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© 2014 American Physical Society. We develop an electrodynamic model based on dyadic Green's functions for analyzing the near-field interactions between a dipolar scatterer (DS) and a plasmonic nanosphere (PN) under external excitation, accounting for multipolar contributions in the evaluation of the scattered fields. In particular, we include all the radiative and nonradiative field interactions between the DS and the PN, particularly the physical mechanism of DS's self-coupling through the PN, which is either neglected or approximated in previous work. Our objective is to show under which conditions self-coupling is important for strong excitation-rate enhancement of the DS and provide a description of the system's properties. We analytically investigate the conditions under which the excitation rate of a DS, such as an organic dye or a quantum dot, is enhanced when located in close proximity to a PN. We show the existence of critical conditions in terms of polarizabilities and distances that lead to large enhancement based on self-coupling and how to predict it.

We provide for the first time the detailed study of the radiation performance of an optical leaky wave antenna (OLWA) integrated into a Fabry-P\'erot resonator. We show that the radiation pattern can be expressed as the one generated by the interference of two leaky waves counter-propagating in the resonator leading to a design procedure for achieving optimized broadside radiation, i.e., normal to the waveguide axis. We thus report a realizable implementation of the OLWA made of semiconductor and dielectric regions. The theoretical modeling is supported by full-wave simulation results, which are found to be in good agreement. We aim to control the radiation intensity in the broadside direction via excess carrier generation in the semiconductor regions. We show that the presence of the resonator can provide an effective way of enhancing the radiation level modulation, which reaches values as high as 13.5 dB, paving the way for novel promising control capabilities that might allow the generation of very fast optical switches, as an example., Comment: 10 pages, 14 figures

Strong light-matter coupling has recently been demonstrated in sub-wavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, and in particular determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit network model whose elements describe energy and dissipation. Because of the subwavelength dimension of each metasurface element, we resort to the quasi-static (electrostatic) description of the near-field and hence define an equivalent capacitance associated to each dipolar element of a flat metasurface, and we show that this is also able to accurately model the phenomenology involved in strong coupling between the metasurface and the intersubband transitions in quantum wells. We show that the spectral properties and stored energy of a metasurface/quantum-well system obtained using our model are in good agreement with both full-wave simulation and experimental results. We then analyze metasurfaces made of three different resonator geometries and observe that the magnitude of the Rabi splitting increases with the resonator capacitance in agreement with our theory, providing a phenomenological explanation for the resonator shape dependence of the strong coupling process., 10 pages, 10 figures

In general, uniaxial materials exhibit ellipsoidal wave vector dispersion relations. Interestingly, under particular conditions, the material dispersion relation may turn into a hyperbola. This fact theoretically imposes no actual maximum bound on the spatial spectrum that is able to propagate within the uniaxial material leading to very interesting physical properties. Recently, there has been emphasis in the development of practical realizations of hyperbolic metamaterials (HMs) that are able to support waves with large transverse wavenumber, which would otherwise be evanescent in free space. It has been shown that homogenized models for HMs are inaccurate for very large spatial spectrum of waves because realistic HMs exhibit a large, but finite, propagating spectrum. Nonetheless, this large spatial spectrum has been shown to enhance the power emitted by impressed dipoles in proximity of the HM surface and to redistribute the radiated power mostly toward the HM (C. Guclu, et al., Phys. Rev. B., 86, 205130, 2012). These physical properties are of key importance and may lead to improvement of bandwidth and angular range of absorption when scattering is created by locating many micro or nano-scatterers at the HM surface, also supported by preliminary experimental data from other researchers. However, the analytical modeling of such structure has not yet been developed and it is of extreme importance as it would provide guidelines for future experimental developments.Therefore, in this paper we analyze the field emitted by a two-dimensional periodic array of impressed sources with a given spatial spectrum on top of a hyperbolic metamaterial. We employ Green's functions and spectral theory to model the system. We calculate the amount of power that is directed inside the HM and study its dependence onto the geometrical parameters of the array of sources, sources' spectra, and the electromagnetic properties of the HM. These results represent a significant advancement to the understanding of the problem of periodic scatterers located on top of a HM.

The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared (~10 μm) to the near-infrared (~1.5 μm). Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10−4 (λ/n)3 from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process., Interactions between material structures and the modes of cavities they are placed in can give rise to strongly coupled light-matter states. Benz et al. show that this regime can be reached using sub-wavelength metamaterial resonators coupled to semiconductor heterostructures in the mid- and near-infrared.

We analyze both theoretically and experimentally subwavelength focusing by using a planar hyperbolic metamaterial (HM) at microwave frequencies. The proposed HM consists of microstrip transmission lines (TLs) loaded by lumped components and exhibits a very flat wave vector iso-frequency dispersion diagram over a wide frequency range, and thus able to transport spectral component with large wavenumbers. This flatness is here exploited to provide subwavelength focusing with a full width half maximum (3-dB power width) of about λg/31 and λg/ 19 at 0.5 and 1 GHz, respectively, where λg is the guided wavelength in the TL microtrip grid. Numerical simulation results are in good agreement with measurement ones. Moreover, we also investigate the capability of the proposed HM to resolve sources with subwavelength distance of about λg / 6 and λg / 3 at 0.5 and 1 GHz, respectively.

We investigate magnetoinductive waves in two-dimensional periodic arrays of split ring resonators or capacitively loaded loops and characterize the modes with real and complex wavenumber excitable in such arrays. Each resonator is modeled as a single magnetic dipole, and the computation of the modal wavenumbers is performed by searching for the zeroes of the homogeneous scalar equation characterizing the field in the array. We provide original developments for the Ewald method applied to the required dyadic periodic Green's function for the array of magnetic dipoles, including the quasi-static case. The Ewald representation is analytically continued into the complex wavenumber space and also provides series with Gaussian convergence rate. In particular, we analyze and classify proper, improper, forward, backward, bound, and leaky magnetoinductive waves varying frequency and compare the fully retarded solution to the quasi-static one. We highlight the importance of accounting for field retardation effects for the prediction of the physical waves excitable in the array when the dimensions of its unit cell are approximately greater than a tenth of the free-space wavelength. The proposed method complements previous investigations and is a powerful tool for the design of waveguiding or radiating structures based on magnetoinductive waves.

We demonstrate experimentally signatures and dispersion control of surface plasmon polaritons from 1 to 1.8 µm using periodic multilayer metallo-dielectric hyperbolic metamaterials. The fabricated structures are comprised of smooth films with very low metal filling factor. The measured dispersion properties of these hyperbolic metamaterials agree well with calculations using transfer matrix, finite-difference time-domain, and effective medium approximation methods despite using only 2.5 periods. The enhancement factor in the local photonic density of states from the studied samples in the near-infrared wavelength region is determined to be 2.5-3.5. Development of this type of metamaterial is relevant to sub-wavelength imaging, spontaneous emission and thermophotovoltaic applications.

Discrete clusters of closely spaced Au nanoparticles can be utilized in devices from photovoltaics to molecular sensors because of the formation of strong local electromagnetic field enhancements when illuminated near their plasmon resonance. In this study, scalable, chemical self-organization methods are shown to produce Au nanoparticle clusters with uniform nanometer interparticle spacing. The performance of two different methods, namely electrophoresis and diffusion, for driving the attachment of Au nanoparticles using a chemical cross-linker on chemically patterned domains of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films are evaluated. Significantly, electrophoresis is found to produce similar surface coverage as diffusion in 1/6th of the processing time with an ∼2-fold increase in the number of Au nanoparticles forming clusters. Furthermore, average interparticle spacing within Au nanoparticle clusters was found to decrease from 2-7 nm for diffusion deposition to approximately 1-2 nm for electrophoresis deposition, and the latter method exhibited better uniformity with most clusters appearing to have about 1 nm spacing between nanoparticles. The advantage of such fabrication capability is supported by calculations of local electric field enhancements using electromagnetic full-wave simulations from which we can estimate surface-enhanced Raman scattering (SERS) enhancements. In particular, full-wave results show that the maximum SERS enhancement, as estimated here as the fourth power of the local electric field, increases by a factor of 100 when the gap goes from 2 to 1 nm, reaching values as large as 1010, strengthening the usage of electrophoresis versus diffusion for the development of molecular sensors. © 2013 American Chemical Society.

We investigate the power emitted by a two-dimensional array of impressed dipoles above a hyperbolic metamaterial (HM). Transmission line formalism of TM/TE spectral plane waves is used to model the scattering from the HM substrate. We show that the HM enhances the amount of emitted power which is mostly absorbed by the HM. © 2013 IEEE.

The power emitted by a dipole at the interface between an indefinite (hyperbolic) medium and an isotropic medium is analyzed. The theoretical estimation is performed by using a transmission line method for time-harmonic fields in layered isotropic materials. The indefinite medium can be constructed in different ways; here we analyze, as an example, the case of alternating two subwavelength layers of isotropic materials. We compute the power spectrum of transverse electric and magnetic (TE and TM) waves and show that the power emitted by the dipole is mainly coupled into the indefinite medium underneath, opening up several application scenarios, such as innovative carpet cloaking implementations and super absorbers. © 2012 IEEE.

We theoretically investigate harmonic generation at ε=0 crossing points where losses are compensated through the inclusion of active photonic materials in metallic nanoshells. This singularity-driven process lowers the thresholds for a plethora of nonlinear optical phenomena. © OSA 2012.

A theoretical investigation of loss-compensation capabilities in composite materials made of plasmonic nanoshells is carried out by considering quantum dots (QDs) as the nanoshells cores. The QD and metal permittivities are modeled according to published experimental data. We determine the modes with real or complex wavenumber able to propagate in a 3D periodic lattice of nanoshells. Mode analysis is also used to assess that only one propagating mode is dominant in the composite material whose optical properties can hence be described via homogenization theory. Therefore, the material effective permittivity is found by comparing different techniques: (i)the mentioned mode analysis, (ii)Maxwell Garnett mixing rule and (iii)the NicolsonRossWeir method based on transmission and reflection when considering a metamaterial of finite thickness. The three methods are in excellent agreement, because the nanoshells considered in this paper are very subwavelength, thus justifying the parameter homogenization. We show that QDs are able to provide loss-compensated ε-near-zero metamaterials and also loss-compensated metamaterials with large negative values of permittivity. Besides compensating for losses, the strong gain via QD can provide optical amplification with particular choices of the nanoshell and lattice dimensions. © 2012 IOP Publishing Ltd.

We propose the control of the radiation of an optical leaky wave antenna through optical pumping schemes. A bi-directional pumping is eventually adopted, and the tunability of the radiation is observed © 2011 IEEE.

We introduce a dual polarized near-field focusing plate (DP-NFFP) with focusing in two dimensions, designed to operate at the near infrared frequency of 193 THz (λ(0) = 1550 nm). Subwavelength focusing in two dimensions, for both incident polarizations, is achieved at a distance of a quarter wavelength from the DP-NFFP. The design procedure is described in detail and the proposed design could be easily scaled to other working frequencies, from microwave to optics. We show that the use of ideal lossless (i.e., perfect electric conductor) or real lossy (i.e., silver) metals provide with subwavelength focusing at 193 THz, indicating that metal losses do not significantly affect the DP-NFFP performance, and thus confirming the design feasibility at the near-infrared frequency. Results are validated by using two distinct full-wave simulators.

In this work we introduce a novel near-field focusing plate able to provide sub-wavelength focusing in both directions, parallel to the plate, as well as for both polarizations of the illuminating plane wave at microwave frequencies. Two similar designs are proposed: the first one is made of a thick metallic plate, whereas the second one is made of a thin copper plate fabricated on top of a Rogers 5880 dielectric substrate. The focusing behavior of these structures is analyzed with Ansys HFSS and CST Microwave Studio full-wave simulators. © 2011 IEEE.

An optical leaky wave antenna (OLWA) is a device that radiates a light wave into the surrounding space from a leaky wave (LW) guided mode or receives optical power from the surrounding space into a guided optical mode. In this work, we propose and provide a 3D analysis of a novel CMOS compatible OLWA made of a silicon nitride (Si3N4) waveguide comprising periodic silicon perturbations which allow electronic tuning capability. The analysis presented here includes the effect of the number of semiconductor perturbations, the antenna radiation pattern and directivity. We show that the number of the silicon perturbations has to be large to provide a long radiating section required to achieve radiation with high directivity. In other words, the proposed structure allows for a very narrow-beam radiation. Preliminary results are confirmed by exploiting leaky wave and antenna array factor theory, as well as verified by means of two full-wave simulators (HFSS and COMSOL). Our purpose is to ultimately use PIN junctions as building blocks for each silicon implantation for the electronic control of the radiation. In particular, the electronic tunability of the optical parameters of silicon (such as refractive index and absorption coefficient) via current injection renders itself the ideal platform for optical antennas that can facilitate electronic beam control, and boost the efficiency of optoelectronic devices such as light-emitting diodes, lasers and solar cells, and bio-chemical sensors. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

In this paper, complex modes in a linear chain of gold nanospheres are analyzed, accounting for metal losses. Dispersion diagrams are computed for travelling modes with both longitudinal and transverse (with respect to the array axis) polarization states. The procedure outlined in this work allows for the description of single mode evolution varying frequency, thus the modal dispersion diagrams are composed by the superposition of all the different modes in the one dimensional array. Each nanoparticle is modeled as an electric dipole, by adopting the single dipole approximation, and the complex zeroes of the homogeneous equation characterizing the field in the periodic structure are computed. The Ewald method is employed to analytically continue the periodic Green's function into the complex spectral domain and to achieve rapid convergence. Full characterization of the modes is provided in terms of their direction of propagation (forward/backward), their guidance and radiation properties (bound/leaky), the position of their wavenumber on the Riemann sheet (proper/improper), and also in terms of their possible physical excitation in the structure by a source in proximity of the array or a defect (physical/nonphysical modes). Understanding the modes excitable in this kind of structures is essential for possible applications in which the linear chain can be employed, from near-field enhancement to SERS, and innovative sensors. © 2011 SPIE.

Resonances of symmetric and antisymmetric polarization states in tightly coupled nanoshell particles made of either a metallic core and a dielectric shell or, vice versa, a dielectric core and a metallic shell were analyzed at optical frequencies. The investigation was performed by using the single dipole approximation (SDA) with all the dynamical retarded field terms included. Furthermore, analytic formulas for the four possible resonances were derived for the first time by retaining only the static (non-retarded) term in the dipolar field expression. The image principle was used to distinguish a priori between symmetric and antisymmetric modes and for full-wave simulations performed to confirm the identification of resonances achieved by the SDA. It was observed that the resonance frequencies of a pair of nanoshells can be tuned over a wide range of wavelength/frequencies by varying the relative dimensions of the core and shell. This makes this kind of particle pairs suited very well to be adopted either as constituents of metamaterials or to enhance local fields when operating frequencies range from the visible to the infrared spectral regions. © 2010 Society of Photo-Optical Instrumentation Engineers.