Consider a bunch of molecules that have been trapped between two mirrors facing each other very closely. In this ‘box’ the light can’t move around freely; it can only exist in certain fixed patterns, somewhat like sound inside a finely tuned musical instrument. Even when all light has been removed from this box, quantum physics causes small flickers of light to pop in and out of existence in fractions of a second. These immutable flickers of the electromagnetic field are sometimes collectively called the vacuum field.
Now, what would happen if the natural motion of matter inside molecules and these confined light patterns become strongly linked?
This isn’t an esoteric thought experiment. Over the last decade, physicists have shown in experiments that when this link is strong, new hybrid states appear that are part light and part matter — and these states can spread over many molecules at once. And in this condition, physicists have found that the materials in which these states occur suddenly have very different properties, all without changing the chemical formulae of their constituent molecules.
A new paper by PhD scholar Subha Biswas and assistant professor Anoop Thomas — both at the Department of Inorganic and Physical Chemistry of the Indian Institute of Science — brought the many findings in this area of research together and argued that the confined vacuum field reshapes the weak forces between molecules, which is the same force that determine how the molecules arrange themselves and how easily energy and charge can move between them. And the duo has shown that this explanation can account for a wide range of surprising findings. Their findings were published in ACS Applied Optical Materials on November 11.
The paper isn’t a single experiment; instead the authors read and compared many published papers about experiments in which other scientists placed molecules inside small mirror cavities and forced them to interact strongly with the trapped light.
Thus, the duo reported, energy could travel quite differently in these cavities. Normally, when energy passes from one molecule to the next, it does so through a series of short hops if the material is disordered. A simple example is an ordinary piece of glass. In a crystal, atoms or molecules sit on a tidy 3D grid, like seats in a stadium. In glass however they’re more like people milling around in a crowd: there’s some short-range order but no long-range one.
When motion inside the molecules is strongly coupled to the trapped light, however, the new light–matter states can extend over many molecules, creating a sort of highway that energy travel on faster, more efficiently, and across much larger regions than physicists generally expect in disordered materials.
For another, the duo reported that some organic materials, especially plastics that usually conduct electricity very poorly, are suddenly able to conduct a lot better when certain internal motions of their molecules are strongly coupled to the trapped light. In some experiments scientists had found that protons also moved faster through water, suggesting the cavity could in some way alter the network of hydrogen bonds in water.
Based on analysing these and other examples, the duo arrived at a central conclusion: that by modifying how light is confined around a material, scientists can influence how its molecules ‘feel’ each other and thus alter its bulk behaviour, even when it’s in its lowest-energy state. (Usually materials become capable of unusual things when they’re imparted more energy.) Put another way the cavity isn’t just a passive container but an active design tool that can reshapes the background electromagnetic field, and with that the landscape of intermolecular forces.
This is interesting because it’s a new way to control molecules in chemistry. Usually when scientists need to change the way a chemical reaction happens, they change the molecules that are in play. For instance they alter the molecules’ structure, add side groups, and/or ‘mix in’ new components. But the new study has found what seems to be an additional handle: scientists may be able to leave the molecules as they are but just place them in a carefully designed optical environment. And by carefully choosing the spacing between the mirrors, the materials that make up the apparatus, and how the cavity resonates with specific molecular motions, they can adjust the reaction rate, favour one crystal form over another, stabilise certain structures, shift the balance between different reaction products, and guide how large molecules assemble into various shapes, etc.
The implications are similarly broad for materials science, where physicists can use these cavities to improve energy transport in thin films, enhance conduction in soft or flexible materials, guide how polymers and other large molecules arrange themselves in devices, and tune how materials crystallise as they’re fabricated. Because the effect comes from light and matter coupling together over a longer range, it might be especially useful in systems that are otherwise disordered and where conventional design tools struggle.
For instance, the molecules of polystyrene are arranged in a messy, haphazard way. This disorder makes it very hard for scientists to ‘engineer’ clean paths within the material through which electrons can flow, so polystyrene is usually an excellent insulator. However, when scientists place thin films of such plastics in a carefully tuned optical cavity, experiments have shown that their ability to conduct electricity increases drastically even though the material’s chemical makeup hasn’t changed.
A simple salt solution, like of table salt dissolved in water, offers another good example. Here the water molecules and the salt ions move around in an incessantly shifting and disordered fashion, and chemists typically control them by changing how much salt or what additives there are. But when the solution is strongly coupled to the confined light, the way water molecules surround and move around the ions changes and the ions travel more easily through the liquid.
The authors have stressed that these effects are selective — which is to say not every material will change dramatically. Instead scientists will have to select details such as how molecules pack together and which internal motion is coupled to the light in the cavity all matter. This in turn means this new vacuum-field engineering won’t altogether replace chemical design but could in fact complement it. In fact the authors have sketched a future in which chemists and materials scientists routinely think together about what molecules to build and the kind of electromagnetic environment to place them in to coax new or improved properties out.
