Optically Probing Electron Spin and Charge in Solid State Nanostructures

Xiaodong Xu
Center for Nanoscale Systems, Cornell University

The optical control of spin and charge are central elements of optically-driven spintronics and optoelectronics. When electrons are confined in solid state nanostructures or at nanoscale interfaces, it opens up opportunities to discover novel physics and to engineer nanoscale devices that miniaturize spintronics and optoelectronics to the single atom level. In this talk, I will present our recent progress in spintronics and optoelectronics, based on quantum dot (QD) nanostructures and graphene field effect transistors, respectively.

A single spin trapped inside a semiconductor QD is a promising candidate for spin-based electronics and quantum logic. One crucial requirement is a long quantum coherence time. However, the electron spin coherence time deteriorates rapidly due to hyperfine coupling with the nuclear environment in III-V materials. We successfully overcome this obstacle by suppressing the nuclear spin fluctuations while coherently manipulating a single electron spin trapped inside a single QD by coherent population trapping spectroscopy. The discovered dynamic nuclear spin polarization feedback process can enhance the quantum coherence of an electron spin by three orders of magnitude compared to its thermal value.

Graphene, a single atomic membrane formed by carbon atoms, is an interesting potential optoelectronic material with unusual electronic, optical, and thermal properties. There is tremendous interest in graphene-based optoelectronic devices. However, due to the unique Dirac cone electronic structure, the physical mechanism giving rise to the optoelectronic responses of zero-bandgap graphene remains elusive. By designing a novel graphene single-bilayer interface junction, we use a scanning photocurrent microscopy (SPM) to demonstrate that the photo-thermoelectric effect dominates the photocurrent generation. We show that this SPM technique can be used as a local probe of the density of states at novel interface nanostructures.