School of Physics - Research Publications

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    A generalized approach to characterize optical properties of natural objects
    Ospina-Rozo, L ; Roberts, A ; Stuart-Fox, D (OXFORD UNIV PRESS, 2022-09-25)
    Abstract To understand the diversity of ways in which natural materials interact with light, it is important to consider how their reflectance changes with the angle of illumination or viewing and to consider wavelengths beyond the visible. Efforts to characterize these optical properties, however, have been hampered by heterogeneity in measurement techniques, parameters and terminology. Here, we propose a standardized set of measurements, parameters and terminology to describe the optical properties of natural objects based on spectrometry, including angle-dependent effects, such as iridescence and specularity. We select a set of existing measurements and parameters that are generalizable to any wavelength range and spectral shape, and we highlight which subsets of measures are relevant to different biological questions. As a case study, we have applied these measures to 30 species of Christmas beetles, in which we observed previously unrealized diversity in visible and near-infrared reflectance. As expected, reflection of short wavelengths was associated with high spectral purity and angle dependence. In contrast to simple, artificial structures, iridescence and specularity were not strongly correlated, highlighting the complexity and modularity of natural materials. Species did not cluster according to spectral parameters or genus, suggesting high lability of optical properties. The proposed standardization of measures and parameters will improve our understanding of biological adaptations for manipulating light by facilitating the systematic comparison of complex optical properties, such as glossy or metallic appearances and visible or near-infrared iridescence.
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    Near-Perfect Absorption of Light by Coherent Plasmon-Exciton States
    Gomez, DE ; Shi, X ; Oshikiri, T ; Roberts, A ; Misawa, H (AMER CHEMICAL SOC, 2021-05-03)
    We experimentally demonstrate and theoretically study the formation of coherent plasmon-exciton states which exhibit absorption of >90% of the incident light (at resonance) and cancellation of absorption. These coherent states result from the interaction between a material supporting an electronic excitation and a plasmonic structure capable of (near) perfect absorption of light. We illustrate the potential implications of these coherent states by measuring the charge separation attainable after photoexcitation. Our study opens the prospect for realizing devices that exploit coherent effects in applications.
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    Real-Time Phase Imaging with an Asymmetric Transfer Function Metasurface
    Wesemann, L ; Rickett, J ; Davis, TJ ; Roberts, A (AMER CHEMICAL SOC, 2022-05-18)
    The conversion of phase variations in an optical wavefield into intensity information is of fundamental importance for optical imaging including the microscopy of biological cells. Recently, meta-optical devices have demonstrated all-optical, ultracompact image processing of optical wavefields but are limited by their symmetric optical response to amplitude and phase gradients. Here, we describe a metasurface that exploits photonic spin-orbit coupling to create an asymmetric optical transfer function for real-time phase imaging. We demonstrate experimentally the effect of the asymmetry with the generation of high contrast pseudo-3D intensity images of phase variations in an optical wavefield without the need for post-processing. This non-interferometric method has potential applications in biological live cell imaging and real-time wavefront sensing.
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    A General Method for Direct Assembly of Single Nanocrystals
    Zhang, H ; Liu, Y ; Ashokan, A ; Gao, C ; Dong, Y ; Kinnear, C ; Kirkwood, N ; Zaman, S ; Maasoumi, F ; James, TD ; Widmer-Cooper, A ; Roberts, A ; Mulvaney, P (WILEY-V C H VERLAG GMBH, 2022-05-28)
    Controlled nanocrystal assembly is a pre-requisite for incorporation of these materials into solid state devices. Many assembly methods have been investigated which target precise nanocrystal positioning, high process controllability, scalability, and universality. However, most methods are unable to achieve all of these goals. Here, surface templated electrophoretic deposition (STED) is presented as a potential assembly method for a wide variety of nanocrystals. Controlled positioning and deposition of a wide range of nanocrystals into arbitrary spatial arrangements − including gold nanocrystals of different shapes and sizes, magnetic nanocrystals, fluorescent organic nanoparticles, and semiconductor quantum dots − is demonstrated. Nanoparticles with diameters <10 nm are unable to be deposited due to their low surface charge and strong Brownian motion (low Péclet number). It is shown that this limit can be circumvented by forming clusters of nanocrystals or by silica coating nanocrystals to increase their effective size.
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    Direct Assembly of Large Area Nanoparticle Arrays
    Zhang, H ; Cadusch, J ; Kinnear, C ; James, T ; Roberts, A ; Mulvaney, P (AMER CHEMICAL SOC, 2018-08-01)
    A major goal of nanotechnology is the assembly of nanoscale building blocks into functional optical, electrical, or chemical devices. Many of these applications depend on an ability to optically or electrically address single nanoparticles. However, positioning large numbers of single nanocrystals with nanometer precision on a substrate for integration into solid-state devices remains a fundamental roadblock. Here, we report fast, scalable assembly of thousands of single nanoparticles using electrophoretic deposition. We demonstrate that gold nanospheres down to 30 nm in size and gold nanorods <100 nm in length can be assembled into predefined patterns on transparent conductive substrates within a few seconds. We find that rod orientation can be preserved during deposition. As proof of high fidelity scale-up, we have created centimeter scale patterns comprising more than 1 million gold nanorods.
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    Optical Janus Effect in Large Area Multilayer Plasmonic Films
    Priscilla, N ; Smith, D ; Della Gaspera, E ; Song, J ; Wesemann, L ; James, T ; Roberts, A (Wiley, 2022)
    Plasmonic and other nanoparticles have attracted considerable interest for their role in structural coloration. The optical “Janus” effect, where the color of light reflected from a partially transmitting film depends on whether the device is viewed from the substrate or the coating side, is observed using a variety of nanostructured films. Herein, the optical Janus effect produced by homogeneous thin-film structures comprising only four layers of three different materials with a total thickness less than 300 nm is demonstrated. An asymmetric Fabry–Perot (FP) nanocavity is formed with a dielectric film bounded by two different metal films of nanoscale thickness. The semitransparent device has a transmitted color that is independent of the viewing direction. A broad color palette is available through the selection of various thicknesses and film materials. In addition to the directional optical effect, the device possesses iridescence properties and can generate images by selective removal of regions of one of the metallic films using simple photolithography. From a manufacturing perspective, this device is scalable and holds significant promise for applications in architecture, producing decorative features, and the development of overt and covert security features.
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    Algorithm-Designed Plasmonic Nanotweezers: Quantitative Comparison by Theory, Cathodoluminescence, and Nanoparticle Trapping
    Li, N ; Cadusch, J ; Liu, A ; Barlow, AJ ; Roberts, A ; Crozier, KB (WILEY-V C H VERLAG GMBH, 2021-07-09)
    Plasmonic apertures permit optical fields to be concentrated into sub-wavelength regions. This enhances the optical gradient force, enabling the precise trapping of nanomaterials such as quantum dots, proteins, and DNA molecules at modest laser powers. Double nanoholes, coaxial apertures, bowtie apertures, and other structures have been studied as plasmonic nanotweezers, with the design process generally comprising intuition followed by electromagnetic simulations with parameter sweeps. Here, instead, a computational algorithm is used to design plasmonic apertures for nanoparticle trapping. The resultant apertures have highly irregular shapes that, in combination with ring couplers also optimized by algorithm, are predicted to generate trapping forces more than an order of magnitude greater than those from the double nanohole design used as the optimization starting point. The designs are realized by fabricating precision apertures with a helium/neon ion microscope and are studied them by cathodoluminescence and optical trapping. It is shown that, at every laser intensity, the algorithm-designed apertures can trap particles more tightly than the double nanohole.
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    Vivid plasmonic color under ambient light
    SHAHIDAN, MFS ; SONG, J ; JAMES, TD ; ROBERTS, A (Optical Society of America (OSA), 2021-12-06)
    We report a novel nanoimprinted polarization-independent plasmonic pixel device utilizing different metals (Ag, Al or Au) exhibiting fade-resistant, vivid coloration under unpolarized light achieved with symmetric cross-shaped nanoantenna-hole structures. The spectral and color responses show minimal sensitivity to the polarization state of the incident light, both in reflection and transmission. The devices also have good tolerance to variations in viewing angle. Various colors are generated by simply adjusting the armlength of the cross and through choice of metal. Among all the devices, those fabricated using Ag demonstrated the best performance with 80% reflection and 12% transmission efficiencies and the production of brighter colors. With the ease of fabrication using a high-throughput NIL method, the plasmonic color devices have significant potential in sensing technology, high-resolution color printing and product-branding applications.
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    Tuning of Plasmonic Resonances in the Near Infrared Spectrum Using a Double Coaxial Aperture Array
    Sun, M ; Kavehei, O ; Beckett, P ; Robert, A ; Shieh, W ; Unnithan, RR (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018-12-01)
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    Directed Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates
    Kinnear, C ; Cadusch, J ; Zhang, H ; Lu, J ; James, TD ; Roberts, A ; Mulvaney, P (AMER CHEMICAL SOC, 2018-06-26)
    The assembly of nanoscale materials into arbitrary, organized structures remains a major challenge in nanotechnology. Herein, we report a general method for creating 2D structures by combining top-down lithography with bottom-up chemical assembly. Under optimal conditions, the assembly of gold nanoparticles was achieved in less than 30 min. Single gold nanoparticles, from 10 to 100 nm, can be placed in predetermined patterns with high fidelity, and higher-order structures can be generated consisting of dimers or trimers. It is shown that the nanoparticle arrays can be transferred to, and embedded within, polymer films. This provides a new method for the large-scale fabrication of nanoparticle arrays onto diverse substrates using wet chemistry.