Roque F. Madrigal

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.

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.

Nonparaxial diffraction analysis of Airy and SAiry beams

We theoretically analyze Airy beams by solving the exact vectorial Helmholtz equation using boundary conditions at a diffraction aperture. As result, the diffracted beams are obtained in the whole space; thus, we demonstrate that the parabolic trajectories are larger than those previously reported, showing that the Airy beams start to form before the Fourier plane. We also demonstrate the possibility of using a new type of Airy beams (SAiry beams) with finite energy that can be generated at the focal plane of the lens due to diffraction by a circular aperture of a spherical wave modified by a cubic phase. The finite energy ensured by the principle of conservation of energy of a diffracted beam.

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.

Multifunctional hybrid materials for combined photo and chemotherapy of cancer

Combined chemo and photothermal therapy in in vitro testing has been achieved by means of multifunctional nanoparticles formed by plasmonic gold nanoclusters with a protecting shell of porous silica that contains an antitumor drug. We propose a therapeutic nanoplatform that associates the optical activity of small gold nanoparticles aggregates with the cytotoxic activity of 20(S)-camptothecin simultaneously released for the efficient destruction of cancer cells. For this purpose, a method was used for the controlled assembly of gold nanoparticles into stable clusters with a tailored absorption cross-section in the vis/NIR spectrum, which involves aggregation in alkaline medium of 15 nm diameter gold colloids protected with a thin silica layer. Clusters were further encapsulated in an ordered homogeneous mesoporous silica coating that provides biocompatibility and stability in physiological fluids. After internalization in 42-MG-BA human glioma cells, these protected gold nanoclusters were able to produce effective photothermolysis under femtosecond pulse laser irradiation of 790 nm. Cell death occurred by combination of a thermal mechanism and mechanical disruption of the membrane cell due to induced generation of micrometer-scale bubbles by vaporizing the water inside the channels of the mesoporous silica coating. Moreover, the incorporation of 20(S)-camptothecin within the pores of the external shell, which was released during the process, provoked significant cell death increase. This therapeutic model could be of interest for application in the treatment and suppression of non-solid tumors.

Vectorial Diffraction Analysis of Near-Field Focusing of Perfect Black Fresnel Zone Plates Under Various Polarization States

Near-field focusing properties of perfect black Fresnel zone plates are analyzed by solving Maxwell equations using the vectorial angular spectrum representation and the equivalent Rayleigh diffraction formulae of the first kind given by the Luneburg vector diffraction theory. Linear and circular polarizations are compared. As a result, an exact analytical expression for axial intensity is obtained showing that, in the near field, the shift of focal length is inherent in the diffraction process and independent of incident field polarization. The numerical intensity distribution of the electromagnetic field is also obtained, which allows us to demonstrate that when vectorial analysis of the system is performed, subwavelength focusing cannot be achieved for very short focal distances. This result is valid for linearly and circularly polarized incident fields. Finally, it is shown that symmetry of the focalized field is strongly dependent on the polarization of the incident field and that the vectorial diffraction theory can be a good alternative to finite-difference time-domain methods.

Near-Field Electromagnetic Analysis of Perfect Black Fresnel Zone Plates Using Radial Polarization

We analyze the near-field focalization properties of perfect black Fresnel Zone Plates (FZPs) through the exact solutions of Maxwell equations for radially polarized fields obtained by means of Luneburg vector diffraction theory. The electromagnetic fields are computed assuming Hermite-Gauss and Bessel-Gauss beams as boundary conditions at the FZP plane z=0, which allows us to demonstrate that the total intensity is necessarily focused at z=0 and thus there is not much improvement of the intensity resolution at the focal plane. On the other hand, the Poynting vector exhibits multiple vortices and saddle points in the near-field region while its z component vanishes at the FZP axis and has a large focal depth as well as several residual contributions outside the focal region. These facts suggest that perfect black FZPs do not focus radially polarized fields at the designed focal length when such length is comparable to the wavelength (or smaller). Our results are very similar to the ones obtained for phase FZPs and they are in good agreement with FDTD simulations.

Coupled wave analysis of holographically induced transparency (HIT) generated by two multiplexed volume gratings

We present a holographic system that can be used to manipulate the group velocity of light pulses. The proposed structure is based on the multiplexing of two sequential holographic volume gratings, one in transmission and the other in reflection geometry, where one of the recording beams must be the same for both structures. As in other systems such as grating induced transparency (GIT) or coupled-resonator-induced transparency (CRIT), by using the coupled wave theory it is shown that this holographic structure represents a classical analogue of the electromagnetically induced transparency (EIT). Analytical expressions were obtained for the transmittance induced at the forbidden band (spectral hole) and conditions where the group velocity was slowed down were analyzed. Moreover, the propagation of Gaussian pulses is analyzed for this system by obtaining, after further approximations, analytical expressions for the distortion of the transmitted field. As a result, we demonstrate the conditions where the transmitted pulse is slowed down and its shape is only slightly distorted. Finally, by comparing with the exact solutions obtained, the range of validity of all the analytical formulae was verified, demonstrating that the error is very low.

Analysis of the diffusion processes in dry photopolymerizable holographic recording materials

In recent years the interest shown in dry photopolymer films as holographic recording materials have increased dramatically. The main reasons for this are the special properties of these media, such as high diffraction efficiency, low noise, real-time processing and low cost. Moreover, these materials are an important option for displays, high density data storage, micro-optical elements and other information processing applications. In these materials the hologram formation mechanism has been broadly studied, and it has been proposed that the temporal behavior of the diffracted beams depends mainly on the diffusion processes. Thus, the spatial modulation of the refractive index and its evolution over time is the result of non-uniform polymerization and the diffusion of monomers. In this work an analysis of the proposed models and the effect of the diffusion coefficient on the behavior of polyvinylalcohol-acrylamide-based compositions is realized.

One-dimensional, two-dimensional, and three-dimensional photonic crystals fabricated with interferometric techniques on ultrafine-grain silver halide emulsions

Holographic techniques have been used for manufacturing multiple band one-dimensional, two-dimensional, and three-dimensional photonic crystals with different configurations, by multiplexing reflection and transmission setups on a single layer of holographic material. The recording material used for storage is an ultra fine grain silver halide emulsion, with an average grain size around 20 nm. The results are a set of photonic crystals with the one-dimensional, two-dimensional, and three-dimensional index modulation structure consisting of silver halide particles embedded in the gelatin layer of the emulsion. The characterisation of the fabricated photonic crystals by measuring their transmission band structures has been done and compared with theoretical calculations.