No abstract

Artificial structures made of stacked two-dimensional crystals have recently been the focus of intense research activity. As in twisted or stacked graphene layers, these structures can show unusual behaviours and new phenomena. Among the various layered compounds that can be exfoliated, transition-metal dichalcogenides exhibit interesting properties governed by their structural symmetry and interlayer coupling, which are highly susceptible to stacking. Here, we obtain-by folding exfoliated MoS2 monolayers-MoS2 bilayers with different stacking orders, as monitored by second harmonic generation and photoluminescence. Appropriate folding can break the inversion symmetry and suppress interlayer hopping, evoking strong valley and spin polarizations that are not achieved in natural MoS2 bilayers of Bernal stacking. It can also enlarge the indirect bandgap by more than 100 meV through a decrease in the interlayer coupling. Our work provides an effective and versatile means to engineer transition-metal dichalcogenide materials with desirable electronic and optical properties.

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (X) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of X states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the X emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe on a gold substrate, we demonstrate ~6 × 10-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 10 of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the possibility to gate-control the chemical potential, which selectively switches on and off one-and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.

Core/shell-like n-type InAs nanowire phototransistors based on majority-carrier-dominated photodetection are investigated. Under optical illumination, electrons generated from the core are excited into the self-assembled near-surface photogating layer, forming a built-in electric field to significantly regulate the core conductance. Anomalous high photoconductive gain and fast response time are obtained at room temperature.