Session J18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- New Fabrication Techniques

Large Area 3D Negative Index Metamaterials Formed by Printing

Negative index metamaterials (NIMS) are man-made structures with values of permittivity and permeability that are simultaneously negative over some range of frequencies. Although advanced lithographic techniques can form the necessary three dimensional (3D) nanoscale features for NIMS, such methods can be applied only over small areas (100's of $\mu $m$^{2})$ on specialized substrates, with low throughput. This talk summarizes a 3D transfer printing method that can yield 3D-NIMs with excellent optical characteristics, in ways that are scalable to arbitrarily large areas and are compatible with manufacturing. We demonstrate 3D-NIMs with 11-layers and sub-micron unit cell dimensions, over areas $>$ 75 cm$^{2}$, corresponding to $>$10$^{5}$x10$^{5}$ unit cells, all with excellent uniformity and minimal defects. These areas and numbers of unit cells both correspond to increases of more than 2x10$^{7}$ times, over previous results. Multiple cycles of printing with a single stamp demonstrate use in a manufacturing mode at throughputs that are $\sim $10$^{8}$ times higher than those possible with state-of-the-art focused-ion beam lithography systems ($\sim $2.5 s per unit cell). Optical measurements show negative index of refraction in the NIR spectral range, with values as large as Re(n) $\sim $ -7 at $\lambda $ = 2.4 $\mu $m and high figures of merit (FOM) of $\sim $8 at $\lambda $ = 1.95 $\mu $m indicating low loss operation Related approaches can be used to form similar classes of 3D-NIMS with operation in the visible regime.   

Optical metamaterials with different metals and wedge demonstration of negative refraction with fishnet metamaterials at visible wavelengths

We investigate the influence of different metals on the electromagnetic response of fishnet metamaterials in the optical regime. We found that, instead of using a Drude model, metals with a dielectric function from experimentally measured data should be applied in order to correctly predict the behavior of optical metamaterials. Through comparison of the performance for fishnet metamaterials made with different metals, i.e., gold, copper, and silver, we found silver is the best choice for the metallic parts compared to other metals, because silver allows for the strongest negative-permeability resonance and, hence, for optical fishnet metamaterials with a high figure-of-merit. We push the negative-index metamaterials to the visible regime and our improved wedge setup provides an unambiguous demonstration of negative refraction for the designed optical metamaterial.   

Subwavelength imaging of plasmonic nano-bubble cavity probed by Cathodoluminescence

Concentrating light into a deep-subwavelength volume is big challenge in conventional optics due to the diffraction limit. In recent years, plasmonics, the interaction of light and metallic nanostructures, offers new opportunities in manipulating light-matter interaction at a subwavelength scale. Because of a strong localized resonant response of the, the field can be confined in a plasmonic cavity with an ultra-small mode volume and a high Purcell factor. Plasmonic light sources at the nanoscale have been demonstrated by utilizing an active medium. However, such light sources are characterized based on either a diffraction-limited technique or a spatially-averaged lifetime measurement, neither of which show subwavelength information. Here, we present a method, cathodoluminescence (CL) that shows a subwavelength resolution image of nanoscale air bubbles trapped in between thin amorphous silicon and silver. A novel multiple-fringe pattern, with a strong dependence on the air gap width, is observed due to an enhanced luminescence. A simple model, based on an oscillating electric dipole, is applied to explain the phenomena. Both the plasmonic and conventional cavity effect of the light interacting with the novel nano-bubble system are considered. The plasmonic nano-bubbles may provide a new approach to generate localized light from a continuous thin film layer with high efficiency, for the application in ultra-compact optical device, molecular imaging, etc.   

Direct laser writing of three dimensional metal nanostructures using a femtosecond laser and various chemistries

Metal nanostructures play important role in various areas such as catalysts or in plasmonics and especially for metamaterial applications. To generate these structures, most fabrication techniques can allow mass production but are either non-controllable, suffer from high cost and low throughput or are limited in two dimensions. Direct laser writing technique resolves these problems but has been mainly used to fabricate polymeric structures. We direct laser write 3D metal structures of tunable dimensions ranging from hundreds of nanometers to micrometers. With computer-controlled translation stage and by utilizing nonlinear optical interactions between chemical precursors and femtosecond pulses, we can limit the metal-ion photo-reduction process to a focused spot smaller than that of the diffraction-limit to create metal nanostructures in a focal volume. We study the chemistry that effects the photo induced metal growth to generate desirable metal structures. By varying the types of solvent, polymer and the concentration ratios of chemicals, we demonstrate our control over the morphology of the resulting metal structures and other features such as flexibility and conductivity. We plan to create diverse metal nanostructures for a wider range of metamaterial applications.   

Self-assembly of metallic nanoparticles into macroscopic, high-density, monolayer films

A vital element of bringing pragmatic optical metamaterials to fruition is the ability to produce and characterize macroscopic, self-assembled, high-density, ensembles of nanoparticles. We have developed a method that functionalizes metallic nanoparticles with thiol-ene ligands, self-assembles the nanoparticles into high-density, monolayer, centimeter size domain films using phase separation, transports the films onto substrates using surface tension gradients, and crosslinks, via click chemistry, the nanoparticles together into a solid film. We have determined the real and imaginary parts of the phase shift for the films using a Mach-Zehnder interferometer and spectrophotometer and compare the measurements to simulations. We discuss the implications of this self-assembly process for the construction of macroscopic optical metamaterials.   

Designing Phoxonic Metamaterials with Fractal Geometry

Recently, the concepts of fractal geometry have been introduced into electromagnetic and plasmonic metamaterials. With their self-similarity, structures based on fractal geometry should exhibit multi-band character with high Q factors due to the scaling law. However, there exist few studies of \textit{phononic }metamaterials based on fractal geometry. We use COMSOL to investigate the wave propagation in two dimensional systems possessing fractal geometries. The simulations of these systems, guided by our recently developed general design framework, help to understand the role of design in determining the phononic properties of the structures. Proposed structures are being fabricated via standard lithographic or 3D printing techniques. The wave behavior of the structures can be characterized using Brillouin Light Scattering, Scanning Acoustic Microscope and Near-field Scanning Optical Microscopy. Due to their sparse spatial distribution, fractal phononic structures show potential fir ``smart skin'', where multifunctional components can be fabricated on the same platform.   

Plasmon Enhanced Transparency of a Metallic Film on Silicon

Low electrical resistivity and high optical transparency are highly desirable for thin films employed in various applications in electronics, optics, solar photovoltaics. In subwavelength scales, plasmon resonance can help electromagnetic waves to propagate through porous metallic films. In this work, we first employ a theory of effective dielectric response, and then quantitative simulations based on finite-difference-frequency-domain (FDFD) and finite-difference-time-domain (FDTD) methods, to understand and demonstrate physics of this effect. We show, that a nanoscopically perforated, yet continuous planar metallic film on silicon, can be designed to be highly transmissive in the entire visible range.   

Enhancing optical gradient forces with metamaterials

The transfer of linear momentum from electromagnetic waves to matter and the associated optical forces allow to dynamically manipulate the geometry of nanophotonic components with electromagnetic fields. In recent years, it has been proposed to use optical gradient forces for all-optical actuation of nanophotonic systems. This would open up the possibility for information processing inside nanoscale devices. Despite many efforts to increase these forces in integrated systems, they remain too small for most practical applications. In this contribution, we demonstrate how optical gradient forces can be enhanced significantly with the use of metamaterials. By implementing the techniques of transformation optics, we show how a metamaterial slab allows for the magnification of optical forces over several magnitudes, even when realistic losses are included.   

Terahertz composite right/left-handed transmission-line metamaterial surfaces

We present terahertz metamaterial waveguides based on the concept of composite right/left-handed (CRLH) transmission-lines implemented in a metal-dielectric-metal geometry. The waveguides are fabricated with spin-coated Benzocyclobutene sandwiched between a ground plane and photolithographically defined top capacitive metal pads. Angle-resolved reflection spectroscopy measurement is used to map the dispersion of this metamaterial surface, which reveals strong resonant absorptions for both right-handed and left-handed (backward wave) propagating modes within the leaky-wave bandwidth. Tuning of the waveguide dispersion is demonstrated by varying the integrated lumped element capacitive geometry. The incident polarization provides selection of different waveguide modes, exhibiting either fully right/left handed, or right-handed only propagation. Analysis based on full-wave finite element method simulations as well as lumped circuit models will be presented.   

Metamaterial Single Polarization Grid: Moving Towards the Dynamic Selective Polarizer

Ongoing interest in active metamaterial devices has increased due to their scalability, tunablity, and, more importantly, the ability to turn them on or off. A dynamic metamaterial polarizer has unique applications for identifying manmade objects anywhere in the IR. This work describes a single layer metamaterial polarization grid. The metamaterial grid is immediately scalable to many different wavelengths. The polarization grid has been designed for conversion to a dynamic metamaterial polarizer with simulated on/off ratios of 9 to 1. To this end, samples have been fabricated using varying doping concentrations of Si in GaAs grown epitaxially on Sapphire. Au metamaterials along with contact grids were then patterned and deposited. This design allows for potential dynamic responses as well as monolithic integration to create an active, metamaterial-stack, selective polarizer.   

A new theoretical model for enhanced optical transmission through thin films

We present a new theoretical approach for modeling the resonant properties and transmission through subwavelength apertures penetrating metal films. We show that standard cavity mode theory can be applied to an effective resonant cavity whose dimensions are determined by the aperture dimensions in conjunction with the evanescent decay lengths of the of diffracted waves. This method predicts the dependence on variation in both the periodicity of the holes and the film thickness over a wide range of values. It further provides a physical mechanism for the enhanced transmission observed in periodic aperture arrays.   

Enhanced second harmonic generation from aperiodic arrays of gold nanoparticles

Second harmonic generation (SHG) from planar arrays of metal nanoparticles has been investigated in the last years for several configurations of particle shape and excitation-collection directions. In particular L-shaped particles have been employed to remove centrosymmetry. In this work we study SHG dependence on the array geometry, by comparing the generation efficiency of periodic arrays with that of deterministic aperiodic ones, namely Fibonacci and Golden Angle Spiral, lacking planar centrosymmetry. Two excitation-collection configurations are employed to test fabricated arrays. The whole range of average particle separation is explored, from closed spaced particles (plasmonic regime) to far spaced particles (photonic regime). We observe that SHG efficiency can be increased by arranging centrosymmetric particles in aperiodic arrays without inversion symmetry, both in the plasmonic regime and in the photonic one. We also demonstrate that Fibonacci and Golden Angle Spiral arrays perform differently in respect of the average interparticle separation and the collection direction, allowing the tunability of SHG extraction.   

Plasmonic Coupling Effects in Large-area High-enhancement, Periodically-Arrayed Nanopillars

Periodically arrayed Si nanopillars, coated with a thin layer of Ag, have been shown to produce large-area ($\sim $1 mm or more), uniform enhancement of the electromagnetic (EM) field near the surface of such arrays, thus suggesting suitability for the development of next-generation chem/bio-sensors. Although short-range plasmonic coupling effects are expected to increase the enhancement factor of these arrays several orders of magnitude, limitations in current lithographic techniques prohibit the fabrication of closely spaced nanopillars where such coupling effects become significant. Here we show experimentally that the use of Atomic Layer Deposition of Ag allows for the fabrication of Ag-coated Si nanopillar arrays with interpillar spacings of a few nanometers ($\sim $2 nm) resulting in 1-2 orders of magnitude increase in EM enhancement observed throughout the whole array area. Experimental observations that provide insight into the nature of the different coupling phenomena contributing to the overall enhancement of the EM field in these systems will also be discussed.