3D Localized Trions in Monolayer WSe2 in a Charge Tunable van der Waals Heterostructure.

Monolayer transition metal dichalcogenides (TMDCs) have recently emerged as a host material for localized optically active quantum emitters that generate single photons. (1-5) Here, we investigate fully localized excitons and trions from such TMDC quantum emitters embedded in a van der Waals heterostructure. We use direct electrostatic doping through the vertical heterostructure device assembly to generate quantum confined trions. Distinct spectral jumps as a function of applied voltage bias, and excitation power-dependent charging, demonstrate the observation of the two different excitonic complexes. We also observe a reduction of the intervalley electron-hole exchange interaction in the confined trion due to the addition of an extra electron, which is manifested by a decrease in its fine structure splitting. We further confirm this decrease of exchange interaction for the case of the charged states by a comparative study of the circular polarization resolved photoluminescence from individual excitonic states. The valley polarization selection rules inherited by the localized trions will provide a pathway toward realizing a localized spin-valley-photon interface.

Single self-assembled InAs/GaAs quantum dots in photonic nanostructures: The role of nanofabrication.

Single self-assembled InAs/GaAs quantum dots are a promising solid-state quantum technology, with which vacuum Rabi splitting, single-photon-level nonlinearities, and bright, pure, and indistinguishable single-photon generation having been demonstrated. For such achievements, nanofabrication is used to create structures in which the quantum dot preferentially interacts with strongly-confined optical modes. An open question is the extent to which such nanofabrication may also have an adverse influence, through the creation of traps and surface states that could induce blinking, spectral diffusion, and dephasing. Here, we use photoluminescence imaging to locate the positions of single InAs/GaAs quantum dots with respect to alignment marks with < 5 nm uncertainty, allowing us to measure their behavior before and after fabrication. We track the quantum dot emission linewidth and photon statistics as a function of distance from an etched surface, and find that the linewidth is significantly broadened (up to several GHz) for etched surfaces within a couple hundred nanometers of the quantum dot. However, we do not observe appreciable reduction of the quantum dot radiative efficiency due to blinking. We also show that atomic layer deposition can stabilize spectral diffusion of the quantum dot emission, and partially recover its linewidth.

Cryogenic photoluminescence imaging system for nanoscale positioning of single quantum emitters.

We report a photoluminescence imaging system for locating single quantum emitters with respect to alignment features. Samples are interrogated in a 4 K closed-cycle cryostat by a high numerical aperture (NA = 0.9, 100× magnification) objective that sits within the cryostat, enabling high efficiency collection of emitted photons without image distortions due to the cryostat windows. The locations of single InAs/GaAs quantum dots within a >50 µm × 50 µm field of view are determined with ≈4.5 nm uncertainty (one standard deviation) in a 1 s long acquisition. The uncertainty is determined through a combination of a maximum likelihood estimate for localizing the quantum dot emission, and a cross correlation method for determining the alignment mark center. This location technique can be an important step in the high-throughput creation of nanophotonic devices that rely upon the interaction of highly confined optical modes with single quantum emitters.

Laser-induced fluorescence from N2(+) ions generated by a corona discharge in ambient air.

In this work, we present the measurement of laser-induced fluorescence from N2(+) ions via the B(2)Σu(+)-X(2)Σg(+) band system in the near-ultraviolet. The ions were generated continuously by a plasma glow discharge in low pressure N2 and by a corona discharge in ambient air. The fluorescence decay time was found to rapidly decrease with increasing pressure leading to an extrapolated decay rate of ≍10(10) s(-1) at atmospheric pressure. In spite of this quenching, we were able to observe laser induced fluorescence in ambient air by means of a time-gated spectral measurement. In the process of comparing the emission signal with that of N2 spontaneous Raman scattering, ion concentrations in ambient air of order 10(8-)10(10) cm(-3) were determined. With moderate increases in laser power and collection efficiency, ion concentrations of less than 10(6) cm(-3) may be measurable, potentially enabling applications in atmospheric standoff detection of ionizing radiation from hazardous radioactive sources.

Feedback-controlled laser fabrication of micromirror substrates.

Short (40-200 µs) single focused CO(2) laser pulses of energy ≳100 µJ were used to fabricate high quality concave micromirror templates on silica and fluoride glass. The ablated features have diameters of ≈20-100 µm and average root-mean-square (RMS) surface microroughness near their center of less than 0.2 nm. Temporally monitoring the fabrication process revealed that it proceeds on a time scale shorter than the laser pulse duration. We implement a fast feedback control loop (≈20 kHz bandwidth) based on the light emitted by the sample that ensures an RMS size dispersion of less than 5% in arrays on chips or in individually fabricated features on an optical fiber tip, a significant improvement over previous approaches using longer pulses and open loop operation.