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This study describes both experimentally and theoretically an important hitherto undiscovered feature of the scattering of micron-sized spherical objects when illuminated with highly focused circularly polarized light. This is a regime of high experimental relevance which has not been described in full detail. The experiments are complemented with the analytical formulas explaining the field scattered directed toward the backward hemispace. In particular, it is proven that this field shows a very regular oscillatory dependency with the optical size. This phenomenon is typically hidden in the total scattered field, as the field is scattered much less toward the backward hemisphere than toward the forward one. These regular oscillations are measured experimentally. It is proven that, by analyzing them, it is possible to determine the index of refraction of isolated micron-sized particles, opening new paths for applications in sensing and metrology.

Artificial neural networks are widely used in many different applications because of their ability to deal with a range of complex problems generally involving massive data sets. These networks are made up of nodes, connections, and nonlinear response connections, which are typically implemented as software code running on ordinary electronic computers. In such systems, electrons, with their advantages and drawbacks, are in charge of storing, processing, and transmitting information. Signal processing in the optical domain can provide ultrafast, parallel operation, nonlinear dynamics, and high energy efficiency, making photonics a suitable technology for the realization of neuroinspired computing platforms. This advantage stimulated the development of photonics neural networks based on single and multiple lasers with classical optical cavities. Recently, networks made of random lasers emerged as a novel concept that uses randomly placed scattering elements to create nonlinearity and complexity in photonics neural networks. In this review paper, we present the general framework for networks of coupled lasers, discuss recent advances in networks of random lasers, and outline future directions in this area. We also examine the challenges and limitations of using random lasers in photonic networks, as well as potential solutions. By harnessing the properties of random lasers, such as their unique spectral characteristics in pulsed emission mode and their robustness against noise, networks of interacting random lasers can explore new and exciting possibilities for photonics technology that could find applications in a variety of fields, including image recognition and encryption., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

The file contains the dataset corresponding to the figures of the article "Characterizing the Backscattered Spectrum of Mie Spheres" written by Martín Molezuelas-Ferreras, Álvaro Nodar, María Barra-Burillo, Jorge Olmos-Trigo, Jon Lasa-Alonso, Iker Gómez-Viloria, Elena Posada, J. J. Miguel Varga, Rubén Esteban, Javier Aizpurua, Luis E. Hueso, Cefe Lopez, and Gabriel Molina-Terriza (DOI: 10.1002/lpor.202300665). The data is organized into different folders, and each folder contains .txt files of the data for each of the figures indicated on its name, together with README instructions on each case.

By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defects.

Random lasers are intriguing devices with promising applications as light sources for imaging, sensing, super-resolution spectral analysis or complex networks engineering. Random lasers can be obtained from optically pumped dyes, optical fibres and crystals or electrically pumped semiconductor heterostructures. Semiconductor random lasers are usually fabricated by introducing scattering defects into the active layer, adding a degree of complexity to the fabrication process and losing the ease of realization potentially offered by disordered structures. The ready availability of electrically pumped random lasers, avoiding a costly fabrication approach, would boost the use of these devices in research and applications. Here we realize an incoherent semiconductor random laser by simply processing the output mirror of an off-the-shelf Fabry–Pérot laser diode via controlled laser ablation. Optical feedback provided by the intact back mirror and the ablated front mirror results in multimode random lasing with low spatial coherence and disordered angular patterns. This result constitutes a proof of principle for future ground-breaking technology developments in the field of random lasers., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

Artificial intelligence (AI) is gaining strength, and materials science can both contribute to and profit from it. In a simultaneous progress race, new materials, systems, and processes can be devised and optimized thanks to machine learning (ML) techniques, and such progress can be turned into innovative computing platforms. Future materials scientists will profit from understanding how ML can boost the conception of advanced materials. This review covers aspects of computation from the fundamentals to directions taken and repercussions produced by computation to account for the origins, procedures, and applications of AI. ML and its methods are reviewed to provide basic knowledge of its implementation and its potential. The materials and systems used to implement AI with electric charges are finding serious competition from other information-carrying and processing agents. The impact these techniques have on the inception of new advanced materials is so deep that a new paradigm is developing where implicit knowledge is being mined to conceive materials and systems for functions instead of finding applications to found materials. How far this trend can be carried is hard to fathom, as exemplified by the power to discover unheard of materials or physical laws buried in data.

Self-assembly of colloidal particles into ordered superstructures is an important strategy to discover new materials, such as catalysts, plasmonic sensing materials, storage systems, and photonic crystals (PhCs). Here we show that porous covalent organic frameworks (COFs) can be used as colloidal building particles to fabricate porous PhCs with an underlying face-centered cubic (fcc) arrangement. We demonstrate that the Bragg reflection of these can be tuned by controlling the size of the COF particles and that species can be adsorbed within the pores of the COF particles, which in turn alters the Bragg reflection. Given the vast number of existing COFs, with their rich properties and broad modularity, we expect that our discovery will enable the development of colloidal PhCs with unprecedented functionality.

Random lasers (RLs) rely on light amplification in a gain material with feedback from multiple scattering in disordered media. They are unconventional light sources characterized by multiple narrow peaks emission with high potential in imaging and sensing applications. At variance with ordinary lasers, optical interaction between single RLs arranged in networks of multiply interconnected resonators remains unexplored. The typical RL architecture where gain material and scatterers are spatially mixed, is unsuitable for the realization of singular devices on the same platform. Here, we use RLs in which gain is restricted to the line between two scattering regions to establish and probe mutual coupling between many RLs. We prove the interaction in the network by detecting the spectral rearrangement of the compound emission as compared to individual RLs. The engineering of coupled RLs sets the basis for building structures with potential to function as optical neural networks., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

The presence of porosity critically affects the performance of solid systems. The pore accessibility to adsorbate molecules and the corresponding adsorption/desorption behavior are crucial aspects to understand the properties of porous materials but are difficult to address, principally when dealing with narrow micropores. A prominent example is colloidal silica (Stöber) spheres whose microporosity, inaccessible for some adsorbates, can be readily filled by water molecules to a large extent but exhibiting a complex adsorption behavior with unexpected hystereses. Here, we perform water adsorption isotherms on Stöber spheres at different temperatures using an original analysis of the Dubinin-Radushkevich representation to examine both the accessibility to the microporosity and the formation of water multilayers on the outer sphere surface. The micropore filling (and emptying) is found to be limited by the kinetic energy of the water molecules, causing low-pressure hysteresis. We further discover that the (temperature-dependent) completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at a high relative pressure. The number of adsorbed water layers is determined, and the adsorption-induced swelling of the spheres is discussed.

[EN] Hypothesis: The wettability of the inner surfaces in a porous network is challenging to be accessed but essential to understand the complex performance of e.g. particulate systems. Experiments: Here we investigate the water behavior in the macroporous (50–100 nm) voids of dense particle packings by performing low-field H NMR relaxometry in solid silica colloid crystals. The systems chosen guarantee a regular, known void size distribution with controllable water affinity (through thermal annealing), where the NMR experiment clearly discriminates the void water from micropore or bound water contributions, allowing separate monitoring. Findings: Analysis of the saturated state indicates that the interparticle voids are completely filled after imbibition, even for less hydrophilic spheres. Due to the interaction with the silica surface, proton relaxation in void water is up to 100 times faster than that in bulk water, serving for assessment of the hydrophilicity within the sample. The relaxation time evolution upon dewetting provides an empirical measurement of the wettability inside the ensemble, revealing a progressively inhomogeneous wetting of increasingly hydrophobic surfaces. Our results provide insight into the imbibition state and dewetting performance in meso- and macroporous systems, with emphasis in the marked influence of the surface nature on the pore wetting distribution.

[EN] Micropores and their interplay with liquids are critical in many solid systems but challenging to investigate, often requiring complex and indirect techniques to even be detected. Here, we perform low-field H NMR transverse relaxometry on submicron Stöber silica spheres to directly detect micropore water. Colloidal crystals with a well-defined sphere configuration are used to facilitate unambiguous distinction of the interparticle void water and enable straightforward micropore volume quantification. We evidence significant microporosity (∼16% of the sphere volume) with high water accessibility and monitor the drying of the micropore water, which exhibits high resilience, evaporating only after complete void drainage. Progressive microporosity closure by thermal annealing above 600 °C is shown, until complete removal at 900 °C. Increasing hydrophobicity of the micropore walls is observed, although it barely affected the water access. Our results prove the capability of NMR relaxometry to quantitatively investigate complex microporosity and the behavior of water confined therein.

Passive daytime radiative cooling has recently become an attractive approach to address the global energy demand associated with modern refrigeration technologies. One technique to increase the radiative cooling performance is to engineer the surface of a polar dielectric material to enhance its emittance atwavelengths in the atmospheric infrared transparency window (8-13 ìm) by outcoupling surface-phonon polaritons (SPhPs) into free-space. Here we present a theoretical investigation of new surface morphologies based upon self-assembled silica photonic crystals (PCs) using an in-house built rigorous coupled-wave analysis (RCWA) code. Simulations predict that silica micro-sphere PCs can reach up to 73 K below ambient temperature, when solar absorption and conductive/convective losses can be neglected. Micro-shell structures are studied to explore the direct outcoupling of the SPhP, resulting in near-unity emittance between 8 and 10 ìm. Additionally, the effect of material composition is explored by simulating soda-lime glass micro-shells, which, in turn, exhibit a temperature reduction of 61 K below ambient temperature. The RCWA code was compared to FTIR measurements of silica micro-spheres, self-assembled on microscope slides.

[EN] The development of new technologies is usually linked to improving society's quality of life. In this sense, the organization of matter and the control of light are possibly two of the most exploited approaches over the years. The importance of the former lies in the fact that it determines most of the physical properties of the system, while the latter represents a technological revolution that is currently underway. Therefore, this doctoral thesis deals with the exploitation of the photonic properties of self-assembled systems in order to characterize two fundamental physical phenomena: phase transitions and disorder in structured materials. Phase transitions are of great interest because of the multitude of physical mechanisms involved. For this reason, Chapter 2 introduced a technique that allows to monitor the transition from a colloidal fluid to a crystal depending on the initial volume fraction. By means of real-time light diffraction at the bottom of a colloidal suspension, the evolution of the natural sedimentation of hard spheres is followed. The appearance of the Bragg peak in the spectra makes it possible to characterize the whole process of self-assembly, i.e. crystallization, compaction, freezing and drying. The phase transition known as percolation is the simplest phase transition since it only depends on the geometry of the system, i.e. how its components are distributed. In Chapter 3, binary photonic crystals are built in order to remove one of the species and to be able to study the statistics of vacancy clusters as a function of the percentage of vacancies introduced into the crystal. For this purpose, the external surfaces of different vacancy-doped photonic crystals are analysed, proving that they follow a 2D statistics described by the percolation theory. It results that the proposed structures prove to be excellent archetypes for designing and fabricating complex systems with random vacancy cluster distributions. The creation and distribu, [ES] El desarrollo de nuevas tecnologías va ligado normalmente a mejorar la calidad de vida de la sociedad. En este sentido, la organización de la materia y el control de la luz son, posiblemente, dos de las vías más explotadas durante el transcurso de los años. La importancia del primero radica en que determina la mayoría de las propiedades físicas del sistema, mientras que el segundo supone una revolución tecnológica la cual vivimos en la actualidad. Por ello, esta tesis doctoral trata sobre la explotación de las propiedades fotónicas de sistemas autoensamblados con el fin de caracterizar dos fenómenos físicos fundamentales: las transiciones de fase y la introducción de desorden en materiales estructurados. Las transiciones de fase suscitan un gran interés por la multitud de mecanismos físicos que involucran. Por este motivo, en el Capítulo 2 de la tesis se investiga una técnica que permite monitorizar la transición de un fluido coloidal a un cristal dependiendo de la fracción de volumen inicial. Por medio de la difracción de luz en tiempo real del fondo de una suspensión coloidal se sigue la evolución de la sedimentación natural de esferas duras. La aparición del pico de Bragg en los espectros permite caracterizar todo el proceso de autoensamblado, esto es, cristalización, compactación, congelación y secado. La transición de fase geométrica, conocida como percolación, es la transición de fase más simple posible ya que depende únicamente de como se distribuyan sus componentes. En el Capítulo 3 se construyen cristales fotónicos binarios con el fin de eliminar una de las especies y poder estudiar la estadística de racimos de vacantes en función del porcentaje de vacantes introducidas en el cristal. Para ello se analizan las superficies externas de diferentes cristales fotónicos dopados con vacantes probando que siguen una estadística 2D descrita por la teoría de la percolación. De esto se desprende que las estructuras propuestas demuestran ser excelentes arquetipos

The presence of porosity critically affects the performance of solid systems. The pore accessibility to adsorbate molecules and the corresponding adsorption/desorption behavior are crucial aspects to understand the properties of porous materials but are difficult to address, principally when dealing with narrow micropores. A prominent example is colloidal silica (Stöber) spheres whose microporosity, inaccessible for some adsorbates, can be readily filled by water molecules to a large extent but exhibiting a complex adsorption behavior with unexpected hystereses. Here, we perform water adsorption isotherms on Stöber spheres at different temperatures using an original analysis of the Dubinin–Radushkevich representation to examine both the accessibility to the microporosity and the formation of water multilayers on the outer sphere surface. The micropore filling (and emptying) is found to be limited by the kinetic energy of the water molecules, causing low-pressure hysteresis. We further discover that the (temperature-dependent) completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at a high relative pressure. The number of adsorbed water layers is determined, and the adsorption-induced swelling of the spheres is discussed.

Sensor de humedad basado en la respuesta óptica de un ópalo artificial a la adsorción de agua. El diseño básico precisa únicamente de la lectura de la banda de difracción Bragg (su intensidad a una longitud de onda preestablecida o su posición) de un ópalo artificial cuya fabricación no necesita de ningún post-procesamiento (funcionalización de superficies, infiltración, curado, etc.). El sensorresponde directamente a la adsorción de moléculas de agua en el ópalo y no requiere, por tanto, de fenómenos intermedios (tales como imbibición, dilatación por hidratación, reacciones químicas, etc.). Opcionalmente, el control de la hidrofilicidad, porosidad, periodicidad o fracción de llenado del ópalo permite ajustar la sensibilidad del sensor y/o región de lecturadel pico Bragg. El elemento responsivo (el ópalo) es inalámbrico, miniaturizable a pocas micras cúbicas y su tiempo de respuesta es típicamente inferior a un segundo. [ES], The invention relates to a moisture sensor based on the optical response of an artificial opal to water adsorption. The basic design only needs to read the Bragg diffraction band (its intensity at a predetermined wavelength or its position) of an artificial opal whose manufacture does not require any post-processing (surface functionalisation, infiltration, curing, etc.). The sensor responds directly to the adsorption of water molecules on the opal and therefore does not require intermediate phenomena (such as imbibition, hydration expansion, chemical reactions, etc.). Optionally, controlling the hydrophilicity, porosity, periodicity or fill fraction of the opal makes it possible to adjust the sensitivity of the sensor and/or Bragg peak reading frame. The responsive element (opal) is wireless, can be miniaturised to a few cubic microns and has a response time of typically less than one second. [EN]

Sensor de humedad basado en la respuesta óptica de un ópalo artificial a la adsorción de agua. El diseño básico precisa únicamente de la lectura de la banda de difracción Bragg (su intensidad a una longitud de onda preestablecida o su posición) de un ópalo artificial cuya fabricación no necesita de ningún post-procesamiento (funcionalización de superficies, infiltración, curado, etc.). El sensor responde directamente a la adsorción de moléculas de agua en el ópalo y no requiere, por tanto, de fenómenos intermedios (tales como imbibición, dilatación por hidratación, reacciones químicas, etc.). Opcionalmente, el control de la hidrofilicidad, porosidad, periodicidad o fracción de llenado del ópalo permite ajustar la sensibilidad del sensor y/o región de lectura del pico Bragg. El elemento responsivo (el ópalo) es inalámbrico, miniaturizable a pocas micras cúbicas y su tiempo de respuesta es típicamente inferior a un segundo.

[EN] Silicon-based materials are needed in cutting-edge technological fields, for which hierarchical porosity and photonic properties help improve performance. In this work, the versatility of several fabrication routes that combine silicon infiltration by chemical vapor deposition (CVD), reactive ion etching (RIE), and carbon calcination, which produce a palette of novel silicon-based material architectures, is demonstrated. Design strategies are discussed and the main features of the processing steps are addressed to provide a variety of new silicon-based materials with high morphological versatility and photonic quality. Three-dimensional hollow carbon opals (HCO) permit control of the porosity of subsequent architectures through the initial silicon infiltration, while open or closed surfaces are achieved primarily as a function of RIE conditions. The composition of derived structures depends on the sequence in which the processes are applied. As a result, pure Si and hybrid C–Si inverse structures can be produced with open or closed spheres in the top layer and with an optional passivation layer. Remarkably, by properly choosing the Si infiltration parameters and the calcination/RIE procedure, final structures are achieved with double-shell spheres.

[EN] Complex systems involving networks have attracted strong multidisciplinary attention since they are predicted to sustain fascinating phase transitions in the proximity of the percolation threshold. Developing stable and compact archetypes that allow one to experimentally study physical properties around the percolation threshold remains a major challenge. In nanoscale systems, this achievement is rare since it is tied to the ability to control the intentional disorder and perform a vast statistical analysis of cluster configurations. Here, a self‐assembly method to fabricate perfectly ordered structures where random defects can be introduced is presented. Building binary crystals from two types of dielectric nanospheres and selectively removing one of them creates vacancies at random lattice positions that form a complex network of clusters. Vacancy content can be easily controlled and raised even beyond the percolation threshold. In these structures, the distribution of cluster sizes as a function of vacancy density is analyzed. For moderate concentrations, it is found to be homogeneous throughout the structure and in good agreement with the assumption of a random vacancy distribution.

High-index dielectric nanostructures have emerged as an appealing complement to plasmonic nanostructures, offering similar light management capabilities at the nanoscale but free from the inherent optical losses. Despite the great interest in these all-dielectric architectures, their fabrication still requires cumbersome fabrication techniques that limit their implementation in many applications. Hence, the great interest in alternative scalable procedures. Among those, the fabrication of silicon spheres is at the forefront, with several routes available in the literature. However, the exploitation of the Mie modes sustained by these silicon resonators is limited over large areas by polydispersity or a lack of long-range order. Here, we present an all-dielectric metamaterial fabricated with a low cost and highly scalable technique: a combination of soft imprinting nanolithography and chemical vapor deposition. The resulting all-dielectric metasurface is composed of an array of silicon hemispheres on top of a high refractive index dielectric substrate. This architecture allows the exploitation of high-quality Mie resonances at a large scale due to the high monodispersity of the hemispheres organized in a single crystal two-dimensional lattice. The optical response of the metasurface can be engineered by the design parameters of the nanoimprinted structure. We further demonstrate the potential of this platform to enhance light emission by coupling dye molecules to the sustained Mie resonances and measuring both an eight-fold amplified signal and a triple lifetime reduction.