Magneto-ionic suppression of magnetic vortices

Magneto-ionics refers to the non-volatile control of the magnetic properties of materials by voltage-driven ion migration. This phenomenon constitutes one of the most important magnetoelectric mechanisms and, so far, it has been employed to modify the magnetic easy axis of thin films, their coercivity or their net magnetization. Herein, a novel magneto-ionic effect is demonstrated: the transition from vortex to coherent rotation states, caused by voltage-induced ion motion, in arrays of patterned nanopillars. Electrolyte-gated Co/GdOx bilayered nanopillars are chosen as a model system. Electron microscopy observations reveal that, upon voltage application, oxygen ions diffuse from GdOx to Co, resulting in the development of paramagnetic oxide phases (CoOx) along sporadic diffusion channels. This breaks up the initial magnetization configuration of the ferromagnetic pillars (i.e. vortex states) and leads to the formation of small ferromagnetic nanoclusters, embedded in the CoOx matrix, which behave as single-domain nanoparticles. As a result, a decrease in the net magnetic moment is observed, together with a drastic change in the shape of the hysteresis loop. Micromagnetic simulations are used to interpret these findings. These results pave the way towards a new potential application of magnetoelectricity: the magneto-ionic control of magnetic vortex states

Managing light in optoelectronic devices with resonant optical nanostructures

Actualment, un dels reptes en l'àmbit de la manipulació de la llum a la nanoescala és la transició del laboratori a aplicacions reals. Tot i el gran potencial demostrat per algunes estructures fotòniques per a incrementar la eficiència de instruments optoelectrònics, la seva implementació de d'aquestes a dispositius de mercat sovint es obstruïda per la necessitat d'usar tècniques de fabricació poc escalables i d'alt cost. Aquesta tesis està dedicada al disseny i implementació de estratègies de manipulació de la llum per a millorar la eficiència en la recol·lecció d'energia de plaques solars i fotodetectors, així com la millora de la emissió en dispositius d'il·luminació, mitjançant mètodes de nanoestructuració escalables com la nano-litografia suau. Aquesta tècnica té la capacitat de produir patrons i estructures amb una resolució de pocs nanòmetres amb gran fidelitat en àrees grans. A més a més, és compatible amb el processament a gran escala mitjançant el sistema de impressió en cadena "roll to roll" (carret-a-carret). També es tracta d'una tecnologia molt versàtil, ja que permet l'ús de diferents tipus de substrats, és poc invasiva i generalment pot ser introduïda en el esquema de fabricació sense haver de modificar cap pas. Amb l'ajuda d'aquesta tècnica de nanofabricació, explorem una varietat de arquitectures fotòniques i les diferents ressonàncies fotòniques que les fan especials. Entre aquestes darreres podem trobar ressonàncies de Mie, modes de Brewster i modes de cristall fotònic, que proveiran al sistema amb una major interacció llum-matèria a la capa activa del dispositiu, podent-ne millorar les seves capacitats òptiques. Primer, hem desenvolupat una estratègia per aconseguir una absorció optima de banda ample en semiconductors ultra-fins, amb menys de 100 nm de gruix, per a totes les energies per sobre de la seva energia de banda prohibida. La sinèrgia de les fortes ressonàncies d'interferència de capes fines presents i els modes del cristall fotònic de l'estructura (amb un alt índex de refracció) fan que l'estructura assoleixi fins a un 81% d'absorció en una ampli rang de longituds d'ona (de 400 a 1500 nm). En segon lloc, hem combinat la litografia suau amb la deposició química de vapor (CVD en anglès) per obtenir una matriu de semiesferes de silici sobre de una guia d'ones d'alt índex de refracció. Hem estudiat les ressonàncies de Mie característiques del substrat, com hibriden amb modes quasi-guiats de la guia d'ones i com això afecta al camp proper de la metasuperfície. Hem anat un pas més enllà estudiant la com la modificació dels paràmetres de disseny de l'estructura afecta les ressonàncies esmentades. Finalment, n'hem demostrat una possible aplicació com a substrat per a incrementar la emissió de llum per part de una molècula emissora. En la tercera part de la tesi, ens hem enfocat en la implementació de estructures de cristall fotònic bidimensional a tres dispositius diferents per a la millora de la seva eficiència. En particular, millorem la eficiència en la recol·lecció de fotons d'infraroig proper en cèl·lules solars de punts quàntics col·loïdals (PbS) i en fotodetectors orgànics (P3HT: PC60BM i PBTTT: PC70BM), i millorem l'emissió de llum de capes de nanofòsfors (nanocristalls de GdVO4:Eu3+). Hem desenvolupat sistemes fotònics adaptats a cada cas i hem fet una caracterització òptica i electrònica de tots els dispositius. La nanoestructuració en forma de cristall fotònic bidimensional proveeix a les capes actives amb propietats de guies d'ona ressonants, millorant les seves propietats de confinament de la llum en les longituds d'ona desitjades, demostrant així la possibilitat d'implementar les arquitectures.Actualmente, uno de los retos en el ámbito de la manipulación de la luz a la nanoescala es la transición del laboratorio a aplicaciones reales. A pesar del gran potencial demostrado por algunas estructuras fotónicas para incrementar la eficiencia de instrumentos optoelectrónicos, su implementación en dispositivos de mercado muchas veces es obstruida por la necesidad de utilizar técnicas de fabricación poco escalables y de alto coste. Esta tesis está dedicada al diseño e implementación de estrategias de manipulación de la luz para mejorar la eficiencia en la recolección de energía de placas solares y fotodetectores, así como la mejora de la emisión en dispositivos de iluminación, mediante métodos de nanoestructuración escalables como la nano-litografía suave. Esta técnica tiene la capacidad de producir patrones y estructures con una resolución de pocos nanómetros con gran fidelidad en áreas grandes. Encima, es compatible con el procesamiento a gran escala mediante el sistema de impresión en cadena "roll-to-roll" (carrete-a-carrete). También se trata de una tecnología muy versátil, puesto que permite el uso de diferentes tipos de sustratos, es poco invasiva y generalmente puede ser introducida en el esquema de fabricación sin tener que modificar ningún paso. Con la ayuda de esta técnica de nanofabricación, exploramos una variedad de arquitecturas fotónicas y las diferentes resonancias fotónicas que las hacen especiales. Entre estas últimas podemos encontrar resonancias de Mie, modos de Brewster y modos de cristal fotónico, que proveerán al sistema con una mayor interacción luz-materia a la capa activa del dispositivo, mejorar sus capacidades ópticas. Primero, hemos desarrollado una estrategia para conseguir una absorción óptima de banda ancha en semiconductores ultra-finos, con menos de 100 nm de grosor, para todas las energías por encima de su energía de banda prohibida. La sinergia de las fuertes resonancias de interferencia de capas finas presentes y los modos del cristal fotónico de la estructura (con un alto índice de refracción) hacen que la estructura logre hasta un 81% de absorción en un amplio rango de longitudes de omda (de 400 a 1500 nm). En segundo lugar, hemos combinado la litografía suave con la deposición química de vapor (CVD en inglés) para obtener una matriz de semiesferas de silicio sobre de una guía de ondas de alto índice de refracción. Hemos estudiado las resonancias de Mie características del sustrato, como hibridan con modos casi-guiados de la guía de olas y como esto afecta en el campo próximo de la metasuperfície. Hemos ido un paso más allá estudiando como la modificación de los parámetros del diseño de la estructura afecta a las resonancias mencionadas. Finalmente, hemos demostrado una posible aplicación como sustrato para incrementar la emisión de luz por parte de una molécula emisora. En la tercera parte de la tesis, nos hemos enfocado en la implementación de estructuras de cristal fotónico bidimensional a tres dispositivos diferentes para la mejora de su eficiencia. En particular, mejoramos la eficiencia en la recolección de fotones de infrarrojo próximo en células solares de puntos cuánticos coloidales (PbS) y en fotodetectores orgánicos (P3HT: PC60BM y PBTTT: PC70BM), y mejoramos la emisión de luz de capas de nanofósforos (nanocristales de GdVO4:Eu3+). Hemos desarrollado sistemas fotónicos adaptados a cada caso y hemos hecho una caracterización óptica y electrónica de todos los dispositivos. La nanoestructuración en forma de cristal fotónico bidimensional provee a las capas activas con propiedades de guías de onda resonantes, mejorando sus propiedades de confinamiento de la luz en las longitudes de onda deseadas, demostrando así la posibilidad de implementar las arquitecturas.Currently, one of the main challenges in light management at the nanoscale is the transition from the laboratory to real applications. Despite the great potential shown by photonic architectures to optically improve the performance of many devices, transitioning into marketable devices is often hampered by the low-throughput and expensive nanofabrication techniques involved. This thesis is devoted to the design and development of subwavelength light managing strategies to improve the light harvesting or out-coupling in solar cells, photodetectors and light emitters while using a scalable nanostructuration such as soft nanoimprint lithography (NIL). This technique has been proven to achieve resolutions down to few tens of nanometers with high fidelity in large areas, being compatible with roll to roll processing. It is also versatile regarding the materials where it can be used, non-invasive, and can be seamlessly introduced in the devices fabrication scheme. With the aid of this technique, we explore a variety of photonic architectures and the different types of resonances sustained, from Brewster modes to Mie resonances, in order to enhance the light-matter interaction with the active layer of the device. First, we develop a strategy to achieve broadband optimal absorption in ultra-thin semiconductor materials (less than 100 nm thick) for all energies above their bandgap. The interplay of strong interference thin film resonances and photonic crystal modes sustained by a high refractive index nanostructure on a gold film renders the system with a 81% total absorption over a broad spectral range (from 400 to 1500 nm). Second, we combine soft NIL and chemical vapor deposition to obtain an array of silicon hemispheres on top of a high refractive index dielectric waveguide. We study the Mie resonances supported by the substrate, how these hybridize with the guided modes of the waveguide and how their interaction influences the electromagnetic near field of the metasurface. We further explore the tunability of such resonances with the design parameters of the structure and we demonstrate a potential application of it as a substrate for enhanced photoluminescence. In the third part of the thesis, we focus on the implementation of 2D photonic structures within the active layer of three different devices to improve performance. In particular we enhance the near infrared (NIR) photon harvesting efficiency in a colloidal quantum dot solar cell (PbS-CQD) and in organic photodetectors (P3HT: PC60BM and PBTTT: PC70BM) and improve the light out coupling from a nanophosphor layer (GdVO4:Eu3+ nanocrystals). We developed photonic systems tailored for each device and provide the complete optical and electronic characterization for each case. The nanostructuration with a 2D periodic arrangement renders the active layers with resonant waveguide properties enhancing its light trapping properties in the targeted spectral ranges, hence demonstrating the possibility to implement photonic schemes within actual devices

Ultrathin semiconductor superabsorbers from the visible to the near infrared

En col·laboració amb la Universitat de Barcelona (UB), la Universitat Autònoma de Barcelona (UAB)i l’Institut de Ciències Fotòniques (ICFO)The design of ultrathin semiconducting materials that achieve broadband absorption is a longsought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either on the use of strong -- but narrowband – or broader --but moderate -- light trapping mechanisms. In this work, we present a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. We demonstrate broadband near-unity absorption in Ge ultrathin films that extend from the visible to the Ge bandgap in the near infrared and robust against angle of incidence variation. Our strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures

Ultrathin semiconductor superabsorbers from the visible to the near infrared

En col·laboració amb la Universitat de Barcelona (UB), la Universitat Autònoma de Barcelona (UAB)i l’Institut de Ciències Fotòniques (ICFO)The design of ultrathin semiconducting materials that achieve broadband absorption is a longsought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either on the use of strong -- but narrowband – or broader --but moderate -- light trapping mechanisms. In this work, we present a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. We demonstrate broadband near-unity absorption in Ge ultrathin films that extend from the visible to the Ge bandgap in the near infrared and robust against angle of incidence variation. Our strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures

Near infrared organic photodetectors based on enhanced charge transfer state absorption by photonic architectures

Near infrared photodetectors are a widespread and fundamental technology in many disciplines, from astronomy and telecommunications to medical sciences. Current technologies are now striving to include new aspects in this technology such as wearability, flexibility and tunability. Organic photodetectors easily offer many of those advantages but their relatively high bandgaps hinder NIR operation. In this work, we demonstrate solution processed organic photodetectors with improved NIR response thanks to a nanostructured active layer in the shape of a photonic crystal. The latter strongly increases the charge transfer state absorption, which is normally weak but broadband, increasing the optical path of light and resulting in remarkable photoresponse significantly below the band gap of the blend. We show responsivities up to 50 mA W−1 at 900 nm for PBTTT:PC71BM based photodetectors. On top of that, by varying the lattice parameter of the photonic crystal structure, the spectral response of the photodetectors can be tuned beyond 1000 nm. Furthermore, our photonic structure can be easily implemented in the device in a single nanoimprinting step, with minimal disruption on the fabrication process, which makes this approach very promising for upscaling.We greatly acknowledge financial support from the Ministerio de Ciencia, Innovación y Universidades MICINN with projects PGC2018-095411-B-I00, MAT2016-79053-P and MAT2015-70850-P and the “Severo Ochoa” excellence program SEV-2015-0496; Generalitat de Catalunya program AGAUR 2017-SGR-00488; and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grants no. CoG648901 and StG637116). P. M. acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF. M. G. R. acknowledges financial support from an FPU grant (no. 16/02631) (2017) of the MICINN (Spain). M. G. R. and P. M. B. acknowledge the departments of Physics, Chemistry and Geology of the Autonomous University of Barcelona (UAB) as coordinators of the PhD programme in Materials Science. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Engineering Plasmonic Colloidal Meta-Molecules for Tunable Photonic Supercrystals

Ordered arrays of metal nanoparticles offer new opportunities to engineer light–matter interactions through the hybridization of Rayleigh anomalies and localized surface plasmons. The generated surface lattice resonances exhibit much higher quality factors compared to those observed in isolated metal nanostructures. Template-induced colloidal self-assembly has already shown a great potential for the scalable fabrication of 2D plasmonic meta-molecule arrays, but the experimental challenge of controlling optical losses within the repeating units has so far prevented this approach to compete with more standard fabrication methods in the production of high-quality factor resonances. In this manuscript, the optical properties of plasmonic arrays are investigated by varying the lattice parameter (between 200 and 600 nm) as well as the diameter of the gold colloidal building-blocks (between 11 ± 1 and 98 ± 6 nm). It is systematically studied how the internal architecture of the repeating gold-nanoparticle meta-molecules influences the optical response of the plasmonic supercrystals. Combining both experimental measurements and simulations, it is demonstrated how, reducing the size of the gold nanoparticles it is possible to switch from strong near-field plasmonic architectures to high-quality factors (>60) for lattice plasmon resonances located in the visible spectral range.P.M. and N.P. contributed equally to this work. This project had received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 637116, ENLIGHTMENT) and the Spanish Ministerio de Ciencia e Innovación through grant, PID2019-106860GB-I00 and FUNFUTURE (CEX2019-000917-S), in the framework of the Spanish Severo Ochoa Centre of Excellence program. L.S. research was supported by the Marie Sklodowska-Curie Actions SHINE (H2020- MSCA-IF-2019, grant agreement no. 894847) and the 2020 Post-doctoral Junior Leader-Incoming Fellowship by “la Caixa” Foundation (ID 100010434, fellow-ship code LCF/BQ/PI20/11760028). L.A.P. thanks the Marie Sklodowska-Curie Actions (H2020-MSCA-IF-2018) for grant agreement no. 839402, PLASMIONICO. P.M. acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF and the UAB under the auspices of the UAB material science doctoral program.Peer reviewe

Enhanced Directional Light Extraction from Patterned Rare‐Earth Phosphor Films

The combination of light‐emitting diodes (LEDs) and rare earth (RE) phosphors as color‐converting layers comprises the basis of solid‐state lighting. Indeed, most LED lamps include a photoluminescent coating made of phosphor material, i.e., crystalline matrix suitably doped with RE elements, to produce white light from a blue or ultraviolet LED chip. Transparent phosphor‐based films constitute starting materials for new refined emitters that allow different photonic designs to be implemented. Among the different photonic strategies typically employed to tune or enhance emission, surface texturing has proved its versatility and feasibility in a wide range of materials and devices. However, most of the nanofabrication techniques cannot be applied to RE phosphors directly because of their chemical stability or because of their cost. The first monolithic patterned structure of down‐shifting nanophosphors with square arrays of nanoholes with different lattice parameters is reported in this study. It is shown that a low‐cost soft‐nanolithography procedure can be applied to red‐emitting nanophosphors (GdVO4:Eu3+ nanocrystals) to tune their emission properties, attaining a twofold directional enhancement of the emitted light at predesigned emission wavelengths in specific directions.This project received funding from the Spanish Ministry of Economy and Competitiveness under grant MAT2017‐88584‐R (AEI/FEDER, UE) and PID2019‐106860GB‐I00 (AEI/FEDER, UE), the excellence program SEV‐2015‐0496, and the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (NANOPHOM, grant agreement no. 715832 and ENLIGHTMENT, grant no. 637116). P.M acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF and the UAB. E.C.O. acknowledges the Spanish Ministry of Universities for the funding through an FPU program (FPU19/00346).Peer reviewe

High-Throughput Nanofabrication of Metasurfaces with Polarization-Dependent Response

Metal nanostructures offer exciting ways to manage light at the nanoscale exploited in fields such as imaging, sensing, energy conversion, and information processing. The optical response of the metallic architectures can be engineered to exhibit photonic properties that span from plasmon resonances to more complex phenomena such as negative refractive index, optical chirality, artificial magnetism, and more. However, the latter optical properties are only observed in intricate architectures, which are highly demanding in terms of nanofabrication and hence less scalable and far away from device implementation. Here, a series of metasurfaces covering centimeter areas and operating in the visible spectrum are presented, which are produced from the combination of nanoimprinting lithography and oblique angle metal evaporation. The potential of this scalable approach is illustrated by easily fabricating metasurfaces engineered to exhibit artificial optical magnetism, tunable linear polarization dependent response, chirality with g‐factor of 0.2, and photoluminescence enhancement of 20 times over a 9 mm2 area.The Spanish Ministerio de Economía, Industria y Competitividad (MINECO) is gratefully acknowledged for its support through Grant No. MAT2016‐79053‐P and through Grant No. SEV‐2015‐0496 in the framework of the Spanish Severo Ochoa Centre of Excellence program. This project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement Nos. 637116, ENLIGHTMENT and 648901, FOREMAT) and the Generalitat de Catalunya program AGAUR 2017‐SGR‐00488. P.M. acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofunded by the ESF. L.A.P. is thankful to the Marie Sklodowska‐Curie Actions (H2020‐MSCA‐IF‐2018) for grant agreement No. 839402, PLASMIONICO.Peer reviewe

Au/TiO2 2D-Photonic Crystals as UV–Visible Photocatalysts for H2 Production

Noble metal decoration of wideband gap semiconductors enables the excitation of surface plasmons in the visible range that upon relaxation generate hot carriers used for catalysis. However, this strategy leads to photocatalytic conversion efficiencies that are still low. Here, a light-trapping scheme is used to amplify the light-harvesting efficiency of the TiO2 semiconductor beyond the UV region by coupling a 2D-photonic crystal to Au decorated titania. This approach is easily scalable using soft nanoimprinting lithography to prepare Au/TiO2 2D-photonic photocatalysts. In a first process, gold nanoparticles (Au NPs) are in situ infiltrated in the superficial 50 nm of a mesoporous titania (mTiO2) scaffold patterned with the photonic structure, while in a second one 2D-photonic crystals with a homogeneous volume distribution of the Au colloids are achieved. The dependence of the optical properties of the photonic crystals on the lattice parameter, geometry, and metal loading is presented through extinction measurements and analyzed through simulations. The improved photocatalytic performance of the substrates is tested for hydrogen production where a maximum of 8.5 mmol gcat−1 h−1 of H2 is recorded and attributed to photonic–plasmonic effects. These results may open new avenues in solar harvesting for hydrogen production using photonic crystals as photocatalysts.M.T. and P.M. contributed equally to this work. This research received funding from the Spanish Ministry of Science, Innovation and Universities, through the RTI2018-096273-B-I00, RTI2018-093996-B-C31, and PID2019-106860GB-I00 projects, the “Severo Ochoa” Programme for Centers of Excellence in R&D grant FUNFUTURE (CEX2019-000917-S), and the Generalitat de Catalunya (2017SGR765 and 2017SGR128 grants). The Spanish Ministry of Education, Culture and Sport, is funding the FPU Fellow of MT (FPU16/05452). P.M. acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF and the UAB. M.T. is enrolled in the Materials Science Ph.D. Program of the UAB (Universitat Autònoma de Barcelona). L.S. is grateful to MICINN Ramon y Cajal program for individual fellowship grant agreement RYC2019-026704-I. J.L. is a Serra Húnter Fellow and is grateful to ICREA Academia Program.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Magneto-ionic suppression of magnetic vortices

Magneto-ionics refers to the non-volatile control of the magnetic properties of materials by voltage-driven ion migration. This phenomenon constitutes one of the most important magnetoelectric mechanisms and, so far, it has been employed to modify the magnetic easy axis of thin films, their coercivity or their net magnetization. Herein, a novel magneto-ionic effect is demonstrated: the transition from vortex to coherent rotation states, caused by voltage-induced ion motion, in arrays of patterned nanopillars. Electrolyte-gated Co/GdOx bilayered nanopillars are chosen as a model system. Electron microscopy observations reveal that, upon voltage application, oxygen ions diffuse from GdOx to Co, resulting in the development of paramagnetic oxide phases (CoOx) along sporadic diffusion channels. This breaks up the initial magnetization configuration of the ferromagnetic pillars (i.e. vortex states) and leads to the formation of small ferromagnetic nanoclusters, embedded in the CoOx matrix, which behave as single-domain nanoparticles. As a result, a decrease in the net magnetic moment is observed, together with a drastic change in the shape of the hysteresis loop. Micromagnetic simulations are used to interpret these findings. These results pave the way towards a new potential application of magnetoelectricity: the magneto-ionic control of magnetic vortex states