Driving Ultrafast Spin and Energy Modulation in Quantum Well Surface States via Photo-Induced Electric Fields

The future of modern optoelectronics and spintronic devices relies on our ability to control the spin and charge degrees of freedom at timescale that can compete with traditional silicon-based devices, operating at speeds >10 GHz.  Rashba spin-split quantum well states, 2D states that develop at the surface of strong spin-orbit coupling materials, are ideal given the easy tunability of their energy and spin states. So far however, most studies have only demonstrated such control in a static way. In this study, we demonstrate ultrafast control of the spin and energy degrees of freedom of surface quantum well states on Bi₂Se₃ at picosecond timescales. By means of a focused laser pulse, we modulate the band bending, producing picosecond time-varying electric fields at the material's surface, thereby reversibly modulating the quantum well spectrum and Rashba Effect. These results open a new pathway for light-driven spintronic devices with ultrafast switching of electronic phases, and offer the interesting prospect to extend this ultrafast photogating technique to a broader host of 2D materials.