POROUS ALUMINIUM METAMATERIALS (POAMS): A VERSATILE, SELF-ASSEMBLED PLASMONIC PLATFORM

The challenges faced by an ever expanding society must be addressed by technological progress. Issues of longstanding importance, such as health and medicine, currently require advances in both treatment and diagnostics that are inexpensive and therefore widely distributable. In order to have minor environmental impact, technological advancements must consider their impact in terms of both energy efficiency, sustainability and cost. To meet these challenges, adapt successfully and fulfil current and future requirements, novel materials designed to have unique and functional properties must be manufactured, investigated and implemented. In the recent past, due to significant research, there has been a revolution in the ability to control the structure and composition of materials at smaller and smaller dimensions. Nowhere has this been as striking as the fabrication of optical metamaterials, where bottom-up material design produces optical properties that are not found in naturally occuring materials. Attention grabbing phenomena such as optical cloaking and perfect lensing, but these demonstrations belie the huge range of possible applications.
At tiny scales, light's interaction with materials provides a wealth of interesting phenomena and despite worldwide research we are only beginning to realise the full potential. One area already delivering healthcare diagnostics to the market is plasmonics, involving the interaction of light with metal surfaces and particles, allowing it to be concentrated and manipulated at ever decreasing length scales. This project aims to explore a recently discovered type of optical metamaterial based on metallic nanoholes. The fabrication begins using thin aluminium layers which are converted into aluminium oxide and simultaneously perforated with holes by a simple electrochemical process. The holes are only a few tens of nanometres in size; the size and separation of these pores may be varied in process. By using simple techniques, a layer of thin aluminium may be left underneath the porous layer. Afterwards, it is a simple step to use the porous template as a mask and expose the system to an argon ion beam. This creates an array of holes, much smaller in size than the wavelength of light, in the underlying aluminium. The process allows broad control over the hole size and separation via the anodisation step, enabling both a new kind of metamaterial to be fabriacted for use from the deep-UV to visible spectral range. A self-assembled process, this method can easily produce large areas of incredible precision and is inexpensive.
Current research primarily uses gold or silver for metamaterials due to their attractive properties despite their expense. This project is instead based on aluminium, the most abundant metal and is suitable for many applications. In IT, overcoming the density and speed limitations facing conventional electronic circuitry requires the use of optical circuitry using optical signals. Aluminium, with excellent properties in the UV can help achieve this, as the wavelength is smaller, so will be the resulting devices, potentially helping to realise new optical circuitry to compete with electronics. Most importantly, and a key objective of this project, is determining the suitability of novel, affordable materials for the detection of chemical and biological agents. This can have huge implications for medical research by assisting diagnosis and prognosis and these materials have the potential to monitor bio-chemical and chemical reactions with high precision, which may be enhanced by UV effects present in biological and organic molecules. These examples highlight the versatility of optical metamaterials that may only minute differences in dimensions and composition.

The project aims to develop a new optical metamaterial platform based on self-assembled arrays of nanoholes in aluminium thin films based on anodisation and broad beam argon-ion milling. To my knowledge, the demonstration of this fabrication method to create either a surface plasmon polaritonic cystals or optical metamaterial design would be a first for not only the UK nano-optical and plasmonic research community but also the world. This project, comprising state-of-the-art fabrication and characterisation presents a unique opportunity to develop and train staff with a wide variety new skills and techniques.
Health, environment, energy and security are demanding areas requiring high quality research from a broad spectrum of scientific disciplines. Currently, challenges in these areas result from increasing global population and industrialisation. Against this backdrop, providing affordable healthcare is a major problem. Solutions, sought from the wider scientific community, look to generate advances in diagnosis and treatment, innovated using new techniques, skills and materials. However, developing new technology faces many constraints such as bio-compatibility, lack of specific targeting, finite knowledge of biological interactions with nanoscale objects and ultimately the interaction of these systems with light at the relevant scales. Despite this, nanotechnology, nano-optical techniques and materials are applied to medical care (~30,000 patents in the last decade) in the field of nanomedicine. Advances can improve patient prognosis by earlier diagnosis and improve treatment methods leading to prolonged life expectancy and quality. In this regard, the proposed material has the promise to revolutionise optical based bio-sensing technology for diagnosis and prognosis.
Additionally, as a future goal outside the scope of this project, developing a multi-faceted plasmonic metamaterial platform, exploiting the UV spectral region, can have a considerable impact on the development of highly integrated, nanoscale, optical and opto-electronic devices. Recently, the marriage of slower but easily integrated electronic devices with faster but much larger optical devices has been a central aspiration of plasmonics research. In this case it is anticipated that such nanoscale devices should be fast and energy efficient, easily fabricated, environmentally sustainable and ideally, inexpensive. These plasmonic systems will be investigated in the UV and infra red, where plasmonic effects can shrink components for telecoms applications. These metamaterials can enhance the non-linear optical properties of plasmonic systems, providing the foundation for the development of nanoscale lasers, modulators and detectors-essential parts of ultra-integrated optical circuitry. Demonstrating operating components would constitute a major scientific and industrial advancement. Climate change and energy security are serious issues and continuing contributions from physicists in this area is paramount. Here, advances in bottom-up material designer optical metamaterials can make significant progress. Enhancing light-matter interactions can help improve the efficiency of solar cells when hybridised with semi-conductors with which aluminium is compatible. Here, such metamaterials have the potential to improve the performance of photovoltaics.
Within this framework, sustainability and affordability are fundamental; inspiring adoption by the wider communities. Aluminium as part of a scalable metamaterial platform is an excellent solution, being affordable, abundant and capable.