"What we've demonstrated is the quantum-interference nature of Raman scattering," Wang says. "It was always there, but it was so hard to see that it was often overlooked."
In a second observation, the researchers found yet another unexpected example of inelastic light scattering. This one, "hot electron luminescence," didn't result from blocked quantum pathways, however.
When a strong voltage is applied and the graphene's Fermi energy is lowered, higher-energy electron states are emptied from the filled band. Electrons that are highly excited by incoming photons, enough to jump to the unfilled band, thus find additional chances to fall back to the now-vacant states in what was the filled band. But these "hot" electrons can only fall back if they emit a photon of the right frequency. The hot electron luminescence observed by the researchers has an integrated intensity a hundred times stronger than the Raman scattering.
The road taken
The poet Robert Frost wrote of coming upon two roads that diverged in a wood, and was sorry he could not travel both. Not only can quantum processes take both roads at once, they can interfere with themselves in doing so.
The research team, working at UC Berkeley and at Berkeley Lab's Advanced Light Source, has shown that inelastic light scattering can be controlled by controlling interference between the intermediate states between photon absorption and emission. Manipulating that interference has enabled new kinds of quantum control of chemical reactions, as well as of "spintronic" states,
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory