Yaroslav A. Urzhumov

Metallic nanoparticle arrays: a common substrate for both surface-enhanced Raman scattering and surface-enhanced infrared absorption.

Nanoshell arrays have recently been found to possess ideal properties as a substrate for combining surface enhanced raman scattering (SERS) and surface enhanced infrared absorption (SEIRA) spectroscopies, with large field enhancements at the same spatial locations on the structure. For small interparticle distances, the multipolar plasmon resonances of individual nanoshells hybridize and form red-shifted bands, a relatively narrow band in the near-infrared (NIR) originating from quadrupolar nanoshell resonances enhancing SERS, and a very broadband in the mid-infrared (MIR) arising from dipolar resonances enhancing SEIRA. The large field enhancements in the MIR and at longer wavelengths are due to the lightning-rod effect and are well described with an electrostatic model.

Probing the Ultimate Limits of Plasmonic Enhancement

Boundaries on Plasmonic Excitations The localization of optical fields within a metal nanostructure can achieve strengths that are orders of magnitude greater than that of the incident field. This focusing and enhancement of the optical field maybe useful in sensing, nonlinear optics, and optical scattering applications. In probing the properties of metallic nanoparticles, Ciracì et al. (p. 1072; see the cover) show that the enhancement is limited by the electronic response of the metal, which has implications for the ultimate performance of nanophotonic systems. The nonlocal dielectric response of metals places a fundamental limit on the performance of plasmonic optical devices. Metals support surface plasmons at optical wavelengths and have the ability to localize light to subwavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. We found that the dominant limiting factor is not the resistive loss of the metal, but rather the intrinsic nonlocality of its dielectric response. A semiclassical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. To demonstrate the accuracy of this model, we studied optical scattering from gold nanoparticles spaced a few angstroms from a gold film. The bounds derived from the models and experiments impose limitations on all nanophotonic systems.

Wide-angle infrared absorber based on a negative-index plasmonic metamaterial

A metamaterial-based approach in making a wide-angle absorber of infrared radiation is described. The technique is based on an anisotropic perfectly impedance-matched negative-index material PIMNIM .I t is shown analytically that a PIMNIM that is subwavelength in all three dimensions enables absorption close to 100% for incidence angles up to 45° to the normal. A specific implementation of such frequency-tunable PIMNIM based on plasmonic metamaterials is presented. Applications to infrared imaging and coherent thermal sources are described.

Leveraging Nanoscale Plasmonic Modes to Achieve Reproducible Enhancement of Light

The strongly enhanced and localized optical fields that occur within the gaps between metallic nanostructures can be leveraged for a wide range of functionality in nanophotonic and optical metamaterial applications. Here, we introduce a means of precise control over these nanoscale gaps through the application of a molecular spacer layer that is self-assembled onto a gold film, upon which gold nanoparticles (NPs) are deposited electrostatically. Simulations using a three-dimensional finite element model and measurements from single NPs confirm that the gaps formed by this process, between the NP and the gold film, are highly reproducible transducers of surface-enhanced resonant Raman scattering. With a spacer layer of roughly 1.6 nm, all NPs exhibit a strong Raman signal that decays rapidly as the spacer layer is increased.

Metamaterial-enhanced coupling between magnetic dipoles for efficient wireless power transfer

Nonradiative coupling between conductive coils is a candidate mechanism for wireless energy transfer applications. In this paper we propose a power relay system based on a near-field metamaterial superlens and present a thorough theoretical analysis of this system. We use time-harmonic circuit formalism to describe all interactions between two coils attached to external circuits and a slab of anisotropic medium with homogeneous permittivity and permeability. The fields of the coils are found in the point-dipole approximation using Sommerfeld integrals which are reduced to standard special functions in the long-wavelength limit. We show that, even with a realistic magnetic loss tangent of order 0.1, the power transfer efficiency with the slab can be an order of magnitude greater than free-space efficiency when the load resistance exceeds a certain threshold value. We also find that the volume occupied by the metamaterial between the coils can be greatly compressed by employing magnetic permeability with a large anisotropy ratio.

Plasmonic nanoclusters: a path towards negative-index metafluids.

We introduce the concept of metafluids-liquid metamaterials based on clusters of metallic nanoparticles which we will term Artificial Plasmonic Molecules (APMs). APMs comprising four nanoparticles in a tetrahedral arrangement have isotropic electric and magnetic responses and are analyzed using the plasmon hybridization (PH) method, an electrostatic eigenvalue equation, and vectorial finite element frequency domain (FEFD) electromagnetic simulations. With the aid of group theory, we identify the resonances that provide the strongest electric and magnetic response and study them as a function of separation between spherical nanoparticles. It is demonstrated that a colloidal solution of plasmonic tetrahedral nanoclusters can act as an optical medium with very large, small, or even negative effective permittivity, epsilon(eff), and substantial effective magnetic susceptibility, Chi(eff) = mu(eff) -1, in the visible or near infrared bands. We suggest paths for increasing the magnetic response, decreasing the damping, and developing a metafluid with simultaneously negative epsilon(eff) and mu(eff).

Engineering the Electromagnetic Properties of Periodic Nanostructures Using Electrostatic Resonances

Electromagnetic properties of periodic two-dimensional subwavelength structures consisting of closely packed inclusions of materials with negative dielectric permittivity epsilon in a dielectric host with positive epsilon(h) can be engineered using the concept of multiple electrostatic resonances. Fully electromagnetic solutions of Maxwells equations reveal multiple wave propagation bands, with the wavelengths much longer than the nanostructure period. Some of these bands are described using the quasistatic theory of the effective dielectric permittivity epsilon(qs). Those bands exhibit multiple cutoffs and resonances which are found to be related to each other through a duality condition. An additional propagation band characterized by a negative magnetic permeability is found. Imaging with subwavelength resolution in that band is demonstrated.

Broadband electromagnetic cloaking with smart metamaterials

The ability to render objects invisible with a cloak that fits all objects and sizes is a long-standing goal for optical devices. Invisibility devices demonstrated so far typically comprise a rigid structure wrapped around an object to which it is fitted. Here we demonstrate smart metamaterial cloaking, wherein the metamaterial device not only transforms electromagnetic fields to make an object invisible, but also acquires its properties automatically from its own elastic deformation. The demonstrated device is a ground-plane microwave cloak composed of an elastic metamaterial with a broad operational band (10-12 GHz) and nearly lossless electromagnetic properties. The metamaterial is uniform, or perfectly periodic, in its undeformed state and acquires the necessary gradient-index profile, mimicking a quasi-conformal transformation, naturally from a boundary load. This easy-to-fabricate hybrid elasto-electromagnetic metamaterial opens the door to implementations of a variety of transformation optics devices based on quasi-conformal maps.

Magnetic metamaterial superlens for increased range wireless power transfer.

The ability to wirelessly power electrical devices is becoming of greater urgency as a component of energy conservation and sustainability efforts. Due to health and safety concerns, most wireless power transfer (WPT) schemes utilize very low frequency, quasi-static, magnetic fields; power transfer occurs via magneto-inductive (MI) coupling between conducting loops serving as transmitter and receiver. At the “long range” regime – referring to distances larger than the diameter of the largest loop – WPT efficiency in free space falls off as (1/d)6; power loss quickly approaches 100% and limits practical implementations of WPT to relatively tight distances between power source and device. A “superlens”, however, can concentrate the magnetic near fields of a source. Here, we demonstrate the impact of a magnetic metamaterial (MM) superlens on long-range near-field WPT, quantitatively confirming in simulation and measurement at 13–16 MHz the conditions under which the superlens can enhance power transfer efficiency compared to the lens-less free-space system.

Magnetic superlens-enhanced inductive coupling for wireless power transfer

We investigate numerically the use of a negative-permeability “perfect lens” for enhancing wireless power transfer between two current carrying coils. The negative permeability slab serves to focus the flux generated in the source coil to the receiver coil, thereby increasing the mutual inductive coupling between the coils. The numerical model is compared with an analytical theory that treats the coils as point dipoles separated by an infinite planar layer of magnetic material [Urzhumov et al., Phys. Rev. B 19, 8312 (2011)]. In the limit of vanishingly small radius of the coils, and large width of the metamaterial slab, the numerical simulations are in excellent agreement with the analytical model. Both the idealized analytical and realistic numerical models predict similar trends with respect to metamaterial loss and anisotropy. Applying the numerical models, we further analyze the impact of finite coil size and finite width of the slab. We find that, even for these less idealized geometries, the presence of the magnetic slab greatly enhances the coupling between the two coils, including cases where significant loss is present in the slab. We therefore conclude that the integration of a metamaterial slab into a wireless power transfer system holds promise for increasing the overall system performance.