Negative Index Modes Research Articles

Broadband negative-index surface-waves on arrays of capped helices

A novel metasurface comprised of capped helices arranged as a hexagonal array that supports a broadband near-isotropic negative-index microwave surface-wave is designed, manufactured, and experimentally characterized. The surface-mode dispersion is studied both numerically and experimentally with the operational band more than double the bandwidth of structures previously reported in the literature. Hence, it is shown how one may provide a structured surface that supports just a negative-index mode over a wide operational band with no forward wave being simultaneously excited.

Ghost resonance in anisotropic materials: negative refractive index and evanescent field enhancement in lossless media

We show that dielectric waveguides formed by materials with strong optical anisotropy support electromagnetic waves that combine the properties of propagating and evanescent fields. These “ghost waves” are created in tangent bifurcations that “annihilate” pairs of positive- and negative-index modes and represent the optical analogue of the “ghost orbits” in the quantum theory of nonintegrable dynamical systems. Ghost waves can be resonantly coupled to the incident evanescent field, which then grows exponentially through the anisotropic media—as in the case of negative index materials. As ghost waves are supported by transparent dielectric media, the proposed approach to electromagnetic field enhancement is free from the “curse” of material loss that is inherent to conventional negative index composites.

Modal evolution in asymmetric three- and four-layer plasmonic waveguides.

Through the employment of a novel approach in solving the dispersion for the three-layer plasmonic waveguides, considering lossy metals, we demonstrate that, besides well-known modes, the complete dispersion always contains high-lossy periodic solutions. Consideration of these solutions is shown to be crucial for the understanding of every aspect of dispersion evolution at broad spectral range when the thickness of the middle layer is varied. In particular, we show that generally considered modes of the three-layer waveguide transform into the single interface modes via interaction with high-lossy periodic solutions. Furthermore, the negative index mode is shown to experience a transition between low- and high-lossy regimes depending on the waveguide's thickness. Our results, avoiding complicated analytical analysis, perfectly integrate and importantly complement past theoretical works.

Non-Hermitian nanophotonic and plasmonic waveguides

Non-Hermitian Hamiltonians arising from parity-time (PT)-symmetric potentials have been extensively explored in optical systems, owing to their ability to generate asymmetric and even nonreciprocal light propagation. In this paper, we investigate such PT potentials in plasmonic systems, demonstrating asymmetric optical propagation in deeply subwavelength waveguides. In particular, we investigate a five layer plasmonic waveguide composed of metallic layers separated by dielectric media containing either loss or gain in equal quantities. Through an analytic solution of Maxwell's equations, we identify the four lowest order modes of the waveguide, including two positive index modes and two negative index modes, and investigate their evolution with increasing but balanced gain and loss, $\ensuremath{\kappa}$. Both the exact analytic approach and an approximate one based on Rayleigh-Schrodinger perturbation theory demonstrate eigenvalue merging and state coalescence with increasing $\ensuremath{\kappa}$, unlike the familiar energy-level splitting observed in conventional coupled systems. The state coalescence always occurs between modes of opposite parity. Also, by changing the coupling between the waveguide layers, state coalescence can occur between modes with opposite refractive indices, resulting in the merging of a positive index mode with a negative index mode at the exceptional point. We use dispersion diagrams and field profiles to illustrate the asymmetric plasmon propagation properties with increasing $\ensuremath{\kappa}$. We also show that at the system's exceptional point, the modal power varies quadratically along the waveguide. This study represents a spectral analysis of deeply subwavelength PT-symmetric plasmonic and multimodal photonic waveguides and provides a foundation for designing asymmetric and unidirectional nanophotonic devices.

Negative refraction of a single-layer metamaterial inserted with dual-coaxial waveguides array

Single-layer metamaterials consisting of a hexagonal close-packed array of Ag/GaP/Ag/GaP/Ag dual coaxial waveguides are investigated by simulation, focusing on both the negative-index property and transmission performance at visible frequencies. Numerical simulations indicate that the negative index mode, the same as the one identified in the Ag/GaP/Ag coaxial waveguides arrays (Nat. Mater. 9, 407, 2010) perfectly conserves and exhibits two negative index bands centered at 400 and 800THz, respectively. The transmission peaks of the Fabry–Perot resonance at 400THz and the transmission intensity of the resonance at 800THz are closely related to the thickness of the silver film. Moreover, the coupling of the two coaxial waveguides can dominate the negative refraction and optical transmission of the constructed metamaterial, only by changing the distance of the two annular apertures. The present configuration can provide more approaches to tune its properties than the single coaxial waveguide array.

Metamaterials: A Broadband Negative Index Metamaterial at Optical Frequencies (Advanced Optical Materials 4/2013)

A. C. Atre et al. theoretically demonstrate a broadband metamaterial with negative indices across hundreds of nano meters in the visible and NIR spectral regime. On page 327, transformation optics is used to conformally map the negative index mode of an infinite plasmonic waveguide to degenerate electric and magnetic dipoles in a nanoscale plasmonic resonator. A periodic array of such resonators exhibits negative refractive indices at optical frequencies, as illustrated in the image of light passing through a micrometer-scale metamaterial prism.

A Broadband Negative Index Metamaterial at Optical Frequencies

A broadband metamaterial presenting negative indices across hundreds of nanometers in the visible and near‐infrared spectral regimes is demonstrated theoretically, using transformation optics to design the metamaterial constituents. The approach begins with an infinite plasmonic waveguide that supports a broadband but dark (i.e, not easily optically accessed) negative index mode. Conformal mapping of this waveguide to a finite split‐ring‐resonator‐type structure transforms this mode into a bright (i.e, efficiently excited) resonance composed of degenerate electric and magnetic dipoles. A periodic array of such resonators exhibits negative refractive indices at optical frequencies in multiple regions exceeding 200 nm in bandwidth. The metamaterial response is confirmed through simulations of plane‐wave refraction through a metamaterial prism. These results illustrate the power of transformation optics for new metamaterial designs and provide a foundation for future broadband metamaterial devices.

Negative index modes in surface plasmon waveguides: a study of the relations between lossless and lossy cases

Surface plasmon modes in structures of metal-insulator-metal (MIM), insulator-insulator-metal (IIM) and insulator-metal-insulator (IMI) are studied theoretically for both lossless and lossy cases. Causality dictates which solutions of Maxwell's equations we accept for these structures. We find that for both lossless and lossy cases, the negative index modes and positive index modes are independent and should be treated separately. For the lossless case, our results differ from some published papers. By studying in detail the lossy case, we demonstrate how the curves should look like.

Negative index Clarricoats-Waldron waveguides for terahertz and far infrared applications

We explore a class of dielectrically loaded metallic waveguides capable of supporting negative index modes in the far infrared and terahertz regime. Principles of operation, modal structure and appropriate coupling schemes are analytically and numerically investigated. The extreme simplicity of the proposed design, along with the non-conventional and counter intuitive electromagnetic properties of this family of waveguides, makes these structures excellent candidates for the practical realization of negative index far infrared and terahertz devices with new and interesting functionalities. Generalizations and extensions of the suggested design are also discussed.

Backward propagating slow light in inverted plasmonic taper

The negative dispersion of the TM1 mode of a thin plasmonic gap, occurring at frequencies exceeding the surface plasmon frequency, is assigned by causality to be a backward wave. This negative index mode, is also slow-light--having small positive group velocity and is exhibiting inverse geometrical cutoff characteristics--namely when the gap width is enhanced.

Are negative index materials achievable with surface plasmon waveguides? A case study of three plasmonic geometries

We present a theoretical analysis of planar plasmonic waveguides that support propagation of positive and negative index modes. Particular attention is given to the modes sustained by metal-insulator-metal (MIM), insulator-metal-insulator (IMI), and insulator-insulator-metal (IIM) geometries at visible and near-infrared frequencies. We find that all three plasmonic structures are characterized by negative indices over a finite range of visible frequencies, with figures of merit approaching 20. Moreover, using finite-difference time-domain simulations, we demonstrate that visible-wavelength light propagating from free space into these waveguides can exhibit negative refraction. Refractive index and figure-of-merit calculations are presented for Ag/GaP and Ag/Si(3)N(4) - based structures with waveguide core dimensions ranging from 5 to 50 nm and excitation wavelengths ranging from 350 nm to 850 nm. Our results provide the design criteria for realization of broadband, visible-frequency negative index materials and transformation-based optical elements for two-dimensional guided waves. These geometries can serve as basic elements of three-dimensional negative-index metamaterials.