Session D18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- What is a good conductor for Metamaterials and Plasmonics

Manipulating Plasmons Using Graphene for One-Atom-Thick Optical Signal Processing

In this talk, we provide an overview of our recent theoretical work on merging the field of graphene with metamaterials and transformation optics. In particular, we show that the control and variation of graphene's conductivity, spatially and temporally, may offer a one-atom-thick platform for manipulation of optical signals propagating along the single sheet of graphene as the surface Plasmon polariton (SPP) surface waves. Since the phase velocity of the SPP surface wave directly depends on the graphene conductivity, tailoring the conductivity can be regarded as a ``knob'' to control and shape the propagation of such SPP across the graphene. We have shown that such inhomogeneous distribution of conductivity across the graphene gives rise to the notion of one-atom-thick transformation optics and metamaterials---essentially thinnest possible metamaterials [A. Vakil, N. Engheta, \textit{Science}, 332, 1291 (2011)]. Additionally we have demonstrated, theoretically and using computer simulations, that one can achieve optical signal processing functions such as Fourier transforming across a single layer of graphene [A. Vakil, N. Engheta, ``Fourier Optics on Graphene'' \textit{ArXiv} 1108.5218 (2011)]. The graphene-based Fourier optics suggests prospects for multilayer signal processing, allowing for design of ultra-compact nanoscale signal processing systems. We are also investigating other optical manipulation, such as waveguiding, field confinement in cavities, graphene antennas, optical spectrometry, and optical mirror reflection, all on one-atom-thick structures. In this talk, we will present some of our results on these topics.   

Graphene, superconductors, and metals: What is a good conductor for metamaterials and plasmonics?

Recent developments in the field of metamaterials and plasmonics have promised a number of exciting applications, in particular at terahertz and optical frequencies. Most metamaterials consist of carefully designed metallic structures that replace atoms in their role as the basic unit of interaction with electromagnetic radiation. Unfortunately, the noble metals are not particularly good conductors at optical frequencies, resulting in significant dissipative loss in metamaterials. In this communication, we address the question of what is a good conductor for use in metamaterials and in plasmonics. We develop a model based on the quasistatic response of the metamaterial constituents to an incident electromagnetic field in order to derive a figure of merit for conductors. We find (1) it is the resistivity of the material-rather than the conductivity or permittivity-that provides direct information on the dissipative loss in the metamaterial, and (2) the dissipative loss depends on certain geometric aspects of the system, such as the layer thickness. Subsequently, we apply the model to graphene, to superconductors (Nb and YBCO), to several noble and transition metals, and to some conducting oxides (like ITO).   

Quantum Plasmonics

Surface plasmon polaritons allow confinement of light to sub-wavelength length-scales. Due to the confinement the electro-magnetic fields involved are stronger, which can be used to enhance optical interactions. We use this fact to realize a plasmonic beam-splitter based on a directional-coupler geometry, i.e. two waveguides close to each other coupled by their evanescent fields. This beam-splitter can be much smaller than conventional dielectric structures. Integrated Niobium-Nitride superconducting single-photon detectors (SSPDs) allow to probe the plasmons directly in the near-field. This makes it possible to study the structure on the quantum level using photon pairs created in a spontaneous parametric down-conversion process. Our aim is to observe Hong-Ou-Mandel interference, a true quantum effect which causes indistinguishable photons arriving at the same time at both inputs to exit through the same port, i.e. bunch. This will prove the quantum nature of surface plasmons and could be used to build sub-wavelength quantum logic gates. We also show that resonant plasmonic antennas can greatly enhance the absorption and therefore detection efficiency of SSPDs.   

Application of metasurface description for multilayered metamaterials and an alternative theory for metamaterial perfect absorber

Recently, the metamaterial perfect absorber has attracted intense interest in metamaterial community. The impedance matching mechanism based on effective bulk permittivity and permeability is widely used to explain such structures. However, this model has difficulties, in particular, because such systems are usually asymmetric, assigning homogenous effective material parameters to these systems may lead to unphysical results. In our work, we use an effective medium model that treats each layer of the metamaterial as a metasurface with unique effective surface electric and magnetic susceptibility, $\chi_{se}$ and $\chi_{sm}$ . We then use a transfer matrix method to analyze the overall EM properties of multilayered metamaterials using the effective material parameters (surface susceptibilities) of each layer. We find that the functional mechanism is the Fabry-Perot interference resulting from the multiple reflections in the cavity bounded by two metamaterial layers. This contrasts with previous explanations based on bulk effective medium theory.   

Active Core-Shell Nanowire Optical Antenna Absorbers

Core-shell dielectric-metal nanoparticles have demonstrated tunability of their absorption properties due to the size- and shape-dependence of the surface plasmon resonance. Recently, the core-shell semiconductor-insulator-metal nanowire was examined as a platform for manipulating the core emitter lifetimes due to the highly confined and intense electromagnetic fields mediated by whispering gallery surface plasmon polariton modes. Combining these two concepts we realize an active semiconductor-insulator-metal optical antenna, which demonstrates a highly tunable absorption spectrum. By directly contacting the semiconductor core, photocurrent data is coupled with simulations to show highly tunable, significant broad-band absorption enhancement; a general result for a range of material systems.   

Genetic Optimization of Optical Nanoantennas

Metal nanostructures can act as plasmonic nanoantennas (PNAs) due to their unique ability to concentrate the light over sub-wavelength spatial regions. However engineering the optimum PNA in terms of a given quality factor or objective function. We propose a novel design strategy of PNAs by coupling a genetic optimization (GA) tool to the analytical multi-particle Mie theory. The positions and radii of metallic nanosphere clusters are found by requiring maximum electric field enhancement at a given focus point. Within the optimization process we introduced several constraints in order to guarantee the physical realizability of the tailored nanostructure with electron-beam lithography (EBL). Our GA optimization results unveil the central role of the radiative coupling in the design of PNA and open up new exciting pathways in the engineering of metal nanostructures. Samples were fabricated using techniques and surface-enhancement Raman scattering measures were performed confirming the theoretical predictions.   

Plasmonic Anti-Hermitian Coupling for Nano-Manipulation of Light

Open quantum systems consisting of coupled bound and continuum states have been studied in a variety of physical systems. In these systems, the effects of the continuum decay channels are accounted for by indirect anti-Hermitian couplings among the bound states. Here we propose a general scheme to control light in a nano-plasmonic system by utilizing the anti-Hermitian coupling between the individually designed resonances of each plasmonic element in the system. As a specific example, we experimentally show a realistic coupled plasmonic dipole antenna array with $\lambda$/15 separations, in which selective excitation of an individual antenna can be achieved by tuning the frequency of the incident light. Without the anti-Hermitian coupling, these antennas are indistinguishable from each other.   

Broadband scattering reduction using a hybrid inertial metamaterial design

The ability to hide an object from an external wave through scattering reduction is one of the most sought-after goals of the metamaterials community. Using transformational optics, Pendry [1] demonstrated that a wave can be bent around an object using a conformal map that reduces the object's scattering cross section to zero. The transformational method has now been extended to transformational acoustics [2], but the traditional inertial method requires infinite mass at the boundary of the hidden object, which cannot be easily approximated in practice. Scattering reduction can also be obtained over a more limited bandwidth using wrapping layers that cancel some of the modal coupling between the object and the exterior environment [3]. Using multiple scattering theory, we demonstrate that a combination of an ``imperfect'' conformal map with a scattering cancellation layer can achieve improved scattering reduction over a broad bandwidth in an aqueous acoustic environment. Our ``hybrid'' design is amenable to a parameter-space constrained to within an order of magnitude of the background fluid in order to obtain a solution with realistic material properties. The introduction of a cancellation layer enables us to optimize performance over targeted frequency bands with only a small impact on the overall size of the design. \\[4pt] [1] J. B. Pendry, D. Schurig, and D. R. Smith, \textit{Science} \textbf{312}, 1780 (2006). \\[0pt] [2] S. A. Cummer and D. Schurig, \textit{New J. Phys.} \textbf{9}, 45 (2007). \\[0pt] [3] A. Al\`{u} and N. Engheta, \textit{Phys. Rev. Lett.} \textbf{100}, 113901 (2008).   

Surface plasmon excitations in multicoaxial metamaterial cables: Zero magnetic field

By using an elegant response function theory, which does not require matching of the messy boundary conditions, we investigate the surface plasmon excitations in the multicoaxial cylindrical cables made up of negative-index metamaterials in the absence of an applied magnetic field. The multicoaxial cables with {\em dispersive} metamaterial components exhibit rather richer (and complex) plasmon spectrum with each interface supporting two modes: one TM and the other TE for (the integer order of the Bessel function) $m \ne 0$. The cables with {\em nondispersive} metamaterial components bear a different tale: they do not support simultaneously both TM and TE modes over the whole range of propagation vector. The computed local and total density of states enable us to substantiate spatial positions of the modes in the spectrum.\footnote{M.S. Kushwaha and B. Djafari-Rouhani, J. Opt. Soc. Am B {\bf 27}, 605 (2010).} Such quasi-one dimensional systems as studied here should prove to be the milestones of the emerging optoelectronics and telecommunications systems.   

Surface Plasmon Based Engineering of Semiconductor Nanowire Optics

Emission from unthermalized (hot) excitons can be observed from high-quality crystals and quantum-well structures due to decreases in the exciton lifetimes but typically with low yields. By employing a plasmonic nanocavity, we observe efficient hot-exciton emission in core-shell CdS-SiO$_{2}$-Ag nanowires with intensities surpassing those from thermalized excitons [1]. These new spectral characteristics are mediated by whispering gallery plasmonic modes that yield highly intense electromagnetic fields. As a result, the exciton radiative lifetime is decreased by several orders of magnitude. The introduction of a high-quality hybrid plasmonic nanocavity structure significantly changes the photophysics of the host material, demonstrating an approach applicable to other material systems. \\[4pt] [1] Chang-Hee Cho, \textit{et al}, Nature Materials, \textbf{10}, 669 (2011).   

Plasmonic nanostructures for multiscale light amplification

We demonstrate experimentally a multiscale plasmonic design for giant light amplification using gold nanoparticles self-assembled in gratings on a metal mirror with thin dielectric spacer. The successive increase of the light enhancement factors upon addition of individual plasmonic elements in the design is tested by measurement of the Raman signal from R6G and benzenethiol molecules on clusters of nanoparticles, their ordered arrays on dielectric, semiconducting, and metal substrates, and on metal substrate with gratings. High fidelity of our structures as SERS substrates are confirmed by areal maps of the Raman response. FDTD numerical calculations are in a good agreement with our experimental measurements.