2D Quantum Materials Group

In monolayer TMDs, broken inversion symmetry and strong spin–orbit coupling make the K/K′ valleys distinct quantum degrees of freedom. Valley–polarization–locked optical selection rules enable selective excitation with circular polarization and coherent K/K′ superpositions with linear polarization.^{[ 1 ]} Using polarization-resolved PL in monolayer MoSe₂, we quantified valley coherence and found that improving sample quality alone does not substantially enhance coherence; the inferred decoherence time is extremely short, suggesting intrinsic (phonon-related) scattering dominates. In type-II heterobilayers, interlayer excitons spatially separate electrons and holes, suppressing intervalley exchange and thereby enabling longer valley-polarization lifetimes (Fig.). In moiré superlattices, additional localization can further stabilize valley states. With time- and polarization-resolved spectroscopy of double-gated WS₂/WSe₂ devices, we demonstrated strong electric-field control of valley relaxation, achieving intervalley scattering times exceeding 700 ns, while coherence remains limited. We are extending these studies to more strongly localized MoSe₂ /WSe₂ using ~100 nm-aperture Au masks to mitigate inhomogeneity, aiming to achieve long-lived, optically addressable valley states for quantum technologies. We are also developing machine-learning-assisted crystal growth methods and studying the fundamental optics of low-dimensional systems.^{[ 2 ],[ 3 ]}