Since the Rashba interaction and QW spectrum are both predicated on the underlying electric potential, tuning the surface potential becomes a powerful method to manipulate both their spin and energy degrees of freedom 7, 16, 17, 18. Combining quantum confinement with strong spin-orbit coupling joins the malleable electronic dispersions of QW states with the robust spin-polarization of Rashba states, making them extremely promising candidates for spin-based devices, such as the Datta Das spin-transistor 14, 15. When these states appear on the surface, the breaking of inversion symmetry removes the spin-degeneracy resulting in spin-polarized QW states 8, 9, 10, 11, 12, 13. Suitable confinement potentials can be achieved by constructing a multilayer heterostructure, but they can also occur spontaneously on surfaces from an adsorbate-induced localized surface potential 5, 6, 7. For these reasons, they have yielded a plethora of applications such as high-efficiency solar cells, infrared lasers, and qubits 1, 2, 3, 4. These states are easy to produce, have tunable band gaps, and are conceptually simple in terms of quantum systems. Quantum well (QW) states in semiconductors are a result of a confining potential that traps electrons (typically in a 2D plane) quantizing their electronic states.
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These results open a pathway for light-driven spintronic devices with ultrafast switching of electronic phases, and offer the interesting prospect to extend this ultrafast photo-gating technique to a broader host of 2D materials. Moreover, we uncover a dynamic quasi-Fermi level, dependent on the Lifshitz transition of the second quantum well band bottom. 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. In this study, we demonstrate control of the spin and energy degrees of freedom of surface quantum well states on Bi 2Se 3 at picosecond timescales. So far, however, most studies have only demonstrated such control in a static way. Rashba spin-split quantum well states, 2D states that develop at the surface of strong spin-orbit coupling materials, are ideal given the tunability of their energy and spin states. The future of modern optoelectronics and spintronic devices relies on our ability to control the spin and charge degrees of freedom at ultrafast timescales.