Conor T. Riley

ZnO/CuO heterojunction branched nanowires for photoelectrochemical hydrogen generation.

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

Multiple pathways guide oxygen diffusion into flavoenzyme active sites

Dioxygen (O2) and other gas molecules have a fundamental role in a variety of enzymatic reactions. However, it is only poorly understood which O2 uptake mechanism enzymes employ to promote efficient catalysis and how general this is. We investigated O2 diffusion pathways into monooxygenase and oxidase flavoenzymes, using an integrated computational and experimental approach. Enhanced-statistics molecular dynamics simulations reveal spontaneous protein-guided O2 diffusion from the bulk solvent to preorganized protein cavities. The predicted protein-guided diffusion paths and the importance of key cavity residues for oxygen diffusion were verified by combining site-directed mutagenesis, rapid kinetics experiments, and high-resolution X-ray structures. This study indicates that monooxygenase and oxidase flavoenzymes employ multiple funnel-shaped diffusion pathways to absorb O2 from the solvent and direct it to the reacting C4a atom of the flavin cofactor. The difference in O2 reactivity among dehydrogenases, monooxygenases, and oxidases ultimately resides in the fine modulation of the local environment embedding the reactive locus of the flavin.

High-Quality, Ultraconformal Aluminum-Doped Zinc Oxide Nanoplasmonic and Hyperbolic Metamaterials.

Aluminum-doped zinc oxide (AZO) is a tunable low-loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Due to its ultrahigh conformality and compatibility with semiconductor processing, atomic layer deposition (ALD) is a powerful tool for many plasmonic applications. However, despite many attempts, high-quality AZO with a plasma frequency below 1550 nm has not yet been realized by ALD. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The highly conformal nature of ALD is also exploited to coat silicon nanopillars to create localized surface plasmon resonances that are tunable by adjusting the aluminum concentration, thermal conditions, and the use of a ZnO buffer layer. The high-quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.

Luminescent hyperbolic metasurfaces

When engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature. Namely, a uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states. Using an unconventional multilayer architecture, we demonstrate luminescent hyperbolic metasurfaces, wherein distributed semiconducting quantum wells display extreme absorption and emission polarization anisotropy. Through normally incident micro-photoluminescence measurements, we observe absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission >0.9. We observe the modification of emission spectra and, by incorporating wavelength-scale gratings, show a controlled reduction of polarization anisotropy. We verify hyperbolic dispersion with numerical simulations that model the metasurface as a composite nanoscale structure and according to the effective medium approximation. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

Near-perfect broadband absorption from hyperbolic metamaterial nanoparticles

Significance The ability to perfectly absorb light with optically thin materials poses a significant challenge for many applications such as camouflage, light detection, and energy harvesting. Current designs require planar reflectors that crack and delaminate after heating or flexing. Moreover, they cannot be transferred to more desirable substrates for mechanically flexible and low-cost applications. Although particulate-based materials overcome these challenges, broadband absorption from standalone systems has not been demonstrated. Here, a class of materials, transferrable hyperbolic metamaterial particles (THMMP), is introduced. When closely packed, these materials show broadband, selective, omnidirectional, perfect absorption. This is demonstrated with nanotubes made on a silicon substrate that exhibit near-perfect absorption at telecommunication wavelengths even after being transferred to a mechanically flexible, visibly transparent polymer. Broadband absorbers are essential components of many light detection, energy harvesting, and camouflage schemes. Current designs are either bulky or use planar films that cause problems in cracking and delamination during flexing or heating. In addition, transferring planar materials to flexible, thin, or low-cost substrates poses a significant challenge. On the other hand, particle-based materials are highly flexible and can be transferred and assembled onto a more desirable substrate but have not shown high performance as an absorber in a standalone system. Here, we introduce a class of particle absorbers called transferable hyperbolic metamaterial particles (THMMP) that display selective, omnidirectional, tunable, broadband absorption when closely packed. This is demonstrated with vertically aligned hyperbolic nanotube (HNT) arrays composed of alternating layers of aluminum-doped zinc oxide and zinc oxide. The broadband absorption measures >87% from 1,200 nm to over 2,200 nm with a maximum absorption of 98.1% at 1,550 nm and remains large for high angles. Furthermore, we show the advantages of particle-based absorbers by transferring the HNTs to a polymer substrate that shows excellent mechanical flexibility and visible transparency while maintaining near-perfect absorption in the telecommunications region. In addition, other material systems and geometries are proposed for a wider range of applications.

Active hyperbolic metasurfaces at telecommunication frequencies

Summary form only given. Hyperbolic metasurfaces (HMS) combine the potential for chip-scale integration of optical metasurfaces with the properties of hyperbolic dispersion. In the ideal, lossless effective medium limit, HMS have an unbound optical density of states (DOS). Thelossless effective medium limit unbound DOS enables infinite mode densities in waveguides and cavities, and, in principal, an infinitely confined modal energy. The finite periodicity of real HMS, however, places an upper momentum limit on the optical DOS. Additionally, dissipation losses in real HMS limit the utility of high momentum states. To assess the possibility of reducing propagation losses in hyperbolic metamaterials, various studies have analyzed the effect of incorporating gain media. While early studies focused on using dye molecules for visible gain, more recently, indium gallium arsenide phosphide (InGaAsP) multiple quantum wells (MQW) has emerged as a viable candidate for gain at telecommunication frequencies. Herein, we report on the demonstration of active hyperbolic metasurfaces (HMS) at telecommunication frequencies. Built from nanostructured silver and InGaAsP MQW, the HMS exhibit hyperbolic dispersion across the near-infrared part of the spectrum. When driven by an external optical pump at 1064 nm, the HMS emit broadband radiation in the 1200 nm-1600nm spectral range. Extreme anisotropy of total emission with respect to the pump polarization is observed, with peak PL differing by more than an order of magnitude for orthogonal pump polarization states. Additionally, we measure degree-of-linear-polarization of the emission as high as 0.9. Experimental results are understood through analytical and numerical modeling, which illustrate the efficacy of the effective medium approximation. Finally, we report on device- and circuit-level applications of the active HMS.

Near-infrared meta-gain media based on hyperbolic metasurfaces(Conference Presentation)

We propose modification to gain spectra of semiconductor quantum heterostructures by incorporation of nanostructured metal, paving the way for tailor made “meta-gain” media. We show that the wavelength dependence of the principal direction of energy propagation in media with hyperbolic dispersion leads to blue-shifting of peak photoluminescence (PL), and thereby optical gain, relative to emission from the bare semiconductor. Additionally we show that emission spectra from metal-semiconductor hyperbolic metasurfaces depends strongly upon the polarization of an external optical pump. The simultaneous co-optimization of pump properties and optical and electronic densities of states provides a platform for not only compensating losses in metallic metamaterials, but also designing emission spectra beyond that provided by the constituent quantum heterostructures.

Conformal plasmonic and hyperbolic metamaterials(Conference Presentation)

The majority of plasmonic and metamaterials research utilizes noble metals such as gold and silver which commonly operate in the visible region. However, these materials are not well suited for many applications due to their low melting temperature and polarization response at longer wavelengths. A viable alternative is aluminum doped zinc oxide (AZO); a high melting point, low loss, visibly transparent conducting oxide which can be tuned to show strong plasmonic behavior in the near-infrared region. Due to it’s ultrahigh conformality, atomic layer deposition (ALD) is a powerful tool for the fabrication of the nanoscale features necessary for many nanoplasmonic and optical metamaterials. Despite many attempts, high quality, low loss AZO has not been achieved with carrier concentrations high enough to support plasmonic behavior at the important telecommunication wavelengths (ca. 1550 nm) by ALD. Here, we present a simple process for synthesizing high carrier concentration, thin film AZO with low losses via ALD that match the highest quality films created by all other methods. We show that this material is tunable by thermal treatment conditions, altering aluminum concentration, and changing buffer layer thickness. The use of this process is demonstrated by creating hyperbolic metamaterials with both a multilayer and embedded nanowire geometry. Hyperbolic dispersion is proven by negative refraction and numerical calculations in agreement with the effective medium approximation. This paves the way for fabricating high quality hyperbolic metamaterial coatings on high aspect ratio nanostructures that cannot be created by any other method.

Gain-enhanced hyperbolic metamaterials at telecommunication frequencies (Presentation Recording)

Using effective medium theory (EMT), Bloch’s theorem (BT), and the transfer matrix method (TMM), we analyze the possibility of gain-enhanced transmission in metamaterials with hyperbolic dispersion at telecommunication frequencies. We compare different combinations of dissipative metals and active dielectrics, including noble metals, transparent conducting oxides (TCO), III-V compounds, and solid-state dopants. We find that both indium gallium arsenide phosphide (InGaAsP) and erbium-doped silica (Er:SiO2), when combined with silver, show promise as a platform for demonstration of pump-dependent transmission. On the other hand, when these active dielectrics are combined with aluminum-doped zinc oxide (AZO), a low-loss TCO, gain-enhanced transmission is negligible. Results based on EMT are compared to the more accurate BT and TMM. When losses are ignored, quantitative agreement between these analytical techniques is observed near the center of the first Brillouin zone of a one-dimensional periodic structure. Including realistic levels of loss and gain, however, EMT predictions become overly optimistic compared to BT and TMM. We also discuss the limitations to assumptions inherent to EMT, BT, and TMM, and suggest avenues for future analysis.