devices is the fact that their dielectric function (i.e., permittivity, ε = ε 1 + iε 2) is predefined. Thus, several research groups, including ours, have recently merged two almost orthogonal fields, photo nics, and metallurgy, to pursue metallic materials with arbitrary permit tivity. [1-5] Alloying is now a burgeoning framework for achieving materials with engineered optical properties, encom passing both nanostructures and thin films, as will be surveyed in this Review. Plasmonics exploits the interaction of incident electromagnetic fields with the col lective motion of free electrons (plasmons) in metals. This phenomenon confines the electromagnetic fields in close vicinity of the metal interfaces, dramatically enhancing the electrical field intensity surrounding the material. [6-8] Plasmons can be divided into two categories: surface plasmon polaritons (SPP) that propagate along the metal and dielectric interface, and localized surface plasmon resonance (LSPR) that are confined in a subwavelength nanostructure. For the latter, their optical scattering or absorp tion (and/or the nearby dielectrics and semiconductors) can be significantly increased. As a result, these enhancement effects are the underpinnings to a range of novel optical processes, and have found countless applications in photovoltaics, [9-11] photo catalysis, [12-14] bio and chemicalsensing, [15,16] electrooptical modu lation, [17,18] and superabsorbers. [19-21] As expected, the features of either type of plasmon are strongly dependent upon the dielectric function of the material, which is somewhat fixed in metals. Concerning materials, coinage metals, such as Au, Ag, or Cu, are widely used in photonics due to their abundant free electrons and chemical stability. [1] Other noble metals including Metallic nanostructures and thin films are fundamental building blocks for next-generation nanophotonics. Yet, the fixed permittivity of pure metals often imposes limitations on the materials employed and/or on device performance. Alternatively, metallic mixtures, or alloys, represent a promising pathway to tailor the optical and electrical properties of devices, enabling further control of the electromagnetic spectrum. In this Review, a survey of recent advances in photonics and plasmonics achieved using metal alloys is presented. An overview of the primary fabrication methods to obtain subwavelength alloyed nanostructures is provided, followed by an in-depth analysis of experimental and theoretical studies of their optical properties, including their correlation with band structure. The broad landscape of optical devices that can benefit from metallic materials with engineered permittivity is also discussed, spanning from superabsorbers and hydrogen sensors to photovoltaics and hot electron devices. This Review concludes with an outlook of potential research directions that would benefit from the on demand optical properties of metallic mixtures, leading to new optoelectronic materials and device opportunities.
The fixed post-manufacturing properties of metal-based photonic devices impose limitations on their adoption in dynamic photonics. Modulation approaches currently available (e.g. mechanical stressing or electrical biasing) tend to render the process cumbersome or energy-inefficient. Here we demonstrate the promise of utilizing magnesium (Mg) in achieving optical tuning in a simple and controllable manner: etching in water. We revealed an evident etch rate modulation with the control of temperature and structural dimensionality. Further, our numerical calculations demonstrate the substantial tuning range of optical resonances spanning the entire visible frequency range with the etching-induced size reduction of several archetypal plasmonic nanostructures. Our work will help to guide the rational design and fabrication of bio-degradable photonic devices with easily tunable optical responses and minimal power footprint.
Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La 0.7 Sr 0.3 FeO 3 (LSFO) thin films and La 0.7 Sr 0.3 MnO 3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.
can be induced on resonance, giving rise to the filtering of distinctive colors. Commonly used resonance structures include Fabry-Pérot (F-P) cavities, [6][7][8][9] plasmonic nanostructures, [10][11][12][13][14] and grating-coupled waveguides. [15][16][17] Compared with conventional approaches using organic dyes or chemical pigments, structural color filtering offers compelling advantages, including durability, environmental friendliness, high resolution, and compatible integration with monolithic fabrication. [2,[18][19][20] One fundamental constraint with those resonance structures is that the generated/ filtered colors are static post-fabrication, yet varieties of modern technologies such as cryptography, data storage, and dynamic color display entail on-demand alteration or vanishment of hue. Although mechanisms to dynamically tailor the optical properties in resonance nano structures have been well established via phasechanging materials, [21,22] electrical biasing/ gating, [23][24][25] mechanical actuation or strain, [3,26,27] methods with lower power consumption and better cost-effectiveness are still called for. From material's perspective, structural color filters consist prevalently of conventional metals (e.g., Au, Ag, and Al) owing to their low optical loss. However, they are either nonearth abundant and CMOS-incompatible (Au, Ag) or nonbiodegradable (Ag, Al), which often restricts their utilization in biosensing [28,29] and augmented reality. [30] Recently, magnesium (Mg) has drawn increasing research interest for nanophotonic and plasmonic applications as an earth-abundant, biodegradable, and CMOS-compatible alternative to conventional metals. [31][32][33][34] Besides, its optical property can be readily modified upon exposure to hydrogen or water, prompting its adoption in dynamic photonic devices. [9,35] Here, we experimentally implement an Mg-based colorfiltering multilayer structure based on two metal-insulatormetal (MIM) F-P cavities connected in tandem. Our devices visually exhibit multiple distinctive tints spanning the CMY reflective color gamut, verified by spectroscopic ellipsometry (SE)-measured reflection spectra featuring marked suppression at resonance wavelengths. We also find that the colors are insensitive to incident angles for most devices, which is a conducive characteristic for color display. In addition, we demonstrate that the colors can vanish within ≈40 s after immersion in water. The fast fading of hue is desirable for securing and protecting optical information as needed. Further, extending
Optical materials based on unconventional plasmonic metals (e.g., magnesium) have lately driven rising research interest for the quest of possibilities in nanophotonic applications. Several favorable attributes of Mg, such as earth abundancy, lightweight, biocompatibility/ biodegradability, and its active reactions with water or hydrogen, have underpinned its emergence as an alternative nanophotonic material. Here, we experimentally demonstrate a thin film-based optical device composed exclusively of earth-abundant and complementary metal-oxide semiconductor (CMOS)-compatible materials (i.e., Mg, a-Si, and SiO 2 ). The devices can exhibit a spectrally selective and tunable near-unity resonant absorption with an ultrathin a-Si absorbing layer due to the strong interference effect in this high-index and lossy film. Alternatively, they can generate diverse reflective colors by appropriate tuning of the a-Si and SiO 2 layer thicknesses, including all the primary colors for RGB (red, green, blue) and CMY (cyan, magenta, yellow) color spaces. In addition, the reflective hues of the devices can be notably altered in a zero power-consumption fashion by immersing them in water due to the resulted dissolution of the Mg back-reflection layer. These compelling features in combination with the lithography-free and scalable fabrication steps may promise their adoption in various photonic applications including solar energy harvesting, optical information security, optical modulation, and filtering as well as structure reuse and recycling.
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