Holographic waveguides in photopolymers.

The possibilities that offer the holographic optical elements for photovoltaic and "see through display" applications open new windows for holographic recording materials. In this sense, some specific characteristics are required for each particular application. Waveguides are one of the key elements for these applications. Photopolymers are one of the most competitive candidates for waveguide fabrication. In this work, we evaluate the performance of one example from each of three families of photopolymer material in fabrication of a 633nm waveguide. Firstly, polyvinyl alcohol acrylamide, PVA/AA, the second one, a nanoparticle-thiol-ene, NPC, and on the last place a penta/hexa-acrylate based polymer with dispersed nematic liquid crystal molecules, PDLC. We study the critical role of the material and in particular, spatial resolution for this application.

Two diffusion photopolymer for sharp diffractive optical elements recording.

Photopolymers as recording media are widely used in optical applications. In such materials, changes in the phase of the transmittance function are generated during exposure due to refractive index and thickness modulations. These changes arise primarily as a consequence of photopolymerization and mass transport processes. Characterizing polymers' performance, for example, quantifying the value of monomer diffusion, is therefore very important. Applying index matching, the volume and surface optical effect are separated in an acrylamide/polyvinylalcohol (AA/PVA) material. Using a simplified model that includes the effects of the holes produced during polymerization, both hole and monomer diffusion are analyzed. The analysis presented indicates higher material sensitivity than previously estimated. The results also indicate the possibility of recording sharper diffractive optical elements profiles, like blazed gratings, having diffraction efficiencies higher than 80%.

Influence of index matching on AA/PVA photopolymers for low spatial frequency recording.

Photopolymers present appealing optical properties for holographic and diffractive applications. They enable modulation of the electrical permittivity and thickness and are self-processing, and layers with a wide range of thicknesses and properties can be fabricated on demand. In order to obtain a complete characterization of the material, low spatial frequency analysis has become a fundamental tool because the motion of the components inside of the material can be measured. We propose to use an index matching component to carry out a complete characterization and to differentiate the "apparent" and the real monomer diffusion. We also have quantified the minimum thickness to obtain the phase modulation of 2π required for the fabrication of many diffractive elements such as lenses, axicons, or blazed gratings. Finally, we have studied the influence of the thermal effects in the thickness variations.

Holographic grating stability: influence of 4,4'-azobis (4-cyanopentanoic acid) on various spatial frequencies.

This paper presents the results obtained when holographic gratings were stored with a spatial frequency of 954 and 2663 lines/mm in transmission geometry and 4600 lines/mm in reflection geometry in a polyvinyl alcohol/acrylamide-based material. Photopolymers are materials that give good results at low frequencies, but their diffraction efficiency (DE) decreases at high frequencies. A chain transfer agent, 4,4'-azobis (4-cyanopentanoic acid) (ACPA) was incorporated in the material composition to improve spatial resolution. Furthermore, a curing process was applied to the stored gratings in order to maintain the DE stable over time. The DE and shrinkage for symmetric holographic transmission and reflection gratings were measured to evaluate their quality and quantify the improvement produced by ACPA.

Relief diffracted elements recorded on absorbent photopolymers.

Relief surface changes provide interesting possibilities for storing diffractive optical elements on photopolymers and are an important source of information for characterizing and understanding the material behavior. In this paper we use a 3-dimensional model, based on direct parameter measurements, for predicting the relief structures generated on without-coverplate photopolymers. We have analyzed different spatial frequency and recording intensity distributions such as binary and blazed periodic patterns. This model was successfully applied to different photopolymers with different values of monomer diffusion.

Surface relief model for photopolymers without cover plating.

Relief surface changes provide interesting possibilities for storing diffractive optical elements on photopolymers and are an important source of information to characterize and understand the material behaviour. In this paper we present a 3-dimensional model based on direct measurements of parameters to predict the relief structures generated on the material. This model is successfully applied to different photopolymers with different values of monomer diffusion. The importance of monomer diffusion in depth is also discussed.

Qualitative disorder measurements from backscattering spectra through an optical fiber.

In the processes related to the development of cancer, there are different genetic and epigenetic events involved that result in structural changes of the affected cells. In the early stages of the disease, these changes occur at the nanoscale, remaining undetectable by conventional light microscopy, due to diffraction-limited resolution (∼250 - 550 nm). In this sense, a technique termed partial wave spectroscopy (PWS) allows the detection of these nanostructural changes by measuring a statistical parameter called disorder strength (L d ). PWS uses a combination of a tunable filter and a camera to acquire the backscattering spectra for each pixel on the image. In this paper, we study and validate the possibility of obtaining a qualitative measurement of the disorder using the spectrum of the averaged spatial information. Instead of using spatial information and measuring sequentially spectral ranges, we measure the backscattered signal gathered by an optical fiber by means of a spectrograph. This will allow this method to be applied in systems where it is not possible to acquire a complete high resolution image for many spectral bands, while significantly enhancing speed.

Linear response deviations during recording of diffraction gratings in photopolymers.

In a holographic recording is advisable that the diffraction efficiency increases linearly with the exposure in a wide zone of the curve of diffraction efficiency versus energetic exposure and the slope of the curve must be approximately constant before saturation in order to improve the energetic sensitivity and to get reproducibility in different recordings with the same kind of photopolymer, although to find examples of deviations to this behavior it is usual. The more important deviation experimentally observed in photopolymers with high thickness happen when the first maximum in the curve is lower than the second one. This effect is opposed to an overmodulation. We present a main hypothesis related to the dye concentration into the layer and with the molecular weight of the polymer chains generated in the polymerization process in order to explain this effect.

New photopolymer holographic recording material with sustainable design.

Photopolymers that absorb in the visible spectrum are useful for different applications such as in the development of holographic memories, holographic optical elements or as holographic recording media. Photopolymers have an undesirable feature, the toxicity of their components and their low environmental compatibility, particularly if we analyse the life cycle of the devices made with these materials and their interaction with the environment. In this work we developed a new photopolymer with photochemical and holographic features similar to those of the standard material but with an improved design from the environmental point of view.

Physical and effective optical thickness of holographic diffraction gratings recorded in photopolymers.

In recent years the interest in thick holographic recording materials for storage applications has increased. In particular, photopolymers are interesting materials for obtaining inexpensive thick dry layers with low noise and high diffraction efficiencies. Nonetheless, as will be demonstrated in this work, the attenuation in depth of light during the recording limits dramatically the effective optical thickness of the material. This effect must be taken into account whenever thick diffraction gratings are recorded in photopolymer materials. In this work the differences between optical and physical thickness are analyzed, applying a method based on the Rigorous Coupled Wave Theory and taking into account the attenuation in depth of the refractive index profile. By doing this the maximum optical thickness that can be achieved can be calculated. When the effective thickness is known, then the real storage capacity of the material can be obtained.

3 Dimensional analysis of holographic photopolymers based memories.

One of the most interesting applications of photopolymers is as holographic recording materials for holographic memories. One of the basic requirements for this application is that the recording material thickness must be 500 microm or thicker. In recent years many 2-dimensional models have been proposed for the analysis of photopolymers. Good agreement between theoretical simulations and experimental results has been obtained for layers thinner than 200 microm. The attenuation of the light inside the material by Beer's law results in an attenuation of the index profile inside the material and in some cases the effective optical thickness of the material is lower than the physical thickness. This is an important and fundamental limitation in achieving high capacity holographic memories using photopolymers and cannot be analyzed using 2-D diffusion models. In this paper a model is proposed to describe the behavior of the photopolymers in 3-D. This model is applied to simulate the formation of profiles in depth for different photopolymer viscosities and different intensity attenuations inside the material.

Temporal evolution of the angular response of a holographic diffraction grating in PVA/acrylamide photopolymer.

In this paper we evaluate the temporal evolution, after exposure, of a diffraction grating stored in a PVA/acrylamide photopolymer. We also study the overmodulation of the refractive index inside the hologram, which gives rise to a particular behaviour of the angular response of diffraction efficiency. This evolution takes place in our photopolymer due to the incorporation of dimethylacrylamide (DMAA), which is a liquid at room temperature and so favours diffusion of the species in solution from the zones of greater concentration to those of lower concentration.

Angular responses of the first and second diffracted orders in transmission diffraction grating recorded on photopolymer material.

Some of the theoretical models in the literature describing the mechanism of hologram formation in photopolymer materials predict the existence of higher harmonics in the Fourier expansion of the recorded refractive index. Nevertheless, quantitative information is only obtained for the first harmonic of the refractive index using Kogelnik's Coupled Wave Theory. In this work we apply the Rigorous Coupled Wave Theory to demonstrate that when recording phase diffraction gratings in PVA/acrylamide photopolymer materials, a second order grating is also recorded in the hologram even when the material is exposed to a sinusoidal interference pattern. The influence of this second order grating on the efficiency of the first order for replay at the first on-Bragg angular replay condition is studied and the size of the 2nd harmonic examined.

Non-local polymerization driven diffusion based model: general dependence of the polymerization rate to the exposure intensity.

The nonlocal diffusion model proposed by Sheridan and coworkers has provided a useful interpretation of the nature of grating formation inside photopolymer materials. This model accounts for some important experimental facts, such as the cut-off of diffraction efficiency for high spatial frequencies. In this article we examine the predictions of the model in the case of a general dependence of the polymerisation rate with respect to the intensity pattern. The effects of this dependence on the different harmonic components of the polymerisation concentration will be investigated. The influence of the visibility on the different harmonic components will also be studied. These effects are compared to the effects of varying RD and sigmaD.

New theoretical matrix formula for intraocular lens calculation using the optimal bending factor.

By applying Gaussian optics we propose a new matrix formula model for intraocular lens (IOL) calculation that yields not only IOL optical power for a particular eye but also an evaluation of the bending factor of different IOL optical designs. This option improves IOL selection and results in the best possible optical image, a function dependent on the IOL's bending factor. This formula is a new approach to IOL calculation and may be useful in designing IOLs for abnormally hyperopic or myopic eyes. Optimal IOL power and design should both be considered in IOL calculation, which may improve the optical and clinical results of IOL implantation in the aphakic patient. The radius of the cornea is assumed to be spherical, and aspherical curves of the cornea may influence the optimal "bending" or shape factor of a lens.

Real-time interferometric characterization of a polyvinyl alcohol based photopolymer at the zero spatial frequency limit.

We characterize the optical modulation properties of a polyvinyl alcohol/acrylamide (PVA/AA) photopolymer at the lowest end of recorded spatial frequencies. To achieve this goal we have constructed a double beam interferometer in combination with the setup to expose the recording material. This is a novel approach since usually holographic recording materials are only characterized at high spatial frequencies. Some benefits are provided by the approach we propose: a direct calculation of the properties of the material is possible, and on the other hand additional information can be obtained since the results are not influenced by diffusion processes. Furthermore, this characterization is needed to optimize the PVA/AA photopolymers for another range of applications, such as recording of diffractive optical elements, where very low spatial frequencies are recorded. Different PVA/AA compositions and layer thicknesses have been analyzed. We have found that, depending on the layer characteristics, we can achieve high values of the phase-shift modulation depth and enhance the sensitivity of the material.

Polarization influences on the efficiency of noise gratings recorded in silver halide holograms.

The efficiency of noise gratings recorded with single-beam exposures in bleached silver halide emulsions is analyzed as a function of the polarization state of the readout wave. Experimental results are presented and discussed on the basis of a theoretical model that considers both a linear and a nonlinear relation between the refractive-index modulation and the exposure. The dependence of the noise gratings' efficiency on the relative polarization between the construction and reconstruction beams is analyzed, and we find that this is represented by a quadratic curve of cos(2) δ, where δ is the polarization angle. The quantitative analysis is carried out with this model based on the coupled-wave theory, and the calculated results obtained by use of the theoretical model agree well with the experimental ones.

Noise gratings in bleached silver halide diffuse-object holograms.

Noise gratings in diffuse-object holograms recorded as volume phase holograms in bleached silver halide emulsion are experimentally analyzed. Measurements of the diffraction efficiency and the noise of the holograms are taken. The experimental results obtained show that there is a drop not only in the diffraction efficiency but also in the noise that is due to the presence of noise gratings.

Entropy-based study of imaging quality in holographic optical elements.

A method for obtaining the best image plane for holographic optical elements by the use of the concept of entropy is described. This method is applied to in-line holographic lenses with different values of spherical aberration. Numerical results show that for holograms with large aberrations the best image plane (obtained by the use of the concept of entropy) is different from the minimum-aberration-variance plane.