Cambridge researchers mix molecules with light

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Researchers from the University of Cambridge have used quantum states to mix a molecule with light at room temperature, which could aid in the exploration of quantum technologies and provide new ways to manipulate the physical and chemical properties of matter. The results were published in Nature in June.

The process was developed by placing single molecules in tiny optical cavities to the point where emitted photons would return to the molecule before they had properly left. Cavities of only one nanometre across had to be constructed in order to trap the light this way. This was done by using a coloured dye molecule and placing it inside the tiny gap between a gold nanoparticle and a mirror.

Energy oscillates back and forth between light and molecule, resulting in a complete mixing of the two. ‘It’s like a hall of mirrors for a molecule, only spaced a hundred thousand times thinner than a human hair,’ said Professor Jeremy Baumberg of the NanoPhotonics Centre at Cambridge’s Cavendish Laboratory, who led the research. 

Previous attempts to do this have been complex to produce, and only achievable at very low temperatures; however, the researchers have developed a method to produce these ‘half-light’ molecules at room temperature. 

These unusual interactions of molecules with light provide new ways to manipulate the physical and chemical properties of matter, and could be used to process quantum information, aid in the understanding of complex processes at work in photosynthesis, or even manipulate the chemical bonds between atoms. 

In order to achieve the molecule-light mixing, the dye molecules needed to be positioned correctly in the tiny gap. To achieve this, the team joined with a team of chemists at Cambridge, led by Professor Oren Scherman, to encapsulate the dyes in hollow barrel-shaped molecular cages called cucurbiturils, which are able to hold the dye molecules in the desired upright position. When assembled together correctly, the molecule scattering spectrum splits into two separated quantum states, which is the signature of this ‘mixing’. This spacing in colour corresponds to photons taking less than a trillionth of a second to come back to the molecule. 

A key advance of the research was to show strong mixing of light and matter was possible for single molecules even with large absorption of light in the metal and at room temperature. ‘Finding single-molecule signatures took months of data collection,’ said said Rohit Chikkaraddy, lead author of the study. The researchers were also able to observe steps in the colour spacing of the states, corresponding to whether one, two, or three molecules were in the gap.

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