Light and matter are usually separate and have distinct properties. However, molecules of matter can emit particles of light called photons. Normally, emitted photons leave the molecule and the two do not mix again.
Now, scientists have trapped a single molecule in such a tiny space that when it emits a photon, the photon cannot escape. This produces an oscillation of energy between the molecule and the photon, creating a mixing of the properties of matter and light.
This unusual interaction of a molecule with light will 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 mixing – called ‘strong coupling’ – was achieved at the University of Cambridge following theoretical simulations by scientists from Imperial College London and Kings College London. The results of the experiment are published today in the journal Nature.
Comparing single-molecule optical cavities.
To trap the photon, researchers created a ‘nanopore’ – an extremely small cavity of only a billionth of a meter (one nanometre) wide. The cavity is formed between a tiny sphere of gold and a gold film, which creates a mirror image of the sphere. In between the sphere and its mirror image, a dye molecule is caught. This molecule emits the photon that becomes trapped.
“The cavity is so small that light doesn’t have a choice but to come together with matter,” said Professor Ortwin Hess from Imperial’s Department of Physics, who led in the theory of how to achieve strong coupling and how to interpret the result.
Photons are packages of light that can behave both like particles and like waves, in a way that is described as ‘quantum’. “The experiment is a test that light is quantum in nature and indeed it showed the quantum effects we predicted,” said Professor Hess. “This is a remarkable crossing of theory and experiment. It’s amazing how much nature behaves like the theory.”
Whole New Experiments
Strong coupling has been achieved before, but it has previously required extreme cooling. This is the first time strong coupling with a single molecule has been achieved at room temperature, making the process “chemically easy to access” according to Professor Hess.
“We can now do a whole range of experiments on matter and light that would have been costly and difficult before,” said Professor Hess. “We could use light to change chemical structures, molecule by molecule.
“It could also be useful in quantum technologies. Light carries quantum information, and we could use this strong coupling to copy the information over to matter and back.”
Professor Hess and his research team at Imperial are exploring the theory of such strong coupling interactions for use in creating the smallest ever lasers by stopping light in in its tracks.
Photon emitters placed in an optical cavity experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter forming building blocks for quantum information systems and for ultralow-power switches and lasers. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complicated fabrication methods, compromising its use. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host–guest chemistry to align one to ten protectively isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from more than 50 such plasmonic nanocavities display characteristic light–matter mixing, with Rabi frequencies of 300 millielectronvolts for ten methylene-blue molecules, decreasing to 90 millielectronvolts for single molecules—matching quantitative models. Statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provides evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis9 and the possibility of manipulating chemical bonds10.
SOURCES- Imperial College of London, Nature