical study of light-induced modulation of the dielectric permittivity in bacteriorhodopsin films has been done (including B--> M and B --> Q transitions). Analysis of dielectric permittivity modulation enables us to determine the fundamental limits of BR to be used in a holographic data storage system, together with the optimum experimental and material conditions. In order to carry out this analysis, the macroscopic dielectric permittivity was related to the microscopic polarizability of the three states of BR considered (B, M and Q). This parameter was calculated using a modelization procedure that includes the effect of ASP85, TRP86, and TYR185 amino acid residues (the B3LYP/6-31+ G(*) method was used for the calculations). Good concordance between theoretical calculations and experimental data was found for the linear optical properties (absorption wavelength, transition dipole moment, and dielectric permittivity modulation). The theoretical upper limits of Deltaepsilson at 750 nm (far from the resonance of the molecule) in a randomly oriented material are about 0.01 and 0.012 for B--> M and B--> Q transitions, respectively. The values of Deltaepsilon obtained were used to simulate diffraction efficiencies (eta) of a volume phase hologram recorded in a BR film. The high absorptive losses at low wavelengths (about 625 nm ) cause an interesting behavior, since the highest Deltaepsilom do not produce the greatest eta. The highest eta is produced for a hologram thickness in the range of 900-1000 microm and working wavelength of 700-750 nm.

Bidimensional chromophores for photorefractive polymers with working wavelength in the near IR.

In this study we show six bidimensional chromophores designed for high Tg photorefractive polymers with a working wavelength in the near IR. The macroscopic optical properties of a poled polymer which contains the designed chromophores were expressed as a function of the microscopic properties of the chromophores, which were calculated using quantum mechanical methods. Later, the diffraction efficiency of a holographic recording and readout experiment was simulated using the Montemezzani equation for anisotropic materials. Results show that high diffraction efficiencies could be obtained for three important working wavelengths (1064, 1300 and 1500 nm) using these chromophores. Of particular interest are the result for the PMC3b derivative at the telecommunication windows of 1300 nm and 1500 nm and the result for PMC1a derivative at the light source wavelength of 1064 nm.

Application of the Fixed Point Theorem for the solution of the 1D wave equation: comparison with exact Mathieu solutions.

A method based in the application of Fixed Point Theorem (FPT) techniques to the solution of the 1D wave equation at normal incidence for materials that present a continuous (real or complex) dielectric constant is presented. As an example, the method is applied for the calculation of the electric field, reflection and transmission spectra in volume holographic gratings. It is shown that the solution obtained using this method agrees with the exact Mathieu solutions also obtained in this paper for volume holographic reflection gratings.

High T(g) photorefractive polymers: influence of the chromophores' beta tensor.

In this paper we study the effect of the chromophores' beta tensor active components on the diffraction efficiency of a high T(g) photorefractive polymer. In particular, we study the two simplest structures with nonvanishing dipole moment, the one-dimension push-pull systems, and the Lambda-shaped chromophores. We have developed a model that relate the diffraction efficiency expression with experimental conditions and microscopic properties of the molecules used. Using this model we determine the optimum experimental conditions for both kinds of chromophores and the criteria for the design of chromophores with improved microscopic properties. The model was also used to evaluate the diffraction efficiency of the chromophore Disperse Red 1 (DR1) with a good agreement with experimental data present in bibliography, and of other chromophores selected with the criteria derived from the model, using quantum mechanical calculations to obtain the microscopic properties. Using the designed chromophores diffraction efficiencies more than one order of magnitude higher than that calculated for DR1 with the experimental conditions has been obtained in simulations. These chromophores also exhibit a low dependency of eta on the electric field polarization in contrast to the DR1 or the low T(g) photoreactive materials.

Acrylamide-N,N'-methylenebisacrylamide silica glass holographic recording material.

In this paper, we describe a photopolymerizable silica glass based on acrylamide (AA) and N,N'-methylenebisacrylamide (BMA) as monomers, triethanolamine (TEA) as coinitiator and yellowish eosin (YE) as photoinitiator. We studied different compositions, analyzing the diffraction efficiency, energetic exposure and effective thickness obtained in the holographic gratings. A diffraction efficiency of 60 % with an energetic exposure of 139 mJ/cm(2) and an effective thickness of 1.1 mm were obtained. Also, by varying the photopolymerizable composition of the material diffraction efficiencies higher than 80 % can be reached with an energetic exposure of 10 mJ/cm(2) and an effective thickness of 113 microm. These values are similar to those obtained in conventional photopolymer systems in polyvinylalcohol and better than the values reached in other sol-gel compositions. Also, 9 holograms were angular multiplexed with diffraction efficiencies between 6 and 12 % and total exposure time shorter than 150 ms, with a dynamic range M/#= 2.4.

Multiplexed holographic gratings for fabricating 3D photonic crystals in BB640 photographic emulsions.

Three multiplexed holographic gratings were used to fabricate 3D photonic crystals without complete band gap using ultra-fine grain holographic emulsion as the storage material. Theoretical and experimental studies were carried out in order to characterize the crystals. Good agreement between theory and experimental results was obtained, showing that it is possible to obtain a band-pass filter for a fixed direction of illumination.

Purple membrane-polyacrilamide films as holographic recording materials.

The holographic parameters of purple membrane-polyacrylamide films obtained from a mutant form of Halobacterium salinarum (originally Halobacterium halobium) were measured. The synthesized films have an absorption of around 2.5 at 532 nm and a pH of 8.65. The results show that diffraction efficiencies of about 1.2 % (measured at 633 nm) can be achieved with writing intensities in the range of 200-400 mW/cm2 (532 nm), and these values remain constant after saturation. Pump-probe experiments were also used to measure the M state lifetime and our PM films were found to have the lowest M state lifetime described at this pH.

Study of angular responses of mixed amplitude--phase holographic gratings: shifted Borrmann effect.

We present theoretical results for angular responses of transmitted and diffracted beams in mixed amplitude-phase holographic gratings. Experimental results for gratings recorded in photographic emulsions and developed without a bleaching bath, with diffraction efficiencies of >20% , are also presented. The model shows an angular shift between minimum transmittance and maximum diffraction efficiency when both index modulation and absorption coefficient modulation are present. Also, the Borrmann effect was found outside the Bragg angle. This method can be extended to a study of the mechanism of image formation in materials such as bacteriorhodopsin and azo-dye-doped polymers that exhibit both types of modulations (index and absorption).

Anomalous D-Log E curve with high contrast developer Kodak D8 on ultra fine grain emulsion BB640.

D-Log E curves, also known as H-D curves, are used since the XIX century as a tool for describing the characteristics of silver halide emulsions. This curve has a very standard shape, with a linear region, a toe, a shoulder and a solarization region. In this work we present a distortion of the usual curve due to the action of a high contrast developer, Kodak D8, on an ultra fine grain emulsion, BB640\cite{ov04}. The solarization effect is replaced by a linear zone where developed densities increase with increasing exposures, until all silver halide present in the emulsion is reduced by developer D8 to metallic silver. Densities higher than 11 have been obtained.

Holography as a technique for the study of photopolymerization kinetics in dry polymeric films with a nonlinear response.

A method is reported that makes use of holography to study the kinetics of the radical photopolymerization of acrylamide in a polyvinyl alcohol when the Kogelnik theory is applied. A mechanism of unimolecular termination by the radicals that initiate the polymerization reaction is postulated to calculate the quantum yield, the molar-extinction coefficient, the index of refraction, and the thickness of the film. The conversion percentage of monomers is obtained along with the ratio of rate constants of the mechanism of polymerization from the nonlinear fit of the transmittance curves, their angular response, and the temporal evolution of diffraction efficiency. Compared with previous holographic techniques, this method has the advantage of predicting these chemical parameters using all the data points of the temporal diffraction efficiency variation rather than being restricted to the linear zone of these curves. In this way the whole reaction process, not just the initial process, is taken into account.

Theoretical and experimental study of the bleaching of a dye in a film-polymerization process.

The quantum efficiency and the molar-absorption coefficients of different phenothiazine dyes are obtained by means of fitting the experimental data of transmittance as a function of time. An analytical expression for the intensity transmitted in a photopolymerizable holographic material is obtained, and good agreement between theory and experience is also achieved. The analysis of these parameters is of fundamental quantities in the photochemical characterization of holographic recording materials.

Optimization of an acrylamide-based dry film used for holographic recording.

A study of the optimization and the characteristics of a dry film photopolymerizable recording material is presented. The effects of intensity, the thickness, and the variation of the concentration of each component have been studied. Diffraction efficiencies of 80%, with energetic sensitivities of 40 mJ/cm(2), have been obtained in photosensitive films of a 35-mum thickness with a spatial frequency of 1000 lines/mm.