Search

Low-toxicity solar concentrator systems represent an important challenge for outstanding photovoltaic (PV) applications. Particularly, multiplexed holographic lenses (MHL) as Holographic Solar Concentrators (HSC) provide insight into promising possibilities for Building-Integrated Concentrating PVs. This technology does not affect crucial ecosystems, and can convert buildings from energy consumers into energy suppliers. They can be used in windows, roofs, or walls, and a high diffraction efficiency and wide acceptance angle are desired. In this work, we presented several designs of MHL of low spatial frequency 525 lines mm−1, based on a low-toxicity photopolymer and supported on a window glass. The average diffraction efficiency of these HSC was evaluated at 633 nm, whereas the acceptance angle was evaluated by measuring the short-circuit current under solar illumination at different incident angles. Versatile and high-efficiency holographic elements have been used to concentrate sunlight from different relative positions during the day, avoiding the need for expensive tracking systems. To the best of our knowledge, this is the best trade-off between high diffraction efficiency (85%) and wide acceptance angle (104∘) in a low-toxicity holographic solar concentrator.

In the present work, the viability of a novel recording geometry to produce reflection holographic couplers is analyzed. Recalling the idea of previous works, photopolymers are used as the recording material because they have been proven to be well-suited for the intended see-through application: the capability to provide a virtual image without compromising the information about the surrounding environment. Moreover, holography fundamentals give us the proper background to examine the proposed design, where no prisms or microlenses arrays are used. Aiming to support the analysis, we provide experimental evidence that the produced reflection holographic gratings exhibit the correct properties to work as a coupler, where the sensitivity of the material and its properties are studied and examined.

It is well known that Spatial Light Modulators (SLM) are of great interest and used in many different areas because of their continuous capability of light modulation and their potential as a linear variable retarder. However, high-definition SLM devices face the challenge of providing high performance under the influence of different phenomena, such as crosstalk between adjacent pixels, fringing fields, and diffraction effects arising from the finite pixel grid pattern. In this work, a numerical analysis that computes the director orientation as a function of the retardance across specific voltages (gray level) is compared to experimental results. More specifically, this work is focused on analysing how previously mentioned phenomena affect the interpretation of the residual twist angle in parallel-aligned (PA) on SLM. This twist angle is computed from the Stokes parameter and compared to the actual orientation of the director distribution.

El espesor de la capa y la rigidez o flexibilidad del polímero formado tras la polimerización son dos factores importantes a tener en cuenta dependiendo del elemento óptico holográfico (EOH) a desarrollar. Para aplicaciones de EOH tipo lente es habitual que el polímero tenga un espesor de capa inferior a 50 micrómetros y una flexibilidad de capa muy limitada. El espesor de la capa influye en el rendimiento en difracción del EOH y por tanto en el rendimiento que consiga el propio elemento al realizar su función durante su uso. Por otra parte la flexibilidad de la capa es un parámetro de gran importancia en el caso de que el EOH vaya unido a soportes con curvatura. Una baja flexibilidad impide la adaptación del EOH al soporte, dificultando o impidiendo su uso en la práctica. En este trabajo se estudian las alternativas en cuanto a monómeros y entrecruzantes que permitan un espesor de capa mayor y una flexibilidad adecuada para adaptarse a soportes curvados.

The behaviour of a parallel-plate capacitor connected to a sinusoidal alternating voltage is analysed and the electric and magnetic fields, as well as the electric and magnetic energies inside the capacitor are obtained. Aspects related to Maxwell's equations, the Poynting vector and the conservation of electromagnetic energy are also discussed. Under certain conditions the electromagnetic energy stored in the capacitor remains constant and the behaviour of the capacitor at high frequencies is like that of an oscillating L-C circuit consisting of an inductor and a capacitor. The general situation is considered, valid for both low and high frequencies, and what is discussed here is of interest to students of an undergraduate electrodynamics course.

The see-through application of holographic optical elements (HOEs) has gained a lot of attention in the scientific literature in recent years. Regarding this kind of holograms recorded in photopolymers, the shrinkage plays an important role in the final properties of the display, as it may alter the Bragg angle. The present work is focused on the characterization of a HOE as a coupler in a waveguide combiner, recorded in holographic transmission geometry without prisms in the experimental set-up. From this point of view, it has been found that for specific geometries where the diffracted beam cannot be trapped a priori, a significant coupling effect exists: this variation in the diffracted energy and the propagation angle inside the glass can be used to measure the shrinkage of the photopolymer for this HOEs recording.

It is a fact that augmented reality or virtual reality has burst into society with force: allowing the creation of programmed video games to be played with virtual reality glasses or the manufacture of glasses that act as mobile devices connected to the Internet ("Google glasses"). Augmented reality devices generally consist of a headset and a display system to show the user virtual information in addition to the real one. The systems used are the transparent optical screen and the mixed video screen. In this way, virtual images can be used that are shown to the user, mixed with reality or projected directly on the screen. On the other hand, the "glasses" have some advantages in relation to a direct viewing mobile screen, for example, hands-free and high privacy character. Among the different models of glasses proposed, transparent glasses are especially important for mobile use because they are safer and easier for the public to accept. However, combining high-quality images with clear visibility throughout the system remains a challenge. In this work, holographic waveguides will be manufactured in photopolymers and the experimental results will be contrasted with the numerical simulations carried out using the RCW models. An important fact that must be taken into account in the recording of the holographic waveguide is the possible recording of noise reflection gratings. In general, noise gratings in other devices extract a small amount of energy from the diffracted order, without any other spurious effects. However, in this particular device it will be shown that noise reflection gratings could create an unwanted afterimage. Since the reconstruction is performed at a different wavelength than the recording, the reflection noise grating may or may not be "visible" during the reconstruction, so it is possible to control the appearance of the gratings, for example, by controlling the thickness of the final waveguide. The gratings to performed the study were recorded

Nowadays, the development of optical systems applied to the generation of clean and renewable energy is one of the great challenges of the 21st century. This work presents a theoretical and experimental study of shrinkage in multiplexed holographic lenses (MHL) stored in a low-toxicity photopolymer named Biophotopol. For this purpose, a study has been carried out using k-space tools. In addition, a study has been performed to optimize the number of MHLs, the hologram thickness, and the angular distance between peaks. To evaluate the efficiency, the short-circuit current has been evaluated under solar illumination with different incident reconstruction angles.

Today, the advancement of optical systems that can harness clean and renewable energy sources is a major focus for researchers and innovators worldwide. As we strive to create a sustainable future, this challenge has become increasingly critical to our success. Fresnel lenses are widely used as traditional concentrators, but they have a small acceptance angle, and the reflective elements need continuous maintenance of the surface reflectivity. Transmitting Holographic Optical Elements (HOEs) are an alternative to conventional lenses because they are more economical and versatile. Their material is usually a flexible photopolymer so that the optical element can be attached to different types of support, depending on whether one type of handling is required or another, and they tend to have low weight and volume, as well as a simple way of manufacturing. In addition, also provide an extended focusing area which helps to protect solar cells from heating damage. A theoretical and experimental study on the shrinkage of multiplexed holographic lenses (MHL) that were stored in a low-toxicity photopolymer was carried out. To accomplish the study, a K-space tool was used. Furthermore, an optimization analysis of the angular distance between peaks was performed. To determine efficiency, an evaluation of the short-circuit current under solar illumination with varying incident reconstruction angles was done.

Holography offers a vital platform for optical information storage and processing, which has a profound impact on many photonic applications, including 3D displays, LiDAR, optical encryption, and artificial intelligence. In this review, we provide a comprehensive overview of optical holography, moving from volume holography based on optically thick holograms to digital holography using ultrathin metasurface holograms in nanophotonics. We review the use of volume holograms for holographic multiplexing through the linear momentum selectivity and other approaches and highlight the emerging use of digital holograms that can be implemented by ultrathin metasurfaces. We will summarize the fabrication of different holographic recording media and digital holograms based on recent advances in flat meta-optics and nanotechnology. We highlight the rapidly developing field of metasurface holography, presenting the use of multi-functional metasurfaces for multiplexing holography in the use of polarization, wavelength, and incident angle of light. In the scope of holographic applications, we will focus on high bandwidth metasurface holograms that offer the strong sensitivity to the orbital angular momentum of light. At the end, we will provide a short summary of this review article and our perspectives on the future development of the vivid holography field.

Holography is a broad field of research that interacts with a wide range of disciplines, from physics and chemistry to art through engineering, as shown by the large number of publications that can be found on the subject. Holography is integrated into our daily lives, often associated with the rainbow-like effect on banknotes, credit cards, and ID cards. Let us also consider that holography involves many optical phenomena (coherence, diffraction, interference, analogue and digital recording devices, lasers, materials, etc.). Therefore, holography is an extraordinarily productive and attractive field as a research area. Furthermore, holographic devices facilitate the development of many applications, some of which are reviewed in the present paper.

Since photopolymers were first used as a holographic recording medium in the 1970s, one of the most important goals to be achieved was a quantitative understanding of their behaviour. In general, photopolymers are composed of one or more monomers, a binder and a dye that absorbs light in a certain region of the spectrum. The absorbed photons initiate polymerisation in the bright regions creating a concentration gradient compensated by the diffusion of chemical molecules described by Fick’s law. The first models to simulate the etching process in this type of materials were proposed in the early 1990s; they were very simple models where the authors assumed a harmonic in the monomer concentration, a constant rate of polymerisation and diffusion without direct influence of the refractive index of the monomer and the polymer. From this point on, improvements started to be made in the so-called “diffusion models”. One of the most important breakthroughs, proposed by John Sheridan’s research group in 2000, is the non-local behaviour of the photopolymer due to the finite size of the polymer chains. This phenomenon affects the limit of holographic recording resolution. Our research group at the University of Alicante (Spain) had the honour to collaborate with John Sheridan’s group developing new advances in the modelling of photopolymers as an optical recording medium, such as the threedimensional expansion of the models, and the explanation of surface variations or models to explain the recording of diffractive optical elements in this type of materials. In this papSer, we review this collaboration to improve diffusion models applied to photopolymers.

Nowadays, the study and optimization of volume holographic lenses (HLs) stored in low-toxicity photopolymers have a great interest. HLs are now a component of optical imaging systems that are mostly used in head-mounted displays for virtual and augmented reality or as non-image systems in light deflectors and concentrators. One of the most important parameters used when working with imaging systems is the resolution of the optical system. In this work, the similarity between the object and image of negative asymmetrical HLs stored in a low-toxicity photopolymer named Biophotopol has been evaluated theoretically and experimentally. For this purpose, the resolution of the HLs was calculated using the Convolution Theorem. A USAF 1951 test was used as an object and the impulse responses of the HLs were obtained with two different sensors: CCD and Hartmann-Shack (HS) wavefront sensor. In addition, the resolution of the HLs has been obtained by two different methods: one using the Convolution Theorem, using both the CCD and the HS wavefront sensor, and the other by forming the USAF test image on the CCD sensor. Finally, a theoretical study of object-image similarity was carried out using the MSE (mean squared error) metric to evaluate the quantitative experimental results.

We print a tunable photopolymer (photopolymer dispersed liquid crystal -PDLC), using the laser-induced direct transfer technique without absorber layer, which was a challenge for this technique given the low absorption and high viscosity of PDLC, and which had not been achieved so far to our knowledge. This makes the LIFT printing process faster and cleaner and achieves a high-quality printed droplet (aspheric profile and low roughness). A femtosecond laser was needed to reach sufficiently peak energies to induce nonlinear absorption and eject the polymer onto a substrate. Only a narrow energy window allows the material to be ejected without spattering.

In the last few years, the interest in storing volume holograms in photopolymers has increased enormously due to their applications in industry, the medical field, security, or renewal energy among others. The production of environmentally compatible photopolymers is one of the main focuses of Holography research. In this work, we have studied how to increase the diffraction efficiency of reflection holograms stored in a low-toxicity PVA-based photopolymer called Biophotopol. The holographic material has been doped with different types of nanoparticles (NPs) to achieve an increase in the refractive index modulation during the recording stage. Metallic NPs, obtained by physical and electrochemical methods have been used. The results obtained with all of them have been compared as a function of the concentration used, the size of the NPs, and the stabilization method used for their synthesis. A considerable increase in diffraction efficiency has been achieved by using NPs in the low-toxicity material. By using high refractive index NPs, the average refractive index of the holographic material increases and consequently the diffraction efficiency.

Photopolymers are designed and engineered with versatile applications including optics and photonics. Holography is one of the classical porpoises that use photopolymers as holographic recording materials. The success of these materials can be seen in the market with the photopolymer fabricated by Covestro. Some of these holographic applications require a long-time life of the holograms recorded in photopolymers. Nevertheless, initial tests of Covestro holograms show significant degradation after less than one year of exposure even after sealing and degradation occurs under solar light exposition. In this sense, it is important to perform deeper studies of the different possibilities for hologram conservation. Usually, the first step after recording is the material cure, with UV or visible light, to eliminate the residual dye and monomer. With this process high efficiency holograms can also be obtained. Afterwards, an index matching technique can be used to cover the material with a glass or it is possible the application of aerosol sealant. In this paper we analyze the introduction of holograms between two glasses linked by pressure, using Bayfol HX 200 from Covestro as the recording material. In order to characterize the process, four different spatial frequencies were tested, which were stored either by transmission or reflection schemes. The data of the reconstruction step has been measured before and after the encapsulation. In addition, multiple holograms have been superposed in the same glass, where we have found that shrinkage is more significant.

En este trabajo presentamos diferentes características que debemos tener en cuenta cuando queremos evaluar el retardo de una lámina retardadora variable, o de cualquier dispositivo que se pueda comportar como una de estas láminas. En nuestra experiencia, y debido a que están involucradas diferentes funciones trigonométricas, hemos encontrado que siempre aparece cierta incertidumbre en el retardo medido que está limitado por los valores obtenidos de dichas funciones. Intentaremos contextualizar los resultados con los diferentes modos de desenrollado que podemos utilizar y ver en qué punto se produce dicho enrollado, así como las consideraciones que deberemos tener en cuenta para evaluar el retardo para que se ajuste a lo que realmente estamos midiendo., In this work we present different aspects that we have to manage when we want to evaluate the retardance introduced by a variable waveplate, or any device that behaves in that way. In our experience, and due to the trigonometric functions that are involved, we have find that we always appears some uncertainty in the measured retardance that are limited by the values obtained by the used trigonometric functions. We will try to contextualize the results with the different methods of unwrapping that we can use and at what point the wrapping occurs. We will make some considerations to consider making the evaluated retardance closer to the real retardance introduced by the waveplate.

The study of liquid crystal (LC) director distribution is an important area of research in materials science and technology. Parallel-aligned liquid crystal (PA-LC) devices have been extensively studied due to their applications in liquid crystal displays, optical devices, and sensors. Estimating the LC director distribution is a critical step in designing and optimising PA-LC devices. This work shows the results derived from apply- ing novel optimisation techniques to estimate the liquid crystal (LC) director distribution in parallel-aligned liquid crystal (PA-LC) devices. Moreover, the genetic algorithm (GA) has been applied and compared with the minimisation of the Frank-Oseen free energy through the Euler-Lagrange equations. The GA is a stochastic optimisation technique that can effectively explore the search space and find the global optimum. Overall, this study’s results demonstrate the GA’s effectiveness in estimating the LC director distribution in PA-LC devices. This approach can improve the performance and design of liquid crystal displays, optical devices, and sensors. Furthermore, it can be extended to other fields where the optimisation of complex systems is required. Further research is needed to optimise the GA parameters and to explore its potential in other applications.

The phase-shift exhibited by liquid crystal on silicon devices (LCoS) depends on the voltage applied and the illumination wavelength. Most of the LCoS used in the labs are digitally addressed using a binary pulse width modulated signal. Usually, these devices are characterized for a very small range of the available binary voltage values and for specific wavelengths. In this work, we consider a commercial parallel-aligned liquid crystal on silicon device (PA-LCoS) in which the binary voltages are accessible through the software of the vendor. We perform a complete averaged Stokes polarimetric characterization of the device where we are able to obtain the absolute unwrapped retardance values for a wide range of voltage parameters and across the visible spectrum. This provides a practical approach to evaluate the whole range of phase modulation possibilities, and to analyze some issues related with the physics of the device.

In the present work, the viability of a novel recording geometry to produce reflection holographic couplers is analyzed. Recalling the idea of previous works, photopolymers are used as the recording material, as they are well-suited for the intended see-through application. Moreover, Kogelnik’s theory fundamentals give us the proper background to examine the proposed design, where no prisms or microlenses arrays are used. Aiming to support the analysis, we provide experimental evidence that the produced gratings exhibit the correct properties to work as a coupler.

In the last decades, new technology fabrication developments have permitted increased resolution and reduced pixel size of Liquid crystal on silicon (LCoS) microdisplays. However, the pixel size reduction triggers the microdisplay performance degradation due to different phenomena, such as the cross-talk between neighbouring pixels, fringing fields, out-of-plane reorientation of the liquid crystal director, and diffraction effects due to the pixelated grid pattern of the microdisplay. In this work, a full 3D simulation model has been applied to predict the liquid crystal director orientation as a function of space and external voltage. The scheme here considered provides the complete vectorial information of the electromagnetic field distribution produced by one single pixel illuminated by plane waves circularly polarised. This analysis is carried on for several pixel and gap sizes for different external voltages. This research focuses on S2 and S3 Stokes parameters and how their behaviour is affected due to the cross-talk phenomena previously presented.