A systematic investigation of plasmonics in the non-classical regime with two-dimensional materials

This proposal aims to tackle important issues in a research field known as 'plasmonics', based on the optics of tiny 'lumps' of metal. These have great potential for many applications relevant to our daily lives, with the promise of significant impact on related sciences, industry (and therefore the economy) and the well-being of our society.

Optics is one of the most ancient sciences, but also one of the most important today. The science of optics has been 'shrinking'. Historically, significant optical phenomena were only noticeable at the macroscale length, which are normally larger than one millimetre (so are distinguishable to the human eye). Today many optoelectronic devices have basic elements at the microscale length (1 micrometre is one thousandth of a millimetre), such as the light-emitting elements in most TV screens, computers and mobile phone displays and the core diameter of optical fibres. Scientists are now studying optics at even smaller lengths, the nanometre scale (1 nanometre is one billionth of a metre. Compared with a metre, a nanometre being the head of a pencil compared with the Earth), termed nano-optics. This proposal aims to tackle some key issues in one very important branch of nano-optics, namely plasmonics, which studies the optical properties of tiny metal 'lumps' approximately a few tens to hundreds of nanometres in diameter (so-called nanoparticles). Metal nanoparticles can exhibit a range of extraordinary optical properties not seen in the bulk material. For example, when two metal nanoparticles are placed very close to each other, leaving only a tiny gap (a few nanometres or less), which can be hundreds of times smaller than the smallest focal spots achievable with a state-of-the-art modern optical microscope. Furthermore, the power of the light inside the nano-gap can be millions of times stronger than that of the incident light. Such an extreme concentration and enhancement of light is at the heart of the research field of plasmonics, central to many very important applications in a wide range of aspects that will have an impact on society, such as energy generation, imaging, data storage, computing, sensing, health, security and defence, to name a few.

However, some key issues concerning how light interacts with metals at such close distances remain unsolved. Classical theories that have been so successful in explaining almost every optical phenomenon in our daily life, have encountered difficulty when trying to properly explain the optics inside nanometre and sub-nanometre scale junctions. At such close distances, quantum mechanical effects and some other peculiar effects called "nonlocality" play significant roles. Although some important new theories have been developed recently, there is an urgent need to validate them by rigorous experiment.

This proposal aims to provide a systematic investigation of how light behaves when squeezed between extremely small gaps. We will create robust junctions that are stable even at distances smaller than one nanometre, through a range of advanced technologies, including using atomically-thin two-dimensional materials, such as graphene. We will even create a tuneable gap at the sub-nanometre-scale distance, an extremely challenging task (imagine that the gap is only a few atoms wide!) but significantly important. This will provide an unprecedented detailed investigation to crack the secret of how light behaves at the deepest level of a nano-world. The proposed research will lead to the development of a number of technologies that are hugely important to our society, such as developing ultrasensitive molecular sensors (which can be used for the detection of tiny amounts of substances, even single molecules, e.g., chemical and biological contaminants in food, viruses in blood for early disease diagnosis, drugs, and explosives etc) and novel optoelectronic industry.

This proposal combines research excellence with high impact for a wide range of beneficiaries across society. Apart from significant academic beneficiaries across a broad variety of disciplines (described in Academic Beneficiaries), this proposal also contains important economic and societal impacts in many public and private sectors.

An important application of the research proposed here is to develop ultra-sensitive molecular sensors based on the extraordinary capabilities of the nanostructure devices to be developed in this proposal, which can massively enhance the optical signals of molecules. More importantly, the devices to be developed will be robust and reproducible (key features that are essential in real applications, but have not been forthcoming in previous studies), for detecting tiny amounts of substances (even single molecules) such as drugs, explosives, pesticides, pollutants, hazard gases, and viruses in blood. Therefore a wide range of public and private sectors will benefit from this research, including the security and health services, the food industry from the field to the table, environment agencies, and mining industries (hazardous gas detection). Specific products can even be developed for household usage, such as convenient devices to detect food contaminants, flu-virus and immune response detection. As such, this research will contribute significantly to improving wellbeing, in terms of health, security, and contributing to a healthier and better environment.

A range of industries will benefit from the proposed research. The plasmonic devices to be developed will foster the development of a range of novel optoelectronic devices based on two-dimensional materials. Two-dimensional materials (a class of extremely thin 'paper' materials, which only have a thickness of one or a few atoms, such as single layer graphene, graphene oxide, and molybdenum disulfide) are 'wonder' materials, which have great potential to develop novel photonic devices. However, due to their atomically-thinness, their optical response is usually low. The plasmonic systems to be developed are ideal platforms to enhance the light-matter interaction in two-dimensional materials (as they provide extremely strong optical fields), which will open up opportunities for developing a range of optoelectronic devices, such as broadband and ultrafast photodetectors, optical modulators, mode-locking lasers and even solar cells. Another important aspect of this research is to develop graphene-based nanometre-scale conductive wires and arrays of nano-dots (which will be orders smaller than the pit size of blue-ray discs) which can be incorporated into future electronic devices (smaller wires will allow us to integrate more transistors to build more powerful circuitry) and data storage devices, such as mobile phones, digital cameras, and LCD displays and computer hard drives, which form substantial portions of the global digital economy. It is obvious that this project has significant commercial potential, which will contribute to the competiveness of the UK economy.

This project also involves the training and development of a number of young researchers, who will later take their acquired knowledge and skill to related sectors and build their careers in academia or industry. The impact to the general public will be demonstrated through actively taking part in events such as Science Festival and University Open Days. These events will raise awareness of the importance of nanoscience to the general public, and promote knowledge and understanding of modern science and its impacts on society and individual lives. These, in turn, will exert a positive influence on long term policy towards science and technology. A young generation will be inspired and attracted to the important fields of nanoscience & nanotechnology that are part of the foundation for the long-term competitiveness of UK.