Javier Alda
Universidad Complutense de Madrid, Optics, Faculty Member
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Summary The perception of Moirefringes is studied with two different tests for binocular and monocular conditions. The results are negative for binocular conditions, and positive for the monocular case when afterimages are used. # 1999... more
Summary The perception of Moirefringes is studied with two different tests for binocular and monocular conditions. The results are negative for binocular conditions, and positive for the monocular case when afterimages are used. # 1999 The College of Optometrists. Published by Elsevier Science Ltd. All rights reserved
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Electrically biased metal nanostructures are at the core of innovative multifunctional integrated devices that control the flow of electrons and photons at the nanoscale. They are based on plasmonic structures that create strongly... more
Electrically biased metal nanostructures are at the core of innovative multifunctional integrated devices that control the flow of electrons and photons at the nanoscale. They are based on plasmonic structures that create strongly confined fields, typically associated with large temperature gradients. These thermal effects may generate artifact responses detrimental to the desired operation. We show here how a biasing polarity and a local optical excitation asymmetry of a generic geometry – a nanoscale constriction – interplay thermally to modify the diffusive electron transport in out-of-equilibrium conditions. Our experimental results are accompanied with computational electromagnetism and multiphysics simulations.
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Real ray tracing is an essential step to obtain numerical quality performances in optical systems and is also involved in its optimization procedure. Real ray tracing does a complete description of the light ray for a selected number of... more
Real ray tracing is an essential step to obtain numerical quality performances in optical systems and is also involved in its optimization procedure. Real ray tracing does a complete description of the light ray for a selected number of rays; moreover, real ray tracing mixes the intuitive approach of ray propagation with the right light. Real ray tracing provides a powerful tool to reach, at the same time, information about an individual light trajectory and about all the optical system. Real ray tracing is based on the knowledge of Snell law and the geometric description of the surfaces used to specify the boundaries of different media glass. Both calculations are difficult to do; so in case of exact calculation, the reliability of a given algorithm or program is strongly based on how many rays are traced from the input to the output plane: the larger the number of rays, the better the results. Although the foundations of ray tracing are the simple rules of geometrical optics, the scientists and engineers involved in optical design have made successive refinements to include both the energy carried out by the light and also the wave nature of the electromagnetic radiation. In this contribution, we present the rules of real ray tracing step-by-step; these rules are explained in a clear language and same examples are included for the first-time readers. Real ray tracing is presented in a selfexplanation form to make this contribution readable in itself. The Snell law is revisited and adapted for its implementation in a numeric algorithm. The spot diagrams resulting from a real ray-tracing calculation are discussed. The wave nature of light propagation can be also approximated by using real ray tracing. Therefore we show the method to calculate a geometrical wave front and also its intrinsic limitations. Linked with the spot diagrams, it is possible to evaluate a point spread function. Also, an optical transfer function and a modulation transfer function are defined within this geometrical approach. The evaluation of the flow of energy through the system is discussed within the ray-tracing framework. Some hints about the inner mechanisms used by the optimization procedures of optical systems are discussed and related with a basic classification of the software packages currently used.
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Research Interests: Philosophy and Humanities
The inactivation of pathogens through the irradiation of ultraviolet light depends on how light propagates within the medium where the microorganism is immersed. A simple geometrical optics analysis, and a fluence evaluation reveal some... more
The inactivation of pathogens through the irradiation of ultraviolet light depends on how light propagates within the medium where the microorganism is immersed. A simple geometrical optics analysis, and a fluence evaluation reveal some reservoirs where the pathogen may hide and be weakly exposed to the incoming radiation. This geometrical hide-outs also generate a tail in the plot of the total inactivation plot vs. the incoming fluence. We have analyzed these facts using geometrical optics principles and illumination engineering computational packages. The results obtained from previous biomedical measurements involving SARS-CoV-2 have been used to evaluate the inactivation degree for an spherical geometry applicable to airborne pathogens, and for an spherical cap geometry similar to that used in biomedical experiments. The case presented here can be seen as the worst-case scenario applicable under collimated illumination.
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In this paper, an optimized design of (FAPbI3) 1-x (MAPbBr3) x perovskite solar cell is numerically investigated using SCAPS-1D software package. A variety of potential charge transport materials are investigated. Cu 2 O as HTL and ZnO as... more
In this paper, an optimized design of (FAPbI3) 1-x (MAPbBr3) x perovskite solar cell is numerically investigated using SCAPS-1D software package. A variety of potential charge transport materials are investigated. Cu 2 O as HTL and ZnO as ETL outperform other choices; they are therefore considered as the best candidates. The impact of the electronic properties of both ZnO/perovskite and Perovskite/Cu 2 O interfaces on the solar cell performance is thoroughly investigated. We discovered that appropriate values of the conduction band offset (CBO + = 0.29) and valence band offset (VBO + = 0.09) assure a "spike-type" band alignment at both interfaces. This choice lowers the unwanted interfacial recombination mechanism, resulting in a challenging PCE. In addition, the impact of the work function of back contact is also investigated. According to simulation findings, Ni back electrodes with a work function of 5.04 eV is appropriate for Zn 0.8 Mg 0.2 O/ (FAPbI 3) 0.85 (MAPbB 3) 0.15 /Cu 2 O perovskite solar cell. The optimized FTO/MgZnO/(FAP-bI 3) 0.85 (MAPbBr 3) 0.15 /Cu 2 O/Ni PSC reaches a conversion efficiency as high as 25.86%. These findings will pave the way for the design of low-cost, high-efficiency solar cells.
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Plasmonic nanoantennas are currently the subject of many theoretical and experimental investigations due to their unique properties and potential applications. In this work, a new type of plasmonic nanoantenna the discrete bowtie... more
Plasmonic nanoantennas are currently the subject of many theoretical and experimental investigations due to their unique properties and potential applications. In this work, a new type of plasmonic nanoantenna the discrete bowtie nanoantenna is introduced, this new type of nanoantenna is composed of discrete elements which are planar metallic nanodisks of different sizes arranged in the shape of a standard bowtie nanoantenna. In this new geometry, we show numerically and experimentally that a large local field enhancement, higher than its classical counterpart, can be obtained. In addition, it was found that the discrete bowtie nanoantenna provides several resonances at different frequencies as a consequence of its structural geometry, these features provide significant improvements and advantages over the standard bowtie nanoantenna which can be useful for certain applications such as Surface Enhanced Raman Spectroscopy (SERS) and other biosensing techniques.
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Boston, Massachusetts (USA), November 29-December 4, 2015; http://www.mrs.org/fall2015/Since several years ago, light is considered as a new way for futuristic computation, overcoming current limitations of electronics [1]. Researchers... more
Boston, Massachusetts (USA), November 29-December 4, 2015; http://www.mrs.org/fall2015/Since several years ago, light is considered as a new way for futuristic computation, overcoming current limitations of electronics [1]. Researchers are currently working in the development and implementation of photonic devices that behave as a counterpart of electronic devices, in the so-called field of Metatronics [2]. One of the main challenges of this field is the implementation of optical circuitry. In this sense, photonic equivalent resistors, capacitors and inductors are needed. Engheta and coworkers [3] showed that nanostructures with convenient effective optical properties present impedance properties similar to that of these lumped elements. However, the main handicap of these devices lies on their complex implementation as well as their passive properties. Current research on semiconductor and semimetal nanostructures is showing new impressive properties, such as optical magnetism [4]. In this work, we focus our attention on the impedance properties of bismuth (Bi) nanospheres. By a simulation tool implementing the finite elements method (COMSOL ©), we observe that these nanoparticles have a remarkable capacitive impedance in the near-infrared part of the spectrum. Changing the particle size, the value of the impedance can be tuned through a wide spectral range. The simplicity of the structure and its fabrication makes it a good candidate as integrated nanocapacitor in potential optical circuits. In addition, by heating the nanoparticle over its melting point (T=2710C), its impedance changes fastly from purely capacitive to a purely inductive. [1] D.A.B. Miller. Are optical transistors the logical next step? Nature Photon. 4, 3-5 (2010). [2] H. Caglayan, S.-H. Hong, B. Edwards, C. R. Kagan, and N. Engheta. Near-Infrared Metatronic Nanocircuits by Design. Phys. Rev. Lett. 111, 073904 (2013). [3] Y. Sun, B. Edwards, A. Alù and N. Engheta. Experimental realization of optical lumped nanocircuits at infrared wavelengths. Nature Mater.11, 208-212 (2012). [4] J.M. Geffrin, B. García.Cámara, R. Gómez- Medina, P. Albella, L.S. Froufe-Pérez, C. Eyraud, A. Litman, R. Vaillon, F. González, M. Nieto-Vesperinas, J.J. Sáenz, F. Moreno. Magnetic and electric coherence in forward-and back-scattered electromagnetic waves by a single dielectric subwavelength sphere. Nature Commum. 3, 1171 (2012).Peer Reviewe
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Infrared antennas and resonant structures take advantage of the successful designs in the radioelectric and microwave spectra. A quite simple and naive approach would be to consider the geometry and shape of those designs and transfer... more
Infrared antennas and resonant structures take advantage of the successful designs in the radioelectric and microwave spectra. A quite simple and naive approach would be to consider the geometry and shape of those designs and transfer them into the optical regime with simple wavelength structure size scaling. However, this translation misses the specific behavior of metals and dielectrics at optical frequencies, and how these characteristics can strongly change the geometries and arrangements of a working antenna in the infrared. Fabrication of optical antennas and resonant structures has been possible only since technology has provided tools and processes able to produce smooth metallic surfaces with sizes ranging in the subwavelength scale for the optical domain. This means that resonant elements are nanophotonic devices and systems. This tiny size also needs an appropriate approach for the design of working elements. The first constraint is related to the physical substrate that the antennas are placed on or embedded in. Most of the antenna-coupled detectors and resonant structures are written on a dielectric substrate or on a dielectric stand-off layer deposited on top of a metallic surface that typically has been evaporated on a dielectric substrate. The thicknesses of these layers are a fraction of a wavelength, and the substrates are dielectric wafers, or plastic flexible substrates. When the device is illuminated from the substrate side, the substrate has to be transparent enough for the given wavelength of operation. In any case, the optical properties of every material involved in the proposed design have to be properly included in the design and modeling of the device. But not only optical properties are important. When thermal effects are at play, thermal conductivity and electrothermal coefficients are also of interest and need to be considered.
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Classical electromagnetism and photonics share a common foundation. Light propagation and interaction, even at the nanoscale, is driven by Maxwell’s equations and macroscopic parameterization of materials through their optical and... more
Classical electromagnetism and photonics share a common foundation. Light propagation and interaction, even at the nanoscale, is driven by Maxwell’s equations and macroscopic parameterization of materials through their optical and electromagnetic properties. These equations relate the spatial and temporal variations of the displacement electric field.
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Refractometric sensors based on plasmonic resonances can be modified to operate using an interrogation method based on the optoelectronic response of the device. In this contribution we show how the bolometric effect can be used to... more
Refractometric sensors based on plasmonic resonances can be modified to operate using an interrogation method based on the optoelectronic response of the device. In this contribution we show how the bolometric effect can be used to generate a signal depending on the change of the refractive index of the analyte. The design has been tuned to sense variations in the range of aqueous media. We have also proposed a modification of a perovskite solar cell to sense variations in the index of refraction of air. These changes in the interrogation method have required a modification of the definitions of the sensitivity and Figure of Merit of such types of sensors. The results show a performance that is competitive with other refractometric sensors and allows an operation method that relies on the measurement of electric, or electronic, parameters.
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Stainless steel is a basic raw material used in many industries. It can be customized by generating laser-induced periodic surface structure (LIPSS) as subwavelength gratings. Here, we present the capabilities of an LIPSS on stainless... more
Stainless steel is a basic raw material used in many industries. It can be customized by generating laser-induced periodic surface structure (LIPSS) as subwavelength gratings. Here, we present the capabilities of an LIPSS on stainless steel to modify the polarization state of the reflected radiation at the IR band. These structures have been modeled using the finite element method and fabricated by femtosecond laser processing. The Stokes parameters have been obtained experimentally and a model for the shape has been used to fit the simulated Stokes values to the experimental data. The birefringence of the LIPSS is analyzed to explain how they modify the polarization state of the incoming light. We find the geometry of the subwavelength grating that makes it work as an optical retarder that transforms a linearly polarized light into a circularly polarized wave. In addition, the geometrical parameters of the LIPSS are tuned to selectively absorb one of the components of the incoming ...
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Optical or infrared antennas are devices that receive optical radiation and transform it into a current within the resonant structure. This high-frequency current is transduced by a given mechanism to a change in voltage or current that... more
Optical or infrared antennas are devices that receive optical radiation and transform it into a current within the resonant structure. This high-frequency current is transduced by a given mechanism to a change in voltage or current that is read by external electronics. From this point of view, optical antennas are light detectors. Over time, several transduction mechanisms have been proposed and realized. One of the first used is the rectification of the currents flowing through the antenna by means of tunnel and Schottky junctions properly placed at the appropriate location, typically where the current density reaches the largest value. These elements are considered generically as diodes. Another simpler transduction mechanism is based on the heating of a bolometric material due to Joule dissipation. In this case, the mechanism is dissipative and needs electronic biasing to sense variations in resistivity of the bolometric element. It happens that most of the metals used in the fabrication of optical antennas have a similar value of the parameter responsible for the change in resistance with temperature, the temperature coefficient of resistance (TCR). Some designs use this effect to make the whole resonant element act as a bolometric transducer, which can be termed a distributed bolometer. Using the change in temperature caused by thermal dissipation of currents, thermoelectric transducers based on the Seebeck effect have also been proposed and tested. In this section we present the basic principles of these transduction mechanisms.
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Diffractive Optical Elements (DOEs) are amplitude and/or phase masks that can be applied to light beams to modify their phase and/or intensity distribution. They are applied in holography, beam shaping, generation of exotic beams... more
Diffractive Optical Elements (DOEs) are amplitude and/or phase masks that can be applied to light beams to modify their phase and/or intensity distribution. They are applied in holography, beam shaping, generation of exotic beams (Hermite-Gauss, Laguerre-Gauss, Gauss-Bessel, accelerating or vortex beams, etc.), generation of custom intensity profiles (top-hat, lines, figures, etc.), atomic physics, quantum optics, etc. They can be implemented using Spatial Light Modulators (SLMs) or micro-structured materials. Femtosecond laser writing is a very promising technique for fabricating photonic and micro-optics devices in metallic and dielectric materials. It consists on the removal (ablation) or modification of the irradiated material. Due to the short pulse duration of fs pulses, the energy is deposited in a localized region by nonlinear absorption mechanisms, allowing a very precise control of the material removal/modification. Compared to other methods, it has many advantages like a reduction of the amount of energy required to fabricate devices, and the absence of pollutant chemicals, becoming one of the most environmentally friendly fabrication techniques. One technique for implementing amplitude modulation DOEs is using dielectric samples covered with a metal thin film (few hundreds of nm thick). Then, the metallic film is selectively removed by laser ablation. This allows the engraving of a binary amplitude mask, where the remaining metallic coating reflects the electric field while the exposed dielectric area supports its transmission. Hence, these masks may work in both transmission and reflection. Although laser processing of DOEs has been successfully proved, some challenges still remain and should be addressed to optimize their behaviour. Several problems may arise during the laser ablation process. One of the problems treated in this contribution is the effect of damage on the dielectric substrate under the metallic coating. This happens since the light used to remove the metallic layer can also affect the dielectric sample, producing damage, variations of the material lattice or compositional changes, thus altering its refractive index. This variation may affect the effect of the DOE when it is used in transmission configuration. Another issue related to the ablation process is the different ablation strategies to engrave a given spatial distribution. Here, the laser is driven to process a matrix of points, or it can work in raster mode across the sample. We will analyze these two effects to properly understand the limitations of the technique and to find some useful strategies to overcome them when engraving DOEs through laser ablation.
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The Minimum Resolvable Temperature Difference (MRTD or MRT) is the most widely accepted and inclusive figure of merit for describing a thermal imaging system's performance. It is the product of analytic mathematical models and... more
The Minimum Resolvable Temperature Difference (MRTD or MRT) is the most widely accepted and inclusive figure of merit for describing a thermal imaging system's performance. It is the product of analytic mathematical models and traditional man-in-loop system hardware ...
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Research Interests: Optics and Focal Length
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The transformation of an hydrogenated amorphous silicon solar cell (aSiH) into an optoelectronic refratometric sensor has been possible through the addition of dielectric bow-tie resonant structures. The indium transparent oxide top... more
The transformation of an hydrogenated amorphous silicon solar cell (aSiH) into an optoelectronic refratometric sensor has been possible through the addition of dielectric bow-tie resonant structures. The indium transparent oxide top electrode is replaced by a thin metallic layer to selectively prevent the direct transmission of light to the active layer of the cell. Then, an array of dielectric bow-tie structures is placed on top of this electrode, to activate the optical absorption through surface plasmon resonance (SPR). The whole device is exposed to the analyte under measure, which is the surrounding medium. Three different dielectric materials with low, medium, and high refractive index were selected for the bow-ties, namely magnesium fluoride (MgF$$_2$$ 2 ), silicon dioxide (SiO$$_2$$ 2 ), and aluminum nitride (AlN) have been tested as coupling structure for SPR excitation. The maximization of the readout/short circuit current has been achieved through the geometrical paramete...
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When considering the pseudo-heterodyne mode for detection of the modulus and phase of the near field from scattering scanning near-field optical microscopy (s-SNOM) measurements, processing only the modulus of the signal may produce an... more
When considering the pseudo-heterodyne mode for detection of the modulus and phase of the near field from scattering scanning near-field optical microscopy (s-SNOM) measurements, processing only the modulus of the signal may produce an undesired constraint in the accessible values of the phase of the near field. A two-dimensional analysis of the signal provided by the data acquisition system makes it possible to obtain phase maps over the whole [0, 2π) range. This requires post-processing of the data to select the best coordinate system in which to represent the data along the direction of maximum variance. The analysis also provides a quantitative parameter describing how much of the total variance is included within the component selected for calculation of the modulus and phase of the near field. The dependence of the pseudo-heterodyne phase on the mean position of the reference mirror is analyzed, and the evolution of the global phase is extracted from the s-SNOM data. The resul...
Research Interests: Mechanical Engineering, Materials Science, Medicine, Near-field scanning optical microscopy, Data acquisition, and 10 moreOptical microscopy, Applied Optics, Optical physics, Data Handling, Post Processing, Optical Data Storage, Electrical And Electronic Engineering, Heterodyning, Data Acquisition System, and Optical Society of America
In the competition of solar cell efficiency, besides top-performance multijunction cells, tandem cells based on perovskites are also breaking efficiency records to enter into the 30% range. Their design takes advantage of the rapid... more
In the competition of solar cell efficiency, besides top-performance multijunction cells, tandem cells based on perovskites are also breaking efficiency records to enter into the 30% range. Their design takes advantage of the rapid development of perovskite cells, and the good sharing of the available spectrum between the perovskite, absorbing at short wavelengths, and the c-Si or similar lower band gap material, working at longer wavelengths. In this paper, we present a novel tandem solar cell that combines crystalline silicon (c-Si) and perovskites cells. We analyzed the device with computational electromagnetism based on the finite element method. Our design arranges the perovskite solar cell as a multilayer 1D grating, which is terminated with a gold thin film (top metallic contact). This multilayer nanostructure is placed on top of the c-Si cell and a thin protective dielectric layer of aluminum nitride covers the whole device. The short-circuit current of the perovskite cell i...
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Diffractive characterization of the vibrational state of an array of microcantilevers