Light relief: Photon and molecular mix

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  • Published: Jun 15, 2016
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Light relief: Photon and molecular mix

Future quanta

Mixing light with dye molecules, trapped in golden gaps. Credit: Yu Ji/University of Cambridge NanoPhotonics

Future quantum information systems and other devices, such as ultralow-power switches and lasers, might rely on the mixing of photons and molecules. Now, researchers in the UK have demonstrated how a single molecule trapped in a tiny optical cavity can emit a photon, but that particle of light re-enters the molecule before it leaves the cavity, mixing molecule and photon completely.

Some molecules are wont to emit photons, but what commonly happens is that the photon is lost to the proverbial ether or else absorbed by some neighbouring entity. What the average molecule does not expect is "return to sender". However, scientists at the University of Cambridge, King’s College and Imperial College London, have trapped the photon emitter, methylene blue in a cavity with a volume of less than 40 cubic nanometres, bounded by a mirror and a gold nanoparticle. By exploiting the molecular recognition phenomenon in host-guest chemistry the team could then control one to ten protectively isolated molecules and see a coupling process even at room temperature and under ambient conditions.

Wouldbe emission

The energy of would be emitted photons oscillates back and forth in the system between light and molecule, representing a mixing of the two otherwise disparate entities. Previous attempts to trap photons in a molecular hall of mirrors have only been possible at cryogenic temperatures, which limits somewhat everyday applications and potential, the UK team's 'half-light' molecules exist at room temperature. Aligning the dye molecules was made possible by Oren Scherman's team at Cambridge which encapsulated the dyes in hollow barrel-shaped molecular cages called cucurbiturils, which keep the dyes upright in the cavity.

"It's like a hall of mirrors for a molecule, only spaced a hundred thousand times thinner than a human hair," explains research leader Jeremy Baumberg of the NanoPhotonics Centre at Cambridge's Cavendish Laboratory. After several months of data collection work, the team's statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provided the evidence of single-molecule strong coupling in the system, the team reports in the journal Nature. The scattering spectrum is split in two quantum states, the signature of the photon-molecule mixing. The separation of peaks suggests that the emitted photon is taking less than a trillionth of a second to return to its origin. The spectra also reveal whether one, two or three molecules were in the gap.

Chemical control

The potential of such a hybrid system lies in the possibility of using them as switches or units of information in a quantum information system or quantum computer. They might also offer insights into natural systems wherein molecules and photons are linked very closely, such as the complex process of photosynthesis in which green plants and algae absorb sunlight and push its photons along an energy trail that ultimately converts water and carbon dioxide from the atmosphere into sugar molecules in the cells. Insights from this and subsequent experiments might also point to novel ways of manipulating matter and in particular making and breaking chemical bonds in a highly controlled way.

Related Links

Nature 2016, online: "Single-molecule strong coupling at room temperature in plasmonic nanocavities"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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