Metal-dielectric Stacks Research Articles

Thermal emitter performance of ultra-broadband solar metamaterial absorber based on metal-insulator-metal device structure

In this paper, we have designed a solar absorber with ultra-broadband absorption. The structure has seven layers with a metal-dielectric periodic film stack and a metallic copper substefficiency. The metal-dielectric periodic membrane stack and the dielectric are continuous to form a metal/dielectric composite microstructure. It has a very high absorption efficiency at 280 nm–2893 nm. The absorption efficiency in this wavelength range is 96.26% and absorption efficiency at AM 1.5 is up to 95.66% by finite difference time domain method (FDTD) simulation. The thermal radiation absorption efficiency of the structure is 93.69% at 1000K and 94.5% at 1200 K, which can also be used in the manufacture of the heat sink due to its high thermal radiation absorption efficiency. Our proposed solar absorber has good incidence angle insensitivity (0°–60°) and good polarization independence (0°–90°). These advantages enable our absorbers to be widely used for solar absorption and emission and offer a variety of design options for ideal absorbers.

Efficient Tandem Quantum‐Dot LEDs Enabled by An Inorganic Semiconductor‐Metal‐Dielectric Interconnecting Layer Stack (Adv. Mater. 4/2022)

Quantum-Dot Light-Emitting Diodes Light-emitting diodes (LEDs) in a tandem configuration offer a strategy to realize high-performance, multi-color devices, but the efficiency of tandem quantum-dot based LEDs (QLEDs) has been limited due to unpassivated interfaces and solvent damage originating from the materials processing requirements of the interconnecting layers (ICLs). In article number 2108150, Edward H. Sargent, Xuyong Yang, and co-workers report an ICL consisting of a semiconductor–metal–dielectric stack that provides facile fabrication, materials stability, and good optoelectronic coupling, making the tandem QLED based on the ICL achieve recording efficiency.

Electrochromic Metamaterials of Metal–Dielectric Stacks for Multicolor Displays with High Color Purity

Inorganic electrochromic (EC) materials with vibrant multicolor change that are compatible with large-scale processing have been at the forefront of EC technology and are crucial in a wide range of applications, such as displays and camouflage. However, limited strategies are available to realize such inorganic materials, and challenges such as low color purity are yet to be overcome. Here, we demonstrate multilayered metal-dielectric metamaterials (MMDMs) as a new family of inorganics-based EC materials to achieve dynamic alternation among multicolors with high contrast and high color purity, which are structurally realized by significantly enhancing the confinement of the incident light in specific optical frequencies. This multilayer structure renders high reflectivity (75%), high quality factor (7.4), and a full width at half-maximum of 60 nm before coloration and presents a color gamut at least 40% wider than that of previously reported metamaterials after coloration, indicating good color quality.

FDTD method study on the effects of geometric parameters on the W- Al2O3 nano-structure thermal emitter

In this paper, a 2D periodic structure made of tungsten (W) grating patch on a thin Al2O3 spacer and an opaque W film is proposed, as a broadband selective thermal emitter. We numerically investigated the spectral emissivity of the structure from the deep by finite-difference time-domain method. Geometric parameters effects on the emissivity are discussed and the mechanisms of magnetic polaritons in the structure are further analyzed in detail. In addition, by adding another metal-dielectric stack upon the pre-existing grating structure, the emissivity of the composite structure can be further enhanced, and the normal emissivity exceeds 0.95 in the wavelength range of 0.65–1.95 μm, some even close to unity. Furthermore, the composite structures are found to exhibit insensitivity to polar angle and polarization.

Epitaxial TiN/MgO multilayers with ultrathin TiN and MgO layers as hyperbolic metamaterials in visible region

Multilayers with metal-dielectric stacks are considered as an effective approach to produce hyperbolic metamaterials (HMMs). However, fabricating ultrathin metal layers with high crystallinity for low-loss HMMs is very challenging. In this work, TiN has been applied as an alternative candidate to the noble metals, and TiN/MgO multilayers with varied numbers of layers and layer thicknesses have been designed. The microstructure study reveals that the thickness of TiN nanolayer is controlled as thin as 2.9 nm, 1.26 nm and 0.9 nm by varying the total number of layers, and the epitaxial quality of the multilayer films is high even with such ultrathin layers. Extraordinary optical properties have been achieved, such as plasmonic resonance in visible range, as well as strong optical anisotropy. In addition, the real part of dielectric permittivity (ε′) shows opposite optical anisotropy in different wavelength ranges, i.e. ε∥′>0, ε⊥′<0 in the range of 350 nm–450 nm, and <0, ε⊥′>0 under higher wavelength, which is a novel property realized in a multilayer system.

Design a Stratiform Metamaterial with Precise Optical Property

In this work, a stratiform metamaterial is arranged as multiple periods of metal-dielectric symmetrical film stack to provide precise equivalent refractive index and admittance. There are multiple solutions of equivalent refractive index retrieved from the characteristic matrix of the film stack. The correct refractive index is derived by connecting different branches of solution at different ranges of wavelength or thickness of the dielectric layer. The refractive index of an Ag-TiO2 five-layered symmetrical film stack shown in previous work is demonstrated to be positive real instead of negative real. The associated type I iso-frequency curve supports negative refraction. In order to extend the operating wavelength of type I metamaterial, the number of the metal-dielectric symmetrical film stack is increased to reduce the thickness of the dielectric film to approach subwavelength requirement.

Effect of metal–dielectric substrates on chemiluminescence kinetics

We studied chemiluminescence of rhodamine 6G (R6G) dye in a series of R6G:DNPO:PMMA films deposited onto a variety of metallic, metal–dielectric, and dielectric substrates. We found that chemiluminescence kinetics on top of Ag and Au films covered by an insulating MgF2 layer had longer decay times than those on top of purely dielectric substrates, MgF2 films on glass. We infer that Ag- and Au-based substrates with an insulating MgF2 layer on top affected the reaction rates by influencing polarization energies of reactants and surrounding molecular dipoles. At the same time, the chemiluminescence kinetics on top of Ag and Au surfaces (without an insulating MgF2 spacer) were substantially shorter than those on top of dielectric substrates, probably because of conventional chemical catalysis. The peak emission intensities on top of metallic substrates (with and without the MgF2 layer on top) were lower than those on top of purely dielectric substrates, likely due to inhibition of the energy transfer to acceptors (R6G dye molecules) known in the vicinity of metals. The further reduction of chemiluminescence intensity on top of lamellar metal–dielectric stacks could be due to preferential emission of light inside the volume of the metamaterial rather than outside.

High performance aperiodic metal-dielectric multilayer stacks for solar energy thermal conversion

A novel spectrally selective Cr/AlCrN/AlCrNO/AlCrN/AlCrNO/AlCrO multilayer coating was deposited on stainless steel substrates using cathodic arc ion plating. In this tandem absorber coating, Cr, AlCrN, AlCrNO and AlCrO act as the reflector layer, semi-absorbing layer, the main absorbing layer and the antireflection layer, respectively. The spectral properties of this multilayer coating during long-term annealing in air have been investigated with the aim of evaluating the thermal stability of this aperiodic metal-dielectric multilayer stacks. An average absorptance of 0.90 and emittance of 0.15 are achieved in this multilayer coatings fabricated with optimized parameters. More importantly, the coating exhibits an outstanding thermal stability with a selectivity of 0.94/0.11 even after annealed at 500 °C for 1000 h in air. The microstructure analysis indicate that the multilayer coating consists of aperiodic AlCrN and AlCrON layers in addition to the Cr and AlCrO layers. After the long-term annealing, small amounts of AlN, CrN and Cr2N nanocrystallites are observed to be homogeneously embedded in the AlCrN and AlCrON amorphous matrices. The nanoparticles in the AlCrN and AlCrON layers can effectively scatter the incident light into a broadband wavelength range, increasing the optical path length in the absorbing layers and thus resulting in a pronounced enhancement in the absorptivity. A handful of Cr2O3 and Al2O3 nanograins are observed to embedd in the amorphous AlCrO antireflection layer, which can efficitively reflect the solar infrared radiation and the thermal emittance from the substrate and thus result in pretty low infrared emissivity. The good thermal stability is attributed to the excellent thermal stability of the cermet-based amorphous matrices and the sluggish atomic diffusion in the nanoparticles, which could effectively slow down the inward diffusion of oxygen and avoid agglomeration of naonparticles. These results suggest that the AlCrON-based selective absorbing coating is a good candidate for photo-thermal conversion at high temperatures.

Slow light in a hyperbolic metamaterial waveguide cladded with arbitrary nonlinear dielectric materials

We have demonstrated a hyperbolic metamaterial (HMM) waveguide cladded with arbitrary nonlinear dielectric materials to explore the properties of slow light. The HMM proposed in this paper is assumed as a metal–dielectric stack which can lead to slow light phenomena while being cladded by dielectric materials. Both the asymmetric (i.e., linear–HMM–linear case and nonlinear–HMM–linear case) and symmetric (i.e., linear–HMM–linear case and nonlinear–HMM–nonlinear case) dielectric structures are considered to examine the properties of slow light in the paper. The dispersion relations which are required to investigate the properties of slow light are derived and presented in detail. The results show that the metal filling factor, the dielectric permittivities, and the arbitrary nonlinearity have significantly changed the characteristics of slow light. Parameter dependence of the effects is calculated and discussed.

Pantoscopic and temperature-controlled dual-band perfect absorber based on strontium titanate material

A nearly perfect dual-band deep sub-wavelength scale and temperature-controlled terahertz absorber is theoretically investigated. The absorber is composed of a periodic metal-dielectric stack array placed on a metallic substrate. Simulation results show that there are two distinct absorption peaks located at frequencies of about 0.140 THz and 0.192 THz in the spectra line under room temperature T = 300 K, and their maximum absorption rates are respectively 99.133% and 98.251%. It is also found that the period of the unit cell is less than 8.97% of the minimum resonant wavelength, which means the deep sub-wavelength absorber structure. Furthermore, it is shown that the proposed absorber is polarization-insensitive, and has good tolerance to the incident angle for both TE and TM wave, i.e., it is pantoscopic for the incident light. Meanwhile, when the temperature changes from 200 K to 400 K, the absorption of the two peaks did not change significantly within the considered frequency range, but the resonant frequency of the two absorption peaks will have a clear blueshift. The effect of structural parameters on the absorbing performance have also been discussed. This work provides a new idea for the design of frequency-agile deep sub-wavelength scale perfect THz absorbers, and the design scheme may be easily extended to the near ultraviolet, optical, infrared, terahertz and millimeter-wave regions.

Biosensing with the singular phase of an ultrathin metal-dielectric nanophotonic cavity

The concept of point of darkness has received much attention for biosensing based on phase-sensitive detection and perfect absorption of light. The maximum phase change is possible at the point of darkness where the reflection is almost zero. To date, this has been experimentally realized using different material systems through the concept of topological darkness. However, complex nanopatterning techniques are required to realize topological darkness. Here, we report an approach to realize perfect absorption and extreme phase singularity using a simple metal-dielectric multilayer thin-film stack. The multilayer stack works on the principle of an asymmetric Fabry–Perot cavity and shows an abrupt phase change at the reflectionless point due to the presence of a highly absorbing ultrathin film of germanium in the stack. In the proof-of-concept phase-sensitive biosensing experiments, we functionalize the film surface with an ultrathin layer of biotin-thiol to capture streptavidin at a low concentration of 1 pM.

Klein tunneling near the Dirac points in metal-dielectric multilayer metamaterials

The Klein tunneling of optical waves near the Dirac points in the metal-dielectric multilayer metamaterials is theoretically investigated and demonstrated through the coupled-mode theory under the tight-binding approximation and the rigorous band structure analysis based on the transfer-matrix method. The optical analogue of Klein tunneling for the relativistic electrons passing across a potential barrier is revealed by the iso-frequency contour analysis and numerical simulation to describe the optical beam propagation and refraction across the interface of two metal-dielectric multilayer metamaterial stacks. The transmission and reflection spectra of the Klein tunneling of optical waves are also explained by the coupled mode theory.

Optical design of Cu/Zr0.2AlN0.8/ZrN/AlN/ZrN/AlN/Al34O62N4 solar selective absorbing coatings

A novel metal–dielectric stacks coating of Cu/Zr0.2Al0.8N/ZrN/AlN/ZrN/AlN/Al34O62N4 as the solar selective absorbing coating was designed. Thin films of Zr0.2AlN0.8, ZrN, AlN and Al34O62N4 deposited by the dual ion beam assisted deposition (DIBAD) were investigated by X-ray diffraction and spectroscopic ellipsometry. XRD results indicate the deposited films of Zr0.2Al0.8N, AlN, and Al34O62N4 are poorly crystallized, whereas, the ZrN film is polycrystalline. The Drude-type dispersion model and Tauc–Lorentz dispersion model are used for determining the complex refractive indices of ZrN and Zr0.2Al0.8N, respectively. For Al34O62N4 and AlN films, Cauchy’s dispersion formula is employed to determine their optical constants. A seven-layer stack with alternating ZrN and AlN layers is optimized by optical simulation programme of TFCalc. The optimized coating obtains the solar absorptance of 0.942 and the emittance of 0.12 at 100°C.

Absorption enhancement of a dual-band metamaterial absorber

In this paper, we propose and fabricate a dual-band metamaterial absorber in 6–24THz region. Electric field distribution reveal that the first absorption band is obtained from localized surface plasmon (LSP) modes which are excited both on inside and outside edges of each circular-patterned metal-dielectric stack, while the second absorption band is excited by LSP modes on outside edges of each stack. Measured results indicate that the absorption band width can be tuned by increasing the radius of circular-patterned layers or reducing the thickness of dielectric spacing layers. Moreover, the designed dual-band metamaterial absorber is independent on circular-patterned dielectric layer combinations.

Localized surface plasmon resonnance induced terahertz broad absorption band

A broad band metamaterial absorber is designed and simulated, which constitutes by double circular-patterned metal–dielectric stacks. A absorption band is obtained from 14.1 to 16.4THz. Electric field distributions reveal that the absorption band is obtained from localized surface plasmon (LSP) modes which are excited both on outside and inside edges of each stack, while the high-frequency absorption peak is excited by LSP modes which are excited only on outside edges. The absorption band width can be tuned by increasing the radius (R) of circular-patterned layers and reducing the thickness of dielectric layers (Hd). Moreover, the designed broad band metamaterial absorber is independent of circular-patterned dielectric layer combination.

Nonlocal effective medium analysis in symmetric metal-dielectric multilayer metamaterials

The optical nonlocality in symmetric metal-dielectric multilayer metamaterials is theoretically and experimentally investigated with respect to transverse-magnetic-polarized incident light. A nonlocal effective medium theory is derived from the transfer-matrix method to determine the nonlocal effective permittivity depending on both the frequency and wave vector in a symmetric metal-dielectric multilayer stack. In contrast to the local effective medium theory, our proposed nonlocal effective medium theory can accurately predict measured incident angle-dependent reflection spectra from a fabricated multilayer stack and provide nonlocal dispersion relations. Moreover, the bulk plasmon polaritons with large wave vectors supported in the multilayer stack are also investigated with the nonlocal effective medium theory through the analysis of the dispersion relation and eigenmode.

Metal-dielectric filters for solar-blind silicon ultraviolet detectors.

We report on the fabrication of metal-dielectric thin film stacks deposited directly onto silicon substrates for use as ultraviolet bandpass filters. Integration of these filters onto silicon improves the admittance matching of the structure when compared to similar designs fabricated on transparent substrates, leading to higher peak transmission or improved out-of-band rejection if used with a Si-based sensor platform. Test structures fabricated with metallic Al and atomic layer deposited Al2O3 were characterized with spectroscopic ellipsometry and agree well with optical models. These models predict transmission as high as 90% the spectral range of 200-300 nm for simple three-layer coatings.

Permittivity evaluation of multilayered hyperbolic metamaterials: Ellipsometry vs. reflectometry

Metal-dielectric nanolaminates represent a class of hyperbolic metamaterials with uniaxial permittivity tensor. In this study, we critically compare permittivity extraction of nanolaminate samples using two techniques: polarized reflectometry vs. spectroscopic anisotropic ellipsometry. Both Au/MgF2 and Ag/MgF2 metal-dielectric stacks are examined. We demonstrate the applicability of the treatment of the multilayered material as a uniaxial medium and compare the derived optical parameters to those expected from the effective medium approximation. We also experimentally compare the effect of varying the material outer layer on the homogenization of the composite. Additionally, we introduce a simple empirical method of extracting the epsilon-near-zero point of the nanolaminates from normal incidence reflectance. The results of this study are useful in accurate determination of the hyperbolic material permittivity and in the ability to tune its optical properties.

Negative index fishnet with nanopillars formed by direct nano-imprint lithography

In this paper we demonstrate the ability to fabricate fishnets by nanoimprinting directly into a pre-deposited three layer metal–dielectric–metal stack, enabling us to pattern large areas in two minutes. We have designed and fabricated two different fishnet structures of varying dimensions using this method and measured their resonant wavelengths in the near-infrared at 1.45 μm and 1.88 μm. An important by-product of directly imprinting into the metal–dielectric stack, without separation from the substrate, is the formation of rectangular nanopillars that sit within the rectangular apertures between the fishnet slabs. Simulations complement our measurements and suggest a negative refractive index real part with a magnitude of 1.6. Further simulations suggest that if the fishnet were to be detached from the supporting substrate a refractive index real part of 5 and FOM of 2.74 could be obtained.

Angle-insensitive and solar-blind ultraviolet bandpass filter.

We present a metal-dielectric stack ultraviolet (UV) bandpass filter that rejects the longer wavelength, visible spectrum and is thin and relatively insensitive to the angle of incidence. Parametric evaluations of the reflection phase shift at the metal-dielectric interface provide insight and design information. This nontrivial phase shift allows coupled Fabry-Perot resonances with subwavelength dielectric film thickness. Furthermore, the total phase shift, with contributions from wave propagation and nontrivial reflection phase shift, is insensitive to the angle of incidence. Filter passbands in the UV can be shifted to visible or longer wavelengths by engineering the dielectric thickness and selecting a metal with an appropriate plasma frequency.