Selective absorption of visible light in film-coupled nanoparticles by exciting magnetic resonance

Harvesting energy from sun, outer space, and soil

While solar power systems have offered a wide variety of electricity generation approaches including photovoltaics, solar thermal power systems, and solar thermoelectric generators, the ability to generate electricity at both the daytime and nighttime with no necessity of energy storage remains challenging. Here, we propose and verify an environment-friendly, sustainable, and cost-effective strategy of harvesting solar energy by solar heating during the daytime and harnessing the coldness of the outer space through radiative cooling to produce electricity at night using a commercial thermoelectric module. It enables electricity generation for 24 h a day. We experimentally demonstrate a peak power density of 37 mW/m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}2 at night and a peak value of 723 mW/m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}2 during the daytime. A theoretical model that accurately predicts the performance of the device is developed and validated. The feature of 24-h electricity generation shows great potential energy applications of off-grid and battery-free lighting and sensing.

Absorption characteristics of a metal-insulator-metal nanodisk for solar thermal applications

Due to their ability to confine light in a sub-wavelength scale and achieve coherent absorption, plasmonic nanostructures have been intensively studied for solar energy harvesting. Although nanoparticles generating localized surface plasmon resonance (LSPR) have been thoroughly studied for application in a direct absorption solar collector (DASC), nanoparticles exciting magnetic polaritons (MP) for use in a DASC have not drawn much attention. In this work, we report a metal-insulator-metal (MIM) nanodisk that can excite MP peaks apart from the LSPR in the solar spectrum. It was found that the MIM nanodisk generates a broader and relatively more uniform absorption band compared to a pure metallic nanodisk. The MP peaks were also found to cause less significant scattering compared to those associated with the LSPR. We finally showed that the peaks induced by the MIM nanodisk are highly tunable by varying the particle dimensions, making the proposed MIM nanodisk a potential candidate for solar thermal applications.

Magnetic and Electric Resonances in Particle-to-Film-Coupled Functional Nanostructures

We investigate the plasmonic coupling of metallic nanoparticles with continuous metal films by studying the effect of the particle-to-film distance, cavity geometry, and particle size. To efficiently screen these parameters, we fabricated a particle-to-film-coupled functional nanostructure for which the particle size and distance vary. We use gold-core/poly(N-isopropylacrylamide)-shell nanoparticles to self-assemble a monolayer of well-separated plasmonic particles, introduce a gradient in the nanoparticle size by an overgrowth process, and finally add a coupling metal film by evaporation. These assemblies are characterized using surface probing and optical methods to show localized magnetic and electric field enhancement. The results are in agreement with finite-difference time-domain modeling methods and calculations of the effective permeability and permittivity. Finally, we provide a proof of concept for dynamic tuning of the cavity size by swelling of the hydrogel layer. Thus, the tunability of the coupled resonance and the macroscopic self-assembly technique provides access to a cost-efficient library for magnetic and electric resonances.

Responses transition in a monolayer Al-Al2O3 nanoparticle-crystal due to oxidation

Nanoparticle is a promising candidate for large scale fabrication of metamaterial. However, optical responses for metamaterial made of abound metal like Al can be thoroughly changed due to oxidization. Especially for nanoparticle whose aspect ratio is extremely high, oxidation usually occurs. So to understand how the responses shift in a nanoparticle system due to oxidization is essential for large scale application of metamaterial. In this paper, we have concluded and quantified two general principles describing this transition in a monolayer Al-Al₂O₃ nanoparticle-crystal, which can be used in a thermophotovoltaic system. Square pattern, in which the unit of changing crystal is a square cell made up of Al and Al₂O₃ particles, is firstly demonstrated. A double oscillators model has been proposed to understand the interference between different absorption modes and their coupling. Using near-field distribution, equivalent inductor-capacitor model and dispersion relationship of surface Plasmon polariton, we have distinguished the resonance modes, concluded the transition principles in a simple case. Then the two principles are applied in a larger cell to verify its university. After detailed demonstration of symmetric square pattern, models and principles are extrapolated to more complex non-symmetric systems. The basic understanding gained here will help the design of robust large-scale metamaterial.

Multiband selective absorbers made of 1D periodic Ag/SiO2/Ag core/shell coaxial cylinders horizontally lying on a planar substrate

In this paper, we present a one-dimensional periodic microstructure for multiband selective absorbers of thermal radiation. The microstructure is made of Ag/SiO₂/Ag core/shell coaxial cylinders horizontally lying on top of a SiO₂ dielectric spacer and an opaque silver substrate. The spectral-directional absorptivity of the proposed structure was numerically investigated with the finite element based Comsol Multiphysics software. Multiband selective absorption in the wavenumber range from 2500 to 20000 cm^-1 for TM-wave incidence was obtained. Physical mechanisms responsible for the multiband selective absorption were elucidated due to the resonance of magnetic polaritons in the SiO₂ spacer shell, excitation of surface plasmon polaritons at the SiO₂/Ag interface, and the effect of Wood's anomaly. Furthermore, the effects of a silver core radius, spacer shell thickness, a confocal elliptical core/shell cylinder on the property of multiband absorption, and the absorptivity of the structure with one core/four shells coaxial cylinders were explored.

Nanoparticle-crystal towards an absorbing meta-coating

In this paper, a double layer nanoparticle-crystal has been proposed, which shown incident and polarization angle, substrate independences for spectral absorptivity. Such phenomenon originates from the near-field light redistribution and excitation of internal collective oscillating. This kind of nanoparticle-crystal can be made of various types of metal with similar optical responses. A three oscillators mode has been proposed in this paper to understand the shift between global and internal collective oscillating, and verify the physical picture demonstrated. That kind of near-field redistribution result in a prototype of novel meta-coating, and facilitates the large scale application of metamaterial.

Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces

We demonstrate a template-assisted colloidal self-assembly approach for magnetic metasurfaces on macroscopic areas. The choice of anisotropic colloidal particle geometry, assembly pattern and metallic film is based on rational design criteria, taking advantage of mirror-charge effects for gold nanorods placed on gold film. Monodisperse gold nanorods prepared utilizing wet-chemistry are arranged with high precision on wrinkled templates to form linear array-type assemblies and subsequently transferred to a thin gold film. Due to the obtained particle-to-film distance of 1.1 nm, the plasmonic mode of the nanorod is able to couple efficiently with the supporting metallic film, giving rise to a magnetic mode in the visible spectrum (721 nm). Conventional UV-vis-NIR measurements in close correlation with electromagnetic simulations provide evidence for the presence of a magnetic resonance on the macroscopic area. The herein presented scalable lithography-free fabrication process paves the road towards colloidal functional metasurfaces with an optical response in the effective magnetic permeability.

Optical coherent thermal emission by excitation of magnetic polariton in multilayer nanoshell trimer

A theoretical demonstration is given of coherent thermal emission via the visible region by exciting magnetic polaritons in isolated metal-dielectric-metal multilayer nanoshells and the collective behavior in a trimer comprising multilayer nanoshells. The dipolar metallic core induces magnetic polaritons in the dielectric shell creating a large enhancement of the emissivity, whose mechanism is different from that of film-coupled metamaterials. The coupling effect of the magnetic polaritons and the electric/magnetic modes of symmetric nanoparticle trimers is discussed to understand the collective behavior in self-assembled nanoparticle clusters with potential solar energy utilizations. The concept of hybridization is employed to understand the collective magnetic polaritons of a multilayer nanoshell trimer. The fundamental understanding gained herein opens up new ways to explore, control, and tailor spectral absorptance, thus facilitating rational design of novel self-assembled nanoclusters for energy harvesting.

Colloidal metasurfaces displaying near-ideal and tunable light absorbance in the infrared

Metasurfaces are ultrathin, two-dimensional arrays of subwavelength resonators that have been demonstrated to control the flow of light in ways that are otherwise unattainable with natural materials. These arrays are typically composed of metallic Ag or Au nanostructures shaped like split rings, nanowire pairs or nanorods (commonly referred to as meta-atoms) that are arranged to produce a collective optical response spanning an impressive range of properties, from the perfect absorption of incident light to superresolution imaging. However, metasurfaces pose major challenges in their fabrication over large areas, which can be prohibitively expensive and time consuming using conventional nanolithography techniques. Here we show that differently shaped colloidal nanocrystals can be organized into metasurface architectures using robust, scalable assembly methods. These metasurfaces exhibit extreme in-plane electromagnetic coupling that is strongly dependent on nanocrystal size, shape and spacing. Colloidal metasurfaces that display near-ideal electromagnetic absorbance can be tuned from the visible into the mid-infrared wavelengths.

Metasurfaces are arrays of subwavelength structures that are tailored to produce specific optical responses. Rozin et al. show that large-area metasurfaces can be readily fabricated by self-assembly of colloidal nanocrystals of different geometries, producing tunable reflectance and absorbance properties.

Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties of Gold Nanorods: From Noninvasive Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges

We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to Au@Ag cuboids. Based on this correlation, a generic methodology for in situ monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethylammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in Au@Ag cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology.