Epsilon-near-zero Metamaterials Research Articles

Si-CMOS compatible epsilon-near-zero metamaterial for two-color ultrafast all-optical switching

Driven by the escalating demands of advanced technologies, developing integration strategies has kept pace with the realization of ultrafast components during the past two decades. Ultrafast all-optical switches enabled by artificial materials are considered at the forefront of the next generation of photonic integration for communications and high-volume data processing. Encouraged by these advancements, applications, and interest have increased toward all-optical switches based on epsilon-near-zero (ENZ) materials. However, some all-optical switches lack CMOS compatibility, require high energy activation, and are limited in switching speed and working wavelength. Here, we propose and demonstrate a multilayered ENZ metamaterial utilizing Si-compatible titanium nitride and indium-tin-oxide materials with two effective working wavelengths in the visible and near-infrared spectrum. This device enables switching time down to a few hundred femtoseconds utilizing minimal energy at the corresponding ENZ regions induced by intraband pumping. Our approach can enhance the adaptability of designing ENZ metamaterials for new hybrid integrated photonic components for low-power ultrafast all-optical terahertz modulation.

Tunable bandpass filter based on epsilon-near-zero metamaterials using liquid crystals

ABSTRACT In the article, a design method of tunable bandpass filter based on epsilon-near-zero (ENZ) metamaterials using liquid crystals (LCs) is proposed. Thanks to the strong and directionally consistent electric field distribution caused by energy tunnelling effect of ENZ metamaterials, the ENZ unit cell with filled LC in the narrow tunnel can theoretically achieve the bandpass filtering characteristic with a large tuning rate. On this basis, a third-order LC-based tunable bandpass filter is constructed by three series unit structures, which achieves obviously tunable bandpass filtering characteristic with different external biased voltages. Finally, one prototype is designed, fabricated and measured, respectively. The measurement results show that the proposed bandpass filter based on ENZ materials with LCs can achieve effective tuning of the operating frequency with different external biased voltages, which verifies the effectiveness of the proposed design method. In addition, the impact of shape variation of the proposed ENZ-based tunable filter on electromagnetic characteristics is also further discussed and analysed in experiments, which is of great significance for achieving RF miniaturisation and high integration.

Pattern Similarity and Gain Enhancement of Dual-Band Antenna Using an ENZ Metamaterial

A dual-band antenna with pattern similarity using the energy-squeezing mechanism of an epsilon-near-zero (ENZ) metamaterial is designed, and experimental results are presented with simulated data. To achieve high gain and pattern similarity, a waveguide-based ENZ metamaterial property is adopted in a planar microstrip form at the upper band. The designed dual-band antenna is resonating at 4.3 GHz and 9.8 GHz frequencies with the desired radiation pattern. A metamaterial unit cell with ENZ characteristics at 9.8 GHz is designed to improve the antenna’s performance. The permittivity and permeability of the unit cell are characterized using the Nicolson–Ross–Weir method. By adding a square ring structure, the antenna geometry exhibits ENZ characteristics. Using the ENZ material’s energy-squeezing mechanism, a 12-dB gain improvement is achieved in the upper band without affecting the lower band radiation characteristics without increasing the complexity. It can be used in C- and X-band applications, such as mobile, military, defense, and radar communication. The dual-band antenna with ENZ material is fabricated, and the measured results show a good match with the simulated results. The proposed antenna size at 4.3 GHz is 0.58λ × 0.58λ × 0.02λ.

Low‐Loss Epsilon‐Near‐Zero Metamaterials

AbstractDifferent from the classical periodic‐resonator‐based metamaterials, epsilon‐near‐zero (ENZ) metamaterials provide a unique paradigm to achieve equivalent electromagnetic characteristics in deep subwavelength scales, exhibiting unprecedented impacts on a broad variety of extreme‐small‐volume applications. By doping regular dielectric rods in the ENZ host, the effective permeability µeff of ENZ metamaterials is properly tuned for desired scattering properties and intrinsic impedance. However, losses in ENZ metamaterials severely limit the tuning range of the real part of µeff and result in an undesired imaginary part. Here, to mitigate the loss issue of ENZ metamaterials, a dielectric‐free approach is theoretically studied and experimentally verified by substituting dielectric dopants with metal‐layer dopants to construct a low‐loss resonant cavity. Therefore, the largest tuning range of µeff is achieved, which advances ENZ metamaterials from ideal cases to more extensive and practical applications. In addition to existing photonics and electronics applications of regular ENZ metamaterials, additional examples are studied to demonstrate the low‐loss benefits of layer‐type ENZ metamaterials, including integrated microfluidic switches and high‐sensitivity sensors with a sensitivity of 11.2% and quality factor of 2800. These examples reveal universal significance for wide‐range applications in extreme‐small‐volume devices and systems, such as integrated circuits, chips, and implanted devices.

Epsilon-near-zero (ENZ)-based optomechanics

Optomechanics deals with the control and applications of mechanical effects of light that stems from the redistribution of photon momenta in light scattering. As an example, light-induced levitation of an infinitesimally small dipolar particle is expected in front of epsilon-near-zero (ENZ) metamaterials. However, a theoretical understanding of these effects on single-material and multi-material larger particles is still lacking. Here, we investigate, analytically and numerically, optical forces on polarizable particles with size ranging from 20 nm to a 1 μm in proximity of ENZ metamaterials. We look at the general features of the repulsive-attractive optomechanics from the nano to the microscale exploiting different theoretical methods (dipole approximation, finite elements calculations, transition (T-)matrix). We discuss the role of realistic layered materials, as our ENZ substrate, on optical forces and analyze the influence of composition and shape by studying a range of complex particles (dielectric, core-shell, plasmonic ellipsoids). Physical insights into the results are discussed and future research directions are forecasted. Our results provide possibilities in exploiting engineered materials and surfaces for the manipulation and tailoring of light-induced forces in optomechanics.

Nonlocality-mediated spatiotemporal optical vortex generation in nanorod-based epsilon-near-zero metamaterials.

Optical vortices have myriad applications in photonics. Very recently, promising concepts of spatiotemporal optical vortex (STOV) pulses based on the phase helicity in the space-time coordinates have attracted much attention owing to their donut shape. We elaborate on the molding of STOV under the transmission of femtosecond pulses through a thin epsilon-near-zero (ENZ) metamaterial slab based on a silver nanorod array in a dielectric host. At the heart of the proposed approach is the interference of the so-called main and additional optical waves enabled by strong optical nonlocality of these ENZ metamaterials, which leads to the appearance of phase singularities in transmission spectra. The cascaded metamaterial structure is proposed for high-order STOV generation.

Design of a gradient epsilon-near-zero refractory metamaterial with temperature-insensitive broadband directional emission

Gradient epsilon-near-zero (ENZ) metamaterials offer broadband directional control over thermal emission. Implementing this approach using materials that remain stable in harsh thermo-chemical environments would allow it to be broadly deployed in thermal photonics. Our prior work showed that heterostructures of rock salt MgO and perovskite BaZr0.5Hf0.5O3 (BZHO) are stable up to 1100 °C in air, with no discernible intermixing. In this work, we design a gradient ENZ metamaterial made from three lattice-matched refractory oxides: MgO, BZHO, and NiO. The miscibility of MgO and NiO makes it possible to linearly vary the ENZ frequency of the metamaterial layers. BZHO is used as a thin, interlayer diffusion barrier. We model the emissivity of our gradient ENZ metamaterial at 25 and 1000 °C to demonstrate that the spectral bandwidth of directional emission is preserved at high temperatures despite changes in the optical properties of each material. Finally, we discuss practical fabrication challenges associated with the back reflector and offer potential solutions based on advancements in hetero-integration. Overall, this work shows a pathway toward gradient ENZ metamaterials with ultrahigh-temperature stability.

Performing calculus with epsilon-near-zero metamaterials.

Calculus is a fundamental subject in mathematics and extensively used in physics and astronomy. Performing calculus operations by analog computing has received much recent research interest because of its high speed and large data throughput; however, current analog calculus frameworks suffer from bulky sizes and relatively low integration densities. In this work, we introduce the concept of an epsilon-near-zero (ENZ) metamaterial processing unit (MPU) that performs differentiation and integration on analog signals to achieve extreme miniaturization at the subwavelength scale by generating desired dispersions of the ENZ metamaterials with photonic doping. To show the feasibility of this proposal, we further build an experimental analog image edge extraction system with a differentiating ENZ-MPU as its compute core. With a computing density theoretically analyzed to be several tera-operations per second and square micrometer, the proposed ENZ-MPU is scalable and configurable for more complex computations, providing an effective solution for analog calculus operators with extreme computing density and data throughput.

Self‐Rolling SiO2/Au Based Epsilon‐Near‐Zero Metamaterials

AbstractWave propagation in epsilon‐near‐zero (ENZ) media offers exciting possibilities in the field of nanophotonics. Here, a thin film self‐rolling technique to fabricate SiO2/Au based 3D multilayer structures is implemented. These cylindrical multilayer metamaterials are utilized to take advantage of the material dispersion as well as the structural dispersion to obtain self‐rolling ENZ metamaterials at the visible to near‐infrared wavelength range. The ENZ features are investigated initially by modelling the dispersion of the effective medium using circular effective medium approximation and further by the characterization of the optical response. Moreover, the potential of these structures for supporting an ENZ waveguide mode is demonstrated. This platform for dispersion engineering and obtaining ENZ metamaterials can provide opportunities for unique applications which cannot be possible with planar systems. It can be used to integrate active materials before the rolling process to actively control the layer properties, or embed single‐layer materials within different layers of ENZ metamaterials with potential applications in quantum nanophotonics.

Performance Improvement of Substrate Integrated Cavity Fed Dipole Array Antenna Using ENZ Metamaterial for 5G Applications.

This paper exhibits a high-gain, low-profile dipole antenna array (DAA) for 5G applications. The dipole element has a semi-triangular shape to realize a simple input impedance regime. To reduce the overall antenna size, a substrate integrated cavity (SIC) is adopted as a power splitter feeding network. The transition between the SIC and the antenna element is achieved by a grounded coplanar waveguide (GCPW) to increase the degree of freedom of impedance matching. Epsilon-near-zero (ENZ) metamaterial technique is exploited for gain enhancement. The ENZ metamaterial unit cells of meander shape are placed in front of each dipole perpendicularly to guide the radiated power into the broadside direction. The prospective antenna has an overall size of 2.58 and operates from 28.5 GHz up to 30.5 GHz. The gain is improved by 5 dB compared to that of the antenna without ENZ unit cells, reaching 11 dBi at the center frequency of 29.5 GHz. Measured and simulated results show a reasonable agreement.

High-performance photonic spin Hall effect in anisotropic epsilon-near-zero metamaterials.

A high-performance photonic spin Hall effect is demonstrated in an anisotropic epsilon-near-zero (ENZ) metamaterial based on the wave-vector-varying Pancharatnam-Berry phase. The giant out-of-plane anisotropy of ENZ metamaterial induces strong spin-orbit coupling. With a small incident angle, photons with opposite spins move along opposite transverse directions gradually. After transmitting through a submicrometer thick ENZ metamaterial, the spin photons are fully separated with a spin separation of 2.7 times beam waist and transmittance of 70.1%, allowing a figure of merit F up to 1.9. A practical ENZ metamaterial consisting of an Ag nanorod array is proposed, whose figure of merit is still up to 0.006. This high-performance photonic spin Hall effect provides an integrated and practical way for the development of spin-photonic devices.

Funneling Spontaneous Emission into Waveguides via Epsilon-Near-Zero Metamaterials.

In this work, we discuss the use of epsilon-near-zero (ENZ) metamaterials to efficiently couple light radiated by a dipolar source to an in-plane waveguide. We exploit both enhanced and directional emission provided by ENZ metamaterials to optimize the injection of light into the waveguide by tuning the metal fill factor. We show that a net increase in intensity injected into the waveguide with respect to the total power radiated by the isolated dipole can be achieved in experimentally feasible conditions. We think the proposed system may open up new opportunities for several optical applications and integrated technologies, especially for those limited by outcoupling efficiency and emission rate.

Effective permittivity of co-evaporated metal-organic mixed films

The combination of organics and metals in a composite film holds promise for combining plasmonic interaction with gain and for the realization of epsilon-near-zero (ENZ) metamaterials. In particular, fluorescent organic dyes can be used to compensate the plasmonic losses of a homogenized metal-organic material. Here, we fabricate such films through thermal co-evaporation of silver and an organic host:guest system and investigate experimentally the resulting linear optical properties for varying metal concentrations. We extract the effective permittivity of the resulting films with ellipsometry measurements and demonstrate the formation of silver nanoparticles, resulting in strongly localized surface plasmon resonances until a percolation threshold is reached. Through enhanced light-matter interaction, we observe a maximum of the photoluminescence for a concentration of 15% in volume of metal in the composite material. These results showcase a variety of growth parameters and will be useful for the future design of gain-compensated plasmonics and ENZ metamaterials.

Phase singularity annihilation in plasmonic nano-apertures via epsilon-near-zero metamaterials

In this work, we performed an extensive theoretical and experimental study to unveil the underlying mechanisms related to the intensified transmittance in epsilon-near-zero (ENZ)-integrated plasmonic nano-apertures. The occurrence of phase singularities at the incident side of plasmonic nano-apertures results in the reduction in transmittance. We show that transmittance enhancement in ENZ-integrated nano-slits is attributed to the annihilation of the phase singularities by the ENZ layer and subsequently the modification in plasmonic fields of metallic apertures. The singularity annihilation via ENZ metamaterials eliminates the abrupt changes in the phase of the Poynting vector underneath the slit, which results in a stronger cavity resonance inside the aperture. Due to this fact, a nano-aperture on top of an ENZ metamaterial is the only effective configuration to enhance the transmittance in ENZ-integrated nano-apertures. Hence, our study reveals the physics of the transmittance enhancement through ENZ-integrated plasmonic nano-apertures.

Non-Periodic Epsilon-Near-Zero Metamaterials at Visible Wavelengths for Efficient Non-Resonant Optical Sensing.

Epsilon-near-zero (ENZ) materials offer unique properties for applications including optical clocking, nonlinear optics, and telecommunication. To date, the fabrication of ENZ materials at visible wavelengths relies mostly on the use of periodic structures, providing some manufacturing and material challenges. Here, we present the engineering of nonperiodic sodium tungsten bronzes (NaxWO3) metamaterials featuring ENZ properties in the visible spectrum. We showcase their use as efficient optical sensors, demonstrating a nonresonant sensing mechanism based on refractive index matching. Our optimized ENZ metamaterials display an unconventional blue-shift of the transmittance maximum to increasing refractive index of the surrounding environment, achieving sensitivity as high as 150 nm/RIU. Our theoretical and experimental investigations provide first insights on this sensing mechanism, establishing guidelines for the future engineering and implementation of efficient ENZ sensors. The unique optoelectronic properties demonstrated by this class of tunable NaxWO3 materials bear potential for various applications ranging from light-harvesting to optical photodetectors.

Adjustable Graphene/Polyolefin Elastomer Epsilon-near-Zero Metamaterials at Radiofrequency Range.

While epsilon-near-zero (ENZ) metamaterials have marvelously shown various application prospects, the way to construct intrinsic ENZ metamaterials and adjust their ENZ properties precisely is still uncovered. The realization of stable and broadband ENZ properties at the radiofrequency range is of great significance. Herein graphene/polyolefin elastomer (POE) intrinsic ENZ metamaterials are initially constructed via the nanohybrid process. The metamaterials possess excellent adjustable ENZ properties by adjusting the content and reduction methods of graphene. The permittivities maintain between -1 and 1 steadily with increasing graphene content, which is attributed to the moderated carrier concentration of the conductive networks in the nanohybrids. Besides, different reduction methods also have significant impacts on ENZ properties. The hydrazine hydrate reduction method increases the maximum ENZ frequency region to 126 MHz. Lorentz type resonance is reasonable for the positive-negative transition in the ENZ frequency regions. As a significant indicator of the emergence of ENZ property, the sudden peak of dielectric loss tangent is observed. This work offers novel routes to construct intrinsic ENZ metamaterials with excellent adjustability in both values of permittivity and ENZ frequency regions.

Gain enhancement of terahertz patch antennas by coating epsilon-near-zero metamaterials

Abstract In this work, we propose a novel approach to enhance the gain of a terahertz patch antenna by coating an epsilon-near-zero (ENZ) metamaterial superstrate. The ENZ metamaterial is composed of indium antimonide (InSb) and silicon dioxide multilayers, of which the out-of-plane component of the effective permittivity is close to zero. Results show that by coating the ENZ superstrate the peak gain of the antenna is increased from 5.37 dB to 7.79 dB, corresponding to 45% gain enhancement and greatly improved radiation directivity. We find that the ENZ frequency of the multilayer metamaterial equals to that of semiconductor InSb and thus it can be tuned dynamically. We expect the concept of adding a multilayer ENZ metamaterial superstrate to enhance the antenna gain will find potential applications in other types of terahertz antennas and in antennas in other frequency regimes.

Deep plasmonic direct writing lithography with ENZ metamaterials and nanoantenna

In this article, we demonstrate a specially designed resonant metamaterial with epsilon-near-zero (ENZ) and nanoantenna to enhance the exposure depth in plasmonic direct writing lithography more than 10 times. The ENZ metamaterial composed of a Ag/Si3N4 multilayer thin film, converts the evanescent field generated by the bowtie aperture nanoantenna to propagating waves with low divergence and high collimation. Deep sub-diffraction limited resolution of less than 65 nm (λ/7) with exposure depth greater than 100 nm is achieved. This work brings plasmonic direct writing lithography one step closer to practical applications.

Superconducting properties of tin-based ENZ and hyperbolic metamaterials

Recent experiments have demonstrated that the superconducting critical temperature may be improved in various metamaterial superconductor geometries. Here, we present the results of a study of tin-based metamaterial superconductors in the epsilon-near-zero (ENZ) and hyperbolic metamaterial configurations. It was observed that Tc enhancement is significantly reduced when the metamaterial structural dimensions exceed 240 nm, the superconducting coherence length in pure tin.

Microstrip Line Impedance Matching Using ENZ Metamaterials, Design, and Application

The idea of this paper is to extend the tunneling effect of epsilon-near-zero (ENZ) narrow channel for matching two microstrip lines with different impedance characteristics. The main advantage of this method is the possibility to design a channel with subwavelength electrical size and obtain similar matching condition when compared with a conventional $\lambda $ /4-transformer. The bandwidth of the structure is directly related to the bandwidth of the ENZ metamaterials. The suggested matching circuit is comprised of a metallic wall and an ENZ narrow channel. To realize the ENZ channel, a rectangular waveguide which operates in TE10 mode is designed and implemented using a substrate integrated waveguide (SIW) technology. A set of vias are also needed for emulating the metallic wall to reduce the ENZ channel cross section. The proposed structure for different impedance values of 50, 100, and $150~\Omega $ was designed, simulated, fabricated, and tested. Moreover, as an important application of a matching network, a microstrip patch antenna has been matched over the desired frequency band. The simulation results based on CST microwave studio have a good agreement with measurements. It is shown that the bandwidth of the circuit is 8%–15%.