Photocurrent Spectroscopy of Dark Magnetic Excitons in 2D Multiferroic NiI.

Two-dimensional (2D) antiferromagnetic (AFM) semiconductors are promising components of opto-spintronic devices due to terahertz operation frequencies and minimal interactions with stray fields. However, the lack of net magnetization significantly limits the number of experimental techniques available to study the relationship between magnetic order and semiconducting properties. Here, they demonstrate conditions under which photocurrent spectroscopy can be employed to study many-body magnetic excitons in the 2D AFM semiconductor NiI.

Dark State Concentration Dependent Emission and Dynamics of CdSe Nanoplatelet Exciton-Polaritons.

The recent surge of interest in polaritons has prompted fundamental questions about the role of dark states in strong light-matter coupling phenomena. Here, we systematically vary the relative number of dark states by controlling the number of stacked CdSe nanoplatelets confined in a Fabry-Pérot cavity. We find the emission spectrum to change significantly with an increasing number of nanoplatelets, with a gradual shift of the dominant emission intensity from the lower polariton branch to a manifold of dark states.

Control of Photoswitching Kinetics with Strong Light-Matter Coupling in a Cavity.

Most photochemistry occurs in the regime of weak light-matter coupling, in which a molecule absorbs a photon and then performs photochemistry from its excited state. In the strong coupling regime, enhanced light-matter interactions between an optical field and multiple molecules lead to collective hybrid light-matter states called polaritons. This strong coupling leads to fundamental changes in the nature of the excited states including multi-molecule delocalized excitations, modified potential energy surfaces, and dramatically altered energy levels relative to non-coupled molecules.

Molecular and solid-state topological polaritons induced by population imbalance.

Strong coupling between electronic excitations in materials and photon modes results in the formation of polaritons, which display larger nonlinearities than their photonic counterparts due to their material component. We theoretically investigate how to optically control the topological properties of molecular and solid-state exciton-polariton systems by exploiting one such nonlinearity: saturation of electronic transitions. We demonstrate modification of the Berry curvature of three different materials when placed within a Fabry-Perot cavity and pumped with circularly polarized light, illustrating the broad applicability of our scheme.

Chemomechanical modification of quantum emission in monolayer WSe.

Two-dimensional (2D) materials have attracted attention for quantum information science due to their ability to host single-photon emitters (SPEs). Although the properties of atomically thin materials are highly sensitive to surface modification, chemical functionalization remains unexplored in the design and control of 2D material SPEs. Here, we report a chemomechanical approach to modify SPEs in monolayer WSe through the synergistic combination of localized mechanical strain and noncovalent surface functionalization with aryl diazonium chemistry.

Simulating molecular polaritons in the collective regime using few-molecule models.

The study of molecular polaritons beyond simple quantum emitter ensemble models (e.g., Tavis-Cummings) is challenging due to the large dimensionality of these systems and the complex interplay of molecular electronic and nuclear degrees of freedom.

Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides.

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.

Mechanistic Investigation of Molybdenum Disulfide Defect Photoluminescence Quenching by Adsorbed Metallophthalocyanines.

Lattice defects play an important role in determining the optical and electrical properties of monolayer semiconductors such as MoS. Although the structures of various defects in monolayer MoS are well studied, little is known about the nature of the fluorescent defect species and their interaction with molecular adsorbates. In this study, the quenching of the low-temperature defect photoluminescence (PL) in MoS is investigated following the deposition of metallophthalocyanines (MPcs).

Spatial Mapping of Electrostatic Fields in 2D Heterostructures.

electron microscopy is an effective tool for understanding the mechanisms driving novel phenomena in 2D structures. However, due to practical challenges, it is difficult to address these technologically relevant 2D heterostructures with electron microscopy. Here, we use the differential phase contrast (DPC) imaging technique to build a methodology for probing local electrostatic fields during electrical operation with nanoscale spatial resolution in such materials.

Valley-selective optical Stark effect of exciton-polaritons in a monolayer semiconductor.

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.

Optimizing photonic ring-resonator filters for OH-suppressed near-infrared astronomy.

Near-infrared wavelength observations are crucial for understanding numerous fields of astrophysics, such as supernova cosmology and positronium annihilation detection. However, current ground-based observations suffer from an enormous background due to OH emission in the upper atmosphere. One promising way to solve this problem is to use ring-resonator filters to suppress OH emission lines.

Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride.

The optoelectronic properties of organic thin films are strongly dependent on their molecular orientation and packing, which in turn is sensitive to the underlying substrate. Hexagonal boron nitride (hBN) and other van der Waals (vdW) materials are known to template different organic thin film growth modalities from conventional inorganic substrates such as SiO. Here, the morphology and temperature-dependent optical properties of pentacene films grown on hBN are reported.

Halide perovskite nanocrystal arrays: Multiplexed synthesis and size-dependent emission.

Halide perovskites have exceptional optoelectronic properties, but a poor understanding of the relationship between crystal dimensions, composition, and properties limits their use in integrated devices. We report a new multiplexed cantilever-free scanning probe method for synthesizing compositionally diverse and size-controlled halide perovskite nanocrystals spanning square centimeter areas. Single-particle photoluminescence studies reveal multiple independent emission modes due to defect-defined band edges with relative intensities that depend on crystal size at a fixed composition.

Direct Visualization of Electric-Field-Induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides.

Layered transition metal dichalcogenides offer many attractive features for next-generation low-dimensional device geometries. Due to the practical and fabrication challenges related to methods, the atomistic dynamics that give rise to realizable macroscopic device properties are often unclear. In this study, transmission electron microscopy techniques are utilized in order to understand the structural dynamics at play, especially at interfaces and defects, in the prototypical film of monolayer MoS under electrical bias.

Electronic Coupling in Metallophthalocyanine-Transition Metal Dichalcogenide Mixed-Dimensional Heterojunctions.

Mixed-dimensional heterojunctions, such as zero-dimensional (0D) organic molecules deposited on two-dimensional (2D) transition metal dichalcogenides (TMDCs), often exhibit interfacial effects that enhance the properties of the individual constituent layers. Here we report a systematic study of interfacial charge transfer in metallophthalocyanine (MPc) - MoS heterojunctions using optical absorption and Raman spectroscopy to elucidate M core (M = first row transition metal), MoS layer number, and excitation wavelength effects. Observed phenomena include the emergence of heterojunction-specific optical absorption transitions and strong Raman enhancement that depends on the M identity.

Electrical Control of Circular Photogalvanic Spin-Valley Photocurrent in a Monolayer Semiconductor.

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.

Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts.

Understanding the electronic transport of monolayer transition metal dichalcogenides (TMDs) and their heterostructures is complicated by the difficulty in achieving electrical contacts that do not perturb the material. Typically, metal deposition on monolayer TMDs leads to hybridization between the TMD and the metal, which produces Schottky barriers at the metal/semiconductor interface. In this work, we apply the recently reported hexagonal boron nitride (h-BN) tunnel contact scheme to probe the junction characteristics of a lateral TMD heterostructure grown via chemical vapor deposition.

Size-tunable Lateral Confinement in Monolayer Semiconductors.

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.

Directional emission from dye-functionalized plasmonic DNA superlattice microcavities.

Three-dimensional plasmonic superlattice microcavities, made from programmable atom equivalents comprising gold nanoparticles functionalized with DNA, are used as a testbed to study directional light emission. DNA-guided nanoparticle colloidal crystallization allows for the formation of micrometer-scale single-crystal body-centered cubic gold nanoparticle superlattices, with dye molecules coupled to the DNA strands that link the particles together, in the form of a rhombic dodecahedron. Encapsulation in silica allows one to create robust architectures with the plasmonically active particles and dye molecules fixed in space.

Coherent optical non-reciprocity in axisymmetric resonators.

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.