Effect of temperature variations on equilibrium distances in levitating parallel dielectric plates interacting through Casimir forces

We study at thermal equilibrium the e ect of temperature deviations around room temperature on the equi- librium distance (deq) at which thin lms made of Te on, silica or polystyrene immersed in glycerol levitate over a silicon substrate due to the balance of Casimir, gravity, and buoyancy forces. We nd that the equi- librium nature (stable or unstable) of deq is preserved under temperature changes, and provide simple rules to predict whether the new equilibrium position will occur closer or further from the substrate at the new temperature. These rules depend on the static permittivities of all materials comprised in the system ("(m) 0 ) and the equilibrium nature of deq. Our designed dielectric con guration is excellent for experimental obser- vation of thermal e ects on the Casimir force indirectly detected through the tunable equilibrium distances (with slab thickness and material properties) in levitation mode.Peer reviewe

Localized surface plasmon effects on the photophysics of perovskite thin films embedding metal nanoparticles

Herein we provide direct experimental evidence that proves that the photophysical properties of thin methylammonium lead iodide perovskite films are significantly enhanced by localized surface plasmon resonances (SPRs). Observations are well supported by rigorous calculations that prove that improved light harvesting can be unequivocally attributed to plasmonic scattering and near field reinforcement effects around silver nanoparticles embedded within the semiconductor layer. Adequate design of the localized SPR allows raising the absorptance of a 300 nm thick film at well-defined spectral regions while minimizing the parasitic absorption from the metallic inclusions. Measured enhancements can be as large as 80% at specific wavelengths and 20% when integrated over the whole range at which SPR occurs, in agreement with theoretical estimations. Simultaneously, the characteristic quenching effect that the vicinity of metals has on the photoluminescence of semiconductors is largely compensated for by the combined effect of the enhanced photoexcitation and the higher local density of photon states occurring at SPR frequencies, with a two fold increase of the perovskite photoemission efficiency being measuredThe research leading to these results has received funding from the Spanish Ministry of Economy and Competitiveness under grant MAT2017-88584-R (AEI/FEDER,UE). A. B. was supported by the U. S. Department of State through the Fulbright Program. S. C. P. is grateful for the support of the AEI under the Juan de la Cierva Incorporación programme (IJCI-2016-28549

Plasmonic Nanoparticles as Light-Harvesting Enhancers in Perovskite Solar Cells: A User’s Guide

In this Perspective we discuss the implications of employing metal particles of different shape, size, and composition as absorption enhancers in methylammonium lead iodide perovskite solar cells, with the aim of establishing some guidelines for the future development of plasmonic resonance-based photovoltaic devices. Hybrid perovskites present an extraordinarily high absorption coefficient which, as we show here, makes it difficult to extrapolate concepts and designs that are applied to other solution-processed photovoltaic materials. In addition, the variability of the optical constants attained from perovskite films of seemingly similar composition further complicates the analysis. We demonstrate that, by means of rigorous design, it is possible to provide a realistic prediction of the magnitude of the absorption enhancement that can be reached for perovskite films embedding metal particles. On the basis of this, we foresee that localized surface plasmon effects will provide a means to attain highly efficient perovskite solar cells using films that are thinner than those usually employed, hence facilitating collection of photocarriers and significantly reducing the amount of potentially toxic lead present in the device.European Union 307081Ministerio de Economía y Competitividad MAT2011-23593, MAT2014-54852-

Absorption enhancement in methylammonium lead iodide perovskite solar cells with embedded arrays of dielectric particles

In the field of hybrid organic-inorganic perovskite based photovoltaics, there is a growing interest in the exploration of novel and smarter ways to improve the cells light harvesting efficiency at targeted wavelength ranges within the minimum volume possible, as well as in the development of colored and/or semitransparent devices that could pave the way both to their architectonic integration and to their use in the flowering field of tandem solar cells. The work herein presented targets these different goals by means of the theoretical optimization of the optical design of standard opaque and semitransparent perovskite solar cells. In order to do so, we focus on the effect of harmless, compatible and commercially available dielectric inclusions within the absorbing material, methylammonium lead iodide (MAPI). Following a gradual and systematic process of analysis, we are capable of identifying the appearance of collective and hybrid (both localized and extended) photonic resonances which allow to significantly improve light harvesting and thus the overall efficiency of the standard device by above 10% with respect to the reference value while keeping the semiconductor film thickness to a minimum. We believe our results will be particularly relevant in the promising field of perovskite solar cell based tandem photovoltaic devices, which has posed new challenges to the solar energy community in order to maximize the performance of semitransparent cells, but also for applications focusing on architectonic integration.European Union 307081Ministerio de Economía y Competitividad MAT2014-54852-R, MAT2017-88584-

Optical interference effects on the Casimir-Lifshitz force in multilayer structures

The Casimir-Lifshitz force F(C−L) between planar objects when one of them is stratified at the nanoscale is herein investigated. Layering results in optical interference effects that give rise to a modification of the optical losses, which, as stated by the fluctuation-dissipation theorem, should affect the Casimir-Lifshitz interaction. On these grounds, we demonstrate that, by nanostructuring the same volume of dielectric materials in diverse multilayer configurations, it is possible to access F(C−L) of attractive or repulsive nature, even getting canceled, at specific separation distances

High-throughput fabrication of resonant metamaterials with ultrasmall coaxial apertures via atomic layer lithography

This is an open access article published under an ACS AuthorChoice License.-- et al.We combine atomic layer lithography and glancingangle ion polishing to create wafer-scale metamaterials composed of dense arrays of ultrasmall coaxial nanocavities in gold films. This new fabrication scheme makes it possible to shrink the diameter and increase the packing density of 2 nm-gap coaxial resonators, an extreme subwavelength structure first manufactured via atomic layer lithography, both by a factor of 100 with respect to previous studies. We demonstrate that the nonpropagating zeroth-order Fabry-Pérot mode, which possesses slow light-like properties at the cutoff resonance, traps infrared light inside 2 nm gaps (gap volume ∼ λ3/106). Notably, the annular gaps cover only 3% or less of the metal surface, while open-area normalized transmission is as high as 1700% at the epsilon-near-zero (ENZ) condition. The resulting energy accumulation alongside extraordinary optical transmission can benefit applications in nonlinear optics, optical trapping, and surface-enhanced spectroscopies. Furthermore, because the resonance wavelength is independent of the cavity length and dramatically red shifts as the gap size is reduced, large-area arrays can be constructed with λresonance ≫ period, making this fabrication method ideal for manufacturing resonant metamaterials.This research was supported by the Office of Naval Research Young Investigator Program (D.Y. and S.-H.O.), Seagate Technology (J.S. and S.-H.O.), and the NIH Biotechnology Training Grant (D.A.M.). N.C.N. and J.P. acknowledge support from the Air Force Office of Scientific Research (AFOSR Grants FA9550-12-1-0357 and FA9550-11-1-0141). L.M.-M. acknowledges support from the Spanish Ministry of Economy and Competitiveness (MAT2014-53432- C5-1-R). T.W.E. acknowledges support from the Agence National de la Recherche (ANR) Equipex Union (ANR-10-EQPX-52-01), the Labex NIE projects (ANR-11-LABX-0058 NIE), and the Investissement d’Avenir program (ANR-10-IDEX- 0002-02).Peer reviewe

Enhancing nonlinear interactions by the superposition of plasmonic lattices on ꭕ(2)-nonlinear photonic crystals

Plasmonic structures have been revealed as efficient units to enhance localized nonlinear phenomena generated at dielectric-metal interfaces. However, their effect on the nonlinear interactions provided by quasi-phase matching processes in ꭕ(2) modulated dielectric crystals have been scarcely addressed, mainly due to the complexity in manufacturing appropriate periodic plasmonic structures overlying the ꭕ(2) dielectric structure. Here, by a simple method we have fabricated a periodic structure based on the combination of two commensurate lattices: a periodic lattice of chains of Ag nanoparticles and a periodic lattice of ꭕ(2)-modulation based on a ferroelectric domains structure. The hybrid system supports multiple surface plasmon lattice resonances (SLRs) at the technologically relevant NIR spectral region, which yield the enhancement of the nonlinear diffraction pattern generated by the ꭕ(2) structure. The superposition of the plasmonic and the ꭕ(2)-modulation lattice results in a 20-fold enhancement of the directional SHG due to the excitation of SLRs by the interacting waves involved in the nonlinear process. The results are obtained in lithium niobate, a widely used crystal in optoelectronics, and demonstrate the potential of the approach to design integrated solid-state platforms for on-chip optical steering, multiplexing or quantum technologiesThis work has been supported by the Spanish Government (Contracts MAT2016-76106-R and PID2019-108257GB-I00/ AEI/10.13039/501100011033 and María de Maeztu “Pro gramme for Units of Excellence in R&D CEX2018-000805-M) and Comunidad de Madrid (Grant SI1/PJI/2019-00105

Modeling weakly scattering random media: a tool to resolve the internal structure of nanoporous materials

Nanoporous media scatter a small fraction of the light propagating through them, even if pore sizes are significantly smaller than the characteristic visible wavelengths. The disordered spatial modulation of the refractive index at the few or few tens of nanometers length scale, resulting from the presence of randomly distributed air bubbles or solid aggregates within a continuous solid background, gives rise to these weak scattering effects. However, standard theoretical approaches to describe this kind of media use effective medium approximations that do not account for diffuse, ballistic, and specular components. Herein, all spectral components and the angular distribution of the scattered light are captured through optical modeling. A Monte Carlo approach, combining scattering Mie theory and Fresnel equations, implemented within a genetic algorithm, allows us to decode the void and aggregate size distribution and hence the internal structure of a nanocrystalline titania (TiO2) film chosen as a paradigmatic example. The approach allows to generically describe the scattering properties of nanoporous materials which, as shown herein, may be used to decipher their internal structure from the fitting of their far-optical field propertiesFinancial support of the Spanish Ministry of Science and Innovation under grant PID2020-116593RB-I00, funded by MCIN/AEI/10.13039/501100011033, and of the Junta de Andalucía under grant P18-RT-2291 (FEDER/UE), is gratefully acknowledged. A.J.-S. gratefully acknowledges Spanish Ministry of Universities for funding through a Beatriz Galindo Research fellowship BG20/0001

Optical trapping and manipulation of plasmonic nanoparticles: fundamentals, applications, and perspectives

This feature article discusses the optical trapping and manipulation of plasmonic nanoparticles, an area of current interest with potential applications in nanofabrication, sensing, analytics, biology and medicine. We give an overview over the basic theoretical concepts relating to optical forces, plasmon resonances and plasmonic heating. We discuss fundamental studies of plasmonic particles in optical traps and the temperature profiles around them. We place a particular emphasis on our own work employing optically trapped plasmonic nanoparticles towards nanofabrication, manipulation of biomimetic objects and sensing

Morphology matters: 0D/2D WO3 nanoparticle-ruthenium oxide nanosheet composites for enhanced photocatalytic oxygen evolution reaction rates

In the field of artificial photosynthesis with semiconductor light harvesters, the default cocatalyst morphologies are isotropic, 0D nanoparticles. Herein, the use of highly anisotropic 2D ruthenium oxide nanosheet (RONS) cocatalysts as an approach to enhance photocatalytic oxygen evolution (OER) rates on commercial WO3 nanoparticles (0D light harvester) is presented. At optimal cocatalyst loadings and identical photocatalysis conditions, WO3 impregnated with RONS (RONS/WO3) shows a fivefold increase in normalized photonic efficiency compared to when it is impregnated with conventional ruthenium oxide (rutile) nanoparticles (RONP/WO3). The superior RONS/WO3 performance is attributed to two special properties of the RONS: i) lower electrochemical water oxidation overpotential for RONS featuring highly active edge sites, and ii) decreased parasitic light absorption on RONS. Evidence is presented that OER photocatalytic performance can be doubled with control of RONS edges and it is shown that compared to WO3 impregnated with RONP, the advantageous optical properties and geometry of RONS decrease the fraction of light absorbed by the cocatalyst, thus reducing the parasitic light absorption on the RONS/WO3 composite. Therefore, the results presented in the current study are expected to promote engineering of cocatalyst morphology as a complementary concept to optimize light harvester-cocatalyst composites for enhanced photocatalytic efficiencyA.G. and S.L. contributed equally to this work. Financial support is gratefully acknowledged from the Max Planck Society, the Cluster of Excellence “e-conversion” (EXC 2089/1–390776260), and the Center for Nanoscience. S.L. is thankful to the Science and Engineering Research Board (SERB), Government of India, for the award of a Ramanujan Fellowship (RJF/2021/000050). A.J.-S. gratefully acknowledges Spanish Ministry of Universities for funding through a Beatriz Galindo Research fellowship BG20/00015. The authors thank Prof. Gisela Schütz (Max Planck Institute for Intelligent Systems, MPI-IS, Stuttgart) for access to XPS analysis at their facilities. The authors are grateful to Dr. Gunther Richter for helpful discussion of XPS data and the MPI-IS for the XPS infrastructure support. The authors thank Andres RodríguezCamargo for FTIR and PXRD measurements and Marie-Luise Schreiber for extensive ICPOES elemental analysis. Open access funding enabled and organized by Projekt DEA