School of Physics - Research Publications

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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.
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    Concealed Structural Colors Uncovered by Light Scattering
    Akinoglu, EM ; Song, J ; Kinnear, C ; Xue, Y ; Zhang, H ; Roberts, A ; Koehler, J ; Mulvaney, P (WILEY-V C H VERLAG GMBH, 2020-10-14)
    Unusual structural colors are demonstrated in thin-film coatings due to a combination of optical interference and light scattering effects. These vivid colors are concealed under ambient illumination but can be observed when light is reflected from the film surface. The origin of the effect is explored computationally and it is shown that, in thin-films of lossless dielectrics coated on near-perfect conductors, incident electromagnetic waves form standing waves. Electric field intensities at the thin film interfaces are maximized for wavelengths that fulfil destructive interference conditions, while nanoscale roughness can enhance scattering at these boundaries. The interplay of these two factors yields vivid, thickness-dependent colors. This approach increases the repertoire of optical effects and perceived colors in thin coatings. When combined with traditional thin-film interference colours, dichromatic images with distinctly changing colors can be generated, which can function as a covert, optical security feature.
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    Direct Assembly of Large Area Nanoparticle Arrays
    Mulvaney, P ; ZHANG, H ; KINNEAR, C ; Cadusch, J ; JAMES, T ; ROBERTS, ANN ( 2018-07-13)
    We describe the fabrication of large area arrays of single nanoparticles using electrophoretic deposition.
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    On-chip optical image processing with plasmonic metasurfaces
    Wesemann, L ; Davis, T ; Roberts, A (IEEE, 2020-01-01)
    We propose the utilization of plasmonic resonant wave-guide gratings for ultracompact image processing in transmission. We experimentally demonstrate two-dimensional edge-detection in amplitude images and contrast enhancement of images of biological samples.
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    Optical Metasurfaces for Processing of Amplitude and Phase Images
    Wesemann, L ; Rickett, J ; Song, J ; Davis, TJ ; Roberts, A (Optica Publishing Group, 2021)
    We investigate the utilization of plasmonic resonant waveguide gratings for all-optical image processing in transmission. We experimentally demonstrate edge-detection in amplitude- as well as phase images and contrast enhancement of images of biological samples.