Optoelectronics based on the spin-valley physics of 2D materials

Two-dimensional (2D) materials are atomically thin crystals with exceptional mechanical, electrical and optical properties. Their unique characteristics originating from quantum confinement in the vertical dimension have attracted a strong interest for scientific research as well as technological applications. Moreover, the weak van der Waals coupling between layers facilitates the stacking of different materials to form heterostructures with new functionalities. This thesis focuses on semiconductor transition metal dichalcogenides (TMDCs), with a direct bandgap in the monolayer limit. The reduced dielectric screening leads to the formation of tightly bound electron-hole pairs (excitons) that are stable at room temperature and dominate the optical spectrum. The first part of the thesis explores the use of TMDC monolayers for developing transparent and tunable exciton light emitting diodes (LEDs) and photodetectors (PDs). The high modulation frequencies and integration capabilities with photonic structures make them promising candidates as optical/electrical interconnects. The properties of TMDC monolayers are determined by those of carriers close to the conduction and valence band extrema, located at the K and -K valleys in reciprocal space. The broken inversion symmetry and strong spin-orbit coupling links the valley and spin degrees of freedom, and enables optical excitation and detection of spin-valley accumulation with circularly polarized light. In the second part of the thesis, we explore the use of electric and magnetic fields to control the spin-valley dynamics of single carriers and excitons in TMDC-based optoelectronic devices. The spin-valley accumulation is probed by photoluminescence, electroluminescene and magneto-optical Kerr rotation. The third part of the thesis focuses on the integration of TMDCs in planar optical cavities. Their large oscillator strength induces a strong exciton-photon coupling, which results in the formation of polaritons. The observed polaritons propagate over long-distances and inherit the spin-valley properties of constituent excitons. By tuning the semiconductor carrier density, we achieve electrical control of exciton-photon coupling and polariton transport. Finally, we demonstrate a cavity integrated polariton light emitting diode where the location, directionality and polarization of emitted light can be electrically controlled at room temperature. Altogether, this thesis investigates the properties of charge carriers and excitons in 2D materials to realize practical optoelectronic devices where the spin-valley degree of freedom can be encoded in the polarization of light.