Metamaterials are artificial optical media composed of sub-wavelength metallic and dielectric building blocks that feature optical phenomena not present in naturally occurring materials. Although they can serve as the basis for unique optical devices that mould the flow of light in unconventional ways, three-dimensional metamaterials suffer from extreme propagation losses. Two-dimensional metamaterials (metasurfaces) such as hyperbolic metasurfaces for propagating surface plasmon polaritons have the potential to alleviate this problem. Because the surface plasmon polaritons are guided at a metal-dielectric interface (rather than passing through metallic components), these hyperbolic metasurfaces have been predicted to suffer much lower propagation loss while still exhibiting optical phenomena akin to those in three-dimensional metamaterials. Moreover, because of their planar nature, these devices enable the construction of integrated metamaterial circuits as well as easy coupling with other optoelectronic elements. Here we report the experimental realization of a visible-frequency hyperbolic metasurface using single-crystal silver nanostructures defined by lithographic and etching techniques. The resulting devices display the characteristic properties of metamaterials, such as negative refraction and diffraction-free propagation, with device performance greatly exceeding those of previous demonstrations. Moreover, hyperbolic metasurfaces exhibit strong, dispersion-dependent spin-orbit coupling, enabling polarization- and wavelength-dependent routeing of surface plasmon polaritons and two-dimensional chiral optical components. These results open the door to realizing integrated optical meta-circuits, with wide-ranging applications in areas from imaging and sensing to quantum optics and quantum information science.

Excitons in semiconductors, bound pairs of excited electrons and holes, can form the basis for new classes of quantum optoelectronic devices. A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. Employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate the transport of neutral interlayer excitons across the whole sample that can be controlled by excitation power and gate electrodes. We also realize the drift motion of charged interlayer excitons using Ohmic-contacted devices. The electrical generation and control of excitons provides a new route for realizing quantum manipulation of bosonic composite particles with complete electrical tunability.As bosonic composite particles, long-lived excitons can be potentially utilized for the realization of coherent quantum many-body systems (1, 2) or as quantum information carriers (3,4). In conventional semiconductors, the exciton lifetime can be increased by constructing double quantum well (DQW) heterostructures, where spatially separated electrons and holes form interlayer excitons (IEs) across the quantum wells (5-10). Strongly bound IEs can also be formed in atomically thin DQW. By stacking two

When an optical dipole is in proximity to a metallic substrate, it can emit light into both far field photons and SPPs. Far-field emission can be measured directly via top-down optical microscopy, whereas SPP emission can be detected by converting SPPs into far-field light via engineered out-coupling structures (Fig. 1a). On a single-crystal silver film, our metal of choice due to its low loss 11 , SPPs are strongly polarized in the out-of-plane (z) direction in the visible frequency range (see Supplementary Information). Consequently, the emission rate into SPPs for an out-of-plane dipole can be as high as 30 times larger than that of an in-plane dipole (Figs. 1b-d, for details of the analysis see Supplementary Information). At the same time, far-field emission of an in-plane dipole is strongly suppressed (Figs. 1b and d) because the in-plane electric field is close to zero near the silver surface. We note that when a point dipole is close to a metal 12 , non-radiative recombination due to ohmic loss can be the dominant decay mechanism.Remarkably, for delocalized excitons in quantum wells and 2D materials, quenching of exciton luminescence by ohmic loss is significantly reduced, even when they are placed 10 nm above a silver surface ( see [ 13 ] and Supplementary Fig. 1 and discussion). Combined together, the net effect of a nearby silver surface is significantly enhanced (suppressed) emission of an out-ofplane (in-plane) dipole into SPPs (far field). (Fig. 1a). The spacing between the monolayer TMD and the silver surface is determined by the bottom hBN thickness, and can easily be controlled by varying hBN thickness. In our devices, the typical spacing is on the order of ten nanometers.Excitons are created using off-resonant 660-nm laser excitation, and the PL spectra are voltages. We normalize both FF and SPP-PL spectra using the intensity of a charged exciton peak X T because it is known to involve a purely in-plane transition dipole moment 16 . The ratio of SPP-PL intensity to the FF-PL intensity after the normalization provides a direct measure of the orientation of the transition dipole for each luminescent species: the unity ratio represents a purely in-plane dipole, while a value larger than one indicates that the transition dipole has some out-of-plane components. Based on our theoretical calculations presented in Fig. 1d and Supplementary Fig. 4, an optical transition with a purely out-of-plane transition dipole should have a normalized coupling ratio of 7 in our device geometry. The experimental results for X D yield a value of 16: this discrepancy between theory and experiment is likely due to small, yet non-negligible absorption of SPPs by charged excitons as they propagate through the WSe 2 , which increases the apparent coupling ratio of X D after normalization (see Supplementary Fig. 5). Indeed, when SPPs propagate through a minimal distance within WSe 2 ( Supplementary Fig. 6), the normalized coupling ratio determined by experiment is close to 7, in good agreement with the theoretical calc...

scattering has emerged as a method for overcoming these limitations and for controlling light at the atomic scale (3-11). For instance, highly reflective mirrors based on individual quantum emitters have been demonstrated by coupling them to optical cavities and nanophotonic waveguides (3-8). Such resonant mirrors feature very unusual properties due to their extraordinary nonlinearity down to the single-photon level (3-8). A two-dimensional (2D) layer of emitters, such as atomic lattices or excitons (9-11), has also been predicted to act as an efficient mirror when the incident light is resonant with the resonance frequency of the system. Such atomically thin mirrors represent the ultimate miniaturization limit of a reflective surface, and could enable unique applications ranging from quantum nonlinear optics (9-11) to topological photonics (12,13).This Report demonstrates that transition metal dichalcogenide (TMD) monolayers can act as atomically thin, electrically switchable, resonant mirrors. These materials are direct-bandgap semiconductors that support tightly bound excitons. Excitonic transitions in TMD monolayers exhibit large oscillator strengths (14-16), resulting in large radiative linewidths compared to excitons in other semiconductor systems. In addition, the excitonic response in monolayers can be controlled electrically via gate-induced doping and by shifting the chemical potential (17)(18)(19).Importantly, these monolayers can be easily integrated with other 2D materials via Van der Waals stacking to improve their quality or add new functionalities. One of the most studied amongst such heterostructures is a TMD monolayer encapsulated by two hexagonal boron nitride 4 (hBN) flakes: this "passivated" monolayer exhibits enhanced carrier mobility (19,20) and reduced photoluminescence linewidth (21,22).Our experiments make use of a device that consists of an hBN-passivated molybdenum diselenide (MoSe 2 ) monolayer placed on an oxide-covered silicon (Si) substrate, and we measure its reflectivity with a normally incident laser beam (Figs. 1A and 1B). The doped Si substrate is used as a gate electrode: by applying a gate voltage (V g ), MoSe 2 monolayers can be made intrinsic or n-doped. When a monochromatic laser beam is tuned to the exciton resonance, we observe substantial reflection from a monolayer device (M1) at V g < 10 V at T = 4 K (Fig. 1C).The reflection contrast between the monolayer region and the substrate disappears at V g > 20 V ( Fig. 1D), indicating that the reflection can be turned off electrically. When we illuminate another monolayer device (M2) with a supercontinuum laser and spectrally resolve the reflection, we find that both the magnitudes and wavelength positions of the reflectance peaks change with V g (Fig. 1E). When the monolayer is intrinsic (V g < 10 V), the reflection is dominated by a peak at the wavelength of the neutral exciton transition. When MoSe 2 is n-doped (V g > 20 V), however, the reflection by the neutral exciton disappears, and a new, weaker, reflectance peak ap...

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices. The unique band structure of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley-spin qubits . 2D TMDs also provide a platform for novel quantum optoelectronic devices due to their large exciton binding energy. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin-valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons.