Three-dimensional confinement allows semiconductor quantum dots to exhibit size-tunable electronic and optical properties that enable a wide range of opto-electronic applications from displays, solar ...cells and bio-medical imaging to single-electron devices. Additional modalities such as spin and valley properties in monolayer transition metal dichalcogenides provide further degrees of freedom requisite for information processing and spintronics. In nanostructures, however, spatial confinement can cause hybridization that inhibits the robustness of these emergent properties. Here, we show that laterally-confined excitons in monolayer MoS
nanodots can be created through top-down nanopatterning with controlled size tunability. Unlike chemically-exfoliated monolayer nanoparticles, the lithographically patterned monolayer semiconductor nanodots down to a radius of 15 nm exhibit the same valley polarization as in a continuous monolayer sheet. The inherited bulk spin and valley properties, the size dependence of excitonic energies, and the ability to fabricate MoS
nanostructures using semiconductor-compatible processing suggest that monolayer semiconductor nanodots have potential to be multimodal building blocks of integrated optoelectronics and spintronics systems.
Abstract
Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states. Though most simply achieved with magnetic fields, polarization-sensitive optical ...methods provide high-speed alternatives. Exploiting the optical selection rules of transition metal dichalcogenide monolayers, the optical Stark effect allows for ultrafast manipulation of valley-coherent excitons. Compared to excitons in these materials, microcavity exciton-polaritons offer a promising alternative for valley manipulation, with longer lifetimes, enhanced valley coherence, and operation across wider temperature ranges. Here, we show valley-selective control of polariton energies in WS
2
using the optical Stark effect, extending coherent valley manipulation to the hybrid light-matter regime. Ultrafast pump-probe measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts. This demonstration of valley degeneracy breaking at picosecond timescales establishes a method for coherent control of valley phenomena in exciton-polaritons.
We describe an approach to optical non-reciprocity that exploits the local helicity of evanescent electric fields in axisymmetric resonators. By interfacing an optical cavity to helicity-sensitive ...transitions, such as Zeeman levels in a quantum dot, light transmission through a waveguide becomes direction-dependent when the state degeneracy is lifted. Using a linearized quantum master equation, we analyze the configurations that exhibit non-reciprocity, and we show that reasonable parameters from existing cavity QED experiments are sufficient to demonstrate a coherent non-reciprocal optical isolator operating at the level of a single photon.
Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer ...transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.
In a monolayer transition metal dichalcogenide (TMDC) that lacks structural inversion symmetry, spin degeneracy is lifted by strong spin–orbit coupling, and a distinctive spin-valley locking allows ...for the creation of valley-locked spin-polarized carriers with a circularly polarized optical excitation. When excited carriers also have net in-plane momentum, spin-polarized photocurrents can be generated at ambient temperature without magnetic fields or materials. The behavior of these spin-polarized photocurrents in monolayer TMDC remains largely unexplored. In this work, we demonstrate the tuning of spin-valley photocurrent generated from the circularly polarized photogalvanic effect in monolayer MoS2, including magnitude and polarization degree, by purely electric means at room temperature. The magnitude of spin-polarized photocurrent can be modulated up to 45 times larger, and the polarization degree of the total photocurrent can be tuned significantly (here from 0.5 to 16.6%) by gate control. Combined with the atomic thickness and wafer-scale growth capabilities of monolayer TMDC, the efficient electrical tuning of spin-valley photocurrent suggests a pathway to achieve spin-logic processing by local gate architectures in monolayer opto-spintronic devices.
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► We have performed a series of simulations of buoyant hydrothermal plumes within Europa’s ocean. ► Plumes in our model are 10–35km in diameter, have temperatures of 10–200μK, and ...velocities of 1–5cm/s. ► Plume parameters are controlled by a single nondimensional parameter, but differ in detail from previous theory.
The liquid water interiors of Europa and other icy moons of the outer Solar System are likely to be driven by geothermal heating from the sea floor, leading to the development of buoyant hydrothermal plumes. These plumes potentially control icy surface geomorphology, and are of interest to astrobiologists. We have performed a series of simulations of these plumes using the MIT GCM ocean circulation model. We assume here that Europa’s ocean is deep (of order 100km) and unstratified, and that plume buoyancy is controlled by temperature, not composition. Our experiments explore a limited region of parameter space, with ocean depth H ranging from 50 to 100km deep, source heat flux Q between 0.1 and 10GW, and Coriolis parameter f corresponding to Europa latitudes between 9° and 47°. As predicted by earlier work, the plumes in our simulations form narrow cylindrical chimneys (a few km across) under the influence of the Coriolis effect. These plumes broaden over time until they become baroclinically unstable, breaking up into cone-shaped eddies when they become 10–35km in diameter; the shed eddies are of a similar size. Large-scale currents in the region of the plume range between 1 and 5cm/s; temperature anomalies in the plume far from the seafloor are tiny, varying between 10 and 180μK. Variations in plume size, shape, speed, and temperature are in excellent agreement with previous laboratory tank experiments, and in rough agreement with theoretical predictions. Plume dynamics and geometry are controlled by a “natural Rossby number” which depends strongly on depth H and Coriolis parameter f, but only weakly on source heat flux Q. However, some specific theoretical predictions are not borne out by these simulations: this may occur because the plumes are “reingesting” their own emissions, a process not considered in our earlier theory.
The explosion of interest in two-dimensional van der Waals materials has been in many ways driven by their layered geometry. This feature makes possible numerous avenues for assembling and ...manipulating the optical and electronic properties of these materials. In the specific case of monolayer transition metal dichalcogenide semiconductors, the direct band gap combined with the flexibility for manipulation of layers has made this class of materials promising for optoelectronics. Here, we review the properties of these layered materials and the various means of engineering these properties for optoelectronics. We summarize approaches for control that modify their structural and chemical environment, and we give particular detail on the integration of these materials into engineered optical fields to control their optical characteristics. This combination of controllability from their layered surface structure and photonic environment provide an expansive landscape for novel optoelectronic phenomena.
The various decay mechanisms of excitons in cuprous oxide (Cu2O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton ...luminescence from cuprous oxide crystals grown via the floating zone method showing that the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation of oxygen and copper vacancies, which leads to a negative correlation between luminescence of unlike vacancies.
•We use luminescence to observe defects in high quality cuprous oxide crystals.•Strain is reduced by annealing.•Annealing causes annihilation of oxygen and copper vacancies.
Energy band realignment at the interfaces between materials in heterostructures can give rise to unique electronic characteristics and non-trivial low-dimensional charge states. In a homojunction of ...monolayer and multilayer MoS2, the thickness-dependent band structure implies the possibility of band realignment and a new interface charge state with properties distinct from the isolated layers. In this report, we probe the interface charge state using scanning photocurrent microscopy and gate-dependent transport with source-drain bias applied along the interface. Enhanced photoresponse observed at the interface is attributed to band bending. The effective conductivity of a material with a monolayer-multilayer interface of MoS2 is demonstrated to be higher than that of independent monolayers or multilayers of MoS2. A classic heterostructure model is constructed to interpret the electrical properties at the interface. Our work reveals that the band engineering at the transition metal dichalcogenides monolayer/multilayer interfaces can enhance the longitudinal conductance and field-effect mobility of the composite monolayer and multilayer devices.
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