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The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge detection capability. Yet, there is still a lack of reconfigurable or programmable schemes, which may drastically enhance the impact of these devices at the system level. Here, we propose and experimentally demonstrate a reconfigurable flat optical image processor using low-loss phase-change nonlocal metasurfaces. The metasurface is configured to realize different transfer functions in spatial frequency space, when transitioning the phase-change material between its amorphous and crystalline phases. This enables edge detection and bright field imaging modes on the same device. The metasurface is compatible with a large numerical aperture of ~0.5, making it suitable for high resolution coherent optical imaging microscopy. The concept of phase-change reconfigurable nonlocal metasurfaces may enable emerging applications of artificial intelligence-assisted imaging and vision devices with switchable multitasking.

Materials of which the refractive indices can be thermally tuned or switched, such as in chalcogenide phase-change alloys, offer a promising path towards the development of active optical metasurfaces for the control of the amplitude, phase, and polarization of light. However, for phase-change metasurfaces to be able to provide viable technology for active light control, in situ electrical switching via resistive heaters integral to or embedded in the metasurface itself is highly desirable. In this context, good electrical conductors (metals) with high melting points (i.e., significantly above the melting point of commonly used phase-change alloys) are required. In addition, such metals should ideally have low plasmonic losses, so as to not degrade metasurface optical performance. This essentially limits the choice to a few noble metals, namely, gold and silver, but these tend to diffuse quite readily into phase-change materials (particularly the archetypal Ge2Sb2Te5 alloy used here), and into dielectric resonators such as Si or Ge. In this work, we introduce a novel hybrid dielectric/plasmonic metasurface architecture, where we incorporated a thin Ge2Sb2Te5 layer into the body of a cubic silicon nanoresonator lying on metallic planes that simultaneously acted as high-efficiency reflectors and resistive heaters. Through systematic studies based on changing the configuration of the bottom metal plane between high-melting-point diffusive and low-melting-point nondiffusive metals (Au and Al, respectively), we explicitly show how thermally activated diffusion can catastrophically and irreversibly degrade the optical performance of chalcogenide phase-change metasurface devices, and how such degradation can be successfully overcome at the design stage via the incorporation of ultrathin Si3N4 barrier layers between the gold plane and the hybrid Si/Ge2Sb2Te5 resonators. Our work clarifies the importance of diffusion of noble metals in thermally tunable metasurfaces and how to overcome it, thus helping phase-change-based metasurface technology move a step closer towards the realization of real-world applications.

This thesis presents the development (design, fabrication and characterisation) of two actively tunable devices for amplitude modulation of light. Chalcogenide phase-change materials, the optical properties of which can be tuned thermally through a variety of methods, are used as the active component. The large contrast in refractive index between the amorphous and crystalline states of these materials allows for substantial change in the optical response of the devices. They also have other desirable properties such as non-volatile operation, good switching endurance and the ability to switch on very small time scales.The first class of devices developed here were novel hybrid metal-dielectric metasurfaces for selective dual-band amplitude modulation at telecommunication frequencies. These devices consist of a metal back plane and an array of nano-cubic resonators consisting of a layer of phase-change material (Ge2Sb2Te5) sandwiched between two layers of silicon. Switching the phase-change material layer between the amorphous and crystalline states changes the resonant properties of the device, shifting the reflectance minimum from the O-band (1310 nm) to the C-band (1550 nm). Devices were designed and optimised using finite element simulations. The final fabricated devices performed well with extinction ratios ranging from -5.0 to -9.4 dB with low insertion losses, ranging from 0.2 to 2.3 dB.A study was conducted on the adverse effects of metal diffusion in these devices using different bottom plane configurations. The use of ultra-thin layers of Si3N4 as barrier layers was investigated as a remedy to diffusion. These barrier layers were found to effectively eliminate the issues without negatively impacting the optical performance of the devices. Further to this, thin film samples with and without barrier layers were fabricated in order to investigate the diffusion of Au into Ge2Sb2Te5. TEM cross-sectional imaging and EDS analysis were used to characterise material constituents through the thin film samples and therefore assess the extent of the diffusion present.The second class of devices were novel thin-film based amplitude-only spatial light modulators. The modulators operate in reflection and were designed to have minimal effect on the phase of the reflected light, while maximising the modulation of the amplitude of the light reflected. The device consists of a Ti bottom layer, a GeTe phase-change material middle layer, and a ITO capping layer. Test spatial crystalline patterns were successfully written to devices using a series of laser scans and the phase response of the device was measured using a purpose built off-axis digital holography interferometer. The experimental results show the modulation depth of these devices reaches 0.38 and the averaged phase difference from an amorphous to crystalline section was found to be less than pi/50, which agrees well with simulation. This design opens up a potential route to ultra-fast spatial light modulation and, when paired with phase-only spatial light modulators, could have widespread applications in wavefront shaping.

Human society is dependent on nature 1 , 2 , but whether our ecological foundations are at risk remains unknown in the absence of systematic monitoring of species’ populations 3 . Knowledge of species fluctuations is particularly inadequate in the marine realm 4 . Here we assess the population trends of 1,057 common shallow reef species from multiple phyla at 1,636 sites around Australia over the past decade. Most populations decreased over this period, including many tropical fishes, temperate invertebrates (particularly echinoderms) and southwestern Australian macroalgae, whereas coral populations remained relatively stable. Population declines typically followed heatwave years, when local water temperatures were more than 0.5 °C above temperatures in 2008. Following heatwaves 5 , 6 , species abundances generally tended to decline near warm range edges, and increase near cool range edges. More than 30% of shallow invertebrate species in cool latitudes exhibited high extinction risk, with rapidly declining populations trapped by deep ocean barriers, preventing poleward retreat as temperatures rise. Greater conservation effort is needed to safeguard temperate marine ecosystems, which are disproportionately threatened and include species with deep evolutionary roots. Fundamental among such efforts, and broader societal needs to efficiently adapt to interacting anthropogenic and natural pressures, is greatly expanded monitoring of species’ population trends 7 , 8 . A systematic census at 1,636 sites around Australia from 2008 to 2021 finds that more than 30% of shallow invertebrate species in cool latitudes exhibit a high extinction risk due to declining populations and oceanic barriers, but tropical coral species remain relatively stable.

The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge-detection capability. Yet, there is still a lack of reconfigurable or programmable schemes, which may drastically enhance the impact of these devices at the system level. Here, we propose and experimentally demonstrate a reconfigurable flat optical image processor using low-loss phase-change nonlocal metasurfaces. The metasurface is configured to realize different transfer functions in spatial frequency space, when transitioning the phase-change material between its amorphous and crystalline phases. This enables edge detection and bright-field imaging modes on the same device. The metasurface is compatible with a large numerical aperture of ~0.5, making it suitable for high resolution coherent optical imaging microscopy. The concept of phase-change reconfigurable nonlocal metasurfaces may enable emerging applications of artificial intelligence-assisted imaging and vision devices with switchable multitasking.

Apps used to be nachos and spinach dip at your local chain restaurant. Now the word \"app\" has moved into the lexicon much like the word \"Web\" did 15 years ago, describing mini software applications that enable mobile devices to become focused, functional and even fun. Given today's tough economic environment, exacerbated by continued pipeline challenges and squeezed margins, one has to wonder if investing in apps is the best use of finite company resources. Apps provide challenges for internal medical, regulatory and legal reviewers that may be using the same lens that they use to evaluate traditional promotional pieces. Apps are not like simple \"once and done\" tactics that are delivered to the customer through existing channels and then forgotten. Apps are like children that need to be fed and nurtured as they grow and change, and cannot be erased from a customer's smartphone each time a new brand manager takes the reigns.