Kenneth M. Goodfellow

Voltage-controlled quantum light from an atomically thin semiconductor

Although semiconductor defects can often be detrimental to device performance, they are also responsible for the breadth of functionality exhibited by modern optoelectronic devices. Artificially engineered defects (so-called quantum dots) or naturally occurring defects in solids are currently being investigated for applications ranging from quantum information science and optoelectronics to high-resolution metrology. In parallel, the quantum confinement exhibited by atomically thin materials (semi-metals, semiconductors and insulators) has ushered in an era of flatland optoelectronics whose full potential is still being articulated. In this Letter we demonstrate the possibility of leveraging the atomically thin semiconductor tungsten diselenide (WSe2) as a host for quantum dot-like defects. We report that this previously unexplored solid-state quantum emitter in WSe2 generates single photons with emission properties that can be controlled via the application of external d.c. electric and magnetic fields. These new optically active quantum dots exhibit excited-state lifetimes on the order of 1 ns and remarkably large excitonic g-factors of 10. It is anticipated that WSe2 quantum dots will provide a novel platform for integrated solid-state quantum photonics and quantum information processing, as well as a rich condensed-matter physics playground with which to explore the coupling of quantum dots and atomically thin semiconductors.

Integrated nanophotonics based on nanowire plasmons and atomically thin material

The continually increasing demands for higher-speed and lower-operating-power devices have resulted in the continued impetus to shrink photonic components. We demonstrate a primitive nanophotonic integrated circuit element composed of a single silver nanowire and single-layer molybdenum disulfide (MoS2) flake. Using scanning confocal fluorescence microscopy and spectroscopy, we find that nanowire plasmons can excite MoS2 photoluminescence and that MoS2 excitons can decay into nanowire plasmons. Finally, we show that the nanowire may serve the dual purpose of both exciting MoS2 photoluminescence via plasmons and recollecting the decaying exciton as nanowire plasmons. The potential for subwavelength light guiding and strong nanoscale light–matter interaction afforded by our device may facilitate compact and efficient on-chip optical processing.

Direct and Scalable Deposition of Atomically Thin Low-Noise MoS2 Membranes on Apertures.

Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a planar freestanding crystal. When the substrate edge is in the form of an aperture, reagent-limited nucleation followed by edge growth facilitate direct and selective growth of freestanding MoS2 membranes. We have found conditions under which MoS2 grows preferentially across micrometer-scale prefabricated solid-state apertures in silicon nitride membranes, resulting in sealed membranes that are one to a few atomic layers thick. We have investigated the structure and purity of our membranes by a combination of atomic-resolution transmission electron microscopy, elemental analysis, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes. Finally, we demonstrate the utility of fabricated ultrathin nanopores in such membranes for single-stranded DNA translocation detection.

Direct On-Chip Optical Plasmon Detection with an Atomically Thin Semiconductor

The determination to develop fast, efficient devices has led to vast studies on photonic circuits but it is difficult to shrink these circuits below the diffraction limit of light. However, the coupling between surface plasmon polaritons and nanostructures in the near-field shows promise in developing next-generation integrated circuitry. In this work, we demonstrate the potential for integrating nanoplasmonic-based light guides with atomically thin materials for on-chip near-field plasmon detection. Specifically, we show near-field electrical detection of silver nanowire plasmons with the atomically thin semiconductor molybdenum disulfide. Unlike graphene, atomically thin semiconductors such as molybdenum disulfide exhibit a bandgap that lends itself for the excitation and detection of plasmons. Our fully integrated plasmon detector exhibits plasmon responsivities of ∼255 mA/W that corresponds to highly efficient plasmon detection (∼0.5 electrons per plasmon).

Distance-dependent energy transfer between CdSe/CdS quantum dots and a two-dimensional semiconductor

Atomically thin semiconductors, such as the transition metal dichalcogenides, show great potential for nanoscale photodetection, energy harvesting, and nanophotonics. Here, we investigate the efficiency of energy transfer between colloidal quantum dots with a cadmium selenide core and cadmium sulfide shell and monolayer molybdenum diselenide (MoSe2). We show that MoSe2 effectively quenches the fluorescence of quantum dots when the two materials are in contact. We then separate the MoSe2 and quantum dots by inserting variable thickness hexagonal boron nitride (h-BN) spacers and show that the efficiency at which the MoSe2 quenches fluorescence decreases as the h-BN thickness is increased. For distances d, this trend can be modeled by a 1/d4 decay, in agreement with theory and recent studies involving graphene.

Optical antenna enhanced graphene photodetector

We report on the integration of an individual, metal-based, plasmonic nano-antenna on a graphene photodetector. The device exhibits an electromagnetic resonance at a wavelength of 580 nm with well-defined polarization sensitivity. With no applied bias voltage, this antenna-coupled graphene photodetector features a responsivity of ∼17 nA/μW, which is four orders of magnitude higher than previously studied single antenna enhanced detectors. Finally, we measure a signal-to-noise ratio of ∼200 in a 1-Hz bandwidth, with an average photocurrent value of 1.2 nA.

Localized emission from defects in MoSe 2 layers

Two-dimensional semiconducting analogues of graphene, like monolayer molybdenum diselenide (MoSe2), provide a rich platform for optoelectronics. We study sharp localized photoluminescence (PL) emission from exfoliated MoSe2 flakes. These emission signatures are present in single layers as well as bi- and few layer flakes at low temperatures. The PL from these defects saturate at a lower power than the neutral and charged excitons of the monolayer MoSe2 and are stable against multiple heating and cooling cycles. We study the Zeeman effect in these emitters through magneto-optical photoluminescence studies and derive a g-factor of around 4 which is similar to the delocalized excitons in MoSe2.

Quantum-Confined Stark Effect of Individual Defects in a van der Waals Heterostructure

The optical properties of atomically thin semiconductor materials have been widely studied because of the isolation of monolayer transition metal dichalcogenides (TMDCs). They have rich optoelectronic properties owing to their large direct bandgap, the interplay between the spin and the valley degree of freedom of charge carriers, and the recently discovered localized excitonic states giving rise to single photon emission. In this Letter, we study the quantum-confined Stark effect of these localized emitters present near the edges of monolayer tungsten diselenide (WSe2). By carefully designing sequences of metallic (graphene), insulating (hexagonal boron nitride), and semiconducting (WSe2) two-dimensional materials, we fabricate a van der Waals heterostructure field effect device with WSe2 hosting quantum emitters that is responsive to external static electric field applied to the device. A very efficient spectral tunability up to 21 meV is demonstrated. Further, evaluation of the spectral shift in the photoluminescence signal as a function of the applied voltage enables us to extract the polarizability volume (up to 2000 Å3) as well as information on the dipole moment of an individual emitter. The Stark shift can be further modulated on application of an external magnetic field, where we observe a flip in the sign of dipole moment possibly due to rearrangement of the position of electron and hole wave functions within the emitter.

Graphene mediated Stark shifting of quantum dot energy levels

We demonstrate an optoelectronic device comprised of single InAs quantum dots in an n-i-Schottky diode where graphene has been used as the Schottky contact. Deterministic electric field tuning is shown using Stark-shifted micro-photoluminescence from single quantum dots. The extracted dipole moments from the Stark shifts are comparable to conventional devices where the Schottky contact is a semi-transparent metal. Neutral and singly charged excitons are also observed in the well-known Coulomb-blockade plateaus. Our results indicate that graphene is a suitable replacement for metal contacts in quantum dot devices which require electric field control.

Integrated nanophotonics based on nanowire plasmons and atomically-thin material

We demonstrate a nanophotonic integrated circuit element composed of a silver nanowire and single-layer molybdenum disulfide (MoS2). We show that the nanowire can both excite MoS2 photoluminescence via plasmons and collect decaying excitons as plasmons.