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ItemNo Preview AvailableModelling structural colour from helicoidal multi-layer thin films with natural disorderDavis, 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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ItemThin film notch filters as platforms for biological image processingSulejman, 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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ItemReal-Time Phase Imaging with an Asymmetric Transfer Function MetasurfaceWesemann, 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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ItemMeta-optical and thin film devices for all-optical information processingWesemann, 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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ItemNanophotonics enhanced coverslip for phase imaging in biologyWesemann, 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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ItemUltracompact Camera Pixel with Integrated Plasmonic Color FiltersPanchenko, 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.
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ItemPlasmene Metasurface Absorbers: Electromagnetic Hot Spots and Hot CarriersShi, Q ; Connell, TU ; Xiao, Q ; Chesman, ASR ; Cheng, W ; Roberts, A ; Davis, TJ ; Gomez, DE (AMER CHEMICAL SOC, 2019-02-01)Light-matter interactions are extremely important, as they sustain life on Earth and can be tailored for diverse applications in areas such as solar energy conversion, chemical sensing, and information storage. One key process of these interactions is the absorption of photons. We demonstrate a novel material capable of absorbing up to 98% of incident visible light. The material comprises a thin sheet of a tightly packed two-dimensional lattice of metal nanoparticles, called plasmene, supported by a thin (subwavelength) dielectric film deposited on top of a mirror. We demonstrate how the resulting metasurface absorbers are useful in surface-enhanced spectroscopy and in the generation of plasmonic hot carriers. These structures hold great promise for applications in structural color, sensing, and photocatalysis.
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ItemLarge-Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot-Carrier ExtractionNg, C ; Zeng, P ; Lloyd, JA ; Chakraborty, D ; Roberts, A ; Smith, TA ; Bach, U ; Sader, JE ; Davis, TJ ; Gomez, DE (AMER CHEMICAL SOC, 2019-03-01)When plasmonic nanoparticles are coupled with semiconductors, highly energetic hot carriers can be extracted from the metal-semiconductor interface for various applications in light energy conversion. However, the current quantum yields for hot-electron extraction are generally low. An approach for increasing the extraction efficiency consists of maximizing the contact area between the surface of the metal nanostructure and the electron-accepting material. In this work, we developed an innovative, simple, and scalable fabrication technique that partially embeds colloidal plasmonic nanostructures within a semiconductor TiO2 layer without utilizing any complex top-down nanofabrication method. The successful embedding is confirmed by scanning electron microscopy and atomic force microscopy imaging. Using visible-pump, near-IR probe transient absorption spectroscopy, we also provide evidence that the increase in the surface contact area between the nanostructures and the electron-accepting material leads to an increase in the amount of hot-electron injection into the TiO2 layer.
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ItemMetasurfaces with Asymmetric Optical Transfer Functions for Optical Signal ProcessingDavis, TJ ; Eftekhari, F ; Gomez, DE ; Roberts, A (AMER PHYSICAL SOC, 2019-07-01)Metasurface thin films created from arrays of structured optical elements have been shown to perform spatial filtering of optical signals. To extend their usefulness it is important that the symmetry of their response with changes to the in-plane wave vector k_{p}→-k_{p} can be tailored or even dynamically tuned. In this Letter we use a general theory of metasurfaces constructed from nondiffracting arrays of coupled metal particles to derive the optical transfer function and identify the physical properties essential for asymmetry. We validate our theory experimentally showing how the asymmetric response of a two-dimensional (planar) metasurface can be optically tuned. Our results set the direction for future developments of metasurfaces for optical signal processing.
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ItemOptical image processing with metasurface dark modesRoberts, A ; Gomez, DE ; Davis, TJ (OPTICAL SOC AMER, 2018-09-01)Here we consider image processing using the optical modes of metasurfaces with an angle-dependent excitation. These spatially dispersive modes can be used to directly manipulate the spatial frequency content of an incident field, suggesting their use as ultra-compact alternatives for analog optical information processing. A general framework for describing the filtering process in terms of the optical transfer functions is provided. In the case where the relevant mode cannot be excited with a normally incident plane wave (a dark mode), high-pass filtering is obtained. We provide examples demonstrating filtering of both amplitude and pure phase objects.
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