Session X17: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- Improved Materials and Applications II

Low-loss multilayered metamaterial exhibiting a negative index of refraction at visible wavelengths

Over the last decade, metamaterials have attracted a great interest thanks to their potential to expand the range of electromagnetic properties found in natural materials. In particular, the possibility of achieving negative refractive index media (NIM) enables us to implement superlenses and optical storing devices. Since the first experimental demonstration at microwave frequencies, much effort has been put in extending negative refraction to the visible spectrum, where we can take full advantage of NIM properties. For instance, the superior imaging ability of NIM would be essential for visible microscopy. The desired features for NIM are low loss and isotropy. This last property includes polarization independence and negative-index behavior in all spatial directions. None of these features have been attained in previous experiments. Thus, the current challenge is to improve such aspects in order to make NIM suitable for practical applications. In this work, we experimentally demonstrate a low-loss multilayer metamaterial exhibiting a double-negative index in the visible spectrum, while presenting polarization independence at normal incidence. This has been achieved by exploiting the properties of a second-order magnetic resonance of the so-called fishnet structure, in contrast to previous works that used first-order magnetic resonances, both related to gap surface plasmon polariton (SPP) modes. The low-loss nature of the employed magnetic resonance, together with the effect of the interacting adjacent layers, results in a figure of merit as high as 3.34. A wide spectral range of negative index is achieved, covering the wavelength region between 620 and 806 nm with only two different designs. The fabricated metamaterials are the first experimental multilayer NIM in the visible spectrum, which entails an important step towards homogeneous NIM in this range. Finally, we found that the SPP modes determining the permeability resonance display weak angular dispersion.   

Photonic-Plasmonic Coupling and Near Field Engineering in Nanoparticle Necklaces

Particle clusters with different degrees of rotational symmetry consisting of circular loops of gold nanoparticles, dubbed nanoplasmonic necklaces, are proposed as a novel, reproducible platform for elastic and inelastic optical sensors with polarization insensitive behavior. Engineering of the necklaces allows for full control of the plasmonic hot-spot locations and near-field strength by coupling photonic resonances to the circular resonator structure. The polarization insensitivity of necklaces guarantees that the plasmonic hot-spots remain excited within the necklaces irrespective of the incident polarization of the excitation field, which is a significant advantage compared to hot-spots in dimer configurations. Near-fields can be further enhanced using radiative coupling in concentric necklaces having integer multiple diameters. Engineering design rules are determined for hot-spot formation, polarization insensitivity, and intensity distribution in necklaces using 3-dimensional Finite-Difference Time-Domain simulations. Plasmonic necklaces of different rotational axes were fabricated using electron-beam lithography and electron-beam deposition of gold films. Surface enhanced Raman scattering measurements were used to experimentally validate our near-field calculations.   

Probing Thermomechanics at the Nanoscale: Impulsively Excited Pseudosurface Acoustic Waves in Hypersonic Phononic Crystals

Hypersonic-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces and are of great interest because of their high sensitivity to the mechanical properties of the material in which they propagate. By modeling nanoscale thermomechanics from first principles, we can calculate a composite system's initial heat-driven response and follow its evolution in time. A spectral decomposition of the response on the calculated eigenmodes of the system allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes, expanding our understanding of surface waves scattering in mesoscale metamaterials, while providing crucial information about non-destructive photoacoustic characterization and imaging of nanostructures for nanoelectronics, nanomedicine and photovoltaic applications. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on 1D and 2D surface phononic crystals, probed using extreme ultraviolet and near-infrared light. Reference: D. Nardi et al., Nano Lett. 11, 4126 (2011).   

Plasmonic nanotips for spectroscopy with nanometer-scale resolution

We theoretically investigate the use of metallic nanotips, i.e. nanowires with a sharp tip, as tools for spectroscopic applications such as surface-enhanced Raman scattering (SERS). Nanotips can provide strong coupling between guided plasmon modes and single emitters such as atoms or molecules. At the same time, the spatial localization of the plasmon response can potentially provide nanometer-scale spatial resolution in a scanning-tip setup. In particular, we will focus on the possibilities of transporting the electromagnetic field to the target through the surface plasmon mode on the wire, and on coupling the emitted radiation into the same mode. We compare the performance of such an approach with more conventional SERS setups, where localized surface plasmons are used to enhance the local field of an incoming laser beam and the emitted free-space radiation. Finally, we will discuss whether single-molecule sensitivity can be reached.   

Structural and Optical Properties of Metals in Photonic Semiconductor Devices

Metals can play important roles in semiconductor photonics if properly incorporated and designed. The mode-coupling coefficient of the periodic waveguide in the device is the key parameter for analyzing optoelectronic performance. This work constructs a modified model to show how the metal gratings on the semiconductor surfaces and how the metal compositions inside the semiconductors can affect the coupling coefficients. Metal gratings with various materials and nano-structures can affect optical interactions at corrugated metal-semiconductor interfaces. Optical effects such as wavelength and wave polarization can affect the optical properties of metals and semiconductors. In addition, metal compositions in semiconductor compounds can change the optical properties of semiconductors. Semiconductors with different optical and structural properties can generate specific wavelengths. Consequently, the above factors related to optical properties and interactions of metals can affect the coupling coefficients. Computational results with physical interpretations provide insights into photonic devices for more applications.   

Cavity Resonances in Plasmonic Patch Nanoantennas

Plasmonic nanoantennas allow for confining and detecting photons at very small length scales. This work presents our recent experimental and theoretical studies of two dimensional periodic arrays of elliptical metal nano-patches on a silver film with a dielectric gap layer. Simulation and theoretical results shows that various cavity modes can be excited with tilted or normal incident light, and that the azimuthal symmetry breaking makes the nanoantennas polarization sensitive due to different resonant frequencies of the even and odd cavity modes. Particularly, it is shown that the cavity modes can be well described by a product of Mathieu functions, providing good agreements with both simulations and experiments. The effects of coupling between the cavity modes and the propagating plasmons will be discussed.   

Efficient calculation for waveguides and for photonic crystal band structures using improved finite elements in 2D

We employ scalar, fifth-order Hermite interpolation polynomials to solve Maxwell's equations in two dimensions in the finite element method. We analyze homogeneous conducting waveguides, inhomogeneous waveguides, and photonic crystals. The Hermite interpolation functions provide greater accuracy than vector finite elements of equal polynomial order, while bypassing the issue of spurious modes observed when using Lagrange polynomials. The scalar Hermite elements offer a level of flexibility which is not seen with vector finite elements, as in multiphysics problems such as coupled Schr\"odinger-Maxwell problems. The use of Hermite elements is an attractive alternative to plane-wave methods for modeling photonic crystals, leading to sparse matrices due to local connectivity of the finite elements, greater flexibility in modeling, and lower computational costs.   

Diffusion of a Plasmon-Exciton Polaron

We consider the motion of an exciton constrained to a quasi-one-dimensional geometry in the vicinity of a metal interface. For weak coupling, the metal causes damping of the center of mass motion, leading to a decrease in the exciton diffusion constant. This can be modeled as non-contact dielectric friction between an oscillating dipole and a substrate, where the frictional force is related to the response of the metal through the fluctuation dissipation theorem [1]. When the exciton frequency is in the neighborhood of the plasma resonance, the interaction can no longer be described by linear response theory, for the exciton and plasmon form a quasiparticle, an exciton-plasmon polaron. We calculate the transmission and reflection coefficients for the exciton-plasmon polaron in the neighborhood of a metal interface, as well as the diffusion rate and radiative lifetime versus coupling strength.\\[4pt] [1] Seppe Kuehn, John A. Marohn, and Roger F. Loring, 110(30) J. Phys. Chem. B, (2006) 1425   

Coherent interband transport in binary zigzag optical waveguide arrays

We have studied the optical oscillation and Zener tunneling of light waves in binary zigzag optical waveguide arrays in which the evanescent coupling in the array is included up to the second order. By tuning the ratio of the first order and second order coupling strengths, there is a miniband-minigap structure in the dispersion diagram. Moreover, by adding a gradient in the propagation constant transverse to propagation, Bloch-Zener oscillation (BZO) and Zener tunneling between two bands can be realized. The occurrence of BZO and Zener tunneling is simulated by the field-evolution analysis using an input Gaussian beam. Through a visual band picture, the simulation results confirm the band structure of the waveguide arrays. A rate equation is proposed to understand the coherent transport behaviors between the two minibands across the gap.   

Tunable rainbow trapped in a self-similar liquid crystal waveguide

We have investigated the optical propagation through a self-similar dielectric waveguide, where a hollow core is surrounded by a coaxial Thue-Morse multilayer. It is found that due to the self-similar furcation feature in the photonic band structure, the transmission multibands are achieved. And different frequency ranges of the transmission modes can be selectively guided and spatially confined along the waveguide. Consequently, a rainbow can be trapped in the waveguide. Then by infiltrating liquid crystal into the cladding layers, the transmission modes and rainbow trapping can be tuned by altering the temperature. And transverse electric (TE) and transverse magnetic (TM) polarizations present different propagating features. The attenuation and energy density distributions of different modes in the waveguide are also discussed. The finding can be applied to designing miniaturized compact photonic devices, such as a spectroscopy on a chip, color-sorters on a chip, and photon sorters for spectral imaging. Reference: Qing Hu, Jin-Zhu Zhao, Ru-Wen Peng, Feng Gao, Rui-Li Zhang, and Mu Wang, Appl. Phys. Lett. (2010) 96, 161101; and Qing Hu, Ru-Wen Peng, Si-Hui Wang, and Mu Wang, manuscript prepared(2011).   

A New Strategy for Designing Broadband Epsilon-Near-Zero Metamaterials

We have developed a new strategy for designing metamaterials in multi-layered film with permittivity being closed to zero over a broad frequency range, which is as known as broadband epsilon-near-zero (ENZ) materials. Milton representation, Bergman-Milton representation and electromagnetic representation of the effective permittivity ($\epsilon_{eff}$) are used, and the strategy consist of the following 3 parts: choosing the operation frequency range, properly placing the poles and zeros into the range, and solving the inverse problem by equating different representations of $\epsilon_{eff}$. Demonstration of the strategy is carried out by zeroth and first order design with several examples. The distribution of electric field inside the designed materials is investigated to reveal the physical principles of the broadband ENZ phenomenon. The study would be further extended to other geometries (e.g. multi-shell cylinder) through conformal transformation. The results obtained are useful for designing ENZ metamaterials.   

Effective permittivity of ensemble-averaged waves in dense random plasmonic composites

Random composites of metallic nanospheres (Ag, Au, Cu, etc.) in transparent media are highly opaque due to absorption resulting from plasmon resonance. A new technique for calculating the effective properties of such dense composites is described. The underlying physical motif is the separation of the space surrounding any inclusion into two regions, one immediately surrounding the particle with the properties of the matrix (the size of this region depends on the static structure factor) and an effective medium. Self consistent closure relations are found for the conditionally averaged fields by solving the vector Helmholtz equations for a layered sphere in an infinite matrix by utilizing a multipole expansion technique. For finitely large $\phi $, the effective permittivity is given by $\varepsilon _{eff} /\varepsilon _m =1+3\beta \phi +\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 4}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$4$}(\beta +4)\beta ^2\phi ^2+O(\phi ^3)$ where $\varepsilon _m $ is the permittivity of the medium and $\beta $ is the particle polarizability per unit volume. For denser systems, the particle and effective medium fields interfere to give rise to a Fano-like line shape for $Im\,(\varepsilon _{eff} )$. The resonance conditions result in a $\phi $ dependent red-shift of the resonance peak. Effects of polydispersity and multiple particle species on $\varepsilon _{eff} $ will be discussed.   

Microfabrication of highly absorbent THz metafilms tuned for specific frequencies

THz imaging has gained increased attention in recent years. The main motivation is the property of THz radiation to penetrate through most non-metalic materials and fabrics, making it attractive for medical and security applications. In addition, the radiation is non-ionizing and therefore does not present a risk for human health. However, THz imaging requires external illumination, often by quantum cascade lasers (QCLs). Metafilm absorbers with nearly 100{\%} absorption, at frequencies matched to appropriate QCLs, have been designed and fabricated using a periodic array of aluminum (Al) squares and an Al ground plane, separated by a thin SiO$_{2}$ dielectric film. All metafilms are below 2 $\mu $m and are suitable for integration with microbolometers or bimaterial sensors for THz imaging. The THz spectral characteristics of structures were probed using FTIR spectrometer. Films with different dielectric layer thicknesses exhibited resonant absorption at close to 100{\%} at respective frequencies. The measured THz reflection, from thin film of both broad-band and resonant metamaterial structures, exhibit excellent agreement with their respective finite element models.