Optically controllable magnetism in atomically thin semiconductors.

We report evidence that ferromagnetic order in electrostatically doped, monolayer transition metal dichalcogenide (TMD) semiconductors can be stabilized and controlled at zero magnetic field by local optical pumping. We use circular dichroism (CD) in reflectivity from excitonic states as a spatially resolved probe of charge-carrier spin polarization. At electron densities ne ~ 1012 cm-2, a diffraction-limited, circularly polarized optical pump breaks symmetry between oppositely polarized magnetic states and stabilizes long-range magnetic order, with carrier polarization exceeding 80% over an 8 µm by 5 µm extent. In time-resolved measurements with pulsed optical excitation, we observe that magnetic interactions amplify the initial pump-induced spin polarization by more than an order of magnitude. The optical control of magnetism with local optical pumps will unlock advancements in spin and optical technologies and provides a versatile tool in the study of correlated phases in two-dimensional electron gases.

All-dielectric multi-resonant bullseye antennas.

Integrated devices that generate multiple optical resonances in the same volume can enhance on-chip nonlinear frequency generation, nonlinear spectroscopy, and quantum sensing. Here, we demonstrate circular Bragg antennas that exhibit multiple spatially overlapping, polarization-selective optical resonances. Using templated atomic layer deposition of TiO2, these devices can be fabricated on arbitrary substrates, making them compatible with a wide range of nonlinear materials and sensing targets, and couple efficiently to underlying films. In this work, we detail the design, simulation, and fabrication of all-dielectric multi-resonant bullseye antennas and characterize their performance using polarized broadband reflection spectroscopy.

Robotic four-dimensional pixel assembly of van der Waals solids.

Van der Waals (vdW) solids can be engineered with atomically precise vertical composition through the assembly of layered two-dimensional materials1,2. However, the artisanal assembly of structures from micromechanically exfoliated flakes3,4 is not compatible with scalable and rapid manufacturing. Further engineering of vdW solids requires precisely designed and controlled composition over all three spatial dimensions and interlayer rotation. Here, we report a robotic four-dimensional pixel assembly method for manufacturing vdW solids with unprecedented speed, deliberate design, large area and angle control. We used the robotic assembly of prepatterned 'pixels' made from atomically thin two-dimensional components. Wafer-scale two-dimensional material films were grown, patterned through a clean, contact-free process and assembled using engineered adhesive stamps actuated by a high-vacuum robot. We fabricated vdW solids with up to 80 individual layers, consisting of 100 × 100 µm2 areas with predesigned patterned shapes, laterally/vertically programmed composition and controlled interlayer angle. This enabled efficient optical spectroscopic assays of the vdW solids, revealing new excitonic and absorbance layer dependencies in MoS2. Furthermore, we fabricated twisted N-layer assemblies, where we observed atomic reconstruction of twisted four-layer WS2 at high interlayer twist angles of ≥4°. Our method enables the rapid manufacturing of atomically resolved quantum materials, which could help realize the full potential of vdW heterostructures as a platform for novel physics2,5,6 and advanced electronic technologies7,8.

Tunable and Transferable Diamond Membranes for Integrated Quantum Technologies.

Color centers in diamond are widely explored as qubits in quantum technologies. However, challenges remain in the effective and efficient integration of these diamond-hosted qubits in device heterostructures. Here, nanoscale-thick uniform diamond membranes are synthesized via "smart-cut" and isotopically (12C) purified overgrowth. These membranes have tunable thicknesses (demonstrated 50 to 250 nm), are deterministically transferable, have bilaterally atomically flat surfaces (Rq ≤ 0.3 nm), and bulk-diamond-like crystallinity. Color centers are synthesized via both implantation and in situ overgrowth incorporation. Within 110-nm-thick membranes, individual germanium-vacancy (GeV-) centers exhibit stable photoluminescence at 5.4 K and average optical transition line widths as low as 125 MHz. The room temperature spin coherence of individual nitrogen-vacancy (NV-) centers shows Ramsey spin dephasing times (T2*) and Hahn echo times (T2) as long as 150 and 400 µs, respectively. This platform enables the straightforward integration of diamond membranes that host coherent color centers into quantum technologies.

High-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2.

Integrating solid-state quantum emitters with nanophotonic resonators is essential for efficient spin-photon interfacing and optical networking applications. While diamond color centers have proven to be excellent candidates for emerging quantum technologies, their integration with optical resonators remains challenging. Conventional approaches based on etching resonators into diamond often negatively impact color center performance and offer low device yield. Here, we developed an integrated photonics platform based on templated atomic layer deposition of TiO2 on diamond membranes. Our fabrication method yields high-performance nanophotonic devices while avoiding etching wavelength-scale features into diamond. Moreover, this technique generates highly reproducible optical resonances and can be iterated on individual diamond samples, a unique processing advantage. Our approach is suitable for a broad range of both wavelengths and substrates and can enable high-cooperativity interfacing between cavity photons and coherent defects in diamond or silicon carbide, rare earth ions, or other material systems.

Electrical control of interlayer exciton dynamics in atomically thin heterostructures.

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. By employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate that neutral interlayer excitons can propagate across the entire sample and that their propagation can be controlled by excitation power and gate electrodes. We also use devices with ohmic contacts to facilitate the drift motion of charged interlayer excitons. The electrical generation and control of excitons provide a route for achieving quantum manipulation of bosonic composite particles with complete electrical tunability.

Electrically Tunable Exciton-Plasmon Coupling in a WSe2 Monolayer Embedded in a Plasmonic Crystal Cavity.

We realize a new electroplasmonic switch based upon electrically tunable exciton-plasmon interactions. The device consists of a hexagonal boron nitride (hBN)-encapsulated tungsten diselenide (WSe2) monolayer on top of a single-crystalline silver substrate. The ultrasmooth silver substrate serves a dual role as the medium to support surface plasmon polaritons (SPPs) and the bottom gate electrode to tune the WSe2 exciton energy and brightness through electrostatic doping. To enhance the exciton-plasmon coupling, we implement a plasmonic crystal cavity on top of the hBN/WSe2/hBN/Ag heterostructure with a quality factor reaching 550. The tight confinement of the SPPs in the plasmonic cavity enables strong coupling between excitons and SPPs when the WSe2 exciton absorption is resonant with the cavity mode, leading to a vacuum Rabi splitting of up to 18 meV. This strong coupling can also be switched off with the application of a modest gate voltage that increases the doping density in the monolayer. This demonstration paves the way for new plasmonic modulators and a general device architecture to enhance light-matter interactions between SPPs and various embedded emitters.

Large Excitonic Reflectivity of Monolayer MoSe_{2} Encapsulated in Hexagonal Boron Nitride.

We demonstrate that a single layer of MoSe_{2} encapsulated by hexagonal boron nitride can act as an electrically switchable mirror at cryogenic temperatures, reflecting up to 85% of incident light at the excitonic resonance. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor. We show that the MoSe_{2} monolayer exhibits power-and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used.

Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons.

Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin-orbit coupling and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication.

Visible-frequency hyperbolic metasurface.

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.