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Metasurfaces have recently emerged as an ultra-compact solution to perform all-optical image processing, including phase contrast imaging. Most metasurfaces used in imaging processing applications operate over a restricted numerical aperture. This limitation imposes constraints on the discernible features that can be effectively visualized and consequently leads to the appearance of undesirable artifacts. Engineering a metasurface that exhibits an asymmetric linear optical transfer function over a relatively large numerical aperture, while maintaining a strong contrast, has proven to be a challenge. In this study, we present a novel approach to designing relatively high numerical aperture and contrast nonlocal metasurfaces (up to a numerical aperture of around 0.5 and an intensity contrast of approximately 50%) with unit cells consisting of several plasmonic nanorods through the use of a rapid, quasi-analytic mode-matching technique, coupled with an optimization algorithm. The combination of these methods facilitates the rapid conceptualization of nonintuitive arrangements of metallic nanoparticles, specifically tailored to perform phase contrast imaging. These designs hold substantial promise in the development of ultra-compact imaging systems.

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.

Meta-optical devices have emerged as promising candidates for all-optical image processing. These devices are of subwavelength size and have the potential to address limitations of current image processing methods including processing speed, energy requirements as well as form factor. We present experimental results demonstrating the use of thin-film absorbers and optical metasurfaces to real-time detection of edges in images and the visualisation of phase objects including human cancer cells. Furthermore, we discuss progress towards the use of meta-optics for ultra-compact wavefront recovery. The findings to be presented have potential for applications in biological live-cell imaging, ultra-compact medical diagnostic tools, and wavefront correction methods.

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.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by the electron.

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.

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.

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.

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.

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.

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.

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.

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.

Spatial frequency filtering is a fundamental enabler of information processing methods in biological and technical imaging. Most filtering methods, however, require either bulky and expensive optical equipment or some degree of computational processing. Here, we experimentally demonstrate real-time, on-chip, all-optical spatial frequency filtering using a thin-film perfect absorber structure. We experimentally demonstrate edge enhancement of an amplitude image and conversion of phase gradients to intensity modulation in an image. The device is used to demonstrate enhancement of an image of pond algae.

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.

Near-complete absorption of light has the potential to underpin advances in photodetection, advanced chemistry, coloration of materials, and energy. This review paper reports recent progress on the development of metasurfaces and thin film structures that produce strong absorption bands in the visible and longer wavelength regions of the electromagnetic spectrum, due in part to the excitation of plasmonic resonances. Proof-of-concept demonstrations are discussed for applications of these in chemical sensing, the generation of structural color, the creation of optoelectronic devices, and photocatalysis. Emerging future applications are also discussed.

Meso-scale plasmons are supported by structures with dimensions on the order of tens of plasmon wavelengths. Metal structures at this length-scale are promising for the design and engineering of structures to direct the flow of optical energy and generate high intensity, localized electric fields. The near-field optical properties of mesoscale crystalline gold plates were examined using near-field scanning optical microscopy with a focus on the effects of modifying morphology and excitation conditions. Excitation of surface plasmon polaritons (SPPs) at plate edges and their subsequent propagation and interference as radial waves across the surface results in nodes of enhancement of the near-field on the plate surface at specific positions within the plate. The spatial position of the near-field enhancement may be directed by controlling either, or both, the boundary conditions (plate shape) and polarization of the excitation light.

Here we present the results of an investigation of resonances of azimuthal trimer arrangements of rectangular slots in a gold film on a glass substrate using cathodoluminescence (CL) as a probe. The variation in the CL signal collected from specific locations on the sample as a function of wavelength and the spatial dependence of emission into different wavelength bands provides considerable insight into the resonant modes, particularly sub-radiant modes, of these apertures. By comparing our experimental results with electromagnetic simulations we are able to identify a Fabry-Pérot mode of these cavities as well as resonances associated with the excitation of surface plasmon polaritons on the air-gold boundary. We obtain evidence for the excitation of dark (also known as sub-radiant) modes of apertures and aperture ensembles.

The interaction of a focused beam with a metasurface supporting dark modes is investigated. We show computationally and experimentally that the excitation of dark modes is accompanied by characteristic changes in the reflected Fourier spectrum. This spatial frequency filtering capability indicates an avenue for the all-optical, on-chip detection of phase gradients for biological and other imaging techniques.

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.

Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin–orbit coupling of the heavy metal atom results in an excited state with significant magnetic-dipole character. The nanostructured architecture supports two distinct optical modes and their assignment achieved with the assistance of numerical simulations. The simulations show that one mode is characterized by strong confinement of the electric field and the other by strong confinement of the magnetic field. These modes elicit drastic changes in the emitter’s photophysical properties, including dominant nanocavity-derived modes observable in the emission spectra along with significant increases in emission intensity and the total decay rate. A combination of simulations and momentum-resolved spectroscopy helps explain the mechanism of the different interactions of each optical mode supported by the metamaterial with the excited state of the emitter.

Dark‐field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction, analogous to true chiral scatterers. Single oligomers, consisting rotationally symmetric arrangements of gold nanorods, are experimentally observed to exhibit circular dichrosim at their maximum scattering with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark‐field normally incident on a glass‐air interface. Owing to this geometric origin, the wavelength of the peak chiral response is experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. This realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark‐field illumination.

Surface plasmons excited by light in metal structures provide a means for manipulating optical energy at the nanoscale. Plasmons are associated with the collective oscillations of conduction electrons in metals and play a role intermediate between photonics and electronics. As such, plasmonic devices have been created that mimic photonic waveguides as well as electrical circuits operating at optical frequencies. We review the plasmon technologies and circuits proposed, modeled, and demonstrated over the past decade that have potential applications in optical computing and optical information processing.

We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes.

Hot charge carrier extraction from metallic nanostructures is a very promising approach for applications in photocatalysis, photovoltaics, and photodetection. One limitation is that many metallic nanostructures support a single plasmon resonance thus restricting the light-to-charge-carrier activity to a spectral band. Here we demonstrate that a monolayer of plasmonic nanoparticles can be assembled on a multistack layered configuration to achieve broadband, near-unit light absorption, which is spatially localized on the nanoparticle layer. We show that this enhanced light absorbance leads to ∼40-fold increases in the photon-to-electron conversion efficiency by the plasmonic nanostructures. We developed a model that successfully captures the essential physics of the plasmonic hot electron charge generation and separation in these structures. This model also allowed us to establish that efficient hot carrier extraction is limited to spectral regions where (i) the photons have energies higher than the Schottky junctions and (ii) the absorption of light is localized on the metal nanoparticles.

Reversible exchange of photons between a material and an optical cavity can lead to the formation of hybrid light-matter states where material properties such as the work function [ Hutchison et al. Adv. Mater. 2013 , 25 , 2481 - 2485 ], chemical reactivity [ Hutchison et al. Angew. Chem., Int. Ed. 2012 , 51 , 1592 - 1596 ], ultrafast energy relaxation [ Salomon et al. Angew. Chem., Int. Ed. 2009 , 48 , 8748 - 8751 ; Gomez et al. J. Phys. Chem. B 2013 , 117 , 4340 - 4346 ], and electrical conductivity [ Orgiu et al. Nat. Mater. 2015 , 14 , 1123 - 1129 ] of matter differ significantly to those of the same material in the absence of strong interactions with the electromagnetic fields. Here we show that strong light-matter coupling between confined photons on a semiconductor waveguide and localized plasmon resonances on metal nanowires modifies the efficiency of the photoinduced charge-transfer rate of plasmonic derived (hot) electrons into accepting states in the semiconductor material. Ultrafast spectroscopy measurements reveal a strong correlation between the amplitude of the transient signals, attributed to electrons residing in the semiconductor and the hybridization of waveguide and plasmon excitations.

We show how a combination of near- and far-field coupling of the localised surface plasmon resonances in aluminium nanoparticles deposited on TiO2 films greatly enhances the visible light photocatalytic activity of the semiconductor material. We demonstrate two orders of magnitude enhancement in the rate of decomposition of methylene blue under visible light illumination when the surface of TiO2 films is decorated with gratings of Al nanoparticle dimers.

Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.