Metal-dielectric Multilayer Structures Research Articles

Highly sensitive short-range mode resonance sensor with multilayer structured hyperbolic metamaterials.

Hyperbolic metamaterial (HMM) based sensors can achieve superior sensing performance than conventional surface plasmon resonance sensors. In this work, the operator approach to effective medium approximation (OEMA) is used to characterize the HMM dielectric constant properties of metal-dielectric multilayer structures, which are classified into short-range (SR) mode and long-range (LR) mode according to the propagation length of the bulk high K waves in HMM. The dispersion relations of SR modes are derived, and a high-sensitivity refractive index sensor is designed for the near-infrared SR mode resonance. The effects of the number of periods, cell thickness, metal fill rate and incidence angle on the SR mode resonance were analyzed for the multilayer structured HMM. Our designed sensing structure achieves a maximum sensitivity of 330 µm/RIU in the near-infrared band with a quality factor of 492 RIU-1. In addition, the simulations show that the SR mode resonance wavelength is flexible and tunable. We believe that the study of HMM-based SR mode resonance sensors offers potential applications for high-sensitivity biochemical detection.

The Broad Band Optical Absorber Designed By Multilayered Hyperbolic Metamaterial

Bu çalışmada, özellikle optik bölgede geniş band soğurma gösteren çok tabakalı bir hiperbolik metamalzeme tasarlanmıştır. Tasarlanan yapıda üç farklı metal (Ag, Au ve Al) ve yarıiletken bir malzeme (InGaAs) kullanılmıştır. Hesaplamalarda Transfer Matris Metodundan (TMM) yararlanılmıştır. Bu yöntem kullanılarak tasarlanan çok tabakalı yapıların optik soğurma ve yansıma spektrumları gelme açısına bağlı olarak incelenmiştir. Metal-dielektrik çok tabakalı yapıların hiperbolik dispersiyon gösterdiği bölgeler belirlenmiş, ayrıca gelme açısına bağlı kontur grafikleri elde edilmiştir. Yapılan hesaplamalar sonucunda, metal olarak gümüşün kullanıldığı heteroyapının en iyi soğurma spektrumu verdiği gözlenmiştir.

Mechanism of emitters coupled with a polymer-based hyperbolic metamaterial.

We study a polymer-based hyperbolic metamaterial (HMM) structure composed of three Au-polymer bilayers with a hyperbolic dispersion relation. Using an effective refractive index retrieval algorithm, we obtain the effective permittivity of the experimentally fabricated polymer-based structure. In particular, the unique polymer-based HMM shows the existence of high-k modes that propagate in the metal-dielectric multilayered structure due to the excitation of bulk plasmon-polaritonic modes. Moreover, we compare the experimental luminescence and fluorescence lifetime results of the multilayered Au and a dye-doped polymer (PMMA) to investigate the dynamics of three different emitters, each incorporated within the unique polymer-based HMM structure. With emitters closer to the epsilon-near-zero region of the HMM, we observed a relatively high shortening of the average lifetime as compared to other emitters either close or far from the epsilon-near-zero region. This served as evidence of coupling between the emitters and the HMM as well as confirmed the increase in the non-radiative recombination rate of the different emitters. We also show that the metallic losses of a passive polymer-based HMM can be greatly compensated by a gain material with an emission wavelength close to the epsilon-near-zero region of the HMM. These results demonstrate the unique potential of an active polymer-based hyperbolic metamaterial in loss compensation, quantum applications, and sub-wavelength imaging techniques.

Anisotropic ɛ-near-zero multilayer structures for omnidirectional bending to the normal

Anisotropic ɛ-near-zero effective medium multilayer structures for omnidirectional bending light to the normal are theoretically studied. A finite element method is presented, using a unit cell with Floquet port master and slave boundaries, for examining metal–dielectric multilayer structures to form the permittivity tensor. Ellipsometry measurements of CdO films are reported, and it is found that film thicknesses of ∼50 nm are adequate for the metallic layer. Omnidirectional bending to the normal is verified for multilayer structures, and this method is shown to simplify the computations over using a typical full space approach.

Fano approximation for coupled modes in metal-dielectric multilayer structures

We study the response of coupled modes in metal-dielectric multilayer planar structures consisting of a metal layer attached to a high-index waveguide by a low-index spacer layer. The structure supports excitation and coupling of a surface plasmon polariton mode and a waveguide mode. A coupled-mode model is developed to analyse the field transfer processes and describe the appearance of Fano resonances as a result of the modes interaction. It is demonstrated that the response of the plasmonic mode can be well fitted to a Fano function that allows extraction of width, height, sensitivity, and field enhancement.

Realization of broadband negative refraction in visible range using vertically stacked hyperbolic metamaterials.

Negative refraction has generated much interest recently with its unprecedented optical phenomenon. However, a broadband negative refraction has been challenging because they mainly involve optical resonances. This paper reports the realization of broadband negative refraction in the visible spectrum by using vertically-stacked metal-dielectric multilayer structures. Such structure exploits the characteristics of the constituent metal and dielectric materials, and does not require resonance to achieve negative refraction. Broadband negative refraction (wavelength 270–1300 nm) is numerically demonstrated. Compared to conventional horizontally-stacked multilayer structures, the vertically-stacked multilayer structure has a broader range of working wavelength in the visible range, with higher transmittance. We also report a variety of material combinations with broad working wavelength. The broadband negative refraction metamaterial provides an effective way to manipulate light and may have applications in super-resolution imaging, and invisibility cloaks.

Giant and highly reflective Goos-Hänchen shift in a metal-dielectric multilayer Fano structure.

We experimentally demonstrated a giant Goos-Hänchen (GH) shift in a metal-dielectric multilayer Fano structure. The observed GH shift was 0.176 mm, which corresponded to (GH shift/λ) = 493, where λ is the incident wavelength. A unique feature of this giant GH shift was that it occurred without attenuation, i.e., reflectivity ∼1, due to Fano interference between surface plasmon polariton and high-Q dielectric waveguide mode. The Q-value is determined by the coupling loss. Therefore, we can enhance the GH shift to an arbitrarily large value by controlling the coupling strength. The unique feature whereby the giant GH shift occurs without attenuation has great potential for real-world applications, such as optical switching, optical filters, and sensors, where the reduction of reflected beam intensity is currently a major drawback.

Fano resonances in near-field absorption in all-dielectric multilayer structures

Extending our previous studies on metal-dielectric multilayer structures, we demonstrate the feasibility of generating and controlling high-Q Fano resonances in all-dielectric multilayer structures. The structures studied consist of two waveguide layers (Ge-doped SiO2 and Al2O3 layers) separated by spacer layers (SiO2 layers) and arranged in the Kretschmann attenuated total reflection geometry. From analyses based on the electromagnetic theory, we reveal that the Fano resonance arises from the interaction between a broad waveguide mode supported by the Ge-doped SiO2 layer and a sharp waveguide mode supported by the Al2O3 layer. This mechanism allows us to generate similar Fano resonances with both the p- and s-polarized incident light. Although the Fano resonances based on radiative modes in a variety of nanostructures and metamaterials have been reported to appear only in the far-field responses, we show that the present Fano resonances appear not only in the far-field reflection spectra but also in the near-field absorption spectra.

Leaky property of the Dyakonov-like wave at the surface of a semi-infinite metal-dielectric multilayered structure.

We investigate the Dyakonov-like surface wave (DLSW) at the interface of a dielectric and a metal-dielectric multilayered (MDM) structure when this MDM structure serves as an elliptic medium according to the effective medium approximation (EMA). Different from the conventional Dyakonov surface waves, we find that this kind of DLSW possesses an unexpected leaky property due to an additional hyperbolic-like wave in the MDM structure, resulting in a significant increase of propagation loss compared to the results estimated by a simple effective model based on the EMA. Moreover, such leaky property is found to be sensitive to the period of the MDM structure. Thus, to diminish this non-negligible leaky loss, one can suppress the amplitude of the leaky component by designing the MDM structure with a smaller period. On the other hand, as the MDM structure sometimes can also support an additional elliptic-like dispersion when it is regarded as a hyperbolic medium, we discuss this condition for completeness. Based on this elliptic-like dispersion, an apparent leaky property is observed in the resultant DLSW. This DLSW propagates with a wider range of propagation direction but suffers from a poor field confinement to the interface it is propagating along.

Electromagnetic modes in different topology regimes modulated by layer thickness of metal–dielectric multilayered structures based on exact transfer-matrix method

The topology regimes of the electromagnetic modes of metal–dielectric multilayered nanostructures have been proven to be dependent on the frequencies and ratios of layer thicknesses indicated by the effective media approximation. Here, we put forward a mechanism by taking the individual layer thickness into consideration. Based on the exact transfer-matrix method with periodic boundary condition, the numerical results reveal that the modes can be modulated dramatically through individual layer thickness, which can not only change dispersion significantly of the modes in the topology regimes predicted by the effective media approximation, but also make modes in the different topology regimes possible. In practice, beam refraction of a TM-polarized Gauss incident light input into a metal–dielectric multilayered structure is also discussed.

Determination of dissipative Dyakonov surface waves using a finite element method based eigenvalue algorithm.

A full-vectorial finite element method is developed to analyze the surface waves propagating at the interface between two media which could be dissipative particularly. The dissipative wave possessing a complex-valued propagation constant can be determined precisely for any given propagation direction and thus the property of losses could be thoroughly analyzed. Besides, by applying a special characteristic of the implicit circular block matrix, we reduce the computational consumptions in the analysis. By utilizing this method, the Dyakonov surface wave (DSW) at the interface between a dielectric and a metal-dielectric multilayered (MDM) structure which serves as a hyperbolic medium is discussed. Its propagation loss is smaller for larger period of the MDM structure but its field becomes less confined to the interface.

Investigation on plasmonic responses in multilayered nanospheres including asymmetry and spatial nonlocal effects

In this work, we present a rigorous approach for analyzing the optical response of multilayered spherical nano-particles comprised of either plasmonic metal or dielectric, when there is no longer radial symmetry and when nonlocality is included. The Lorenz–Mie theory is applied, and a linearized hydrodynamic Drude model as well as the general nonlocal optical response model for the metals are employed. Additional boundary conditions, viz., the continuity of normal components of polarization current density and the continuity of first-order pressure of free electron density, respectively, are incorporated when handling interfaces involving metals. The application of spherical addition theorems, enables us to express a spherical harmonic about one origin to spherical harmonics about a different origin, and leads to a linear system of equations for the inward- and outward-field modal coefficients for all the layers in the nanoparticle. Scattering matrices at interfaces are obtained and cascaded to obtain the expansion coefficients, to yield the final solution. Through extensive modelling of stratified concentric and eccentric metal-involved spherical nanoshells illuminating by a plane wave, we show that, within a nonlocal description, significant modifications of plasmonic response appear, e.g. a blue-shift in the extinction / scattering spectrum and a broadening spectrum of the resonance. In addition, it has been demonstrated that core–shell nanostructures provide an option for tunable Fano-resonance generators. The proposed method shows its capability and flexibility to analyze the nonlocal response of eccentric hybrid metal–dielectric multilayer structures as well as adjoined metal-involved nanoparticles, even when the number of layers is large.

Line shape engineering of sharp Fano resonance in Al-based metal-dielectric multilayer structure

A systematic experimental study was performed on the Fano line shape exhibited by multilayer structures consisting of an Al layer, a SiO2 spacer layer, and an Al2O3 waveguide layer. In the structures studied, a sharp Fano resonance appears on the background of broad asymmetric resonance attributed to the excitation of a surface plasmon polariton at the Al/SiO2 interface. It is shown that the background asymmetric surface plasmon resonance can be well fitted to a single Fano function, and the sharp Fano line shape can be well fitted to a double Fano function expressed as a product of two single Fano functions. The results of measurements performed by varying the spacer layer thickness indicate that the width (Q factor) of the sharp Fano resonance decreases (increases) monotonously as the thickness increases. The Q factor achieved in the present study is as high as ∼1500. A comparison with the results of electromagnetic calculations suggests that not only the spacer layer thickness but also the imaginary part of the dielectric constant of the waveguide layer plays an important role in the Fano line shape engineering.

Polarization effects in light-tunable Fano resonance in metal-dielectric multilayer structures

A planar metal-dielectric multilayer structure containing a waveguide layer doped with azo-dye molecules exhibits a sharp Fano resonance that can be shifted by pump light irradiation owing to a photoinduced change of the refractive index of the waveguide layer by photoisomerization of the azo-dye molecules. Azo-dye-containing materials are polarization sensitive and the photoinduced shift of the Fano resonance is expected to depend on the pump polarization. Previous pump-probe attenuated-total-reflection measurements performed with a normal incidence of pump light exhibited intriguingly no dependence on the pump polarization. In this work, we extend our measurements to an oblique incidence, to optimize the overlap between the pump and probe polarizations, and we demonstrate a strong dependence on the pump polarization. To analyze the experimental results for both the normal and oblique pump conditions, we introduce phenomenological equations, based on the angular hole burning model of the photoisomerization of the azo-dye molecules. The equations allow us to describe the dependence of the refractive index changes in the waveguide layer on the pump angle and polarization. Using the equations, we could obtain good theoretical fits to the experimental data. The results of fitting suggest that the overlap between the pump and probe polarizations plays an important role in determining the amount of the photoinduced shift of the Fano resonance.

Ellipsometric and magneto-optical study of nanosized ferromagnetic metal-dielectric structures [Co/TiO2]n/Si

Ferromagnetic metal-dielectric multilayer structures [Co/TiO2]n with layer thicknesses of 2–4nm were fabricated on the (001)-surface of Si substrates by the ion-beam deposition technique. The complex dielectric function was studied in the spectral range of 0.6–5.6eV using the optical ellipsometry and analyzed using the optical matrix technique for reflection response of lossy isotropic multilayer dielectric structures and an anisotropic effective medium approximation. Magnetic characteristics of the structures were determined by using the magneto-optical Kerr effect. Obtained results allow us to conclude that these nanosized ferromagnetic metal-dielectric structures may be regarded as artificial uniaxial media exhibiting a strong optical and magnetic anisotropy at room temperature.

Optical response of cylindrical multilayers in the context of hydrodynamic convection-diffusion model

In this work, we generalize the cascading scattering matrix algorithm for calculating the optical response of concentric multilayered structures comprised of either plasmonic metal or dielectric, within the framework of hydrodynamic convection-diffusion model of electrodynamics. Two additional boundary conditions, namely, the continuity of first order pressure of free electron density and the continuity of normal components of free charge velocity, respectively, are adopted in order to handle the behaviour at interfaces involving metals. Scattering matrices at interfaces can be readily obtained and cascaded to obtain the modal coefficients in each layer by expanding electromagnetic waves in harmonic modes with cylindrical vector wave functions. We have validated the proposed method by analyzing the optical responses of several configurations of nanostructures, including a bi-metallic nanocylinder and a hyperlens. We found that nonlocal effects can be important for small structures, when the characteristic size is comparable to the Fermi wavelength. The proposed method shows its capability and flexibility to solve hybrid metal-dielectric multilayer structures even when the number of layers is large. Although we have discussed our method in the context of the retarded radiation regime, it can be applied in quasi-static scenarios without any difficulties. Furthermore, it may be extended to solve similar problems in other areas of physics, such as acoustics.

Light-tunable Fano resonance in metal-dielectric multilayer structures.

High-Q optical Fano resonances realized in a variety of plasmonic nanostructures and metamaterials are very much promising for the development of new potent photonic devices, such as optical sensors and switches. One of the key issues in the development is to establish ways to effectively modulate the Fano resonance by external perturbations. Dynamic tuning of the Fano resonance applying the mechanical stress and electric fields has already been demonstrated. Here, we demonstrate another way of tuning, i.e., photo-tuning of the Fano resonance. We use a simple metal-dielectric multilayer structure that exhibits a sharp Fano resonance originating from coupling between a surface plasmon polariton mode and a planar waveguide mode. Using a dielectric waveguide doped with azo dye molecules that undergo photoisomerization, we succeeded in shifting the Fano resonance thorough photo-modulation of the propagation constant of the waveguide mode. The present work demonstrates the feasibility of photo-tuning of the Fano resonance and opens a new avenue towards potential applications of the Fano resonance.

Active tuning of epsilon-near-zero point of hyperbolic metamaterial at visible and near-infrared regimes

An active method of continuously tuning the effective permittivity of a hyperbolic metamaterial at visible and near-infrared spectra is introduced in this letter. A transparent conducting oxide, whose optical properties can be altered accurately by applying an external voltage, is inserted into a traditional metal–dielectric multilayer structure to increase the degree of freedom for tuning the epsilon-near-zero point by shifting the topological transition point. The tuning effect is explained in detail by combining the relationship between permittivity and voltage with wavevector analysis. This may promote other active applications of hyperbolic metamaterials and ultrafast tunable optoelectronic devices.

A Photoexcited Broadband Absorption Based on Au Nano Cross Elliptical Disks Structure with Polarization-Insensitive and Wide-Angle for Visible Light

We present the design and numerical simulations of a polarization-insensitive and wide-angle photoexcited broadband metamaterial absorption in the visible region base on Au nano cross elliptical disks structure. The Au nano cross elliptical disks structure comprises a periodic array of metal-dielectric multilayered structure and a no transmission metallic functional layer on the bottom. Utilizing 3D finite difference time domain (FDTD) algorithm, the hybrid surface plasmon and the polarization-insensitive absorption of above 90 % for a wide range of incident angles of 40° are observed in the whole visible light (3.8–7.8 × 1014 Hz).

Dielectric singularity in hyperbolic metamaterials: the inversion point of coexisting anisotropies.

Hyperbolic Metamaterials are artificially engineered materials whose optical properties can be specifically tailored to manifest an extremely high level of anisotropy. Due to this remarkable anisotropy they represent a unique opportunity to realize effective bulk meta-structure with extraordinary optical properties in the visible range. A simultaneous dielectric singularity in the in plane permittivity, with respect to the propagation direction, has to lead to a complete sign inversion of the same permittivity for that specific visible frequency. Such a drastic phase change has been theoretically highlighted in the past as the major challenge to be overcome in order to unlock many remarkable optical properties not present artificial optical systems. In this paper we experimentally demonstrate the realization of a metal-dielectric multilayer structure showing an inversion point of coexisting anisotropies at a specified wavelength in the visible range, rising from the particular design and fabrication process. Theoretical models and numerical simulations are in very good agreement with experimental data. Ellipsometrical experiments and optical modeling demonstrate the drastic type I/type II transition. Supercollimation effect has been achieved at the inversion point of the coexisting extreme anisotropies, whereas at the epsilon near zero and pole frequency the perfect lens behavior has been observed.