Nano 'hall of mirrors' causes molecules to mix with light

When a molecule emits a blink of light, it doesn't expect it to ever come back. However researchers have now managed to place single molecules in such a tiny optical cavity that emitted photons, or particles of light, return to the molecule before they have properly left. The energy oscillates back and forth between light and molecule, resulting in a complete mixing of the two.

Previous attempts to mix molecules with light have been complex to produce and only achievable at very low temperatures, but the researchers, led by the University of Cambridge, 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. The results are reported in the journal Nature.

To use single molecules in this way, the researchers had to reliably construct cavities only a billionth of a metre (one nanometre) across in order to trap light. They used the tiny gap between a gold nanoparticle and a mirror, and placed a coloured dye molecule inside.

"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.

In order to achieve the molecule-light mixing, the dye molecules needed to be correctly positioned in the tiny gap. "Our molecules like to lie down flat on the gold, and it was really hard to persuade them to stand up straight," said Rohit Chikkaraddy, lead author of the study.

To solve 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 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 Chikkaraddy.

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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The Cambridge team collaborated with theorists Professor Ortwin Hess at the Blackett Laboratory, Imperial College London and Dr Edina Rosta at Kings College London to understand the confinement and interaction of light in such tiny gaps, matching experiments amazingly well.

Reference:

Single-molecule strong coupling at room temperature in plasmonic nanocavities
Nature (2016) doi:10.1038/nature17974

University of Cambridge

Nanoplasmonics makes the impossible possible

Over a five-year period, Alexander Dmitriev and his research team at Chalmers will take on a task that until now has been deemed impossible: creating strong interaction between light and magnetic fields and determining ways to control light with magnetism on the nanoscale. The Harnessing light and spins through plasmons at the nanoscale project has received close to SEK 38 million from the Knut and Alice Wallenberg Foundation, and may eventually lead to more effective ways to process and store information with light and create different types of optical elements.

"The entire field is still fairly unknown, and we are one of only a few research teams in the world currently looking specifically into light as nanoplasmonic resonances combined with magnetic nanostructures," says Alexander Dmitriev, associate professor of physics at Chalmers.

For a long time it has been deemed impossible to combine light and magnetism because of a frequency gap where light moves 10,000 times faster than magnetism reacts, which means they do not feel each other and cannot integrate. By capturing the light in what are known as nanoantennas, which are built over a surface, it is possible for the two to interact on the nanoscale. There are nanoplasmons in this artificially created surface of nanoantennas – in other words small units of electrons that when exposed to visible light, move or oscillate collectively and thus create enhanced and localised electromagnetic fields that can then be connected with magnetic materials via different types of magneto-optical effects.  

"We want to attempt to force the light to become steerable using magnetism, and vice versa, and thus eliminate the frequency gap," says Alexander Dmitriev.

Steerable optical components

When the project ends in five years, the team hopes to have obtained a fundamental understanding of the field and be better equipped to build the specific nanostructures needed to achieve the desired properties. By bringing internationally leading research teams from Chalmers and the universities in Uppsala and Gothenburg together, it will be possible to utilise expertise within both theoretical and experimental physics in nanoplasmonics, nanomagnetism and spintronics. However, even if the project has a purely fundamental character, Alexander Dmitriev sees clear areas of application where it will hopefully be possible to use the methods in the future.

"This technology could enable steerable and adaptable optical components that are not easily controlled with electric current, for example three-dimensional holograms that move in real time. Thanks to the enhanced interaction we want to create between light and magnetism on the nanoscale, it will be possible to use low-intensity magnetic fields similar to those found in regular refrigerator magnets, and it will be quick, energy-efficient and easy to integrate with electronics.  

Chalmers University of Technology