Our world is made of molecules. Our goal was to explore a new way for molecules to interact with each other, and thus enable new opportunities in chemistry and materials science. Traditionally molecules interact through their electric charge. The new approach was to find ways to link molecules not with electric charge, but with light, something that produces two big changes. First, molecular energy levels can be modified, for example molecules that would naturally absorb blue light can be transformed into ones that absorb green light. Second, energy can be de-localised, for example the energy absorbed by one molecule could be made available not just on the nanometre scale – as in photosynthesis – but over centimetres, perhaps enabling alternative light-harvesting technologies.
Linking molecules with light in this way is known as strong coupling. At the start of the photmat project many ground-breaking results were still un-confirmed, and many of the mechanisms and underlying science poorly understood. The aim of photmat was to develop a significantly deeper scientific understanding, and to highlight more clearly where the potential transformative developments lie. Significant progress has been made, and a clearer picture of what still needs to be understood is now emerging.
Linking molecules with light is surprisingly simple, but rather subtle. It is achieved by confining light within a small volume of space, and then placing optically active molecules within this volume. In many experiments the light is confined by placing two metal mirrors close together to form a cavity, a sort of planar sandwich, the mirrors are the slices of bread, the molecules form the sandwich filling. If now we think of light inside the cavity in the form of a photon, it can be absorbed by the molecules, then released into the cavity. The light cannot escape because the cavity confines the light, instead it may again be absorbed by the molecules etc. etc. It is this exchange of the photon's energy between the molecules and the cavity that lies at the heart of strong coupling, it is how we link molecules with light. The subtlety is that we use virtual photons, thereby harnessing some of the weirdness of quantum mechanics.
The importance for society is that linking molecules with light potentially offers new ways to do chemistry, and opens new routes for materials science. Many desirable chemical reactions are difficult to achieve, often requiring high temperatures and/or catalysts that use scarce materials etc. Strong coupling offers the prospect of controlling some chemical reaction pathways ‘simply’ by placing the reactants in an optical cavity. Another example is organic electronics, where device efficiencies ore often limited by wasteful triplet states. Strong coupling offers a way to bypass these states, thus improving efficiency, e.g. for display device.
