Xiulai Xu

Observation of Strong Interlayer Coupling in MoS2/WS2 Heterostructures

Epitaxial growth of A-A and A-B stacking MoS2 on WS2 via a two-step chemical vapor deposition method is reported. These epitaxial heterostructures show an atomic clean interface and a strong interlayer coupling, as evidenced by systematic characterization. Low-frequency Raman breathing and shear modes are observed in commensurate stacking bilayers for the first time; these can serve as persuasive fingerprints for interfacial quality and stacking configurations.

Structural and optical properties of ZnO thin films produced by filtered cathodic vacuum arc

Polycrystalline ZnO films have been deposited by filtered cathodic vacuum arc with various substrate temperatures and bias voltages. The films deposited at room temperature are amorphous. The films grown at 230 and 430°C oriented in the (002) and (103) directions. Strong near-band edge room temperature photoluminescence emission has been observed in the film deposited at 230°C under floating bias. This is attributed to the reduced oxygen vacancies as determined by Raman spectroscopy. The bias voltage and high substrate temperature (up to 430°C) will induce more defects, resulting in broad band tails near the band edge. The optimized ZnO film has a transmittance which is over 80%.

Electrically pumped single-photon sources in lateral p-i-n junctions

Electrically pumped single-photon sources using semiconductor quantum dots are of interest as they can be integrated with other semiconductor devices, using standard processing techniques. In this letter, we report electroluminescence from single quantum dots in a lateral p-i-n junction. Exciton and biexciton emission from a single quantum dot can be achieved under different electrical bias conditions. Antibunching effects from exciton and biexciton emission are observed using cw and pulsed electrical injection, indicating single-photon emission; this can be used for quantum information processing.

Strongly coupled single quantum dot in a photonic crystal waveguide cavity

Cavities embedded in photonic crystal waveguides offer a promising route toward large scale integration of coupled resonators for quantum electrodynamics applications. In this letter, we demonstrate a strongly coupled system formed by a single quantum dot and such a photonic crystal cavity. The resonance originating from the cavity is clearly identified from the photoluminescence mapping of the out-of-plane scattered signal along the photonic crystal waveguide. The quantum dot exciton is tuned toward the cavity mode by temperature control. A vacuum Rabi splitting of ∼140 μeV is observed at resonance.

Blue electroluminescence from tris- 8-hydroxyquinoline aluminum thin film

We report the fabrication of symmetrically configured alternating-current (ac) light-emitting devices based on tris-(8-hydroxyquinoline) aluminum (Alq3). The devices consist of an emissive layer of Alq3 sandwiched between two layers of silicon dioxide. A blue emission at 457 nm could be obtained from the devices under the ac driving. The blue emission corresponds to the direct recombination from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO) of Alq3. According to the dependence of luminescent intensity on the voltage and frequency, we ascribed this direct recombination to the dissociation of Frenkel excitons and the charge accumulation at the interface of SiO2 and Alq3.

Spin-injection Hall effect in a planar photovoltaic cell

A technique that allows the electrical detection of spin-polarized transport in semiconductors without disturbing the spin-polarized current or using magnetic elements has now been demonstrated. The approach could lead to the integration of spintronics elements into semiconductor microelectronic circuits.

“Plug and play” single-photon sources

The authors report a “plug and play” source of single photons, with full integration to a single-mode optical fiber. One end of the fiber is attached to the top of an InGaAs∕GaAs quantum dot wafer. The other end is connected via a wavelength-division multiplexing system to two separate fibers: one for carrying excitation light and the other for emitted light. A Hanbury-Brown and Twiss [Nature (London) 77, 27 (1956)] measurement was performed on the emission from single excitons recombining in the quantum dots. A second-order correlation function at zero time delay of approximately 0.01 indicates a nearly ideal source of single photons. The maximum variation of peak position over 24days is less than 0.1nm.

Quantum Interference Induced Photon Blockade in a Coupled Single Quantum Dot-Cavity System

We propose an experimental scheme to implement a strong photon blockade with a single quantum dot coupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantum dot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay g(2)(0) in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantum dot and the pumping field. Moreover, the photon blockade scheme based on quantum interference mechanism does not require a strong coupling strength between the cavity and the quantum dot, even with the pure dephasing of the system. This simple proposal provides an effective way for potential applications in solid state quantum computation and quantum information processing.

Plug and play single photons at 1.3 μm approaching gigahertz operation

We report a “plug and play” single photon source, fully integrated with an optical fiber, emitting at 1.3μm. Micropillars were patterned on a single layer InAs quantum dot wafer to guarantee a single pillar per fiber core. The single exciton peak filtered with a tunable optical filter was fed to a Hanbury Brown and Twiss interferometer, and the second order correlation function at zero delay was less than 0.5, indicating single photon emission. The measured decay dynamics under double-pulse excitation show that the single photon device can be operated at speeds greater than 0.5GHz.

Splitting of excitons and biexcitons in coupled InAs quantum dot molecules

Coupling in laterally coupled InAs quantum dot molecules has been observed by conventional microscopic photoluminescence spectroscopy. At low excitation intensity, splitting of exciton emission is observed, and this is ascribed to recombination of bonding and antibonding states when two quantum dots are coupled. With increasing excitation intensity, splitting of biexciton emission because of coupling is observed on both sides of the exciton peaks. The splitting of both exciton and biexciton peaks increases with increasing energy, which implies a large wave function overlap at high energy.