Theoretical studies of coupled quantum well excitons and microcavity dipolaritons, their transport dynamics and applications in optical devices

In the last 100 years, the advent of quantum mechanics has spawned rapid technological advances and renewed growth in our understanding of the world around us. A few decades ago, quantum wells - layers of material just tens of atoms thick - made possible the study of a new two-dimensional (2D) world where quantum effects are exceptionally prominent.

The proposed work entails the study of particles known as excitons trapped inside quantum wells. An exciton is comprised of a negatively charged electron and a positively charged hole that are bound by the attractive forces between them. Laser light excites an electron so that it freely roams the 2D quantum well plane. This leaves an empty space - a hole - that the electron can eventually re-occupy. The hole also traverses the quantum well and can be treated as a real particle with mass and charge. A fascinating variety, known as indirect excitons, is the main focus. These are where electrons and holes are separated into closely spaced wells so that excitons acquire a dipole orientation. In particular, their 2D transport and ways to control their motion are studied.

Excitons are short lived and decay to emit light. In their brief existence, they display a dramatic variety of physical phenomena. One such phenomenon is the macroscopically ordered exciton state. In this state, excitons spontaneously organise themselves into clusters, equally spaced and uniform in size. The exact cause of this has been heavily debated since its discovery more than a decade ago. An explanation of this effect in terms of the intricate interplay of forces between charges will be sought.

Excitons also give rise to particles known as polaritons. Light can be absorbed to make an exciton which later decays to emit light. However, that light gets reabsorbed to make another exciton. The perpetual cycle continues at such a rapid pace that we no longer think in terms of an exciton and light but rather a new mixed state called a polariton. When quantum wells are placed between two mirrors to trap the light, microcavity polaritons are realised. These have their own unique properties and are neither like excitons or light. They display striking features such as Bose-Einstein condensation - an exotic state of matter predicted by Bose and Einstein almost a century ago. Excitons and polaritons provide a means to study the beauty of quantum mechanics in a whole new way and are among the best tools to craft the outer limits of human understanding. In this work, a new breed of polariton will be studied where the polariton's exciton part is a dipolar indirect exciton. The motion of these dipolaritons can be controlled both electrically and optically. They enable new types of experiment and new ways to manipulate light.

The ultimate goal of the work is to employ the remarkable nature of excitons and polaritons in the development of new optical technology. Devices such as optical transistors have the potential to revolutionise the communication era. Currently, optical fibres transfer information at high speed using light whilst information processing is done using electronic transistors. The conversion between optical and electronic signals leads to bottlenecks in communication networks. Optical transistors will solve this problem and will become an integral part of future communication systems. The goal is to identify new ways to create optical transistors mediated by excitons and polaritons. The success of this work will contribute to a globally emerging industry.

Economic impact - The long term economic impact of this fellowship lies in the potential of the studied exciton and polariton systems to contribute to future technology. The rapidly expanding optoelectronics industry is driven by breakthroughs in the developments of these kinds of technology. Devices based on exciting new quantum well and microcavity structures can radically improve optical communications technology and lead to new ways to process information optically, rather than electrically. The eventual success of these activities will lead to the creation and development of new products and an industry to manufacture them. Therefore, there is clear potential for the UK economy to benefit from this research.

Societal impact - A sizeable fraction of the general public have an enthusiastic interest in the latest developments in science and wish to know what technology the future will bring. The proposed work has a strong potential to satisfy and fuel such interest. To facilitate this, written descriptions of the work will be posted on the internet. They will include articles in online science and technology magazines which reach a broad audience. Summaries of the work will also be posted on blogs where people can post comments and feedback. The articles and blogs will keep readers up to date with the latest breakthroughs during the project's lifetime. Descriptions will be focussed around technological advances so as to gain a wide interest. In addition, the articles will aim to capture interest in the remarkable physics of quantum well excitons and microcavity polaritons. These topics will be described in the most accessible way that encourages readers towards a basic conceptual understanding. The societal impact of the work is, therefore, to raise public awareness and engage interest in a cutting edge area of science which they may have previously been unaware of. Such awareness changes the way people think about science and is an important type of societal impact.