Photon Emission Rate Enhanced in Fluorescent Molecules

Fluorescent molecules can be made to emit photons more quickly by sandwiching them between metal nanocubes and a gold film.

This film-coupled metal nanocube system features emitters embedded in the dielectric gap region. Its developers at Duke University said it meets the requirements necessary “to move nanophotonic devices such as lasers and single-photon sources into the practical realm” — directional emission, room-temperature and broadband operation, high radiative quantum efficiency and a large spontaneous emission rate.

“One of the applications we’re targeting with this research is ultrafast LEDs,” said Dr. Maiken Mikkelsen, assistant professor of electrical and computer engineering and physics at Duke. “While future devices might not use this exact approach, the underlying physics will be crucial.”

In the study, the researchers manufactured 75-nm silver nanocubes and trapped light between them to increase the light’s intensity. When fluorescent molecules were placed near the intensified light, they emitted photons at a faster rate through Purcell enhancement.

Using computer simulations, the team determined the gap needed to be about 20 atoms wide to resonate with the fluorescent molecules.

“We can select cubes with just the right size and make the gaps literally with nanometer precision,” said Gleb Akselrod, a postdoctoral scholar at Duke. “When we have the cube size and gap perfectly calibrated to the molecule, that’s when we see the record 1,000-fold increase in fluorescence speed.”

The researchers plan to take their work a step further by designing a system that precisely places individual fluorescent molecules under a single nanotube. Even higher fluorescence rates could be achieved by standing the molecules up on edge, at the corners of the cube.

“If we can precisely place molecules like this, it could be used in many more applications than just fast LEDs,” Akselrod said. “We could also make fast sources of single photons that could be used for quantum cryptography. This technology would allow immediate communications across vast distances that could not be hacked, at least not without breaking the laws of physics.”

The work was funded by the Lord Foundation of North Carolina and the U.S. Air Force Office of Scientific Research. The research was published in Nature Photonics (doi: 10.1038/nphoton.2014.228).

For more information, visit www.duke.edu.