Metal-dielectric Multilayer Structures Research Articles

Dependence of Purcell effect on fluorescence wavelength in dye molecules on metal–dielectric multilayer hyperbolic metamaterials

Recently, the enhancement of spontaneous emission, i.e., broadband Purcell effect, has been achieved using hyperbolic metamaterials. Hyperbolic metamaterials, which can be realized using a metal–dielectric multilayer structure, have an extremely large optical anisotropy of permittivity in both the parallel and perpendicular directions to the propagation of light, especially when the signs of permittivities in both directions differ. In this study, we investigated the conditions for realizing the broadband Purcell effect using dye molecules with different fluorescence wavelengths. Our fabricated metal–dielectric multilayer structure exhibited hyperbolic dispersion at wavelengths beyond 500 nm. In the case of coumarin 500 whose fluorescence peak is located at 500 nm, no broadband Purcell effect was observed. However, in the case of pyridine 1 whose fluorescence peak is located at 650 nm, we observed the successfull fluorescence lifetime shortening, i.e., the broadband Purcell effect.

Broadband light absorption by tapered metal-dielectric multilayered grating structures

We present the design and numerical simulations of a polarization-insensitive broadband infrared absorber with a tapered metal-dielectric multilayered grating structure. The absorber exhibits perfect absorption simultaneously at multiple frequencies close to one another, leading to a broadband absorption. Polarization-insensitive absorption of above 90% is observed at wavelengths of 2749–4966nm, and the broadband absorption remains high for a wide range of incident angles. The electromagnetic field distributions at several selected wavelengths for both TE and TM polarizations are investigated to disclose the broadband absorption mechanism. It is anticipated that the designed broadband absorber can be applied in the field of thermal emitter, IR cloaking and solar power harvesting.

Enhancement of spontaneous emission in metal-dielectric multilayer structures accounting losses

We study the emission rate enhancement of the dipole emitter centred in the stratified metal-dielectric metamaterial, characterized by the hyperbolic isofrequency surface. We find out a limited enhancement of the Purcell factor in the layered metamaterial. We demonstrate that the radiative decay rate is strongly depends on a ratio of the thickness of layers and is affected by the level of losses in metal.

Wave scattering by metal-dielectric multilayer structures with gain

We study wave scattering by multilayer metal-dielectric structures of a finite extent characterized by a hyperbolic-like dispersion. We find the regimes of wave propagation when substantial compensation of losses becomes possible with the use of a gain medium with realistic parameters. We discuss the Purcell effect in these structures and its possible implications on the performance of the loss compensation.

Novel metal-dielectric metameric optical filters for optical security devices

Optical security devices play an essential role in the fight against counterfeiting. In this paper, we study and design a pair of metal-dielectric optical filters based on metameric effect, which offer a hidden image effect by the color shift at a specific angle of observation. Compared with all-dielectric multilayer system, the metal-dielectric multilayer structure has larger color shift with varying incident angle, higher color saturation and fewer layers. Finally, the stacks with 5 layers and 7 layers are achieved, and the color difference index is only 0.71, which shows good metameric matching effect. Simultaneously, the sensitivity of filters to deposition errors is analyzed when the thickness deviation is ±2%, and the results show that the two filters have good manufacturability.

Metal-dielectric multilayer structure supporting surface plasmons: electromagnetic modelling by the method of single expression

Surface plasmons’ (SPs) excitation in Kretschmann configuration, where the thin metallic layer is substituted by a metal-dielectric multilayer (MDML) structure is analysed numerically. For numerical analysis the method of single expression (MSE) is used. Derivation of MSE equations for analyzing TM polarized wave incident on a multilayer structure is presented. First, for validation, an excitation of SPs in conventional Kretschmann configuration with single silver layer is analysed. Further, four possible combinations of MDML $$\hbox {(silver/SiO}_\mathrm{2})$$ structures of different alternation of outermost layers are analysed on the subject of SPs excitation. It is obtained that MDML structures can support SPs at specific number of layers, when the total thickness of metallic layers in MDML structure is less than the thickness of single metallic layer in conventional Kretschmann configuration. It is found, that specific angles of SPs excitation for MDML structures are greater than that for conventional Kretschmann configuration.

Multi-resonator structure based on continuous silver thin films for transparent conductors

A type of metal-dielectric multilayered structures is investigated theoretically and experimentally for achieving optical transparency with a high electrical conductivity. The structure in our demonstrated case comprises of two coupled metal-dielectric-metal planar optical resonators with metal-layer thicknesses near to its skin depth. Simulations show that the maximum transmittance for visible light can easily reach 90% for silver-based structures. Experimentally, the sample fabricated exhibits a transmission window with a bandwidth of 150 nm and a maximum transmittance of 76% around 643 nm wavelength at normal incidence. Its sheet resistance is measured to be less than 10Ω/□, much smaller than that of common indium-tin-oxide films. Transparent conductors functioning for blue light and even for the whole visible light are also shown to be theoretically possible. Owing to their simple fabrication procedure as well as design flexibility, such a layered structure can serve as a compelling alternative as transparent conductors for optoelectronic devices, especially for liquid-crystal displays and light-emitting diodes.

Strong coupling in hyperbolic metamaterials

Nanoscale light-matter interaction in the weak-coupling regime has been achieved with unique hyperbolic metamaterial modes possessing a high density of states. Here, we show strong coupling between intersubband transitions (ISBTs) of a multiple quantum well (MQW) slab and the bulk polariton modes of a hyperbolic metamaterial (HMM). These HMM modes have large wave vectors (high-k modes) and are normally evanescent in conventional materials. We analyze a metal-dielectric practical multilayer HMM structure consisting of a highly doped semiconductor acting as a metallic layer and an active multiple quantum well dielectric slab. We observe delocalized metamaterial mode interaction with the active materials distributed throughout the structure. Strong coupling and characteristic anticrossing with a maximum Rabi splitting (RS) energy of up to 52 meV is predicted between the high-k mode of the HMMand the ISBT, a value approximately 10.5 times greater than the ISBT linewidth and 4.5 times greater than the material loss of the structure. The scalability and tunability of the RS energy in an active semiconductor metamaterial device have potential applications in quantum well infrared photodetectors and intersubband light-emitting devices.

Experiment and Theory of the Broadband Absorption by a Tapered Hyperbolic Metamaterial Array

A broadband absorber based on a tapered, alternating metal–dielectric multilayered structure was realized in the visible and IR range. The structure is fabricated by using a versatile method easily scalable to large areas by taking advantage of the line width reduction that occurs naturally in masked evaporation processes. The multilayered structure can be treated as a hyperbolic metamaterial (HMM), and the tapered structure can be regarded as a HMM waveguide with varying width. Light couples into the tapered structure most strongly at the mode cutoff position in the waveguide due to the hyperbolic dispersion and the phase matching condition. Resonant cavities are formed between the top of the tapered structure and the cutoff level, producing strong absorption peaks. These resonances are closely spaced spectrally due to the high k modes in HMM. Finally a broad absorption band is formed due to the broadening of the resonances from an array of coupled HMM tapered structures.

Solar-blind ultraviolet band-pass filter based on metal—dielectric multilayer structures

Solar-blind ultraviolet (UV) band-pass filter has significant value in many scientific, commercial, and military applications, in which the detection of weak UV signal against a strong background of solar radiation is required. In this work, a solar-blind filter is designed based on the concept of “transparent metal”. The filter consisting of Al/SiO2 multilayers could exhibit a high transmission in the solar-blind wavelength region and a wide stopband extending from near-ultraviolet to infrared wavelength range. The central wavelength, bandwidth, Q factor, and rejection ratio of the passband are numerically studied as a function of individual layer thickness and multilayer period.

Eliminating Angular Dispersion in Microcavity by Employing Metamaterials With Hyperbolic Dispersion as Reflectors

Microcavity typically exhibits a blue shift of the resonance with the angle increasing, which may bring disadvantages in related applications. Herein, we propose a design concept of eliminating the angular dispersion in microcavity by employing metamaterials with hyperbolic dispersion as reflectors. By exploiting optimal constitutive parameters of the metamaterials, the reflection phase from the cavity upon both metamaterial reflectors increases drastically with the incident angle, and can nearly compensate the decrement of the phase accumulation inside the cavity for each angle. A microcavity without angular dispersion is therefore established, in which the metamaterials may be realized by employing metal-dielectric multilayered structures in practice.

Indefinite by Nature: From Ultraviolet to Terahertz

A class of strongly anisotropic materials having their principal elements of dielectric permittivity or magnetic permeability tensors of opposite signs, so-called indefinite or hyperbolic materials, has recently attracted significant attention. These materials enabled such novel properties and potential applications as all-angle negative refraction, high density of states, and imaging beyond the diffraction limit using a so-called hyperlens. While several studies identified a few examples of negative refractions in birefringent crystals existing in nature, the majority of optical materials with hyperbolic dispersion relations known to date are engineered composite materials, “metamaterials”, such as metal-dielectric subwavelength multilayered structures or metal nanowires in a dielectric matrix. In this paper, we investigate naturally existing hyperbolic materials with indefinite permittivity for a range of frequencies from terahertz to ultraviolet. These include graphite, MgB2, cuprate, and ruthenate. Sp...

Loss-compensated broadband epsilon-near-zero metamaterials with gain media

The concept of loss-compensated broadband epsilon-near-zero metamaterials consisting of step-like metal-dielectric multilayer structures doped with gain media is proposed based on the combination of the Milton representation of the effective permittivity and the optical nonlocality due to the metal-dielectric multilayer structures. With the loss compensation by gain media, broadband epsilon-near-zero metamaterials possesses significantly low material loss in optical frequency range, leading to superior broadband electromagnetic properties for realizing unique functional optical devices, such as the demonstrated prisms for broadband directional emission and S-shaped lenses for phase front shaping.

Anomalous optical coupling between two silicon wires of a slot waveguide in epsilon-near-zero metamaterials

Anomalous optical coupling properties between two silicon wires in a silicon slot waveguide embedded in epsilon-near-zero (ENZ) metamaterials are proposed and demonstrated. The dependences of optical field enhancement in the slot region and transverse optical force on the slot size and the permittivity of surrounding material are studied in details. It is demonstrated that the optical field in the slot region is significantly enhanced due to the giant index contrast at the slot interface between silicon wires and ENZ metamaterials, but the optical mode coupling between silicon wires is greatly reduced so that the transverse optical force is suppressed into almost zero. Moreover, metal-dielectric multilayer structures are designed to realize ENZ metamaterials in the slot region for achieving the electric field enhancement.

Colored solar selective absorbing coatings with metal Ti and dielectric AlN multilayer structure

The colored solar–thermal collectors have the advantages of architectural integration and color appearance. The solar selective absorbing coatings with a metal–dielectric multilayer structure can show different colors by changing the layer number and thickness. In this work, five colored coatings with a metal titanium (Ti) and dielectric aluminum nitride (AlN) multilayer structure were designed by the optical multilayer software and fabricated by magnetron sputtering. The color of the five coatings is black, purple, yellowish green, red, and yellowish orange. The energy performance, chromaticity, and brightness of the coatings were studied and compared. The results show that the solar absorbance of these coatings is between 0.82 and 0.94, the thermal emittance is between 0.05 and 0.27, and the brightness is in the range of 0.65–8.89%. These colored coatings can be produced by a commercial production process, and is suitable for the application of building integration.

Broadband epsilon-near-zero metamaterials with steplike metal-dielectric multilayer structures

The concept of the broadband epsilon-near-zero meta-atom consisting of layered stacks with specified metallic filling ratio and thickness is proposed based on the Bergman spectral representation of the effective permittivity. The steplike metal-dielectric multilayer structures are designed to achieve realistic broadband epsilon-near-zero meta-atoms in optical frequency range. These meta-atoms can be integrated as building blocks for unconventional optical components with exotic electromagnetic properties over a wide frequency range, such as the demonstrated broadband directional emission and phase front shaping.

Nonlocality-Induced Negative Refraction and Subwavelength Imaging by Parabolic Dispersions in Metal–Dielectric Multilayered Structures with Effective Zero Permittivity

We investigate metal–dielectric multilayered structures with an effectively zero permittivity. Nonlocality induced by the surface plasmons in such structures can produce intriguing dispersions characterized by two crossing branches of parabolas. We obtain the critical conditions to set the two branches of parabolas apart, and reverse the direction of group velocity such that the system becomes capable of negative refraction as well as subwavelength imaging. Such phenomena theoretically exist in the quasistatic limit.

Giant transverse optical forces in nanoscale slot waveguides of hyperbolic metamaterials

Here we demonstrate that giant transverse optical forces can be generated in nanoscale slot waveguides of hyperbolic metamaterials, with more than two orders of magnitude stronger compared to the force created in conventional silicon slot waveguides, due to the nanoscale optical field enhancement and the extreme optical energy compression within the air slot region. Both numerical simulation and analytical treatment are carried out to study the dependence of the optical forces on the waveguide geometries and the metamaterial permittivity tensors, including the attractive optical forces for the symmetric modes and the repulsive optical forces for the anti-symmetric modes. The significantly enhanced transverse optical forces result from the strong optical mode coupling strength between two metamaterial waveguides, which can be explained with an explicit relation derived from the coupled mode theory. Moreover, the calculation on realistic metal-dielectric multilayer structures indicates that the predicted giant optical forces are achievable in experiments, which will open the door for various optomechanical applications in nanoscale, such as optical nanoelectromechanical systems, optical sensors and actuators.

Nanoscale metamaterial optical waveguides with ultrahigh refractive indices

We propose deep-subwavelength optical waveguides based on metal-dielectric multilayer indefinite metamaterials with ultrahigh effective refractive indices. Waveguide modes with different mode orders are systematically analyzed with numerical simulations based on both metal-dielectric multilayer structures and the effective medium approach. The dependences of waveguide mode indices, propagation lengths and mode areas on different mode orders, free space wavelengths and sizes of waveguide cross sections are studied. Furthermore, waveguide modes are also illustrated with iso-frequency contours in the wave vector space in order to investigate the mechanism of waveguide mode cutoff for high order modes. The deep-subwavelength optical waveguide with a size smaller than {\lambda}0/50 and a mode area in the order of 10-4 {\lambda}02 is realized, and an ultrahigh effective refractive index up to 62.0 is achieved at the telecommunication wavelength. This new type of metamaterial optical waveguide opens up opportunities for various applications in enhanced light-matter interactions.

Optical field enhancement in nanoscale slot waveguides of hyperbolic metamaterials

Nanoscale slot waveguides of hyperbolic metamaterials are proposed and demonstrated for achieving large optical field enhancement. The dependence of the enhanced electric field within the air slot on waveguide mode coupling and permittivity tensors of hyperbolic metamaterials is analyzed both numerically and analytically. Optical intensity in the metamaterial slot waveguide can be more than 25 times stronger than that in a conventional silicon slot waveguide, due to tight optical mode confinement enabled by the ultrahigh refractive indices supported in hyperbolic metamaterials. The electric field enhancement effects are also verified with the realistic metal-dielectric multilayer waveguide structure.