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We numerically investigated the propagation characteristics of Ga:ZnO (GZO) thin films embedded in a ZnWO4 background in the epsilon near zero (ENZ) region. We found that, for GZO layer thickness ranging between 2 - 100 nm (∼ 1/600 - 1/12 of ENZ wavelength), such structure supports a novel non-radiating mode with its real part of effective index lower than surrounding refractive index or even less than 1. Such a mode has its dispersion curve lying to the left of the light line in the background region. However, the calculated electromagnetic fields display non-radiating nature contrary to the Berreman mode, because the transverse component of the wave vector is complex, ensuring a decaying field. Furthermore, while the considered structure supports confined and highly lossy TM modes in the ENZ region, no TE mode is supported. Subsequently, we studied the propagation characteristics of a multilayer structure constituting an array of GZO layers in the ZnWO4 matrix considering the modal field’s excitation using the end-fire coupling. Such a multilayer structure is analyzed using high-precision rigorous coupled-wave analysis and shows strong polarization selective and resonant absorption/emission, the spectral location and bandwidth of which can be tuned by judiciously selecting the thickness of the GZO layer and other geometrical parameters.

Energy efficiency in many illumination devices is far from optimal. For instance, in liquid crystal displays, a large fraction of the emitted light is lost provided absorbing polarizers are used for the image formation. Herein, a light‐guiding structure to achieve diffuse polarized light emission from the top of the guide, combined with light harvesting and conversion back into electricity of the light with the unwanted polarization is proposed. Key in achieving such a double goal is the use of a half‐cylinder photonic plate for an ergodic light‐guided propagation incorporating a nonabsorbing reflective multilayer light polarizer and perovskite (PVK) solar cells. In the complex combination of micro‐ and nanometric dimensions of the proposed structure, light propagation was resolved using ray optics tracing interlaced with a full wave vector inverse integration approach. A broadband polarization extinction ratio of 0.1 for the light diffused from the top surface is demonstrated, while close to 90% of the s‐polarized light remains trapped and can be recycled back to electricity by the PVK solar cells placed on the front and back ends of the guide. PVK cells are shown to be optimal in such conversion with an overall efficiency ranging from 38 to 52%.

12 pags., 6 figs.; 1 tab. 1 app., The active and analog tuning of the phase of light by metamaterials is needed to boost the switching performance of photonic devices. However, demonstrations of this type of tuning in the pivotal visible spectral region are still scarce. Herein, we report the active analog tuning of the phase of visible light reflected by a bismuth (Bi)-based metamaterial, enabled by a reversible solid-liquid transition. This metamaterial, fabricated by following a lithography-free approach, consists of two-dimensional assemblies of polydisperse plasmonic Bi nanostructures embedded in a refractory and transparent aluminum oxide matrix. The analog tuning of the phase is achieved by the controlled heating of the metamaterial to melt a fraction of the nanostructures. A maximum tuning of 320° (1.8 π) is observed upon the complete melting of the nanostructures at 230°C. This tuning is reversible by cooling to 25°C. In addition, it presents a wide hysteretic character due to liquid Bi undercooling. This enables the phase achieved by this analog approach to remain stable over a broad temperature range upon cooling and until re-solidification occurs around 100°C. Therefore, Bi-based metamaterials are endowed with analog optical memory capabilities, which are appealing for a wide range of applications, including optical data storage with enhanced information density or bistable photonic switching with a tunable "on" state., This research was supported by Spanish grants RTI2018-096498-B-I00, Funder Id: http://dx.doi.org/10.13039/100014440 (MCIU/AEI/ FEDER, UE) and LINKA20044, Funder Id: http://dx.doi. org/10.13039/501100003339 (CSIC). AKPL is grateful to the National Science Foundation under Collaborative Grant #DMR 1600837, Funder Id: http://dx.doi. org/10.13039/100000001 for funding. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357, Funder Id: http://dx.doi. org/10.13039/100006151. E.N.P. acknowledges funding from Comunidad de Madrid (Spain), Garantia Juvenil contract PEJ-2018-AI/IND-10888.

6 pags., 1 fig., Although plasmonics and high refractive index photonics have experienced very fast growth thanks to classical physics concepts, there is an increasing interest in harnessing quantum physics concepts for further pushing the frontiers of these fields. In this context, this perspective highlights the importance of some quantum nanostructures for building nanomaterials and metamaterials with enhanced plasmonic and high refractive index properties. Two types of nanostructures displaying quantum properties are considered: (a) quantum confined nanostructures consisting of noble metals or standard semiconductors, (b) nanostructures built from alternative materials whose dielectric function and optical properties are driven by (possibly tailored) giant interband electronic transitions. A special emphasis is made on the potential of this latter type of nanostructures for achieving outstanding effects for applications, such as ultrabroadband light harvesting, giant refractive index, coupling between dielectric, low-loss plasmonic and magnetic properties, compositionally or externally tuneable optical response. Possible future developments to the field are discussed.

Optical guide system for broadband polarized diffuse light emission, incorporating light recycling back into electricity, based on a combination of periodic order in a half-cylinder photonic plate and randomness in a multilayer reflective polarizer.

11 pags., 5 figs., Random metasurfaces based on disordered phase-change nanostructures with broadband ultraviolet–visible plasmonic properties are fantastic platforms for the development of active photonic components due to their versatility. Here optical ON–OFF switching upon nanosecond laser irradiation of Bi-based random metasurfaces is demonstrated profiting from the solid–liquid phase transition of the Bi nanostructures. The fast switching time is revealed by using single-pulse real-time reflectivity measurements with sub-nanosecond temporal resolution. A sudden reflectivity decrease is observed upon laser excitation, triggered by heating and melting of the embedded Bi nanostructures of the metasurface. This transient state of low reflectivity remains constant until the Bi solidifies upon cooling, recovering the initial reflectivity level, and showing full reversibility of the process. The duration of this ON-state, defined by characteristic low reflectivity values, can be tuned from 10 to >700 ns by adjusting the nanostructure components of the metasurface and laser fluence. Furthermore, it is demonstrated that this ON–OFF switch process is repeatable for more than 100 000 times without observable degradation of the metasurface. These results pave the way for the development of a new generation of fast, robust, and integrable nanophotonic switching devices., This research was funded by the Spanish Research Agency (AEI, Ministry of Research and Innovation) and the European Regional Development Fund (ERDF) under grants TEC2017-82464-R and RTI 2018-096498-B-I00, and the CSIC PIE-201850E057 and PIE-202050E195 projects. M.G.-P. acknowledges funding from PTA2019-016763-I (AEI/MICINN).

As the efficiency of a solar cell approaches its limits, photonic considerations to further enhance its performance overtake electronic ones. It has been theoretically shown for GaAs solar cells that with the combined effects of a surface random texturing and a perfectly reflecting rear mirror, efficiencies close to the Shockley–Queisser limit can be reached, even when the absorber layer is very thin. In here, we demonstrate a method for taking advantage of surface random texturing to enhance the efficiency of planar perovskite solar cells. By naturally transferring the perovskite random nanotexturing to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, backscattering reduces light escape from the solar cell structure. This leads to a close to optimal light absorption that allows bringing the cell efficiency from 19.3% to 19.8%. Such a path we opened toward an ergodic behavior for maximum light absorption in perovskite cells may lead to...

10 pags., 4 figs., Plasmonic metasurfaces tailored at the few-nanometers scale and fabricated by high-throughput lithography-free methods are needed to efficiently produce optical components with optimal performance and small footprint. Herein, the design of such metasurfaces and their fabrication over cm areas by self-assembly are reported. They present an optical cavity structure, in which a 2D array of parallel Au nanorods is coupled to a metal mirror through a thin spacer. The nanorods present a quasi-monodisperse ≈10 nm diameter and separation, and a broad polydisperse length distribution. These remarkable nanostructural features, combining periodicity in one direction with randomness in another, endow the nanorod array with unconventional plasmonic and anisotropic effective optical properties. Thanks to these properties, for light polarized along the nanorods, the metasurface displays strong destructive optical interference and thus near-perfect absorption over a broad range of angles of incidence and of wavelengths in the near infrared. In contrast, for light polarized perpendicular to the nanorods, no such interference occurs and thus the metasurface behaves as a metal mirror. In sum, the metasurface can be used as broadband omnidirectional polarizing mirror, an original thin and monolithic optical component merging the functionalities of a metal mirror and of a wire-grid polarizer., This research was funded by the Spanish Research Agency (AEI, Ministry of Research and Innovation) and the European Regional Development Fund (ERDF) under grant RTI 2018‐096498‐B‐I00, and the CSIC (PIE-202050E195 project). The work of P.G.-R. was supported by the Autonomous Community of Madrid and the European Social Fund (contract E-28‐2019‐0767805)

13 pags., 9 figs., 1 tab., Functional materials with a tailored optical response are needed for applications including coloring, optical instrumentation, data encryption, thermal radiation shaping, energy harvesting, and environmental or biological sensing. A given application requires the material to absorb, transmit, reflect, or scatter light in a suitable way, in a specific, narrow, or broad spectral range selected in the visible or infrared. To achieve the optical response and functionality needed, it is appealing to design nanomaterials with a suitable composition and a properly engineered structure, and this should ideally be realized with high-throughput and cost-effective fabrication methods. Herein, the aim is to provide a view on some very recently reported functional nanomaterials showing an optical response tailored for applications by lithography-free methods. It is illustrated how assembling tailor-made nanostructures based on the expanding library of optical materials (and their specific wavelength-dependent dielectric functions) enables harnessing a variety of optical resonances (plasmonic, epsilon near zero, high refractive index Mie, film interference) to tailor the nanomaterials' optical response. It is also shown that building the nanostructures from novel optical materials brings special functionalities, such as a switchable response. Examples of applications are put forward.

Liquid crystal display (LCD) is the most widely used technology to obtain high resolution full color images in current electronic devices. Unfortunately, such high quality images come with no consideration for any kind of energy management. Indeed, in current portable electronics or large TV monitor screens based on LCDs, 95% of the light emitted by the LEDs does not reach the user's eye but instead it is lost as heat in the device.

In an optical medium slab, a randomization of the interfaces will lead to an ergodic light propagation, implying a kind of light trapping that sets an upper limit for a broadband absorption enhancement factor. However, in thin film solar cells where such light trapping is most needed, it is not straightforward how the implementation of a surface randomization should be done. In addition, such randomization may also severely limit the electronic device performance. Here, a cylindrical periodic corrugation, which is named half-cylinder photonic plate, is introduced at the light entering interface which leads to an ergodic light propagation. Advantage of such ergodicity is taken to enhance light absorption in a perovskite cell, physically separated by a planar glass substrate from such half-cylinder photonic plate. Theoretically and experimentally it is demonstrated that the enhancement light absorption factor measured is close to the maximum possible for light trapping schemes using one interface periodic corrugations. This upper limit results from the ergodic light propagation that is achieved with a half-cylinder photonic plate made from an optical transparent material with a low refractive index that minimizes light rejection by reflectivity., SCOPUS: ar.j, info:eu-repo/semantics/published

Sputtered ultrathin silver-based multilayer electrodes have emerged as promising ITO-free candidates for lightweight, roll-to-roll processable flexible organic photovoltaics due to their high conductivity, low parasitic absorption, and excellent mechanical flexibility. However, Ag-based flexible solar cells normally yield inferior device performance when compared to conventional glass/ITO analogues. In this work, we report the use of a two-resonance tapping cavity (TRTC) based transparent electrode for achieving high-performance flexible PBDBT:ITIC solar cells. To reach optimal light harvesting without significantly sacrificing the high open-circuit voltage, the non-fullerene acceptor was partially substituted by PC71BM to transform the binary blend into a ternary blend. The combination of open-circuit voltages as high as 0.88 V with short-circuit currents close to 18 mA cm−2 leads to power conversion efficiencies higher than 11% for optimal TRTC rigid cells under AM 1.5G 1-sun illumination. Remarkably, when the TRTC was integrated onto flexible PET substrates, a 10.6% efficiency was achieved for the cells, while a robust mechanical flexibility was preserved. This is the highest efficiency value ever achieved for flexible organic solar cells.

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of largecations to obtain a perovskite with larger lattice parameter together with a large crystalsize to minimize interface nonradiative recombination. When applying the two-stepmethod for a better crystal control, it is rather challenging to fabricate perovskites withFA+cations, given the small penetration depth of such large ions into a compact PbI2film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl-anions achieving,via a solvent annealing-controlled dissolution-recrystallization, larger than 1µmperovskite crystals in a solar cell. This solar cell, with a largely increasedfluorescencequantum yield, exhibited an open-circuit voltage equivalent to 93% of thecorresponding radiative limit one. This, together with the low band gap achieved(1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.

In this talk we will discuss the potential of oxides for novel plasmonic and photonic devices. For this purpose, we have designed and produced highly functional thin film oxide metamaterials by either embedding non-oxide nanoparticles which act as optical resonators, or by growing pure nanostructured oxides with a fine control of their composition. In the former case we build robust plasmonic and sensing platforms that profit from the coupling between plasmonic and photonic modes[1-3]. And in the latter case we control the band-gap and thus the optical response tuning of the nanostructured oxide, thus creating a platform for integrated photonics [4]. In this context we will show the light emission properties of Europium oxides that can range from wide white band to narrow red emission, and how the coupling with metallic nanostructures enhances and modifies their spectral response. [1]. J. Toudert, R. Serna, I. Camps, J. Wojcik, P. Mascher, E. Rebollar, T. Ezquerra, J. Phys. Chem. C 121, 3511 ( 2017). [2] G. Baraldi, M.García Pardo, J. Gonzalo, R. Serna and J. Toudert, Adv. Mater. Interfaces 5, 1870058 (2018). [3] J. Toudert, R. Serna, M. Pardo, N. Ramos, R. Peláez, and B. Maté, Opt. Express 26, 34043 (2018). [4] A. Mariscal, A. Quesada, A. Tarazaga Martín-Luengo, Miguel A. García, A. Bonanni, J.F. Fernández and R. Serna, Appl. Surf. Science 456,980 (2018).

13 pags., 9 digs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0, Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n~10) and a negative ultraviolet-visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infrared-to-ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For t > 15 nm, the films display a continuous structure and their dielectric function is comparable with that of bulk bismuth. For t < 15 nm, the film structure is discontinuous, and the dielectric function differs markedly from that of bulk bismuth. It is proposed from FDTD simulations that this marked difference arises mainly from effective medium effects induced by the discontinuous film structure, where quantum electronic confinement does not play a dominant role. This suggests that ultrathin and continuous bismuth films should present the same outstanding optical properties as bulk bismuth for high performance nanophotonic devices., Ministerio de Ciencia, Innovación y Universidades (MICINN) (MICINN RTI2018-096498-B-I00, MINECO/FEDER TEC2015-69916-C2-1-R).

17 pags., 12 figs., Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength λ. However, in current state-of-the-art designs, a still large absorbing film thickness (∼λ/50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus, really nanoscale films, with a broad wavelength tenability by using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than λ. With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub - λ/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 μm., European Social Fund (ESF); Spanish Ministry for Economy and Competitiveness (MINECO) (MINECO/ERDF TEC2015-69916-C2-1-R); European Regional Development Fund (ERDF) (MINECO/ERDF TEC2015-69916-C2-1-R).

7 págs.; 7 figs., Materials, and particularly glasses, with a tailor-made spectral optical response that can be switched in a controlled way using external stimuli excitation are of interest for the development of active optical devices, such as active filters or all-optical switching components. Based on the dielectric functions of solid and liquid bismuth (Bi), it is expected that both solid and liquid Bi nanostructures will show optical resonances with relevant different features in the near ultraviolet to near infrared. In addition, Bi has a low melting point that enables the solid–liquid phase change under moderate heating. Profiting of these singular properties we present and review in this paper the main results that have been obtained in our laboratory concerning the thermo-optical response of bulk germanate glasses and amorphous Al2O3 thin films with embedded Bi nanoparticles (NPs). In both materials the optical transmission as a function of the temperature shows broad hysteresis loops. The optical contrast and width of these cycles depend on the Bi content and NP size. From the successful modelling of the optical properties of the Bi NPs-doped glasses, it is proposed that the observed material thermo-optical response arises from the change in the dielectric function of the Bi NPs upon melting, which induces relevant changes in the resonant optical behaviour of the Bi NPs. These results suggest that glasses with embedded Bi NPs are promising for the design of spectral-selective thermo-optical devices. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved., This work has been partially supported by FP7-NMP-2010-EuMexico Grant agreement no. 263878: Functionalities of Bismuth based Nanostructures (BisNano) and by the Spanish Ministry of Economy and Competitiveness under Project AMALIE TEC2012-38901-C02-01. J. T. acknowledges a Juan de la Cierva Grant number JCI-2009–05098.

7 pags., 7 figs., We demonstrate successful nanometric conformal coverage by pulsed laser deposition (PLD) of Ag nanoparticle (NP) arrays with a transparent amorphous AlO layer. The Ag NPs-arrays are formed by a bimodal distribution of well separated spheroidal Ag NPs with an average in-plane diameter of 40 or 90 nm and a maximum height of ∼70 nm. The cover layer induces no relevant alteration of the morphology and organization of the underlying Ag NPs-arrays. The conformal nature of the AlO cover layer is confirmed by its surface topography that shows the presence of characteristic nanodomes, with sizes determined by those of the underlying Ag NPs. The distinctive tapered shape of the nanodomes is related to the angular dependence with respect to the NPs surface normal of the deposition rate and the self-sputtering induced by energetic Al ions present in the laser generated plasma. The presence of the amorphous AlO cover layer red-shifts the characteristic dipolar and quadrupolar Surface Plasmon Resonances of Ag NPs and increases the total and diffuse reflectance of the NPs arrays., M.M-O. ac-knowledges thefinancial support of the Autonomous Community of Madrid and the European Social Fund [contract E-28-2017-0679526] and G.B. a CSIC-JAE predoctoral grant, cofounded by the European Social Fund. This work was partially funded by the Spanish Ministry of Science, Innovation and Universities [projects MINECO/FEDERTEC2015-69916-C2-1-R and MICINN RTI2018-096498-B-I00]

Aquest article pot ser utilitzat per a fins no comercials, d'acord amb els termes i condicions d’auto-arxiu de Wiley. Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies which are presented in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, a robust nanocrystalline ordering is induced in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC71BM cells are shown to exhibit T80 lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.

For many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two ¿/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day. Q.L., L.G., F.B., J.T., and J.M. acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) through the “Severo Ochoa” program for Centres of Excellence in R&D (SEV2015-0522), from Fundació Privada Cellex, from Generalitat de Catalunya through the Impulsa-Llavor (Llav-00073) and CERCA programs, and also acknowledge financial support from MINECO through project MAT2017-89522-R. T.L. and X.Z. thank the NSFC (No. 21734001). F.B. also thanks the CONACyT international grants program.

23 págs., Spectroscopic ellipsometry (SE) is a powerful technique for the characterization of materials, which is able to probe in a sensitive way their nanostructure as well as to get rich information about their dielectric properties, through the interaction of polarized light with matter. In the present trend of developing functional advanced materials of increasing complexity for a wide range of technological applications, works involving SE have flourished and have been reported in an increasing number of articles, reviews, and books. In this context, the aim of this paper is to provide for those among material scientists who are not SE specialists, a concise and updated overview of the capabilities of SE for the characterization of the so-called nano- and metamaterials, especially those presenting active functionalities. Key aspects for a reliable material characterization by SE are given: choice of the setup and measurement conditions, measurement accuracy, definition of a model, sensitivity to parameters. Also, very recent works involving SE are highlighted, especially those dealing with the development of building block materials for optimized or active plasmonics applications, the still ongoing exploration of small-size effects on the dielectric response of matter, the characterization of metamaterials, and the design of detectors with improved accuracy based on coupling of the phase sensitivity of SE with metamaterials engineering.

11 pags., 6 figs., For years bismuth (Bi) has appealed to a broad community of scientists due to its peculiar electronic, optical, and more recently plasmonic and photocatalytic properties, which enable both the understanding of basic science phenomena and the development of a wide range of applications. In spite of this interest, a comprehensive spectral analysis of the dielectric function (¿ = ¿1 + j¿2) of bulk Bi from the far infrared (IR) to the ultraviolet (UV) region is not available. So far, the data have been reported in limited spectral ranges and show a wide dispersion that is especially notorious for the IR region. In this work we report ¿ for Bi in a wide spectral range from 0.05 to 4.7 eV (24.8 to 0.3 ¿m, far IR to UV). ¿ is extracted from spectroscopic ellipsometry measurements of excellent quality (dense and smooth) Bi films by using the transfer matrix formalism and Kramers¿Kronig consistent analysis. The higher quality and accuracy of the obtained ¿ compared with the literature data is demonstrated. The analysis and use of this reference bulk dielectric function provides crucial information for the exploration and understanding of the optical, plasmonic, and photocatalytic properties of Bi nanostructures. From its analysis, it is evidenced that the optical properties of Bi in the mid wave IR-to-UV are driven only by interband transitions, which are responsible for the dominant absorption band peaking at about 0.8 eV. Therefore, the plasmonic behavior and the photocatalytic performance of Bi nanostructures in the visible and UV are likely driven by these interband transitions that make ¿1 turn negative in this region without the need of exciting free carriers. Furthermore, classical electrodynamic simulations using the obtained ¿ show a strong size dependence for the optical extinction of Bi nanospheres in the far IR-to-near IR with Mie-like resonances broadly tunable across this region., This research received funding from the European Community Seven Framework Programme (FP7-NMP-2010-EU-MEXICO) under Grant Agreement No. 263878. We also acknowledge funding from the Spanish Ministry for Economy and Competitiveness through and TEC2012-38901-C02-01 and TEC 2015-69916-C2-1-R projects. E.R. acknowledges funding from the Spanish Ministry for Economy and Competitiveness through a Ramón y Cajal Fellowship (RYC-2011-08069). In Canada, this work was supported by the Discovery Grants program of the Natural Science and Engineering Research Council (NSERC).

This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. An optimal photon absorption in thin film photovoltaic technologies can only be reached by effectively trapping the light in the absorber layer provided a considerable portion of the photons is rejected or scattered out of such layer. Here, a new optical cavity is proposed that can be made to have a resonant character at two different nonharmonic frequencies when adjusting the materials or geometry configurations of the partially transmitting cavity layers. Specific configurations are found where a reminiscence of such two fundamental resonances coexists leading to a broadband light trapping. When a PTB7-Th:PC71BM organic cell is integrated within such cavity, a power conversion efficiency of 11.1% is measured. This study also demonstrates that when materials alternative to organics are used in the photoactive cell layer, a similar cavity can be implemented to also obtain the largest light absorption possible. Indeed, when it is applied to perovskite cells, an external quantum efficiency is predicted that closely matches its corresponding internal one for a broad wavelength range.

14 págs.; 4 figs. ; OCIS codes: (160.0160) Materials; (160.4760) Optical properties; (250.5403) Plasmonics; (260.5740) Resonance; (240.5420) Polaritons; (300.0300) Spectroscopy, We investigate the origin of the ultraviolet - visible plasmonic properties of three elemental materials from the p-block: Bi, Sb and Ga, which has not been discussed so far despite growing interest in these materials for plasmonic applications. We review and analyze a broad range of optically-determined dielectric functions ε = ε + jε of these elemental materials available in the literature, covering a wide photon energy range (from 0.03 to 24 eV). It is shown that the contribution of free carriers to ε in the ultraviolet - visible is negligible for Bi and Sb and small for Ga. In contrast, the interband transitions of these elemental materials show a high oscillator strength that yields a strong negative contribution to ε in the ultraviolet - visible. Therefore it is proposed that, in nanostructures made of these elemental materials, the interband transitions induce localized surface plasmon-like resonances in the ultraviolet - visible. It is exemplified how these resonances are sensitive to the size and environment of the nanostructures. Furthermore, ultraviolet - visible plasmonic properties achieved through interband transitions, without free carrier excitation, are especially appealing because they might be tuned through the tailoring of the band structure and by the occupancy of electronic states. Therefore they are promising for the development of broadly tunable nanostructures and metamaterials. ©2016 Optical Society of America, We acknowledge the Spanish Ministry for Economy and Competitiveness (Projects TEC 2012-38901-C02-01 and TEC2015-69916-C2-1-R).

12 págs.; 4 figs., In the quest to unveil alternative plasmonic elements overcoming noble metals for selected applications in photonics, we investigate, by numerical simulations, the near ultraviolet (UV)-to-near infrared optical response of solid and liquid Bi nanospheres embedded in a dielectric matrix. We also determine the resulting transmission contrast upon reversible solid-liquid phase transition to evaluate their potential for switchable optical filtering. The optical response of the solid (liquid) Bi nanospheres is ruled by localized polaritonic (plasmonic) resonances tunable by controlling the diameter. For a selected diameter between 20 and 50 nm, both solid and liquid nanospheres present a dipolar resonance, inducing a strong peak extinction in the near UV, however, at different photon energies. This enables a high transmission contrast at selected near UV photon energies. It is estimated that a 2-D assembly of 30-nm solid Bi nanospheres with a surface coverage of 32% will almost totally render extinct (transmission of 2%) a near UV 3.45-eV (359 nm) light beam, whereas upon phase transition, the resulting liquid Bi nanospheres will show a transmission of 30%. This paper is appealing for the fabrication of locally reconfigurable optical metamaterials for integrated switchable near-UV optics. Ó 2016 IEEE, This work was supported in part by the European Community Seventh Framework Programme (FP7-NMP-2010-EU-MEXICO) under Grant 263878 and in part by the Spanish Ministry for Economy and Competitiveness under Project TEC2012-38901-C02-01 and Project TEC2015-69916-C2-1-R.

The reflectivity variations of a dielectric thin film during the deposition of a disordered monolayer of metallic nanoparticles are studied. We present experimental results and provide theoretical physical insight into the behavior of the reflectivity signal and its dependence on the dielectric thin-film thickness and structural features of the monolayer of nanoparticles. A closed-form expression is used to describe the reflectivity of a disordered monolayer of particles on a flat substrate within the frame of a coherent-scattering model approach. It is shown that the model reproduces qualitatively the behavior of the reflectivity signal during the experiment. Finally we study the optical response in the limit of small particles for low surface coverage fractions of the monolayer to evidence the main parameters that dictate the evolution of the reflectivity signal during the growth of a monolayer of nanoparticles. © Springer-Verlag 2012

Authors thank the Projects FUNCOAT CONSOLIDER-INGENIO CDS2008 – 0023, MAT2007 – 65764, MAT2010-18447, MAT2010-21228, CEN2007 – 2014, P09-TEP-5283, Domingo Martinez Foundation, and Juan de la Cierva Grant Nº JCI-2009 – 05098 for J.T.

Nanoporous ZnO thin films presenting a tunable nanostructure and photoluminescence (PL) were grown by plasma enhanced vapor deposition on surface oxidized Si substrates. These films consist of c-ax...

7 pags., 3 figs., Sensing devices for environment, safety, healthcare, and optoelectronic applications require an accurate and noninvasive monitoring of their tem- perature, because its variations markedly affect the overall response of the device. Monitoring the optical response of temperature-sensitive integrated photonic elements, such as microresonators or microinterferometers, is an appealing solution in this context. However, achieving high-resolution optical thermometry with such elements is unpractical and costly as this requires lithography processing, highly monochromatic laser sources, complex light coupling strategies. Here, a photonic-plasmonic metasurface design that ena- bles practical optical thermometry with a sub-10¿3 °C resolution is proposed. It is based on a self-assembled nanostructured material implemented with a lithography-free process. The optical response of the temperature-sensitive metasurface is probed using a white light source and by monitoring the optical phase in a standard reflectance configuration. This facile, yet pow- erful, sensing scheme stands on the effective response of the metasurface, which involves the hybridization of thin film interference and low-quality- factor plasmon resonances to enable a quasi-darkness response with a sharp spectral variation (jump) of the optical phase. Such jump is equivalent with a high-quality-factor resonator that yields a high sensor responsivity and thus enables high-resolution optical thermometry., M.G.P. acknowledges support from the European Social Fund and G.B. a CSIC-JAE predoctoral grant, cofunded by the European Social Fund. This work was funded in part by the Spanish Ministry for Economy and Competitiveness through the project MINECO/FEDER TEC2015-69916-C2-1-R.

This work reports an easy-to-handle method for growing two-dimensional assemblies of Ag nanostructures presenting a tunable in-plane optical anisotropy. Ag is deposited by DC sputtering in an Ar plasma at room temperature onto bundled nanocolumnar SiO2 thin films grown by glancing angle physical vapor deposition. In contrast with previously reported processes involving the grazing angle deposition of the metal, DC sputtering is performed at normal incidence. By varying the deposition angle of SiO2 and the Ar pressure, it was possible to tune the deposited amount of Ag and thus the topology of the Ag deposit from isolated spherical Ag nanoparticles with isotropic optical properties to strongly dichroic Ag nanostripes oriented along the bundling direction of the SiO2 nanocolumns. Based on simple calculations taking into account the shadowing effects during metal deposition, it is proposed that the width and shape of the tip of the bundled SiO2 nanocolumns influence significantly the metal local atom flux arriving to them and thus the final structure of the deposit. © 2010 Springer Science+Business Media, LLC., The authors thank the European Union (STREP Project PHODYE, contract no. 033793), the Spanish Ministry of Research and Innovation (Juan de la Cierva Grant JCI-2009-05098 for J.T., projects MAT2007-65764 and CONSOLIDER INGENIO 2010- CSD2008-00023), and the Junta de Andalucia (project TEP2275) for financial support.

The morphology and optical response of Au and Ag nanoparticles capped with dielectric amorphous Si 3 N 4 , BN, and Al 2 O 3 layers are studied. Quantitative analysis of high-angle annular dark-field images obtained in a scanning transmission electron microscope and simulations of optical transmittance spectra are used to investigate the dependence of the morphology of the nanoparticles on the particle size and on the capping material. It is demonstrated that capping-layer effects induce a size-dependent and cap-dependent reshaping of the nanoparticles.

5 pages, 5 figures.-- Presented as communication to: E-MRS 2007 Spring Meeting, Symposium C: Rare Earth Ion Doping for Photonics: Materials, Mechanisms and Devices (Strasbourg, May 28-Jun 1, 2007).-- Printed version published on Jan 2008., Enhancement of the 1540 nm emission in nanostructured Er(3+)-doped a-Si/a-Al2O3 thin films prepared by pulsed laser deposition is studied. This paper is focused on a symmetric film structure formed by a central Er(3+)-doped Al2O3 layer sandwiched between two a-Si layers, which are isolated from the silicon substrate and the ambient medium by non-doped Al2O3 buffer layers. The layer thicknesses have been chosen in order to minimize the reflectance of the film at the pumping wavelength of 514.5 nm and thus to increase the effective excitation of the Er(3+) ions. The Er3+ ions within the central Er(3+)-doped Al2O3 layer have been distributed in layers separated by about 5 nm in order to minimize clustering and concentration quenching effects. Upon excitation at 514.5 nm, the film annealed at 700°C shows the characteristic emission of Er(3+) ions at 1540 nm in addition to a continuous emission band, which is also observed upon excitation at 476 nm while no Er(3+) related emission can be evidenced. This band may thus be linked to emission from the Si layers. It has been determined that the net Er(3+) emission intensity enhancement at 1540 nm is of a factor 1.5 when compared to a film prepared with no Si layers. A two-component luminescence decay is observed upon excitation at 514.5 nm: first, a fast decay regime that can be associated to the broad emission band from the Si layers, and then a slower decay regime linked to the Er(3+) emission and characterized by a time constant of 5.7 ms. This lifetime value is 2.5 times higher than for the film without silicon layers (2.2 ms)., This work has been supported by CICYT (Spain) under project TEC2006-04538-MIC. S. N-S. Acknowledges the Ministerio de Educacion y Ciencia (Spain) for support from a FPU grant.

Plasmonic nanostructures can be described by equivalent impedance, allowing the analysis of their properties and performance by an RLC circuit equivalent model. In this letter, the equivalent impedance has been obtained from a power budget calculation that considers the dielectric functions of the matrix and the nanostructured material, the nanostructure geometry (in this case, spheres), the light wavelength, and its polarization. When Bi or Ga nanospheres are considered, the equivalent circuit undergoes a switching from a capacitive to and inductive response when the materials change their phase from solid to liquid. Finally, this approach is applied to the design of reconfigurable metamaterials. This work was supported by the Ministerio de Economía y Competitividad of Spain, project 32 of CEMIE-Solar from Secretara de Energía-Sustentabilidad Energética and the Terahertz Science and Technology National Lab from CONACYT (México) under Grant TEC2012-38901-C02-01, Grant TEC2015-69916-C2-1-R, Explora Grant TEC2013-50138-EXP, Grant TEC2013-40442, and Grant PR2015-00063. Publicado

Nanoplasmonic materials are intensively studied due to the advantages they bring in various applied fields such as photonics, optoelectronics, photovoltaics and medicine. However, their large-scale fabrication and tunability are still a challenge. One of the promising ways of combining these two is to use the self-organization mechanism and after-growth engineering as annealing for tuning the properties. This paper reports the development of a bulk nanoplasmonic, Bi2O3-Ag eutectic-based metamaterial with a tunable plasmonic resonance between orange and green wavelengths. The material, obtained by a simple growth technique, exhibits a silver nanoparticle-related localized surface plasmon resonance (LSPR) in the visible wavelength range. We demonstrate the tunability of the LSPR (spectral position, width and intensity) as a function of the annealing temperature, time and the atmosphere. The critical role of the annealing atmosphere is underlined, annealing in vacuum being the most effective option for a broad control of the LSPR. The various potential mechanisms responsible for tuning the localized surface plasmon resonance upon annealing are discussed in relation to the nanostructures of the obtained materials.

14 págs.; 5 figs.; 1 tab.; OCIS codes: (160.4236) Nanomaterials; (160.3918) Metamaterials; (250.5403) Plasmonics., Nanoplasmonic materials are intensively studied due to the advantages they bring in various applied fields such as photonics, optoelectronics, photovoltaics and medicine. However, their large-scale fabrication and tunability are still a challenge. One of the promising ways of combining these two is to use the self-organization mechanism and aftergrowth engineering as annealing for tuning the properties. This paper reports the development of a bulk nanoplasmonic, Bi2O3-Ag eutectic-based metamaterial with a tunable plasmonic resonance between orange and green wavelengths. The material, obtained by a simple growth technique, exhibits a silver nanoparticle-related localized surface plasmon resonance (LSPR) in the visible wavelength range. We demonstrate the tunability of the LSPR (spectral position, width and intensity) as a function of the annealing temperature, time and the atmosphere. The critical role of the annealing atmosphere is underlined, annealing in vacuum being the most effective option for a broad control of the LSPR. The various potential mechanisms responsible for tuning the localized surface plasmon resonance upon annealing are discussed in relation to the nanostructures of the obtained materials. ©2015 Optical Society of America, The authors thank the Maestro Project 2011/02/A/ST5/00471 and the Preludium Project 2012/07/N/ST5/02428 from the National Science Centre, and the U.S. Air Force Office of Scientific Research under Grant FA9550-14-1-0061 for support of this work.

9 págs.; 6 figs., © 2015 American Chemical Society.We fabricate an Er3+/nano-Si ultrathin (≈ 4 nm) layer and explore its optical response from the near-UV to the near-IR, in the linear and nonlinear regimes. This nanohybrid layer combines the tunable broad-band light harvesting properties of nano-Si with the robust and sharp Er3+ light emission. Its unique nanostructure enables efficient nanometer-range transfer of the harvested energy to the Er3+ ions. Therefore, clear 1.54 μm Er3+ photoluminescence (PL) is observed under excitation at any photon energy (Eexc) from the visible to the near-UV, despite the small amount of Er3+ ions in the layer (, This work has been partly funded by the Spanish Ministry of Economy and Competitiveness, in the framework of the AMALIE TEC2012-38901-C02-01 and MAT2010-16407 research projects of the national R&D plan. J.T. acknowledges funding from the former Spanish Ministry for Research, Development and Innovation (Juan de la Cierva Fellowship JCI-2009-05098). J.M.R. thanks the CSIC for a JAE predoc grant.

Dual ion-beam sputtering has been used to achieve the growth of silver and iron nanoclusters embedded within graphite-like carbon and boron nitride thin films. Encapsulation of the nanoclusters in carbon nanocages has been realized when the deposition was performed at 500 °C whereas a temperature of 200 °C was sufficient to form hexagonal BN layers surrounding the nanoclusters. We show that, thanks to the enhanced surface diffusion of the deposited species, self-organized arrays of nanoclusters elongated along the growth direction can be produced by argon-assisted co-deposition of the metal and the matrix material. Vertical ordering of the nanoclusters, which develops from a topology-induced self-organization, can also be obtained by alternate deposition of very thin metallic layers separated by BN layers, as shown by high-resolution transmission electron microscopy and grazing incidence small-angle X-ray scattering.

The variation of the optical properties of embedded metallic nanoclusters induced by changes of their morphology is investigated by an approach combining electron microscopy, grazing incidence small angle X-ray scattering and transmission measurements. For this purpose, silver nanoclusters with various shapes (from prolate to oblate) and embedded in an amorphous Si3N4 matrix are elaborated by ion beam sputtering using either a co-deposition process or an alternate sputtering. Moreover, a post-irradiation with Ar+ ions (at different fluences and low incident energy of 75–100 keV) is used to complement the fabrication process in order to tailor the nanoclusters shape. Prolate and oblate nanoclusters become then more spherical leading to a spectral shift of their surface plasmon resonance.

Ag–Si 3 N 4 multilayers with nominal Ag thickness of 1 nm and Si 3 N 4 thickness ranging from 3 to 12 nm were elaborated by dual ion beam sputtering. The nanocomposite films were analyzed by transmission electron microscopy, grazing incidence small-angle X-ray scattering and spectroscopic ellipsometry. Optical properties exhibit surface plasmon resonance (SPR) coming from an enhancement of the electrical field inside Ag nanoclusters. The SPR spectral position has been related to the oblate shape of the nanoclusters. Furthermore, it is shown that a modification of the magnitude of the multipolar electromagnetic interactions among clusters occurs when the in-depth dielectric separation of the Ag layers varies from H to 2 H , where H is the height of the nanoclusters.

8 págs.; 10 figs.; 2 tabs., Extinction of the reflection of the p-polarized wave by the surface of a semi-infinite transparent medium is a well-known phenomenon exploited in Brewster angle microscopy. For absorbing media, however, described by a complex refractive index of refraction, the Brewster extinction condition does not exist anymore since the real and imaginary parts of the reflection coefficient cannot be canceled simultaneously. The exact zero reflection condition can be restored if a thin film of suitable complex index is deposited on top of a semi-infinite substrate. In such case, whatever the accuracy of the experimental setup, the existence of a true perfect cancelation of the reflection is guaranteed by the Jordan topological theorem. In the immediate vicinity of the zero reflection point, the phase of the reflected wave undergoes an extremely steep variation that enables ultra-sensitive sensing. The existence of a topological darkness condition and its application to the detection of dilute biological molecules were first demonstrated on plasmonic meta-surfaces made of two-dimensional arrays of gold nanopillars deposited on a glass substrate by electron beam lithography. The effective complex refractive index of the homogenized equivalent film indeed satisfies the topological darkness condition. In this paper, we show that several plasmonic systems prepared by the physical chemistry methods of colloids exhibit topological darkness and may provide good substrates for ultra-sensitive sensing at lower cost., This work was initiated in the framework of the FP7 collaborative project METACHEM under Grant #228762 and benefited from financial support from the Amadeus program under grants ANR-10-LABX-0042-AMADEUS and ANR-10-IDEX-0003- 02.

We report a powerful and general concept for the design of three-dimensional metamaterials suitable for optically probed nanoscale sensing, combining the environmental sensitivity of localized surface plasmon resonances (LSPRs) and a low absorption of the probe light, even when the LSPRs are supported by lossy plasmonic elements. This concept is based on the tuning of the plasmon-induced Brewster conditions (angle of incidence and photon energy) of the material by suitable tailoring of its structure. This tuning is made possible by the interplay between LSPRs and optical interferences, i.e., the so-called plasmonic optical interferences. The plasmon-induced Brewster photon energy can be set outside the spectral range of the LSPR, thus in a spectral region of low optical absorption. Such off-resonant' plasmon-induced Brewster conditions are examplified in a self-assembled lamellar three-dimensional metamaterial consisting of stacked polymer layers doped with gold nanoparticles alternating with undoped polymer layers. At the off-resonant' plasmon-induced Brewster conditions, an abrupt jump of the ellipsometric phase angle δ occurs in the photon energy space. In the spectral vicinity of this jump, δ shows strong sensitivity to the spectral features of the LSPR, potentially allowing the detection of infinitesimal LSPR energy shifts (

13 pags., 9 figs., 3 tabs., In this work, we report on the production of regular (SiGe/SiO) multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (t ) from 3.5-2 nm results in a transition from continuous SiGe crystalline layer (t ∼ 3.5 nm) to layers consisting of isolated nanocrystals (t ∼ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ∼3-8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t ∼ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K)hotoluminescence measurements performed on annealed SiGe/SiO nanostructures show an emission band located between 0.7-0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix., This work was partially funded by: (i) FEDER funds through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011; (ii) the transnational access framework of the ANNA Eu Project (Contract No. 026134-RII3) through the funding of the ANNA_- TA_UC9_RP006 proposal; (iii) UID/CTM/50025/2013 and (iv) UID/FIS/04650/2013. EMFV is grateful for financial support through the FCT and POPH of the grant SFRH/ BPD/95905/2013. The authors would like also to thank engineer José Santos for technical support at the Thin Film Laboratory

8 pags. ; 5 figs. © 2014 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/)., The aim of this work is to assess the evolution of the structural and optical properties of BixTiyOz films grown by rf magnetron sputtering upon post-deposition annealing treatments in order to obtain good quality films with large grain size, low defect density and high refractive index similar to that of single crystals. Films with thickness in the range of 220–250 nm have been successfully grown. After annealing treatment at 600 °C the films show excellent transparency and full crystallization. It is shown that to achieve larger crystallite sizes, up to 17 nm, it is better to carry the annealing under dry air than under oxygen atmosphere, probably because the nucleation rate is reduced. The refractive index of the films is similar under both atmospheres and it is very high (n =2.5 at 589 nm). However it is still slightly lower than that of the single crystal value due to the polycrystalline morphology of the thin films., This research received funding from the European Community Seven Framework Programme (FP7-NMP-2010-EU-MEXICO) under grant agreement No. 263878 and from the Spanish Ministry of Economy and Competitively under project TEC2012-38901-C02-01 and founds of patrimonio autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas. The authors would also like to thank CINVESTAV for the technical support given in the Atomic Force Microscoscopy (AFM) measurements., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).