Way to unlock polaritons’ ‘magic power’ discovered

By tuning the disorder in a reaction, researchers in the US have found a way to control the unusual light–matter hybrid particles known as polaritons. The finding is a step towards understanding the mysterious mechanisms underlying polaritonic chemistry, in which compounds placed in an optical cavity – a tiny box with mirrored walls – can behave in unexpected ways. This could one day be used to drive difficult reactions, create more efficient solar cells – or make lightsabers.

Optical cavities can trap light so that it interacts with the vibrations or electronic transitions of molecules inside the cavity. This strong light–matter coupling creates polaritons, particles that are part photon, part molecule.

Sometimes, compounds confined in a cavity experience astonishing changes: reactions can have altered product distributions and non-conducting polymers can show electrical conductance. Other times, nothing happens. ‘We defined new criteria for when strong coupling can show some of its magic power,’ says project co-leader Wei Xiong from the University of California San Diego, US.

The key to manipulating polaritons is controlling the system’s disorder. Xiong’s team had previously calculated that highly ordered structures could make materials more susceptible to polaritonic interactions.

Now, they’ve shown this in the laboratory for the first time. Their phenol derivative switches between different conformations three times faster when confined inside a cavity – but only when used in its solid form. A saturated solution of the phenol showed no rate acceleration, even though polaritons still formed.

That’s because polaritons don’t take well to disorder. In the solid, there’s little disorder: each molecule experiences a very similar environment. This unleashes polaritons’ power of transferring energy between far-away molecules. In a solution, disorder is high, with subsets of molecules sitting in different surroundings. Here, polaritons are unable to do long-range energy transfer.

Reducing disorder

‘This is the first case I know of where we have a very clear design principle, something that we can pick up on,’ says polariton chemist Andrew Musser from Cornell University, US.

In practice, reducing disorder is difficult, says Pengfei (Frank) Huo, who works on light–matter coupling at the University of Rochester, US. Sometimes, using solids or pure liquids might do the trick. There are also ideas about ‘antenna’ solvents that funnel strong coupling into a dissolved compound. The easier alternative, he says, is to massively crank up the light–matter coupling strength. This could be done by increasing concentration, using a smaller cavity or completely changing the cavity design.

Both Huo and Musser highlight that this method currently applies to light-induced cavity reactions only. Some polaritonic chemistry is done in dark cavities. ‘We are no closer to really understanding how those work,’ Musser says.

Chemists are keen to learn how to control polaritons and use them for synthesis. ‘If somebody could come up with, for example, a stereoselective reaction, that could have huge implications for high-value chemicals,’ Musser says.

There are also potential applications in light-harvesting devices, optical computing – and lightsabers. ‘The reason we don’t have lightsabers is because photons don’t interact with each other’ Huo says. ‘Having polaritons instead may help you achieve that.’