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  1. Abstract

    Triangular cross-section silicon carbide (SiC) photonic devices have been studied as an efficient and scalable route for integration of color centers into quantum hardware. In this work, we explore efficient collection and detection of color center emission in a triangular cross-section SiC waveguide by introducing a photonic crystal mirror on its one side and a superconducting nanowire single photon detector (SNSPD) on the other. Our modeled triangular cross-section devices with a randomly positioned emitter have a maximum coupling efficiency of 89% into the desired optical mode and a high coupling efficiency (>75%) in more than half of the configurations. For the first time, NbTiN thin films were sputtered on 4H-SiC and the electrical and optical properties of the thin films were measured. We found that the transport properties are similar to the case of NbTiN on SiO2substrates, while the extinction coefficient is up to 50% higher for 1680 nm wavelength. Finally, we performed finite-difference time-domain simulations of triangular cross-section waveguide integrated with an SNSPD to identify optimal nanowire geometries for efficient detection of light from transverse electric and transverse magnetic polarized modes.

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  2. Abstract

    We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots (QDs) on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III–V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factorsQ17000representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of400MHzresonators is achieved to temperaturesT>7Kand electrical radio frequency powersPrf>+9dBm. Heterogeneously integrated single QDs couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid QD—surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the QD. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.

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  3. Abstract

    The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS2, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS2/graphene stacks employing first-principles calculations and find that the lower conduction band of WS2at theK/Kvalleys (theCBband) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at theQpoint of WS2mediates the coupling betweenCBand graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

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  4. Abstract

    GaAs‐AlGaAs based nanowire (NW) lasers hold great potential for on‐chip photonic applications, where lasing metrics have steadily improved over the years by optimizing resonator design and surface passivation methods. The factor that will ultimately limit the performance will depend on material properties, such as native‐ or impurity‐induced point defects and their impact on non‐radiative recombination. Here, the role of impurity‐induced point defects on the lasing performance of low‐threshold GaAs(Sb)‐AlGaAs NW‐lasers is evaluated, particularly by exploring Si‐dopants and their associated vacancy complexes. Si‐induced point defects and their self‐compensating nature are identified using correlated atom probe tomography, resonant Raman scattering, and photoluminescence experiments. Under pulsed optical excitation the lasing threshold is remarkably low (<10 µJ cm−2) and insensitive to impurity defects over a wide range of Si doping densities, while excess doping ([Si]>1019 cm−3) imposes increased threshold at low temperature. These characteristics coincide with increased Shockley‐Read‐Hall recombination, reflected by shorter carrier lifetimes, and reduced internal quantum efficiencies (IQE) . Remarkably, despite the lower IQE the presence of self‐compensating Si‐vacancy defects provides an improved temperature stability in lasing threshold with higher characteristic temperature and room‐temperature lasing. These findings highlight an overall large tolerance of lasing metrics to impurity defects in GaAs‐AlGaAs based NW‐lasers.

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  6. Abstract

    Janus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long‐lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, second and third harmonic generation in MoSSe and WSSe Janus monolayers is studied. Polarization‐resolved spectroscopy is used to map the full second‐order susceptibility tensor of MoSSe, including its out‐of‐plane components. In addition, the effective third‐order susceptibility and the second‐order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe are measured at room and cryogenic temperatures. This work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next‐generation on‐chip multifaceted photonic devices.

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  7. Abstract

    High-quality sources of single photons are of paramount importance for quantum communication, sensing, and metrology. To these ends, resonantly excited two-level systems based on self-assembled quantum dots have recently generated widespread interest. Nevertheless, we have recently shown that for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity. Here, we demonstrate that generating single photons from self-assembled quantum dots with a scheme based on two-photon excitation of the biexciton strongly suppresses the re-excitation. Specifically, the pulse-length dependence of the multi-photon error rate reveals a quadratic dependence in contrast to the linear dependence of resonantly excited two-level systems, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.

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  8. Abstract

    Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin–orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2and via plasma stripping followed thermal annealing of MoS2. However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room‐temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low‐energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room‐temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

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