Team member Rachel Segalman, an associate professor of chemical engineering at UC Berkeley and a faculty scientist in Berkeley Lab's Materials Sciences Division, provided the ion gel that was key to the experimental device. An ion gel confines a strongly conducting liquid in a polymer matrix. The gel was laid over a flake of graphene, grown on copper and transferred onto an insulating substrate. The charge in the graphene was adjusted by the gate voltage on the ion gel.
"So by cranking up the voltage we lowered the graphene's Fermi energy, sequentially getting rid of the higher energy electrons," says Wang. Eliminating electrons, from the highest energies on down, effectively eliminated the pathways that, when impinged upon by incoming photons, could absorb them and then emit Raman-scattered photons.
What comes of interference, constructive and destructive
"People have always known that quantum interference is important in Raman scattering, but it's been hard to see," says Wang. "Here it's really easy to see the contribution of each state."
Removing quantum pathways one by one alters the ways they can interfere. The changes are visible in the Raman-scattering intensity emitted by the experimental device when it was illuminated by a beam of near-infrared laser light. Although the glow from scattering is much fainter than the near-infrared excitation, changes in its brightness can be measured precisely.
"In classical physics, you'd expect to see the scattered light get dimmer as you remove excitation pathways," says Wang, but the results of the experimenter came as a surprise to everyone. "Instead the signal got stronger!"
The scattered light grew brighter as the excitation pathways were reduced what Wang calls "a canonical signature of destructive quantum interference."
Why "destructively?" Because phonons and scattered photons can be excited by many different,
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory