Session Q17: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- Improved Materials and Applications I

Metamaterials and Plasmonics: Improved Material Building Blocks

Optical metamaterials are rationally designed and manufactured materials built of nanostructured unit cells, or ``artificial atoms'' much smaller than the wavelength of operating light. These materials can be engineered to exhibit optical properties beyond any naturally occurring materials. This field has been gaining momentum over the past several years, as it continues to provide new fascinating ideas promising a variety of exciting applications including for example super-resolution microscopes, extremely efficient light concentrators and invisibility cloaks. In most metamaterial devices, noble metals (primarily gold and silver) have been used as the constituent material to make subwavelength building blocks. But metals suffer from high optical losses that are much too large to create practical and robust metamaterials devices. A recent approach that could unlock the technological potential of plasmonics and optical metamaterials is to look for better plasmonic materials that have a negative real part of dielectric permittivity. Here we provide an overview of two classes of alternative plasmonic materials - doped semiconductors and intermetallics - that could allow realization of novel transformation optics and metamaterial devices with greatly improved performance operating at near infrared and visible frequencies.   

Bianisotropy Compensation in Metamaterials

The potential scientific and technological applications for metamaterials abound and continue to multiply at an unprecedented rate. However, with applications come non-trivial design challenges. For instance, many metamaterial designs exhibit the phenomenon of bianisotropy, the ability for an incident electric field to excite a magnetic response (and vice versa) in the metamaterial. In many applications, this bianisotropic response is considered a parasitic effect to be avoided whenever possible. Metamaterials can be designed to eliminate bianisotropy at the unit cell level, but the presence of a substrate will inevitably reintroduce bianisotropy into the system. Here, through a judicious choice of unit cell geometry, we have compensated for and removed the effects of substrate-induced bianisotropy in broadside coupled split-ring resonators on a GaAs substrate. We present numerical simulation results, parameter extraction, and experimental measurements at terahertz frequencies to validate this claim.   

Indium Tin Oxide Nanorod Building Blocks for Near-Infra-Red Filter Metamaterials

We report arrays of indium tin oxide (ITO) nanorods function as near-infra-red (NIR) filter metamaterials. We fabricated arrays of rectangular cross-section, plasmonic ITO nanords of varying pitch, width, gap, and height of the nanorods using nanoimprint lithography and chlorine based inductively coupled plasma (ICP) etching processes. The transmission spectrum of the periodic nanorod arrays may be spectrally tuned in the NIR by the geometry of the arrays and the optical response depends on the polarization of the incident light. The nanorod array behaves as an optical nanocircuit. For illumination by an E-field vector parallel to the nanorod array, the array functions as a parallel L-C circuit, acting as a bandpass filter. For illumination by an E-field vector perpendicular to the nanorod array, the array functions as a series L-C circuit, acting as a bandstop filter. We show good agreement between optical measurements of fabricated nanorod arrays and simulations using equivalent circuit theory and finite-difference time-domain (FDTD) methods. The optical properties of the ITO nanorod circuit may be further tuned by filling the gap between the nanorods using various dielectric materials.   

Measurement of Resonant Frequencies and Modes of Freestanding Nanoparticle Monolayers

We recently showed that freestanding membranes of ligated nanoparticles can be assembled in a one-step drying-mediated process [1]. These 10nm thin membranes can stretch over holes up to 100 microns in diameter and are supported by a substrate only along their outer edge, thereby freely suspending of the order of 100 million close-packed particles [2]. Previous work has focused on quasi-static mechanical properties [1-3]. Here we present the first investigation of the full dynamic response of freely suspended nanoparticle membranes, utilizing a high frequency laser interferometer with picometer sensitivity. This instrument allows us to rapidly measure the dynamical properties of freestanding nanoparticle monolayers for the first time including resonant frequencies, quality factors, and images of different modes.\\[4pt] [1] Klara E. Mueggenburg et al., ``Elastic membranes of close-packed nanoparticle arrays,'' Nature Materials 6, 656-660 (2007). \\[0pt] [2] Jinbo He et al., ``Fabrication and Mechanical properties of large-scale freestanding nanoparticle membranes,'' Small 6, 1449-1456 (2010).\\[0pt] [3] Pongsakorn Kanjanaboos et al., ``Strain Patterning and Direct Measurement of Poisson's Ratio in Nanoparticle Monolayer Sheets,'' Nano Letters 11, 2567-2571 (2011).   

Metamaterial Based Terahertz Detector

We have designed, fabricated, and characterized metamaterial enhanced bimaterial cantilever pixels for far-infrared detection. Local heating due to absorption from split ring resonators (SRRs) incorporated directly onto the cantilever pixels leads to mechanical deflection which is readily detected with visible light. Highly responsive pixels have been fabricated for detection at 95 GHz and 693 GHz, demonstrating the frequency agility of our technique, and their subwavelength nature enables their use as a focal plane array (FPA) to image near the diffraction limit. We have obtained single pixel responsivities as high as 16,500 V/W and noise equivalent powers of 10-8~W/Hz1/2~with these first-generation devices, which were achieved at room temperature and pressure. Consequently, MMs hold great promise for facilitating the development of a ``versatile'' THz detector which can a) strongly absorb THz radiation; b) operate at room temperature; c) function as a multi-pixel array for imaging applications; and d) be lightweight and low cost.   

Experimental observation of strong microwave-induced force in parallel-plate metallic cavity

It has long been known that light induced forces have an impact on matter. These forces, albeit small, have found various applications in physics, chemisty and biology. The magnitude of this induced force is directly related to the momentum carried by light. With a much longer wavelength than visible light, microwave rediation is commonly regarded to having negligible mechanical impact on macroscopic objects. Here we present the first experimental demonstration of a strong repulsive force induced by incident microwave radiation in parallel centimeter-sized metallic plates. We found that the microwave radiation induced force can be significantly stronger than the usual photon pressure exerted by the incident beam when the cavity is excited at resonance, as strong electromagnetic energy is trapped inside the cavity walls. There is good agreement between experimental measurement and calculations using a boundary element method and the Maxwell stress tensor formalism. Our effort may bring new applications of microwave manipulations in the microwave and metamaterial communities.   

Geometrical structure, multifractal spectra and localized optical modes of aperiodic Vogel spirals.

We present a numerical study of the structural properties, photonic density of states and bandedge modes of Vogel spiral arrays of dielectric cylinders in air. Specifically, we systematically investigate different types of Vogel spirals obtained by the modulation of the divergence angle parameter above and below the golden angle value ($\approx $137.507 degrees). We found that these arrays exhibit large fluctuations in the distribution of neighboring particles characterized by multifractal singularity spectra and pair correlation functions that can be tuned between amorphous and random structures. We also show that the rich structural complexity of Vogel spirals results in a multifractal photonic mode density and isotropic bandedge modes with distinctive spatial localization character. Vogel spiral structures offer the opportunity to create a novel photonic devices that leverage radially localized and isotropic bandedge modes to enhance light-matter coupling, such as optical sensors, light sources, concentrators, and broadband optical couplers.   

Geometrically Structured Dynamic Shadowing Lithography: Structural Growth and Control

Sphere lithography (SL), or nanosphere lithography (NSL), stands out as a versatile technique capable of producing 2D periodic micro- and nanostructures using colloidal crystal as deposition mask. Many of the fundamental aspects of the features produced by SL have been extensively investigated, including the optical, magnetic, electronic, and catalytic behaviors with emphasis toward applications in biosensing, ultrasensitive spectroscopy, and nanodevice fabrication. Previous work has primarily focused on 2D patterning, however, with little attention paid to vertical growth of the SL features. In this work, we will demonstrate the 3D structural evolution of metal dot arrays by SL-based geometrically structured dynamic shadowing lithography (GSDSL). The resulting structure is highly dependent on the nature of the metal that is used as evaporative source. We will specifically focus on the difference in the grain size of several typical metals and illustrate the ability in control of the structural growth through experiment and modeling. We believe that knowledge of the detailed geometry will enable us to understand further information on the physical and chemical properties of the SL substrates.   

Omni-directional active pillar arrays for emission extraction and on-chip generation of optical vortices at 1.55$\mu$m

We engineer deterministic aperiodic structures (DAS) for omni-directional light extraction, emission profile shaping and direct orbital angular momentum generation from Si-based light emitting devices operating at telecom wavelengths. Omni-directional diffraction is achieved through the use of structures with circularly symmetric Fourier space and is well suited for extracting light from devices, such as LEDs or lasers. To exploit the unique light scattering properties of these structures we have fabricated active pillar arrays of Erbium (Er) doped Silicon-Rich Nitride (SRN) using electron beam lithography (EBL) and reactive ion etching (RIE) while varying geometry to optimize extraction enhancement around 1.55$\mu$m. We find that aperiodic spiral arrays with continuous circular Fourier space give over 10 times extraction enhancement and outperform Archimedean lattices, which are the standard structures commonly utilized for omni-directional extraction. Additionally we directly image the real and reciprocal space of the emitted radiation at 1.55$\mu$m and demonstrate direct generation of optical vortices with well-defined values of orbital momentum. These results offer the opportunity to engineer novel optical devices that leverage the control of structured light on optical chips, such as novel laser sources, broadband optical couplers and concentrators.   

Synthesis and Structural Characterization of Orthorhombic Vanadium Oxide Nanorods

Nanorod structures for Li storage are of interest for rechargeable battery applications. Vanadium pentoxide is a promising battery cathode material and in this work we report on the synthesis of V$_{2}$O$_{5}$ orthorhombic single crystal and polycrystalline nanorods by the sol-gel polymerizing acryl amide method via ethylenediamine tetra acetic acid EDTA assisted hydrothermal process. In order to determine the thermodynamic stability of nanostructured polymorphs vanadates, heat treatments were performed from 450 \r{ }C to 500 $^{\circ}$C with annealing times ranging from 48 to 72 h. The morphologies and structures of the nanorods were characterized by XRD, SEM and HRTEM. Thermo Gravimetric Analysis (TGA) was employed to monitor reaction mass losses during the course of the synthesis. Nanorod diameters ranging from 50 to 150 nm were observed. The lengths and diameter of the rods depended on the conditions of the preparation, such as concentration, and reaction time.   

Nonlinear metamaterials through enhanced impact ionization in GaAs at terahertz frequencies

We report the experimental observation of a nonlinear response in metamaterial split ring resonators on semi-insulating GaAs at terahertz frequencies. Using metamaterials with narrow gaps (down to 1$\mu $m), a local in-gap THz field of 3.5 MV/cm was achieved with a field enhancement of $\sim $20 on resonance. With increasing THz electric field the metamaterial resonance is gradually quenched as the capacitive gap is shorted due to a large change in the local conductivity in the gap. This indicates an increase of the local carrier density by ten orders of magnitude resulting from impact ionization. Hybrid metamaterials with intense local electric fields not only have the potential to serve as a new tool to study nonequilibrium transport phenomena in materials, but also provide a new way to explore nonlinear metamaterials at terahertz frequencies.   

Mode nonorthogonality in nonhermitian PT-symmetric optical resonators

PT -symmetric optical resonators combine absorbing regions with active, amplifying regions. The latter are the source of radiation generated via spontaneous and stimulated emission, which embodies quantum noise and can result in lasing. We calculate the frequency-resolved output radiation intensity of such systems and relate it to a suitable measure of excess noise and mode nonorthogonality The lineshape differs depending on whether the emission lines are isolated (as for weakly amplifying, almost hermitian systems) or overlapping (as for the almost degenerate resonances in the vicinity of exceptional points associated to spontaneous PT -symmetry breaking). The calculations are carried out in the scattering input-output formalism, and are illustrated for a quasi one-dimensional resonator set-up.   

Elastic metamaterials and effective medium theory

The unusual properties of a metamaterial are induced by the resonance in its building blocks. We derived an effective medium theory for elastic metamaterials in two dimensions, which is capable of predicting the wave propagation behavior inside the metamaterial near resonances of the building-block. It reveals the connection between resonances and negativities in effective medium parameters. Based on the EMT, we design two types of elastic metamaterials consisting of different resonance structures in their building blocks that can exhibit multiple negative dispersion bands with special characteristics. One is able to produce negative shear modulus and negative mass density simultaneously over a frequency regime, and the other is super-anisotropic. All of these unusual properties are demonstrated by multiple-scattering theory or finite element simulations.