Inside a photon prison, a light-and-matter hybrid is born

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Trapped inside a tiny cavity between a gold film and a gold nanoparticle, a photon and a blue dye molecule are forced to continuously interact with each other, forming a "strong coupling" at room temperature(Credit: R Chikkaraddy/J Baumberg)

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Scientists at Cambridge University and the Imperial College London have trapped photons inside a tiny gold cavity, forcing it to interact with matter to form a hybrid state. This unique mixture – or "strong coupling" – of light and matter, achieved for the first time at room temperature, will help scientists develop better on-chip communications, manipulate quantum information, or even tweak the chemical bonds of single molecules.

When we manipulate matter to produce light – be it by rubbing sticks to light a fire, heating a tungsten filament, or combining electric charges – photons normally leave their point of origin at the speed of light, never to return. But if those photons are somehow trapped and forced to interact with matter over and over (as was done in this study), light and matter can achieve "strong coupling," a unique hybrid state where a photon and a molecule are in a continuous interchange.

"It is a very strange interchange, indeed, because if you would compare the molecule and the light to a system such as tea and milk you would see the tea and the milk mixing when you pour the milk into the cup, but no-one would really expect that tea and milk would in time continuously de-mix and mix again," Prof. Ortwin Hess, the study's lead author, told Gizmag. "In the strongly coupled state, however, this happens to light and the molecule."

The effect has been achieved before in the lab, but only using complicated and expensive setups that involved working at temperatures near absolute zero. Now, the teams of scientists led by professors Jeremy Baumberg, Oren Schermann, and Hess have made the study of strong coupling much cheaper and more convenient by achieving it at room temperature and localizing it within a single molecule.

Their "photon trap" is made of a thin gold film that acts as a mirror, on top of which sits a cucurbit[n]uril molecule, shaped like an empty barrel. The molecule acts as a spacer and container for methylene-blue dye molecules, keeping them separated from each other and oriented in the right direction. On top of the spacer sits a gold nanoparticle.

This structure forms a tiny "nanocavity" between the gold particle and the film below which is only 40 cubic nanometers in volume – so small that, once the blue dye molecule is made to emit a photon, the photon is trapped and forced to interact with the dye molecule over and over, achieving strong coupling.

Hess and colleagues found that theory and practice were in remarkable agreement, despite the amount of interference that normally comes with performing quantum experiments at room temperature.

"Strong coupling can be understood on a variety of levels – classical, semi-classical and quantum," Hess told us. "While in theory one generally seeks to explain natural phenomena such as strong coupling in 'clean' principles, nature has at room temperature a strong tendency to behave more as a classical system rather than reveal quantum properties. It is therefore so remarkable that the particular system shows such behavior in the presence of all the extra 'noise' at room temperature."

Hess and colleagues say they will now focus on using this advance to create tiny lasers that work by exploiting the effect of trapped photons rather than the optical cavity (essentially, a clever mirror arrangement) used in larger lasers.

"About two years ago we predicted the 'stopped-light laser' – literally a cavity-free nanolaser that would work on the nanoscale (much smaller than the wavelength of the light it emits) by feedback generated from preventing light from moving away from where it had been created," Hess revealed.

Such a device, Hess says, could be used to generate laser light on site, without having to transport it there through a fiber. It could also be used to create more effective systems of on-chip communication, or to copy quantum information over to matter and back.

A paper further detailing the study was published in the journal Nature.

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