Momentum-Space Observation of Optically Excited Nonthermal Electrons in Graphene with Persistent Pseudospin Polarization.

The unique optical properties of graphene, with broadband absorption and ultrafast response, make it a critical component of optoelectronic and spintronic devices. Using time-resolved momentum microscopy with high data rate and high dynamic range, we report momentum-space measurements of electrons promoted to the graphene conduction band with visible light and their subsequent relaxation. We observe a pronounced nonthermal distribution of nascent photoexcited electrons with lattice pseudospin polarization in remarkable agreement with results of simple tight-binding theory. By varying the excitation fluence, we vary the relative importance of electron-electron vs electron-phonon scattering in the relaxation of the initial distribution. Increasing the excitation fluence results in increased noncollinear electron-electron scattering and reduced pseudospin polarization, although up-scattered electrons retain a degree of polarization. These detailed momentum-resolved electron dynamics in graphene demonstrate the capabilities of high-performance time-resolved momentum microscopy in the study of 2D materials and can inform the design of graphene devices.

Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS_{2}.

Using time- and angle-resolved photoemission, we present momentum- and energy-resolved measurements of exciton coupling in monolayer WS_{2}. We observe strong intravalley coupling between the B_{1s} exciton and A_{n>1} states. Our measurements indicate that the dominant valley depolarization mechanism conserves the exciton binding energy and momentum. While this conservation is consistent with Coulomb exchange-driven valley depolarization, we do not observe a momentum or energy dependence to the depolarization rate as would be expected for the exchange-based mechanism.

Quantum plasmonic two-dimensional WS2-MoS2 heterojunction.

Two-dimensional heterostructures have recently gained broad interest due to potential applications in optoelectronic devices. Their reduced dimensionality leads to novel physical effects beyond conventional bulk electronics. However, the optical properties of the 2D lateral heterojunctions have not been completely characterized due to the limited spatial resolution, requiring nano-optical techniques beyond the diffraction limit. Here, we investigate lateral monolayer WS2-MoS2 heterostructures in a plasmonic Au-Au tip-substrate picocavity using subdiffraction limited tip-enhanced photoluminescence (TEPL) spectroscopy with sub-nanometer tip-sample distance control. We observed more than 3 orders of magnitude PL enhancement by placing a plasmonic Au-coated tip at the resonantly excited heterojunction. We developed a theoretical model of the quantum plasmonic 2D heterojunction, where tunneling of hot electrons between the Au tip and MoS2 leads to the quenching of the MoS2 PL, while simultaneously increasing the WS2 PL, in contrast to the non-resonant reverse transfer. Our simulations show good agreement with the experiments, revealing a range of parameters and enhancement factors corresponding to the switching between the classical and quantum regimes. The controllable photoresponse of the 2D heterojunction can be used in novel nanodevices.

Coupling nanobubbles in 2D lateral heterostructures.

Two-dimensional transition metal dichalcogenides provide flexible platforms for nanophotonic engineering due to their exceptional mechanical and optoelectronic properties. For example, continuous band gap tunability has been achieved in 2D TMDs by elastic strain engineering. Localized elastic deformations in nanobubbles behave as "artificial atoms" with a spatially varying band gap resulting in funnelling of excitons and photocarriers. Here we present a new method of nanobubble fabrication in monolayer 2D lateral heterostructures using high temperature superacid treatment. We fabricated MoS2 and WS2 nanobubbles and performed near-field imaging with nanoscale resolution using tip-enhanced photoluminescence (TEPL) spectroscopy. TEPL nanoimaging revealed the coupling between MoS2 and WS2 nanobubbles with a large synergistic PL enhancement due to the plasmonic tip, hot electrons, and exciton funnelling. We investigated the contributions of different enhancement mechanisms, and developed a quantum plasmonic model, in good agreement with the experiments. Our work opens new avenues in exploration of novel nanophotonic coupling schemes.