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.