Tuning a Circular p-n Junction in Graphene from Quantum Confinement to Optical Guiding

The photon-like propagation of the Dirac-electrons in graphene together with the record-high electronic mobility can lead to applications based on ultra-fast electronic response and low dissipation. But the chiral nature of the charge-carriers which is responsible for the high mobility also makes it difficult to control their motion and prevents electronic switching. Here we show how to manipulate the charge-carriers by using a circular p-n junction whose size can be continuously tuned from the nanometer to the micrometer scale. The junction size is controlled with a dual-gate device consisting of a planar back-gate and a point-like top-gate made by decorating an STM tip with an Au nanowire. The nanometer-scale junction is defined by a deep potential well created by the tip-induced charge. It traps the Dirac-electrons in quantum-confined states which are the graphene equivalent of the atomic collapse states predicted to occur at super-critically charged nuclei. As the junction size increases, the transition to the optical regime is signaled by the emergence of whispering-gallery modes¹.

¹Y. Jiang, et al, Nature Nanotechnology (2017) doi:10.1038/nnano.2017.181