School of Physics - Research Publications

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    Modelling structural colour from helicoidal multi-layer thin films with natural disorder
    Davis, TJ ; Ospina-rozo, L ; Stuart-Fox, D ; Roberts, A (Optica Publishing Group, 2023-10-23)
    A coupled mode theory based on Takagi-Taupin equations describing electromagnetic scattering from distorted periodic arrays is applied to the problem of light scattering from beetles. We extend the method to include perturbations in the permittivity tensor to helicoidal arrays seen in many species of scarab beetle and optically anisotropic layered materials more generally. This extension permits analysis of typical dislocations arising from the biological assembly process and the presence of other structures in the elytra. We show that by extracting structural information from transmission electron microscopy data, including characteristic disorder parameters, good agreement with spectral specular and non-specular reflectance measurements is obtained.
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    Thin film notch filters as platforms for biological image processing
    Sulejman, SB ; Priscilla, N ; Wesemann, L ; Lee, WSL ; Lou, J ; Hinde, E ; Davis, TJ ; Roberts, A (NATURE PORTFOLIO, 2023-03-18)
    Many image processing operations involve the modification of the spatial frequency content of images. Here we demonstrate object-plane spatial frequency filtering utilizing the angular sensitivity of a commercial spectral bandstop filter. This approach to all-optical image processing is shown to generate real-time pseudo-3D images of transparent biological and other samples, such as human cervical cancer cells. This work demonstrates the potential of non-local, non-interferometric approaches to image processing for uses in label-free biological cell imaging and dynamical monitoring.
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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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    On-chip optical image processing with plasmonic metasurfaces
    Wesemann, L ; Davis, T ; Roberts, A (IEEE, 2020)
    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.
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    Meta-optical and thin film devices for all-optical information processing
    Wesemann, L ; Davis, TJ ; Roberts, A (American Institute of Physics, 2021-09-01)
    All-optical spatial frequency filtering has a long history with many applications now commonly replaced with digital alternatives. Although optical approaches are attractive in that they minimize energy requirements and images can be manipulated in real time, they are relatively bulky compared to the compact electronic devices that are now ubiquitous. With emerging interest in nanophotonic approaches to all-optical information processing, these approaches to enhancing images and performing phase visualization are attracting significant interest. Metasurfaces have been demonstrated as tailored alternatives to conventional spatial filters, but utilizing the spatial frequency sensitivity of these and thin film devices also has the potential to form the basis for ultracompact approaches to image processing. There are, however, significant challenges remaining to realize this promise. This review summarizes the current status of research in this rapidly growing field, places it in the context of the history of all-optical spatial filtering, and assesses prospects for future directions.
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    Nanophotonics enhanced coverslip for phase imaging in biology
    Wesemann, L ; Rickett, J ; Song, J ; Lou, J ; Hinde, E ; Davis, TJ ; Roberts, A (SPRINGERNATURE, 2021-05-08)
    The ability to visualise transparent objects such as live cells is central to understanding biological processes. Here we experimentally demonstrate a novel nanostructured coverslip that converts phase information to high-contrast intensity images. This compact device enables real-time, all-optical generation of pseudo three-dimensional images of phase objects on transmission. We show that by placing unstained human cancer cells on the device, the internal structure within the cells can be clearly seen. Our research demonstrates the significant potential of nanophotonic devices for integration into compact imaging and medical diagnostic devices. The nanophotonics enhanced coverslip (NEC) enables ultra-compact phase imaging of samples placed directly on top of the device. Visualisation of artificial phase objects and unstained biological cells is demonstrated.
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    Plasmonic Metasurfaces for Optical Information Processing
    Wesemann, L ; Panchenko, E ; Singh, K ; Gomez, DE ; Davis, TJ ; Roberts, A ; Simpson, MC ; Juodkazis, S (SPIE-INT SOC OPTICAL ENGINEERING, 2019-01-01)
    Optical spatial frequency filtering is a key method for information processing in biological and technical imaging. While conventional approaches rely on bulky components to access and filter the Fourier plane content of a wavefield, nanophotonic approaches for spatial frequency filtering have recently gained attention. Here computational and experimental progress towards the design and demonstration of metasurfaces with spatial frequency filtering capability for optical image processing will be presented. Using the example of a metasurface consisting of radial rod trimers we demonstrate its potential to perform edge enhancement in an amplitude image and conversion of phase gradients in a wavefield into intensity modulations. The presented results indicate a potential avenue for ultra-compact image processing devices with applications in biological live-cell imaging.
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    Tuning the asymmetric response of metasurfaces for optical spatial filtering
    Davis, TJ ; Eftekhari, F ; Gomez, DE ; Roberts, A ; Mitchell, A ; RubinszteinDunlop, H (SPIE-INT SOC OPTICAL ENGINEERING, 2019-01-01)
    The spatial filtering of optical signals has been demonstrated previously with metasurface thin-films created from arrays of structured optical elements. We consider the problem of changing the symmetry of their response with changes to the in-plane wavevector kI→-kI and show it can be tailored or even dynamically tuned. Our work is based on a general theory of metasurfaces constructed from non-diffracting arrays of coupled metal particles. We present the optical transfer function of such a metasurface, identify the physical properties essential for asymmetry and demonstrate its behaviour experimentally.
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    Ultracompact Camera Pixel with Integrated Plasmonic Color Filters
    Panchenko, E ; Wesemann, L ; Gomez, DE ; James, TD ; Davis, T ; Roberts, A (WILEY-V C H VERLAG GMBH, 2019-09-17)
    Photodetector size imposes a fundamental limit on the amount of information that can be recorded by an image sensor. Compact, high-resolution sensors are generally preferred for portable electronic devices such as mobile phones and digital cameras, and as a result, a significant effort has been invested in improving the image quality provided by small-area image sensors. Reducing photodetector size, however, still faces challenges in implementation requiring improvements in current technology to meet the demand for ultracompact imaging systems such as cameras. An issue with a decrease in size is associated with photodetectors utilizing color filters. In most commonly used camera designs these filters are made of dyes or pigments and incompatible with the complementary metal-oxide-semiconductor fabrication process. They are, therefore, fabricated in two different technological processes and require subsequent alignment. As the pixel size decreases, the alignment of these layers becomes challenging. Furthermore, dye-based filters need to have a thickness of the order of micrometers to ensure sufficient absorption. Here a compact, low-cost color sensor is proposed and experimentally demonstrated utilizing monolithically integrated plasmonic antennas that have a nanoscale thickness and are fabricated in the same technological process with photodetector matrix.