Near-zero Refractive Index Research Articles

Tunable phase shifter with zero refractive index photonic crystal

In this work, a phase shifter composed of cascaded photonic crystals (PCs) was proposed. The PCs with the property of near-zero refractive index at and near Dirac point frequency have zero phase delay. By cascading two identical PCs, an arbitrarily tunable 360° phase-shifter which can be tuned linearly is demonstrated. The properties of such presented PC structure are well described by comprehensive numerical simulations.

Numerical study of a near-zero-index acoustic metamaterial

This Letter studies a two-dimensional, membrane-based acoustic metamaterial with a near-zero refractive index. It yields a frequency-dependent effective density that is near-zero at a narrow frequency band centered around its first resonant frequency. This effective density results in its near-zero refractive index. Numerical simulations are shown which demonstrate that the phase in this metamaterial undergoes small changes, and the metamaterial functions as an angular filter such that only a wave with a near-zero incident angle can transmit. Its ability to tailor acoustic phase pattern is also discussed in this Letter.

Superradiance in media with a near-zero refractive index

The dependence of the efficiency of collective spontaneous emission in a spherical medium on the modulus of a complex refractive index approaching zero and on the ordering of optical centers is studied.

A Spatial Beam Splitter Consisting of a Near-Zero Refractive Index Medium

A novel metamaterial-based beam splitter is demonstrated in this communication. The metamaterial is a three-dimensional (3 D) fishnet structure arranged in a two-dimensional (2 D) square lattice. According to Snell's law, the refracted wave from medium into air tends to be normal to the interface if the effective refractive index of medium is approximately zero. In light of this concept, the study presents a metamaterial with near-zero effective refractive index and an internal line source inside as excitation. Metallic flares with specific tilting angle were installed at four sides of the beam splitter to reduce reflection coefficient and match the impedance of the fishnet structure and free space. In addition to calculating the phase relation of eigenwaves in the metamaterial, this study measures the radiation patterns to verify its spatially beam-splitting characteristics.

Zero-index metamaterial based on double-negative structure

We present a new kind of zero-index metamaterial composed of periodic array of dendritic cells with continuous long pole, and investigate its electromagnetic characteristics. First, we study the one-level dendritic cell with continuous long pole. The retrieved effective permittivity and permeability of the unit cell simultaneously tend to be zero from their negative values at a frequency of 9.5 GHz, which leads to a near-zero refractive index. Then by changing the geometric parameters of the unit, we find the influences of the branch length and the separation angle on the electromagnetic parameters. Finally, increasing the level number of the branch gives rise to two-level and three-level dendritic cells respectively with continuous long pole. Adjusting the geometric parameters of each unit cell to appropriate values, the zero-index matematerials with zero permittivity and permeability can be obtained at the same frequency.

Spectral response and far-field pattern of a dipole nano-antenna on metamaterial substrates having near-zero and negative indices of refraction

In this work the resonance, frequency response, and far-field patterns of an optical nano-antenna placed on an interface between air and a metamaterial substrate is obtained through finite element calculations. The metamaterial is characterized by an effective, macroscopic index of refraction which can take negative and near-zero values, or by published values of the effective permittivity and permeability for metamaterials. The results show that the resonant wavelength and response can be fitted to analytical functions that are even functions of the index of refraction, this is consistent with the knowledge that negative indices of refraction allows for wave propagation in the same magnitude but opposite direction observed with positive indices of refraction. The simulations also show that substrates with near-zero index of refraction will enhance the antenna response by 62% compared to substrates with n>1. Lossy metamaterials are also considered in the simulations. The far-field pattern of the antenna, obtained through a near-field to far-field transformation, behaves the same independently of the sign of the index of refraction, also the far-field pattern for the emission towards near-zero substrates is nearly constant and independent of the angle for the evaluated angular range.

Experimental characterization of the dispersive behavior in a uniaxial metamaterial around plasma frequency

In this paper, the dispersive behavior around the plasma frequency in a magnetically uniaxial metamaterial is experimentally investigated. We show by theoretical analysis, parameter retrieval and experiment that when material loss is considered, while the plasma frequency is defined by the frequency where the real part of permeability approaches zero, ultra fast phase velocity actually appears at a slightly lower frequency, due to the change of the dispersion diagram. Both parameter retrieval and experimental data show that within a narrow frequency band to the left of the plasma frequency, the inherent loss keeps finite and is much less than that in the corresponding resonant region. In a real metamaterial sample, an ultra fast phase velocity of 24,440 times the speed of light in free space is measured, and negative phase propagation due to the only negative permeability is observed. The existence of such ultra fast phase velocity with finite loss perfectly explains how the highly directivity antennas based on near-zero refractive index metamaterial work, and can be further used in other applications such as in-phase wave divider and coherent wave sources.

Analysis of Antenna Gain Enhancement with a New Planar Metamaterial Superstrate: an Effective Medium and a Fabry-Pérot Resonance Approach

We have compared gain enhancement behavior of patch and horn antennas from two different points of view: namely, effective medium analysis and a Fabry-Perot cavity resonance approach. To examine how a near-zero refractive index (n) affects the performance of antennas, we designed a new planar metamaterial (MTM) superstrate, which can produce negative, zero, and positive values of n at around 2 GHz. We placed the MTM superstrate very close to the patch and horn antennas to see whether an n value that is effectively near zero can collimate antenna beams and increase antenna gain, which provided opposite gain behavior for the two antennas. To explain the dissimilar enhancement in the gain of the patch and horn antennas, we retrieved constitutive parameters from the proposed MTM superstrate and discussed the effect of various incidence angles upon the superstrate. In addition, to increase the gain further, we examined appropriate resonant heights between the antennas and the superstrate. Consequently, with the help of Fabry-Perot cavity resonance, we obtained relatively high antenna gain. Moreover, the results of the prediction are in good agreement with the results of the measurement.

Highly collimated emission from a left-handed photonic crystal with a quasi-cavity

We study the collimated emission characteristics from a dipole source inside a negative-effective-refractive-index photonic crystal with a quasi-cavity constructed by a concave photonic crystal reflector. The emissions along the ±X and −Y directions are forbidden by the quasi-cavity, so that most emissions propagate along the +Y direction. Simulation results show that a narrow collimated beam is achieved due to the near-zero negative effective refractive index. Moreover, the half-power beam width of such a collimated beam can be reduced to 3.48° by optimizing the size of the source area. Such a compact structure would have potential applications in micro-optical devices.

Confirmation of Zero-N Behavior in a High Gain Grid Structure at Millimeter-Wave Frequencies

A multilayer grid structure that has been used previously to create an antenna with high directivity and gain and that has been explained as having a near zero index of refraction is extended to millimeter-wave frequencies. Alternate explanations for the associated phenomena are examined. Further confirmation of this zero-n behavior is presented.

Parametrically Shielding Electromagnetic Fields by Nonlinear Metamaterials

An analytical theory is developed for parametric interactions in metamaterial multilayer structures with simultaneous nonlinear electronic and magnetic responses and with a near-zero refractive index. We demonstrate theoretically that electromagnetic fields of certain frequencies can be parametrically shielded by a nonlinear left-handed material slab, where the permittivity and permeability are both negative. The skin depth is tunable, and even in the absence of material absorption, can be much less than the wavelength of the electromagnetic field being shielded. This exotic behavior is a consequence of the intricate nonlinear response in the left-handed materials and vanishing optical refractive index at the pump frequency.