Slobodan Vuković

Negative Refractive Index Metasurfaces for Enhanced Biosensing

In this paper we review some metasurfaces with negative values of effective refractive index, as scaffolds for a new generation of surface plasmon polariton-based biological or chemical sensors. The electromagnetic properties of a metasurface may be tuned by its full immersion into analyte, or by the adsorption of a thin layer on it, both of which change its properties as a plasmonic guide. We consider various simple forms of plasmonic crystals suitable for this purpose. We start with the basic case of a freestanding, electromagnetically symmetrical plasmonic slab and analyze different ultrathin, multilayer structures, to finally consider some two-dimensional “wallpaper” geometries like split ring resonator arrays and fishnet structures. A part of the text is dedicated to the possibility of multifunctionalization where a metasurface structure is simultaneously utilized both for sensing and for selectivity enhancement. Finally we give an overview of surface-bound intrinsic electromagnetic noise phenomena that limits the ultimate performance of a metasurfaces sensor.

Surface Bloch waves in metamaterial and metal-dielectric superlattices

We study the properties of electromagnetic Bloch waves in semi-infinite periodic structures created by alternating metamaterial and dielectric layers. We derive and analyze the dispersion relations in the long-wavelength limit for both TE- and TM-polarized surface Bloch modes for magnetic metamaterials with negative refraction and metal-dielectric plasmonic superlattices. We reveal that in the subwavelength regime, the bulk modes are characterized by three different refractive indices (“trirefringence”), while the surface modes can propagate parallel to the Bloch wavevector and along the interface between superlattice and semi-infinite dielectric.

Engineered surface waves in hyperbolic metamaterials

We analyzed surface-wave propagation that takes place at the boundary between a semi-infinite dielectric and a multilayered metamaterial, the latter with indefinite permittivity and cut normally to the layers. Known hyperbolization of the dispersion curve is discussed within distinct spectral regimes, including the role of the surrounding material. Hybridization of surface waves enable tighter confinement near the interface in comparison with pure-TM surface-plasmon polaritons. We demonstrate that the effective-medium approach deviates severely in practical implementations. By using the finite-element method, we predict the existence of long-range oblique surface waves.

Nondiffracting Bessel plasmons

We report on the existence of nondiffracting Bessel surface plasmon polaritons (SPPs), advancing at either superluminal or subluminal phase velocities. These wave fields feature deep subwavelength FWHM, but are supported by high-order homogeneous SPPs of a metal/dielectric (MD) superlattice. The beam axis can be relocated to any MD interface, by interfering multiple converging SPPs with controlled phase matching. Dissipative effects in metals lead to a diffraction-free regime that is limited by the energy attenuation length. However, the ultra-localization of the diffracted wave field might still be maintained by more than one order of magnitude.

Oblique surface waves at an interface between a metal–dielectric superlattice and an isotropic dielectric

We investigate the existence and dispersion characteristics of surface waves that propagate at an interface between a metal–dielectric superlattice and an isotropic dielectric. Within the long-wavelength limit, when the effective-medium (EM) approximation is valid, the superlattice behaves like a uniaxial plasmonic crystal with the main optical axes perpendicular to the metal–dielectric interfaces. We demonstrate that if such a semi-infinite plasmonic crystal is cut normally to the layer interfaces and brought into contact with a semi-infinite dielectric, a new type of surface mode can appear. Such modes can propagate obliquely to the optical axes if favorable conditions regarding the thickness of the layers and the dielectric permittivities of the constituent materials are met. We show that losses within the metallic layers can be substantially reduced by making the layers sufficiently thin. At the same time, a dramatic enlargement of the range of angles for oblique propagation of the new surface modes is observed. This can lead, however, to field non-locality and consequently to failure of the EM approximation.

Propagation of Dyakonon Wave-Packets at the Boundary of Metallodielectric Lattices

We rigorously analyze the propagation of localized surface waves that takes place at the boundary between a semi-infinite layered metal-dielectric (MD) nanostructure cut normally to the layers and a isotropic medium. It is demonstrated that Dyakonov-like surface waves (also coined dyakonons) with hybrid polarization may propagate in a wide angular range. As a consequence, dyakonon-based wave-packets (DWPs) may feature subwavelength beamwidths. Due to the hyperbolic-dispersion regime in plasmonic crystals, supported DWPs are still in the canalization regime. The apparent quadratic beam spreading, however, is driven by dissipation effects in metal.

Plasmon modes on laminated nanomembrane-based waveguides

We studied the propagation of plasmonic modes along planar multilayer metal- dielectric structures with finite number of bi-layer unit cells. The dispersion relations for various investigated waveguide structures with the multilayer core and symmetric or asymmetric cladding have been analyzed. In the case of symmetric metallic cladding we have found both TE and TM modes within the light cone, while TM modes only exist outside the light cone. Both symmetric and asymmetric dielectric claddings support modes outside the light cone and of TM-polarization only. Formation of photonic bands and gaps, the structure of their edge lines, and the behavior of modes that cross the edge lines has been investigated. In the subwavelength regime, we have found ordinary surface plasmon polariton dispersion in the forbidden gap that is created via coalescence of the two modes that cross the neighboring band-gap edges. One of those modes can exhibit negative group velocity.

Nanomembrane-based plasmonics

This paper reviews the main properties and applications of nanomembrane-based plasmonic structures, including some results presented here for the first time. Artificial nanomembranes are a novel building block in micro- and nanosystems technologies. They represent quasi-two-dimensional (2D) freestanding structures thinner than 100 nm and with giant aspect ratios that often exceed 1,000,000. They may be fabricated as various quasi-2D metal-dielectric nanocomposites with tailorable properties; they are fully symmetric in an electromagnetic sense and support long-range surface plasmon polaritons. This makes nanomembranes a convenient platform for different plasmonic structures such as subwavelength plasmonic crystals and metamaterials and applications such as plasmon waveguides and ultrasensitive bio/chemical sensors. Among other advantages of nanomembrane plasmonics is the feasibility to fabricate flexible, transferable plasmonic guides applicable to different substrates and dynamically tunable through stretching. There are various approaches to multifunctionalization of nanomembranes for plasmonics, including the use of transparent conductive oxide nanoparticles, but also the incorporation of switchable ion channels. Since the natural counterpart of the artificial nanomembranes are cell membranes, the multifunctionalization of synthetic nanomembranes ensures the introduction of bionic principles into plasmonics, at the same time extending the toolbox of the available nanostructures, materials and functions.

Uniaxial epsilon-near-zero metamaterials: from superlensing to double refraction

Abstract. We investigated optical properties of nanostructured metal-dielectric multilayered lattices under the conditions of epsilon-near-zero (ENZ), a concept derived from the effective-medium approach (EMA). We theoretically found that the periodic array of metallic nanolayers may exhibit either superlensing driven by broadband canalization from point emitters or single-polarization double refraction, and conventional positive as well as negative, even at subwavelength regimes. For the latter case, we formulated a modified EMA, and subsequently a generalized refraction law, that describes both refractive behaviors concurrently. The modal coupling of plasmonic lattice resonances, and nonlocality induced by partial screening across the nanolayer length, are responsible for these distinct effects. Numerical simulations show that deep-subwavelength lensing along the optical axis of Ag-GaAs metamaterial is clearly enhanced at optical wavelengths. On the other hand, transverse-magnetic-polarized radiation that is obliquely incident on the ENZ periodic nanostructures with the same materials in the infrared (around 1.55  μm) undergoes double refraction neighboring 50/50 beamsplitting.

Substantial enlargement of angular existence range for Dyakonov-like surface waves at semi-infinite metal-dielectric superlattice

Abstract. We investigated surface waves guided by the boundary of a semi-infinite layered metal-dielectric nanostructure cut normally to the layers and a semi-infinite dielectric material. Using the Floquet-Bloch formalism, we found that Dyakonov-like surface waves with hybrid polarization can propagate in dramatically enhanced angular range compared to conventional birefringent materials. Our numerical simulations for an Ag-GaAs stack in contact with glass show a low to moderate influence of losses.