Core/Shell-Like Localized Emission at Atomically Thin Semiconductor-Au Interface.

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

Self-Hybridized Polaritonic Emission from Layered Perovskites.

Light-matter coupling in excitonic materials has been the subject of intense recent investigations due to emergence of new materials. Two-dimensional layered hybrid organic/inorganic perovskites (2D HOIPs) support strongly bound excitons at room temperature with some of the highest oscillator strengths and electric loss tangents among the known excitonic materials. Here, we report strong light-matter coupling in Ruddlesden-Popper phase 2D HOIP crystals without the necessity of an external cavity. We report the concurrent occurrence of multiple orders of hybrid light-matter states via both reflectance and luminescence spectroscopy in thick (>100 nm) crystals and near-unity absorption in thin (<20 nm) crystals. We observe resonances with quality factors of >250 in hybridized exciton-polaritons and identify a linear correlation between exciton-polariton mode splitting and extinction coefficient of the various 2D HOIPs. Our work opens the door to studying polariton dynamics in self-hybridized and open cavity systems with broad applications in optoelectronics and photochemistry.

Dynamics of self-hybridized exciton-polaritons in 2D halide perovskites.

Excitons, bound electron-hole pairs, in two-dimensional hybrid organic inorganic perovskites (2D HOIPs) are capable of forming hybrid light-matter states known as exciton-polaritons (E-Ps) when the excitonic medium is confined in an optical cavity. In the case of 2D HOIPs, they can self-hybridize into E-Ps at specific thicknesses of the HOIP crystals that form a resonant optical cavity with the excitons. However, the fundamental properties of these self-hybridized E-Ps in 2D HOIPs, including their role in ultrafast energy and/or charge transfer at interfaces, remain unclear. Here, we demonstrate that >0.5 µm thick 2D HOIP crystals on Au substrates are capable of supporting multiple-orders of self-hybridized E-P modes. These E-Ps have high Q factors (>100) and modulate the optical dispersion for the crystal to enhance sub-gap absorption and emission. Through varying excitation energy and ultrafast measurements, we also confirm energy transfer from higher energy E-Ps to lower energy E-Ps. Finally, we also demonstrate that E-Ps are capable of charge transport and transfer at interfaces. Our findings provide new insights into charge and energy transfer in E-Ps opening new opportunities towards their manipulation for polaritonic devices.

Tunable Localized Charge Transfer Excitons in Nanoplatelet-2D Chalcogenide van der Waals Heterostructures.

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D-2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate-2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe2 and WSe2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

Valley-Polarized Interlayer Excitons in 2D Chalcogenide-Halide Perovskite-van der Waals Heterostructures.

Interlayer excitons (IXs) in two-dimensional (2D) heterostructures provide an exciting avenue for exploring optoelectronic and valleytronic phenomena. Presently, valleytronic research is limited to transition metal dichalcogenide (TMD) based 2D heterostructure samples, which require strict lattice (mis) match and interlayer twist angle requirements. Here, we explore a 2D heterostructure system with experimental observation of spin-valley layer coupling to realize helicity-resolved IXs, without the requirement of a specific geometric arrangement, i.e., twist angle or specific thermal annealing treatment of the samples in 2D Ruddlesden-Popper (2DRP) halide perovskite/2D TMD heterostructures. Using first-principle calculations, time-resolved and circularly polarized luminescence measurements, we demonstrate that Rashba spin-splitting in 2D perovskites and strongly coupled spin-valley physics in monolayer TMDs render spin-valley-dependent optical selection rules to the IXs. Consequently, a robust valley polarization of ∼14% with a long exciton lifetime of ∼22 ns is obtained in type-II band aligned 2DRP/TMD heterostructure at ∼1.54 eV measured at 80 K. Our work expands the scope for studying spin-valley physics in heterostructures of disparate classes of 2D semiconductors.

Enhancing the Purity of Deterministically Placed Quantum Emitters in Monolayer WSe2.

We present a method utilizing an applied electrostatic potential for suppressing the broad defect bound excitonic emission in two-dimensional materials (2DMs) which otherwise inhibits the purity of strain induced single photon emitters (SPEs). Our heterostructure consists of a WSe2 monolayer on a polymer in which strain has been deterministically introduced via an atomic force microscope (AFM) tip. We show that by applying an electrostatic potential, the broad defect bound background is suppressed at cryogenic temperatures, resulting in a substantial improvement in single photon purity demonstrated by a 10-fold reduction of the correlation function g(2)(0) value from 0.73 to 0.07. In addition, we see a 2-fold increase in the intensity of the SPEs as well as the ability to activate/deactivate the emitters at certain wavelengths. Finally, we present an increase in the operating temperature of the SPE up to 110 K, a 50 K increase when compared with the results when no electrostatic potential is present.

High-Density, Localized Quantum Emitters in Strained 2D Semiconductors.

Two-dimensional chalcogenide semiconductors have recently emerged as a host material for quantum emitters of single photons. While several reports on defect- and strain-induced single-photon emission from 2D chalcogenides exist, a bottom-up, lithography-free approach to producing a high density of emitters remains elusive. Further, the physical properties of quantum emission in the case of strained 2D semiconductors are far from being understood. Here, we demonstrate a bottom-up, scalable, and lithography-free approach for creating large areas of localized emitters with high density (∼150 emitters/um2) in a WSe2 monolayer. We induce strain inside the WSe2 monolayer with high spatial density by conformally placing the WSe2 monolayer over a uniform array of Pt nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable luminescence measurements combined with near-field luminescence spectroscopy suggest the formation of localized states in strained regions that emit single photons with a high spatial density. Our approach of using a metal nanoparticle array to generate a high density of strained quantum emitters will be applied to scalable, tunable, and versatile quantum light sources.

Coherent two-dimensional Fourier transform spectroscopy using a 25 Tesla resistive magnet.

We performed nonlinear optical two-dimensional Fourier transform spectroscopy measurements using an optical resistive high-field magnet on GaAs quantum wells. Magnetic fields up to 25 T can be achieved using the split helix resistive magnet. Two-dimensional spectroscopy measurements based on the coherent four-wave mixing signal require phase stability. Therefore, these measurements are difficult to perform in environments prone to mechanical vibrations. Large resistive magnets use extensive quantities of cooling water, which causes mechanical vibrations, making two-dimensional Fourier transform spectroscopy very challenging. Here, we report on the strategies we used to overcome these challenges and maintain the required phase-stability throughout the measurement. A self-contained portable platform was used to set up the experiments within the time frame provided by a user facility. Furthermore, this platform was floated above the optical table in order to isolate it from vibrations originating from the resistive magnet. Finally, we present two-dimensional Fourier transform spectra obtained from GaAs quantum wells at magnetic fields up to 25 T and demonstrate the utility of this technique in providing important details, which are obscured in one dimensional spectroscopy.

Biexcitons in monolayer transition metal dichalcogenides tuned by magnetic fields.

We present time-integrated four-wave mixing measurements on monolayer MoSe2 in magnetic fields up to 25 T. The experimental data together with time-dependent density function theory calculations provide interesting insights into the biexciton formation and dynamics. In the presence of magnetic fields the coherence at negative and positive time delays is dominated by intervalley biexcitons. We demonstrate that magnetic fields can serve as a control to enhance the biexciton formation and help search for more exotic states of matter, including the creation of multiple exciton complexes and excitonic condensates.

Progressive osseous heteroplasia in the face of a child.

We describe a rare case of progressive osseous heteroplasia of the face in a child. Biopsy showed osteoma cutis superficially with ectopic bone formation in the deeper tissues including skeletal muscle. Analysis of DNA from peripheral blood leukocytes showed mutations in the gene encoding the alpha subunit of the stimulatory G protein of adenylyl cyclase (GNAS1), confirming the diagnosis of progressive osseous heteroplasia.