ENZ Material Research Articles

Broadband multiple resonant metasurface for mixture surface-enhanced infrared absorption based on polarization-sensitive folded nanoantennas

Multiple resonant metasurfaces for mixture component detection by surface-enhanced infrared absorption in a broadband spectrum region are still highly desirable. This paper presents a polarization-sensitive multi-resonant broadband metasurface in the mid-infrared (MIR) region based on an odd symmetric folded antenna array. Six major resonant modes are effectively excited in 6–15 μm covering the absorption peak of multiple gases and biomolecules. In addition, the calculation indicates the maximum electric field intensity enhancement reaches 3607 and the average electric enhancement factors are dominantly amplified to a maximum of 75.08. The excited multiple hotspots provide more spatial overlap with analytes and would be beneficial for label-free detection. Moreover, owing to the contributions of the intrinsic phonons and polaritons in the ENZ material, the spectroscopy features are relatively stable when changing the geometry parameters and the incident angles, allowing more fabrication tolerance and experiment flexibility. Furthermore, the multiple resonant metasurface is also effectively enabled under orthogonal circular polarizations. Finally, the sensing abilities of the designed metasurface for polyethylene terephthalate and isopropanol molecules have been computationally validated. These results indicate the potential for nanophotonic detection and mixture spectroscopy analysis.

Dual-wavelength switchable perfect infrared absorber based on multiple ENZ materials

Dual-wavelength switchable perfect infrared absorber based on multiple ENZ materials

Graphene Oxides as Epsilon‐Near‐Zero Platforms Operating in Terahertz Frequency Range

AbstractEpsilon‐near‐zero (ENZ) materials, with their unique light manipulation capabilities, have fascinating applications such as cloaking, super‐resolution imaging, energy harvesting, and sensing. Herein, free‐standing graphene oxide films are fabricated whose dielectric constant can be engineered by thermal reduction, resulting in ENZ characteristics at the transition between insulating and metallic phases. The ENZ frequency is found to be 0.4–0.5 THz under an annealing temperature of 200 °C, whereas another ENZ phase appeared at 360 °C. ENZ film can be transferred to curved metal surfaces, serving as superabsorbers that suppress wave reflection from underneath metal surfaces. Hyper‐resolution in nondestructive THz imaging is also possible, owing to superior beaming. Spatial resolution is improved two‐fold by adding the ENZ film, even when the metal pattern is enclosed by a paint layer. Lastly, hybrid sensors are developed by incorporating the meta‐pattern on the free‐standing ENZ substrate. The energy splitting of metamaterial resonance originating from the coupling with ENZ mode is observed. Hybridized devices have proven effective in detecting microorganisms with a low refractive index, owing to the unique dielectric configuration offered by ENZ material. Using the novel ENZ platform, a 15‐fold improvement in microbial sensitivity is achieved compared with the conventional sensors.

Magnetic‐Driven Broadband Epsilon‐Near‐Zero Materials at Radio Frequency

AbstractEpsilon‐near‐zero (ENZ) materials, exhibiting unique physical characteristics such as near‐zero refraction, have aroused extensive interest and exhibit great potentials in novel applications of perfect absorbers, high‐harmonic generation, and nonlinear optical response. Here, for the first time, magnetic‐driven broadband ENZ materials are designed by fabricating polyvinyl alcohol (PVA)/Ni@carbon nanotubes (CNTs) films. Dielectric properties including real permittivity (ɛ′), imaginary permittivity (ɛ″), dielectric loss (tanδ), and impedance (Z) are investigated. When Ni@CNTs content reached 30 wt.%, negative permittivity transferred to positive permittivity at ≈11.5 MHz, and epsilon‐near‐zero (|ɛ′| < 1) is realized from ≈9 to 14 MHz, exhibiting broad ENZ bandwidth of ≈5 MHz. Theory calculations confirm that delocalized electrons are introduced from CNTs, which improve the carrier mobility and achieve low frequency dispersion behavior. Longer interfacial polarization electric fields between PVA and CNTs are also demonstrated by theory calculations, enhancing the positive permittivity response to offset negative permittivity response from Ni@CNTs. These two mechanisms result in broadband ENZ at radio frequency. This film also exhibits excellent magnetic actuation ability under magnetic field, broadening applications from ENZ materials to novel fields such as magnetically actuated robots with perfect absorption, magnetic‐driven biomimetic aircrafts with shielding ability, magnetic‐driven photodetectors, etc.

Enhanced Magneto‐Optical Effects in Epsilon‐Near‐Zero Indium Tin Oxide at Telecommunication Wavelengths

AbstractEpsilon‐near‐zero (ENZ) materials exhibit near‐zero permittivity and induce various intriguing linear and nonlinear optical phenomena. Magneto‐optical (MO) effects, which are notoriously weak in the optical domain, have been predicted to be significantly enhanced in ENZ materials, which can facilitate the realization of compact nonreciprocal devices. However, to date, enhanced MO effects have been predominantly observed in effective ENZ media, which are commonly based on complex combinations of photonic nanostructures. It is difficult for these effective media to achieve isotropic ENZ responses, which severely limits their use in the development of ENZ‐MO devices. Here, enhanced MO effects in pristine indium tin oxide (ITO) with ENZ properties at technologically important telecommunication wavelengths are demonstrated. MO transmission (reflection) spectroscopy of ITO films with different ENZ wavelengths reveal Faraday (Kerr) rotation peaks around the respective ENZ (EN‐one) wavelengths, demonstrating that these observations are due to the intrinsic properties of the ITO materials. The demonstrated mechanism of the MO effect enhancement is universal and can be applied to various ENZ materials, including those incorporating ferromagnetic materials. Native ENZ materials with enhanced MO responses will greatly expand the opportunities for the development of novel nonreciprocal nanophotonic devices, such as on‐chip optical isolators and one‐way topological waveguides.

Complex analytic dependence on the dielectric permittivity in ENZ materials: The photonic doping example

AbstractMotivated by the physics literature on “photonic doping” of scatterers made from “epsilon‐near‐zero” (ENZ) materials, we consider how the scattering of time‐harmonic TM electromagnetic waves by a cylindrical ENZ region is affected by the presence of a “dopant” in which the dielectric permittivity is not near zero. Mathematically, this reduces to analysis of a 2D Helmholtz equation with a piecewise‐constant, complex valued coefficient a that is nearly infinite (say with ) in . We show (under suitable hypotheses) that the solution u depends analytically on δ near 0, and we give a simple PDE characterization of the terms in its Taylor expansion. For the application to photonic doping, it is the leading‐order corrections in δ that are most interesting: they explain why photonic doping is only mildly affected by the presence of losses, and why it is seen even at frequencies where the dielectric permittivity is merely small. Equally important: our results include a PDE characterization of the leading‐order electric field in the ENZ region as , whereas the existing literature on photonic doping provides only the leading‐order magnetic field.

Tunable Organic ENZ Materials with Large Optical Nonlinearity

Tunable Organic ENZ Materials with Large Optical Nonlinearity

Enhanced epsilon-near-zero structures for photonics

We present an experimental realization of a novel layered metamaterial we label enhanced epsilon-near-zero (eENZ). The structure is a stack of alternating thin films made of ENZ– and dielectric material and it can be designed for desired refractive/reflective properties by appropriately tuning the film thicknesses. The structure supports thin film resonances, guided modes and Ferrel-Berreman plasmon modes and the performance of the structure shows a large improvement to many currently available bulk ENZ materials. Additionally, we recently demonstrated the possible use of eENZ for coherence switching in lasers [1]. We demonstrate the design, fabrication and characterization of the optical properties of the eENZ stack and compare the measured transmission properties with transfer matrix method (TMM) simulations.

Second harmonic generation enhancement of ITO-based ENZ materials and metasurfaces

Second harmonic generation enhancement of ITO-based ENZ materials and metasurfaces

ENZ material filled microstructured optical fiber with high birefringence and near-zero flattened dispersion at terahertz frequency

为了实现太赫兹波的保偏波导传输，设计了一种含有纤芯缺陷孔和椭圆形包层空气孔的高双折射微结构光纤。通过在包层空气孔中选择性地填充太赫兹近零介电常量（epsilon-near-zero, ENZ）材料，引入了几何结构和材料分布的双重不对称性，破坏了2个偏振基模的简并以获得高双折射特性。应用有限元方法研究了光纤的双折射、损耗和色散等传输特性随结构参数的变化规律。在0.5 THz~2 THz的宽频段范围内获得了大于0.01的高双折射。<i>x</i>和<i>y</i>偏振基模的损耗在0.8 THz附近具有最小值，分别为0.903 dB·cm⁻¹和0.851 dB·cm⁻¹。纤芯缺陷孔可以有效调节色散特性，<i>y</i>偏振基模在1 THz~1.8 THz范围内具有 (0±0.054) ps·THz⁻¹·cm⁻¹近零平坦色散特性。光纤的传输特性对ENZ材料的折射率变化不敏感。研究结论为研制太赫兹保偏光纤提供了理论参考。

Adiabatic Frequency Conversion Using a Time-Varying Epsilon-Near-Zero Metasurface

A time-dependent change in the refractive index of a material leads to a change in the frequency of an optical beam passing through that medium. Here, we experimentally demonstrate that this effect-known as adiabatic frequency conversion (AFC)-can be significantly enhanced by a nonlinear epsilon-near-zero-based (ENZ-based) plasmonic metasurface. Specifically, by using a 63-nm-thick metasurface, we demonstrate a large, tunable, and broadband frequency shift of up to ∼11.2 THz with a pump intensity of 4 GW/cm2. Our results represent a decrease of ∼10 times in device thickness and 120 times in pump peak intensity compared with the cases of bare, thicker ENZ materials for the similar amount of frequency shift. Our findings might potentially provide insights for designing efficient time-varying metasurfaces for the manipulation of ultrafast pulses.

Modeling interaction of ultrashort pulses with ENZ materials

In this work, the split-step Fourier method for beam propagation is used to investigate the interaction of ultra-short pulses with epsilon-near-zero materials. The propagation of pulses is governed by the nonlinear Schrödinger equation (NLSE) containing dispersion, gain-bandwidth, self-phase modulation, self-steepening, and absorption parameters. It is found that the intensity profile of the pulse is broadened and the phase of the pulse is shifted by dispersion phenomena. The gain/loss related to the imaginary part of the refractive index causes an increase or decrease in intensity and pulse edge effects. These effects do not favor the steady propagation of the pulse. The self-phase modulation is not noted to appreciably affect the intensity pulse profile. The self-steepening modifies the phase and energy of the pulse during propagation, as well as absorption, which influences the losses by both the linear and nonlinear effects.

ENZ materials and anisotropy: enhancing nonlinear optical interactions at the nanoscale.

Epsilon-near-zero materials are exceptional candidates for studying electrodynamics and nonlinear optical processes at the nanoscale. We demonstrate that by alternating a metal and a highly doped conducting-oxide, the epsilon-near-zero regime may be accessed resulting in an anisotropic, composite nanostructure that significantly improves nonlinear interactions. The investigation of the multilayer nanostructure reveals the actual role of the anisotropy, showing that high degrees of anisotropy might be necessary to effectively boost nonlinear processes. Moreover, using a microscopic, hydrodynamic approach we shed light on the roles of two competing contributions that are for the most part overlooked but that can significantly modify linear and nonlinear responses of the structure: nonlocal effects, which blueshift the resulting resonance, and the hot electrons nonlinearity, which redshifts the plasma frequency as the effective mass of free electrons increases as a function of incident power density and enhances the nonlinear signal by several orders of magnitude. Finally, we show that, even in the absence of second order bulk nonlinearity, second order nonlinear processes are also significantly enhanced by the layered structure.

Gausson parameter dynamics in ENZ-material based waveguides using moment method

In this article, we study the dynamics of optical soliton (Gausson) pulses in waveguides based on ENZ-materials. The theoretical approach used is based on the handling of the non-linear Schrödinger's equation using the moment method. The physical system studied is dissipative. The loss of energy in the waveguide, which can be explained by the existence of imaginary parts of the refractive index of the ENZ-material, is negligible because permittivity of ENZ–materials is low. In addition, we show that self-phase modulation results in the increase of chirp during the propagation of such Gaussons.

Optical density of states near planar ENZ materials.

We study the local density of optical states (LDOS) for lossy dielectric substrates whose electric permittivity has a vanishing real part, approaching zero from the positive side of the real axis. A criterion for evaluating the threshold height above (below) which radiative (non-radiative) processes dominate for a dipole emitter is established. We focus on the case of a vertical dipole above the ϵ-near-zero (ENZ) substrate and show that, in the lossless case, complete LDOS cancellation originates from radiative modes in its near field. We evaluate the performance of commercially available ENZ materials and quantify the limits of such cancellation effects with the intrinsic losses of the substrate.

Reflection by and transmission through an ENZ interface

We have studied the reflection and transmission of traveling and evanescent plane waves, incident upon an ENZ material. The Fresnel reflection and transmission coefficients were obtained in the ENZ limit. For a p polarized incident wave, the transmission coefficient vanishes, except very close to normal incidence. The reflection coefficient is -1 for both traveling and evanescent waves. It is shown, however, that there is a finite electric field in the ENZ material, even though the transmission coefficient is zero. This field is either linearly polarized or circularly polarized. The magnetic field in the medium for p polarized illumination is zero, and therefore there can be no energy flow through the material. For s polarization, the magnetic field in the medium is circularly polarized, and energy can flow through the material, parallel to the interface.

Giant field enhancement in longitudinal epsilon-near-zero films

We report that a longitudinal epsilon-near-zero (LENZ) film leads to giant field enhancement and strong radiation emission of sources in it and that these features are superior to what found in previous studies related to isotropic ENZ. LENZ films are uniaxially anisotropic films where relative permittivity along the normal direction to the film is much smaller than unity, while the permittivity in the transverse plane of the film is not vanishing. It has been shown previously that realistic isotropic ENZ films do not provide large field enhancement due to material losses, however, we show the loss effects can be overcome using LENZ films. We also prove that in comparison to the (isotropic) ENZ case, the LENZ film field enhancement is not only remarkably larger but it also occurs for a wider range of angles of incidence. Importantly, the field enhancement near the interface of the LENZ film is almost independent of the thickness unlike for the isotropic ENZ case where extremely small thickness is required. We show that for a LENZ structure consisting of a multilayer of dysprosium-doped cadmium oxide and silicon accounting for realistic losses, field intensity enhancement of 30 is obtained which is almost 10 times larger than that obtained with realistic ENZ materials

Optical response of dipole antennas on an epsilon-near-zero substrate

Materials with vanishing permittivity (epsilon-near-zero or ENZ materials) show unconventional optical behavior. Here we show that plasmonic dipole antennas on an ultrathin ENZ substrate have properties significantly different from antennas on a traditional substrate. Specifically, the presence of a 23-nm-thick ENZ material strongly modifies the linear response of plasmonic antennas and, as a result, the resonant wavelength is independent of the linear dimensions of the dipole antenna.

Field-effect optical modulation based on epsilon-near-zero conductive oxide

Epsilon-near-zero (ENZ, dielectric constant εr≈0) materials have attracted significant research interest, however, their applications in the near-infrared regime are very limited. Conductive oxide (COx), owing to its moderate carrier concentration, is a candidate for ENZ material at telecom wavelengths based on the Drude model. Herein, we report an indium tin oxide (ITO) thin film as an ENZ material with cross-over wavelength, where real part of permittivity crosses zero, and enhanced light absorption at telecom wavelengths. We also report the investigation of electro-absorption modulation based on a metal-oxide-semiconductor (MOS)-like structure, more specifically a metal-oxide-ITO stack, where ITO works around ENZ. Based on the attenuated total reflectance (ATR) configuration, our test shows great promise for future electro-optical (EO) modulators. The operation speed of the MOS-like structure is limited mainly by RC delay.

Loss-Induced Omnidirectional Bending to the Normal inϵ-Near-Zero Metamaterials

Contrary to conventional wisdom that light bends away from the normal when it passes from high to low refractive index media, here we demonstrate an exotic phenomenon that the direction of electromagnetic power can bend toward the normal when light is incident from an arbitrary high refractive index medium (or air) to a ϵ-near-zero (ENZ) metamaterial. Moreover, the direction of the transmission is close to the normal for all angles of incidence. This anti-Snell's law refraction results from the interplay between ENZ and material loss. The loss can increase the transmission at the air-ENZ interface and collimate the beam inside the ENZ medium. Furthermore, in an ideal loss configuration, the propagation loss in anisotropic ENZ materials can approach zero when the material loss goes to infinity.