Multilayer Hyperbolic Metamaterials Research Articles

Magnetically Tunable Near-Field Radiative Heat Transfer in Hyperbolic Metamaterials

Active control of radiative heat transfer still remains open due to its exciting application potential. In view of this, we present a theoretical demonstration of dynamically tunable near-field radiative heat transfer (NFRHT) between two multilayer hyperbolic metamaterials (consisting of alternating layers of a magneto-optical (MO) material and a dielectric) using an external magnetic field. We show that magnetization-induced hyperbolic modes play a significant role in radiative heat transfer and allow a highly tunable heat flux. Moreover, the hybridization of intrinsic and magnetization-induced hyperbolic modes significantly enhances the radiative heat transfer. In particular, we find that polarization conversion due to nonzero off-diagonal elements enables the spectral heat transfer coefficient for the TE polarization to be as large as that for the TM polarization, which is vastly different from the case of two closely spaced bulk MO plates. In addition, to quantitatively characterize the thermal modulation, we show a negative thermal magnetoresistance effect in this system, which approaches $\ensuremath{-}59.6\mathrm{%}$ when the magnetic field intensity reaches 10 T. Our findings may be beneficial for active noncontact thermal management at the nanoscale, and facilitate a deeper understanding of the mechanisms of MO hyperbolic metamaterials in terms of NFRHT.

Specific Fabry–Perot resonant transmission property of graphene-dielectric multilayered hyperbolic metamaterials

We theoretically investigate the characteristics of the multilayered hyperbolic metamaterial (HMM) composed of graphene and discuss the transmission properties from another angle of Fabry–Perot (F–P) resonance analysis. Dispersion characteristics of graphene-dielectric multilayered hyperbolic metamaterials (GDM HMMs) can be adjusted by changing the chemical potential of graphene. Transfer matrix method is improved to adapt the condition of large tangential vectors, and transmission properties are analyzed numerically. Calculated results indicate that dielectric material and graphene codetermine the dispersion properties of the HMMs, and the optical properties can be dynamically adjusted due to the introduction of graphene. Transmission spectra exhibit F–P resonance properties and discussions prove the validity of the F–P cavity theory. However, the transmission characteristics of GDM HMMs are different from the phenomena and laws of the traditional F–P cavity. Further analysis reveals that the mechanism originates from the contribution of graphene and high- k waves in HMMs. We present an innovative perspective for investigating and understanding transmission properties of GDM HMMs and provide references for design of HMMs and other related photonic devices.

Broadband tunable nonlinear optical response in plasmonic metamaterials -INVITED

Nanostructured materials with tunable nonlinear optical response are of great interest for different applications in nanophotonics. In this work we report the results of a comprehensive study on the nonlinear optical properties of two kinds of plasmonic metamaterials, i.e., honeycomb nanoprism arrays and multilayer hyperbolic metamaterials, which proved to have a very rich spectrum of parameters (as metamaterials morphology and composition, wavelength and polarization of an input beam) to exploit for controlling their nonlinear response over a broad spectral range.

Mode-resolved directional enhancement of spontaneous emission inside/outside finite multilayer hyperbolic metamaterials

The concept of large photonic density of states associated with high-k modes of hyperbolic metamaterials (HMMs) plays a significant role in the interests behind the HMM notion. Despite the theoretical predictions, it is proved that photonic local density of states (LDOS) is not infinite in practical multilayer HMMs. In the present study, a developed theory based on the effective medium approximation is used to probe the finite number of high-k modes in finite multilayer HMMs. The high-k modes are classified in two categories as short-range and long-range propagating modes, strongly dependent on the number of periods. Introducing a mode-resolved transition rate enhancement factor, emission coupling of a quantum emitter, placed inside and in near vicinity of HMM slabs, to the high-k modes is calculated. The results show that the short-range and long-range modes have the commanding influence on transition rate enhancement outside and inside the slabs, respectively. The results point out that HMM slabs having lower number of periods provide higher total LDOS giving rise to larger transition rate enhancement. The results prove the capability of engineering photonic density of states in finite HMMs and pave the way for application-orientated HMMs design.

Tunable Anisotropic Absorption in Hyperbolic Metamaterials Based on Black Phosphorous/Dielectric Multilayer Structures

Black phosphorus (BP) as a promising two-dimensional material with extraordinary optical properties constitutes an excellent building block in multilayer hyperbolic metamaterials. In this work, we design a multilayer structure composed of BP/dielectric layer stacking unit cells patterned on a gold mirror and theoretically demonstrate the tunable anisotropic absorption in the infrared regime. The electric dipole resonance between the adjacent unit cells drives the structure in the critical coupling state, and impedance of the structure matches to that of free space, showing the perfect absorption for one polarization direction, while the impedance mismatch for the other polarization direction leads to only 8.2$\%$ absorption at the same wavelength. The anisotropic absorption response of the proposed structure can be attributed to the intrinsic anisotropy of BP, which exhibits few dependence on the incident angle. Furthermore, we investigate the tunable optical absorption of the proposed structure with the electron doping of BP and the geometric parameters. These results demonstrate great potentials of BP in constituting multilayer hyperbolic metamaterials, and open up avenues in designing anisotropic metadevices with tunable spectral and polarization selectivity in the infrared regime.

Existence Conditions of High‐k Modes in Finite Hyperbolic Metamaterials

AbstractThe capability to support optical waves with very large wave vectors (high‐k) is one of the principle features of hyperbolic metamaterials (HMMs). These waves play the key role in HMM applications such as imaging and lifetime engineering. Effective medium approximation (EMA) as widely used analytical method to predict HMMs behavior, has shortcomings in calculating high‐k modes of practical structures. EMA is applicable to a subwavelength unit cell of implicitly infinite periodic structures. Using conventional EMA, in the present paper, boundary effects and spatial dispersion are taken into consideration to properly compute the high‐k modes of finite‐thickness multilayer HMMs. Applying nonlocal homogenization to stacks of alternating metal‐dielectric layers, the corresponding effective medium is examined as a high‐k waveguide sandwiched between the substrate and an ambient superstrate. The developed theory enables us to recognize two types of bulk waves coined as short‐range and long‐range propagating modes. Number of such modes as well as their cutoff conditions are quantified for the first time. Validity of the developed theory is verified both numerically by rigorous simulations of the multilayer structures with the transfer matrix method and experimentally by optical characterization of the HMMs in infrared regime.

Optimization of Nanopatterned Multilayer Hyperbolic Metamaterials for Spontaneous Light Emission Enhancement

Light emission processes can be modified through the Purcell effect near a metallic surface. Such effect relies on the matching between spontaneous emission spectra and surface plasmon resonances, and the latter is typically limited by the existing metal properties. Nanopatterned multilayer hyperbolic metamaterials (HMMs) with engineerable material property are promising in enhancing spontaneous emission rates at desired frequencies with improved far‐field radiative power. In this work, the authors study the optimization process for spontaneous emission enhancement by using nanopatterned HMMs. By theoretically investigating the Purcell effect on HMMs compared with traditional metals, the authors choose better material combinations for stronger Purcell enhancement. Different decay channels in the HMM are analyzed against the emitter distance and their wavelengths. Systematic optimization of achieving large emission intensity is demonstrated by comparing performance of nanopatterned HMMs with different geometry parameters. The promise in achieving light emission with both high decay rates and brightness has various potential applications including light‐emitting devices, single‐molecule detection, and surface‐enhanced Raman scattering.

A Highly-Efficient Single Segment White Random Laser.

Production of multicolor or multiple wavelength lasers over the full visible-color spectrum from a single chip device has widespread applications, such as superbright solid-state lighting, color laser displays, light-based version of Wi-Fi (Li-Fi), and bioimaging, etc. However, designing such lasing devices remains a challenging issue owing to the material requirements for producing multicolor emissions and sophisticated design for producing laser action. Here we demonstrate a simple design and highly efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multicolor lasing emitted from core-shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables decreasing the energy consumption of photon propagation. As a result, the energy upconversion emission is enhanced by ∼50 times with a drastic reduction of the lasing threshold. The multiple scatterings arising from the inherent nature of the disordered nanoparticle matrix provide a convenient way for the formation of closed feedback loops, which is beneficial for the coherent laser action. The experimental results were supported by the electromagnetic simulations derived from the finite-difference time-domain (FDTD) method. The approach shown here can greatly simplify the design of laser structures with color-tunable emissions, which can be extended to many other material systems. Together with the characteristics of angle free laser action, our device provides a promising solution toward the realization of many laser-based practical applications.

Experimental Demonstration of Hyperbolic Metamaterial Assisted Illumination Nanoscopy

An optical metamaterial is capable of manipulating light in nanometer scale that goes beyond what is possible with conventional materials. Taking advantage of this special property, metamaterial-assisted illumination nanoscopy (MAIN) possesses tremendous potential to extend the resolution far beyond conventional structured illumination microscopy. Among the available MAIN designs, hyperstructured illumination that utilizes strong dispersion of a hyperbolic metamaterial (HMM) is one of the most promising and practical approaches, but it is only theoretically studied. In this paper, we experimentally demonstrate the concept of hyperstructured illumination. A ∼80 nm resolution has been achieved in a well-known Ag/SiO2 multilayer HMM system by using a low numerical aperture objective (NA = 0.5), representing a 6-fold resolution enhancement of the diffraction limit. The resolution can be significantly improved by further material optimization.

Strong Coupling and Exceptional Points in Optically Pumped Active Hyperbolic Metamaterials

We investigate the interaction of light in gain-enhanced multilayered hyperbolic metamaterials in the strong interaction regime. Pumping the dye in the dielectric layers from inside the light cone, while emission occurs into the lower hyperbolic band outside the light cone, eases the problem of light incoupling. In the strong coupling regime both emission and absorption lines cause a distortion of the plasmonic modes due to Rabi splitting and a PT-symmetry-broken phase, with generation of exceptional points at loss–gain compensation frequencies. We derive a semiclassical model that describes these phenomena for finite and infinite devices in detail, requiring only the overlap factor and the complex frequencies of the dye transition and the optical mode.

Surface waves on multilayer hyperbolic metamaterials: Operator approach to effective medium approximation

Surface waves on multilayer hyperbolic metamaterials: Operator approach to effective medium approximation

Nanostructuring Multilayer Hyperbolic Metamaterials for Ultrafast and Bright Green InGaN Quantum Wells.

Semiconductor quantum well (QW) light-emitting diodes (LEDs) have limited temporal modulation bandwidth of a few hundred MHz due to the long carrier recombination lifetime. Material doping and structure engineering typically leads to incremental change in the carrier recombination rate, whereas the plasmonic-based Purcell effect enables dramatic improvement for modulation frequency beyond the GHz limit. By stacking Ag-Si multilayers, the resulting hyperbolic metamaterials (HMMs) have shown tunability in the plasmonic density of states for enhancing light emission at various wavelengths. Here, nanopatterned Ag-Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160-fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high-speed optoelectronic devices, optical wireless communications, and light-fidelity networks.

Anomalous scaling laws of hyperbolic metamaterials in a tubular geometry

Hyperbolic metamaterials (HMM) can be used to control light propagations in emerging meta-devices and thus lead to various functionalities (e.g., hyperlens and cloaking devices). Here we propose a kind of exotic tubular cavity by using multilayered HMM, which contrasts with traditional materials with elliptical dispersion. In such tubular microcavities, the calculations reveal that they have anomalous scaling laws, such as that the higher-order resonance mode oscillates at a longer wavelength and the resonant wavelengths hold their positions with changing the tube wall thickness and diameter. These findings can help the understanding of tubular metamaterials and could inspire interesting optical experiments and metadevices.

Graphene oxide on magnetron sputtered silver thin films for SERS and metamaterial applications

Abstract In the last years the potential of combining the attractive materials characteristics of graphene related materials and silver nanostructures for SERS and metamaterials has emerged. Here, we report of graphene oxide thin films deposited by dip-coating on magnetron sputtered silver thin films. Our work represents a novelty in the field of the study of graphene oxide- silver composites, since magnetron sputtering deposition is an alternative way to silver thin films fabrication; previous works used instead silver nitrate aqueous solution mixed with the graphene oxide. Micro-Raman technique, morphological analysis and variable angle spectroscopic ellipsometry were performed. The final SERS signal intensity was investigated and we found Raman peaks dependent on the intensity of the laser and the thickness of silver and GO films. These results could open somestudies on plasmonics and on the reduction of graphene oxide mediated by silver thin films. Moreover, effective medium theory calculations show the possible use of these graphene oxide/silver thin films in multilayer hyperbolic metamaterials for optical applications.

Limits of imaging with multilayer hyperbolic metamaterials.

The multilayer hyperbolic metamaterials are known to be capable of imaging with sub-wavelength resolution. In this work performance of these "hyperbolic lenses" is analyzed in depth by employing commonly used transfer matrix method as well as the eigen-mode approach, the latter offering a clear physical insight into the operation of hyperbolic imagers and revealing their fundamental limitations. The resolution of multilayer structures is shown to decrease with the number of layers not only due to increased loss but also because of the severe suppression of large spatial frequencies caused by the cancellation between symmetric and antisymmetric eigen-modes. Additionally, the resolution is strongly affected by the granularity and fill ratio. In the end, hyperbolic metamaterials can create an image with subwavelength resolution only at very close distance to the object and hence limiting their utility.

Near-Field Heat Transfer between Multilayer Hyperbolic Metamaterials

Abstract We review the near-field radiative heat flux between hyperbolic materials focusing on multilayer hyperbolic meta-materials. We discuss the formation of the hyperbolic bands, the impact of ordering of the multilayer slabs, as well as the impact of the first single layer on the heat transfer. Furthermore, we compare the contribution of surface modes to that of hyperbolic modes. Finally, we also compare the exact results with predictions from effective medium theory.

Fano resonance engineering in slanted cavities with hyperbolic metamaterials

We present the possibility to engineer Fano resonances using multilayered hyperbolic metamaterials. The proposed cavity designs are composed of multilayers with a central slanted part. The highly tunable resonances originate from the interference between a propagating and an evanescent mode inside the slanted section. The propagating mode can reach an extremely high effective index, making the realization of deeply subwavelength cavities possible, as small as 5 nm. The evanescent mode is rarely analyzed but plays an important role here, as its contribution determines the particular shape of the cavity characteristic. Moreover, these phenomena cannot be described using effective medium theory, and we provide a more rigorous analysis. The reported resonances are very sensitive to any structural changes and could be used for sensing applications.

Observation of enhanced thermal emission from patterned hyperbolic metamaterials

Hyperbolic metamaterials (HMMs) are known to theoretically possess an unbounded range of wave vectors to which radiation can couple due to the hyperbolic shape of the HMM isofrequency surface. Thus, in practice, there exists a large range of modes that can be excited by thermal fluctuations that can then reradiate by coupling to high spatial frequency features. Herein, we show direct observation of enhanced thermal emission in an alternating gallium-doped zinc oxide (GZO) and zinc oxide (GZO/ZnO) multilayer HMM by utilizing a FLIR IR camera. This observation is supported by numerical simulations performed in CST MWS using index of refraction and extinction coefficient parameters obtained from an IR VASE.

Nonlocal optical effects on the Goos–Hänchen shifts at multilayered hyperbolic metamaterials

The lateral beam shift of light incident on a multilayered hyperbolic metamaterial (HMM) is investigated using a theoretical model which emphasizes the nonlocal optical response of the indefinite material. By applying an effective local response theory formulated recently in the literature, it is found that nonlocal effects only affect p polarized light in this Goos–Hänchen (GH) shift of the incident beam; leading to a blue-shifted peak for positive shifts at high frequencies and red-shifted dip for negative shifts at low frequencies in the GH shift spectrum. An account for the observed phenomenon is given by referring to the ‘Brewster condition’ for the reflected wave from the HMM. This observation thus provides a relatively direct probe for the nonlocal response of the HMM.

Stimulated Emission of Surface Plasmons on Top of Metamaterials with Hyperbolic Dispersion

We have demonstrated that the stimulated emission of surface plasmons propagating on top of multilayered hyperbolic metamaterials coated with dye-doped polymeric films has a much lower (2.5–7 times) threshold than that on top of thick Ag films. This observation correlates with more than 2-fold shortening of the spontaneous emission kinetics on top of a lamellar metamaterial as compared to that on top of silver. The propagation of surface plasmons on top of the metamaterial and silver substrates was modeled (in the Otto geometry) using COMSOL Multiphysics. It has been shown that at given system parameters, the plasmon propagation loss in a metamaterial is smaller than that in silver by ∼14% and cannot explain the dramatic reduction of the lasing threshold observed experimentally in the metamaterial samples. We infer that the stimulated emission of propagating plasmons on top of a metamaterial is enhanced by the nonlocal dielectric environment and high local density of photonic states.