Design of plasmonic nanostructures for an enhanced control over their ultrafast nonlinear optical response.

After a decade of existence, and driven by a remarkable expansion in research and development, plasmonics -the technology that exploit the unique optical properties of metallic nanostructures to enable routing and actively manipulating light at the nanoscale- has entered a defining period in which researchers will seek to answer a critical question: can plasmonics provide a viable technological platform which includes both passive and active nanodevices?
The design of these devices is driven by a two-fold objective: 1) to manipulate electromagnetic energy at the nanoscale, including harvesting, guiding and transferring energy, with high lateral confinement down to a few tens of nanometers, and 2) to generate ultrafast (a few femtoseconds) and strong non-linear effects with low operating powers to produce basic active functions such as transistor or lasing actions. Utilizing the resonant properties -field enhancement and spectral sensitivity- of Surface Plasmons Polaritons (SPPs) is generally thought to represent a practical avenue to achieving this objective.
However, our ability to control and manipulate light at this scale dynamically -i.e. to produce active functionalities- and in real-time through low-energy external control signals is a missing link in our aim to develop a fully integrated sub-wavelength optical platform. To date, active plasmonic systems, including thermo- and electro-optic media, quantum dots, and photochromic molecules, are achieving sensitive progress in switching and modulation applications. However, high switching times (>nanosecond) or the need for relatively strong control energy (~microJ/cm^2) to observe sensible signal modulation (35% to 80%), limit the practical use of such structures as signal processing or other active opto-electronic nanodevices.
In this context, this research aims to assess the potential for defects to enhance the non-linear optical properties of hybrid plasmonic crystals. The objective is to integrate defects, made of plasmonic cavities, in plasmonic crystals to create a focal point for electromagnetic energy stored in surface plasmon waves at the crystal's interfaces. The role of the defect is then to transfer this energy to a neighbouring non-linear material in order to change its optical properties at the femtosecond timescale, thus creating an active functionality. This research, largely based on ultrafast time-resolved near-field optical microscopy, is also expected to enhance our understanding of ultrafast energy transfers at the nanoscale- a critical expertise in designing nanodevices.

Energy, health, and education are unarguably areas that need immediate attention with strong commitment in research, development, and training in order to build a sustainable society. These are the main areas this work aims to impact. This will occur directly or through selected intermediaries such as the information and technology industry. In particular, the Physics Department at King's has a strong ongoing partnership with several major players in this industry including INTEL, Seagate, Ericsson, Oxonica, and IMEC, with whom exchange and transfer of scientific and technical expertise take place on a continuous basis. How will this industry benefit from this work is a consequence of the crucial stage this industry has recently entered where the integration of optical devices (fast but bulky) and electronic devices (slow but highly integrable) would provide a giant leap in the performance of opto-electronic devices. These hybrid nanodevices are to be fast, strongly integrated, and not least, energy efficient. The research proposed here should therefore provide valuable knowledge for progress toward developing this new technology in terms of systems studied -plasmonic systems will play a pivotal role on the development of nanometer scale devices- and in terms of characterization technique that is resolved temporally, spatially, and energetically. In particular, enhancing the non-linear optical properties of plasmonic systems will provide grounds for the development of low-power consumption lasers and modulators, as well as ultra-sensitive detectors-all of which are integral components of an ultra-integrated circuitry platform. Understanding energy flow at the nanoscale will also allow us to improve the efficiency of solar cells and ultimately help design artificial systems that will overcome the theoretical efficiency limitations dictated by currently used materials such as silicon, to name but one. The potential benefits in terms of energy self-sufficiency and subsequent environmental impact are therefore immense. The health care industry will be a natural beneficiary from this research as well. Nanotechnology is increasingly applied to medical care (~30,000 patents in the last decade) from the development of diagnostic devices to the treatment of patients. However, applications are often constrained by bio-compatibility, lack of targeted (where is the treatment needed and what is it reacting with) treatment, and our finite knowledge of the complex nanoscale machinery of the human body limits the extend to which nanotechnology impacts this field today. Here again, our understanding of the driving forces governing the inner-workings of nanoscale systems and how these systems interact and exchange energy with their environment is central to the development of bio-compatible devices, will offer the capabilities to manufacture efficient and targeted diagnostics test and treatments, leading to better decease prevention and decreased hospital stays. On the educational front, the constant decline in interest for science witnessed in UK classrooms over the past decade is amongst the factors threatening the competitiveness of this country on the world stage. This has a direct impact on the quality of the research carried out and the innovative output of this research, whether in academia or in industry. This is particularly evident when comparing the EU's patent output -the UK situation is not an artefact- with the USA's and increasingly Asian countries. In this context, the proposed research will positively impact UK's competitiveness by implementing an original and state-of-the-art research program in nanosciences at King's. This will directly benefit KCL's students in their training but with the proper outreach strategy, should also be beneficial to the broader public and help attract, train and advise the next generations of scientists and engineers -a crucial element for UK's increased and sustained competitiveness.