KEYWORDS: Holographic materials, Solar thermal energy, Gelatin, Photovoltaics, Solar radiation, Sunlight, Solar cells, Lenses, Holography, Solar energy
This paper provides a broad overview of historical and current developments of holographic lenses (HLs) in concentrating solar power. The review focuses on the recording material typically employed for HLs in concentrating solar photovoltaic and/or concentrating solar thermal collectors. This review shows that the use of HLs for energy transformation, achieves high performance efficiency in the designed physical systems, furthermore, some important elements to consider for future designs are presented, especially those related to the etching material of the HLs. Finally, the article outlines future recommendations, emphasizing potential research opportunities and challenges for researchers entering the field of photovoltaic and/or concentrating solar thermal collectors based on HLs.
Nowadays, one of the challenges in obtaining competitive photovoltaics is to achieve lightweight, low-cost, and free-tracking systems. It is also desired to eliminate wavelengths that can damage the photovoltaic cell by overheating. To this end, volume holographic lenses (HLs) allow controlling the solar radiation that hits the photocell, avoiding harmful radiation that can damage photocells, such as infrared radiation, which heats the solar cells, but does not efficiently convert solar energy into electrical energy. In addition, holographic solar concentrators based on multiplexed HLs have the advantage that they do not require any solar tracker. In this work, we present the optimization of the relevant conditions in the fabrication of multiplexed HLs stored in Biophotopol. These parameters refer to both the material factors (optimal concentrations of dye and monomer, thickness) and the optical recording factors (optimal number of multiplexed HLs, angular distance peak-to-peak, and exposure times). Finally, a theoretical study of the exposure times has been done using the exposure schedule method (ESM) to improve the average diffraction efficiency.
The development of new holographic materials represents a field of research in continuous evolution. The design and synthesis strategy of these materials must respond to the desirable characteristics of the photonic devices that need to be manufactured. Hydrogels are used as binders in applications in which the material must retain its structural integrity in liquid media. In applications where it is required to store volume holograms in hydrogels, it is necessary to carry out a first incubation process in solutions containing the necessary components. The general aim of this work has been to optimize the composition of incubator solutions composed of acrylamide as monomer, N,N′-methylenebis(acrylamide) as crosslinker, triethanolamine and yellow eosin as photoinitiator system and DMSO/water as solvent. The effect of varying the monomer and crosslinker on the diffraction efficiency of stored holograms has been investigated.
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.
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.
The storage of time-stable holographic gratings in photohydrogels when the material is immersed in liquid media represents a great challenge at present. A very important stage in the process of storing holograms in photohydrogels are the washing stages to eliminate the remains of the components that have not reacted in the photochemical reaction. The main goal of this work is focusing on the study of the optimization of the washing stages of the photohydrogels based on acrylamide and N,N’-methylenebis(acrylamide) once unslanted transmission holograms have been stored. For the purpose of determining the compositions of the wash solutions, High-Performance Liquid Chromatography and UV-visible measurements have been employed in our system. PBST and DMSO:H2O 6:4 (v/v) are used as solvents in the washing stages. The diffraction efficiencies are measured during the washing stages and after the storing of the holograms during several days in PBST. Maximum diffraction efficiencies of 38.0 and 27.6% are reached when PBST and DMSO:H2O 6:4 are employed, respectively.
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.
KEYWORDS: Holographic concentrators, Multiplexing, Diffraction, Solar cells, Holography, Sunlight, Solar concentrators, Photovoltaics, Holographic optical elements, Solar energy
Solar concentrator systems represent an important challenge in our society for outstanding photovoltaic (PV) applications. Fresnel lenses or parabolic mirrors concentrate sunlight in a small solar cell surface. On the one hand, Fresnel lenses have an exceedingly small acceptance angle and require expensive tracking systems to follow the path of the Sun. On the other hand, conventional parabolic mirrors need periodic maintenance of the surface reflectivity. Holographic optical elements (HOEs) represent a suitable alternative to Fresnel lenses and solar reflectors, they are cheaper and more versatile. Particularly, multiplexed holographic solar concentrators (HSCs) give an insight into promising possibilities for Building-Integrated Concentrating PV (BICPV). A good trade-off between wide acceptance angle and high diffraction efficiency represents an important milestone in the area. Our research group obtained the higher acceptance angle in a multiplexed HSC design (Morales et. al. Opt. Express 30, 25366 (2022)). This design was composed of seven holographic multiplexed lenses in Biophotopol material with thick thickness, 197 μm. In the present work, more efficient holographic solar concentrators than previous works are shown. As far as we know, it has been obtained the best trade-off between high efficiency and wide acceptance angle HSC-PV solar cell systems.
The technology we are developing consists of the use of coordination compounds with metals to carry out this detection of Acetic acid. This compound normally reacts with acetic acid changing its colour, making it a suitable compound for use as a detector. The proposed method allows detecting acetic acid in any medium, whether in solution, in the gas phase, in the solid phase, or in any combination of these. Upon contact with the acid, a colour change occurs that can be detected visually or through optical means. After its use, the active medium can be regenerated by a simple procedure and be available again for new use. This allows the creation of simple and intuitive detection devices, usable by non-experts and that can be regenerated and reused. The main advantage of this sensor is to allow the specific detection of acetic acid and quantification of its concentration, using coordination compounds with metals that are present in the yellow dye.
The study of the optical properties and behaviour of holograms stored in hydrogel matrices when the material is immersed in liquid medium represent a very important challenge currently. Hydrogels are 3D polymer networks capable of undergoing reversible volume changes. These hydrogels can be chemically modified to obtain materials with different properties such as to be sensitive to a range of relevant analytes. Emergent applications require that the holograms stored in hydrogels be time-stable in a liquid medium. One of the most important applications of this type of system are holographic sensors. Holographic sensors have advantages over other types of sensors such as the possibility of miniaturization due to the use of holographic techniques, the ability to produce three-dimensional images, real-time quantification, possibility of low-cost mass manufacturing and label-free analyte- responsive. Due to these advantages, these sensors have great potential to be used in different areas such as environmental detection, veterinary testing, pharmaceutical bioassays and medical diagnosis. Therefore, the optical behaviour of the holograms and the optimization of the hydrogel’s matrices must be well studied. When volume phase holograms are stored in hydrogels matrices in liquid medium, the holographic planes can undergo a bending process that give rise to asymmetries in the lateral lobes around the Bragg angle. This bending affect to the diffraction efficiency, wavelength of maximum diffraction efficiency and the angular sensibility. These parameters are used as signal transducers in holographic sensors in transmission mode. The general aim of this work has been study the bending that is produced in the holographic planes when unslanted transmission volume phase holographic grating with a frequency of 1200 lines/mm have been stored in hydrogel matrices based on acrylamide (AA) and N,N'-methylenebisacrylamide (MBA) using different solvents (dimethyl sulfoxide, water and buffer solution) during the manufacturing process. Considering previous works on bending, and grating attenuated hologram.
Parallel-aligned liquid crystal on silicon devices (PA-LCoS) can be found nowadays in most of the advanced areas in optics and photonics. Many works have been dedicated to their characterization for optimum utilization in applications. However, usual techniques are based on diffractive or interferometric measurements. Recently, we proposed the use of Stokes polarimetry for a versatile yet easy to implement characterization. We show that the LCoS can modelled as a nonabsorbent reciprocal device which, combined with time-average Stokes polarimetry, enables to demonstrate robust measurements across the whole applied voltage range for the retardance and its flicker. One of the main novelties is that we also obtain the director orientation, which we show that changes across the voltage range, especially at larger applied voltages. This might affect in very sensitive applications. It might also provide a deeper insight into the internal dynamics in the LC layer.
In this work it is shown the first characterization of holographic solar concentrators recorded in Biophotopol - one of the greenest photopolymers. Biophotopol is an acrylate-based and water-soluble photopolymer with good recycling properties. The composition of this photopolymer and their thickness are easily changeable, which implies an important advantage vs. others commercialized photopolymers. Good diffraction efficiency and wide acceptance angles are achieved on phase volume transmission holograms by using an optimized composition and thin layers. A curing stage with a white incoherent light has been performed to obtain high temporal stability together with a good diffraction efficiency. Finally, the performance of the holographic lenses as holographic solar concentrators has been evaluated with an electronic setup connected to a polycrystalline silicon photovoltaic cell and a high intensity solar simulator emitting a standard solar spectrum (AM1.5G).
The development and optimization of photopolymers in which time-stable holographic grating can be stored when the material is immersed in an aqueous medium represent a challenge at present. In this sense, the aim of this work was the fabrication of unslanted transmission gratings in a hydrogel matrix which incorporates in its molecular structure a monomer (2-Methacryloyloxyethyl phosphorylcholine) that can act as an anti-fouling agent. The dependence of the diffraction efficiency on the thickness of the hydrogel matrix and the stability of the transmission gratings immersed in water were also studied. An increment in the diffraction efficiency up to 47 % was observed after a washing stage. After two days immersed in water, the holograms showed high stability even though the diffraction efficiency decreased to 37%. The optical parameters were obtained by fitting procedure through Kogelnik’s coupled wave theory.
Phase spatial light modulators and, in particular, parallel-aligned liquid crystal on silicon (PA-LCoS) microdisplays are widely used to display programmable diffractive optical elements (DOEs). These are pixelated elements with inherent different characteristics when compared with DOEs produced with micro-optics fabrication techniques. Specifically, programmable DOEs may be affected by the fill factor, time-flicker, fringing-field and interpixel cross talk effects, and limited and quantized modulation depth of the LCoS device. Among the multilevel DOEs, we focus on the important case of the blazed gratings. We develop the corresponding analytical expressions for the diffracted field where, as novelties of this work, fill factor and flicker are introduced together with phase depth and the number of quantization levels. Different experimental-based normalizations are considered, which may lead to wrong conclusions if the fill factor is not considered in the expressions. We also analyze the differences arising between one- and two-dimensional pixelated devices. When compared with numerical procedures, our approach provides an analytical expression that facilitates the design, prediction, and discussion of experiments. As an application, we prove, for the limiting case of no interpixel cross talk, that multiorder DOEs cannot be more efficient than the equivalent single-order DOE. We also show how the results for DOEs with a unit fill factor can be adapted to DOEs with a fill factor smaller than one with a very efficient procedure.
Holographic gratings stored in low-toxicity photopolymer, Biophotopol, have been analyzed to achieve stable and efficient holograms. A curing process allows the hologram stabilization, but at the same time, it could produce a diffraction efficiency (DE) reduction. Here, a detailed low-cost LED curing protocol is shown to stabilize over time 1205 l/mm transmission holograms, and at the same time, a 33% DE increment (with respect non-curing holograms) have been demonstrated. Finally, to obtain a better understanding of DE change, a theoretical fit of our experimental result, based on Kogelnik’s coupled wave theory was carried out and discussed.
Parallel-aligned liquid crystal on silicon (PA-LCoS) microdisplays are widely used in spatial light modulation applications, especially in those requiring phase-only modulation. One such application area is programmable diffractive optics which plays a very important role in modern optical imaging systems or in optical interconnections for optical telecommunications. Among the multilevel diffractive optical elements (DOEs) we focus on the important case of the blazed gratings. We develop the corresponding analytical expressions for the diffracted field where, as one of the novelties in the work, an analytical expression including the fill factor and the flicker is obtained. This enables to have a model against to compare the experimental results in a number of situations where fill factor, flicker, period, and number of quantization levels are the variables. This also enables to design appropriate compensation techniques to enhance the performance of the blazed gratings.
Thin film organic lasers (TFOLs) represent a new generation of inexpensive, mechanically flexible devices with demonstrated applicability in numerous applications in the fields of spectroscopy, optical communications and sensing requiring an organic, efficient, stable, wavelength-tunable and solution-processable laser material. A distributed feedback (DFB) laser is a particularly attractive TFOL because it shows single mode emission, low pump energy, easy integration with other devices, mechanical flexibility and potentially low production cost. Here, amplified spontaneous emission (ASE) and DFB laser applications of novel high performing perylene dyes and p-phenylenevinylene (PV) oligomers, both dispersed in thermoplastic polymers, used as passive matrixes, are reported. Second-order DFB lasers based on these materials show single mode emission, wavelength tunability across the visible spectrum, operational lifetimes of >105 pump pulses, larger than previously reported PV oligomers or polymers, and thresholds close to pumping requirements with light-emitting diodes.
Distributed feedback (DFB) laser sensors with active films consisting of a highly efficient and photostable perylenediimide dye (perylene orange, PDI-O) dispersed in polysytrene (PS), used as passive matrix, are reported. PDIdoped- PS DFB lasers show an excellent operational durability under ambient conditions, superior to those of previously reported DFBs used for sensing purposes. Their bulk refractive index sensing capabilities, under exposure to liquids of different refractive index, have been determined from changes in their emission wavelength. The role of the active film thickness on both, the laser and the sensing performance, has been explored. The use of a thick active film (850 nm) allows obtaining the lowest possible threshold and highest operational lifetime for this type of device although the sensor sensitivity is lower than that achievable with a thin film (160 nm). It is also shown that the inclusion of a high refractive index TiO2 layer on top of the sensor structure allows improving the sensor sensitivity by around two times.
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