Anomalous circular photogalvanic effect in p-GaAs.

The anomalous circular photogalvanic effect (ACPGE) is observed in p-GaAs with a thickness of 2 µm at room temperature, in which circularly polarized light is used to inject spin-polarized carriers and the spin diffusion can generate a macroscopic detectable charge current due to the inverse spin Hall effect. The normalized ACPGE signals show first increasing and then decreasing with increasing the doping concentration. The role of the doping impurities is discussed by both extrinsic and intrinsic models, and both can well explain the variation of ACPGE with the doping concentration.

Elimination of Bimodal Size in InAs/GaAs Quantum Dots for Preparation of 1.3-µm Quantum Dot Lasers.

The device characteristics of semiconductor quantum dot lasers have been improved with progress in active layer structures. Self-assembly formed InAs quantum dots grown on GaAs had been intensively promoted in order to achieve quantum dot lasers with superior device performances. In the process of growing high-density InAs/GaAs quantum dots, bimodal size occurs due to large mismatch and other factors. The bimodal size in the InAs/GaAs quantum dot system is eliminated by the method of high-temperature annealing and optimized the in situ annealing temperature. The annealing temperature is taken as the key optimization parameters, and the optimal annealing temperature of 680 °C was obtained. In this process, quantum dot growth temperature, InAs deposition, and arsenic (As) pressure are optimized to improve quantum dot quality and emission wavelength. A 1.3-µm high-performance F-P quantum dot laser with a threshold current density of 110 A/cm2 was demonstrated.

Structural discontinuity induced surface second harmonic generation in single, thin zinc-blende GaAs nanowires.

We investigate optical second harmonic generation (SHG) from individual self-catalyzed zinc-blende (ZB) GaAs nanowires (NWs), where the polarimetry strongly depends on the NW diameter. We report a direct observation on the SHG induced by surface nonlinear susceptibilities in a single, ultra-thin GaAs NW. By considering the contributions from both optical field and structural discontinuities in our theoretical model, we can well explain the optical SHG polarimetry from NWs with different diameters. We also show that the optical in-coupling coefficient arising from the depolarization electromagnetic field can determine the polarization of the SHG. The results open perspectives for further geometry-based studies on the origin and control of SHG in small nanostructures.

Precise characterization of self-catalyzed III-V nanowire heterostructures via optical second harmonic generation.

We demonstrate the utility of optical second harmonic generation (SHG) polarimetry to perform structural characterization of self-assembled zinc-blende/wurtzite III-V nanowire heterostructures. By analyzing four anisotropic SHG polarimetric patterns, we distinguish between wurtzite (WZ), zinc-blende (ZB) and ZB/WZ mixing III-V semiconducting crystal structures in nanowire systems. By neglecting the surface contributions and treating the bulk crystal within the quasi-static approximation, we can well explain the optical SHG polarimetry from the NWs with diameter from 200-600 nm. We show that the optical in-coupling and out-coupling coefficients arising from depolarization field can determine the polarization of the SHG. We also demonstrate micro-photoluminescence of GaAs quantum dots in related ZB and ZB/WZ mixing sections of core-shell NW structure, in agreement with the SHG polarimetry results. The ability to perform in situ SHG-based crystallographic study of semiconducting single and multi-crystalline nanowire heterostructures will be useful in displaying structure-property relationships of nanodevices.

Bright Single-Photon Source at 1.3 µm Based on InAs Bilayer Quantum Dot in Micropillar.

A pronounced high count rate of single-photon emission at the wavelength of 1.3 µm that is capable of fiber-based quantum communication from InAs/GaAs bilayer quantum dots coupled with a micropillar (diameter ~3 µm) cavity of distributed Bragg reflectors was investigated, whose photon extraction efficiency has achieved 3.3%. Cavity mode and Purcell enhancement have been observed clearly in microphotoluminescence spectra. At the detection end of Hanbury-Brown and Twiss setup, the two avalanched single-photon counting modules record a total count rate of ~62,000/s; the time coincidence counting measurement demonstrates single-photon emission, with the multi-photon emission possibility, i.e., g 2(0), of only 0.14.

Large optical Stark shifts in single quantum dots coupled to core-shell GaAs/AlGaAs nanowires.

Nanowire quantum dots (NW-QDs) can be used for future compact and efficient optoelectronic devices. Many efforts have been made to control the QD states by inserting the QDs in doped structures and applying an electric field in a nanowire system. In this paper, we use down-conversion and up-conversion photoluminescence excitations to explore the optical and electronic properties of single quantum dots in GaAs/AlGaAs core-shell nanowires. We investigate a large optical Stark shift in this system as a new method to tune the QD states. When the tunable laser lies within the spectral bandwidth of ZB/WZ GaAs (780 nm-860 nm), we observe an extremely large optical Stark shift of 1.3 nm (0.5 nm) with increasing excitation power at a resonant wavelength of 800 nm (840 nm) in GaAs states. The ability to in situ control the energy states of self-catalyzed NW-QDs should open a new way for quantum light sources and nonlinear optics in a nanowire system.

Measurement of the inhomogeneous broadening of a bi-exciton state in a quantum dot using Franson-type nonlocal interference.

The inhomogeneous broadening of the bi-exciton state in quantum dots, i.e., the inhomogeneous broadening of the upper level of the cascade process, is not only a fundamental problem in quantum dots, but also closely related with the coherent control of this complex system and the quality of the entangled photon pairs, especially the time-bin entangled photon pairs. This inhomogeneous broadening is inherently a two-photon correlated phenomenon. In this work, we construct a genuine Franson-type nonlocal interference process to measure the inhomogeneous broadening of the bi-exciton state. The results show that the inhomogeneous broadening of the bi-exciton state is considerably smaller than that of the exciton state, that is why the entangled photon pairs can be generated by the cascade process in the quantum dot.

Telecommunication Wavelength-Band Single-Photon Emission from Single Large InAs Quantum Dots Nucleated on Low-Density Seed Quantum Dots.

Single-photon emission in the telecommunication wavelength band is realized with self-assembled strain-coupled bilayer InAs quantum dots (QDs) embedded in a planar microcavity on GaAs substrate. Low-density large QDs in the upper layer active for ~1.3 µm emission are fabricated by precisely controlling the indium deposition amount and applying a gradient indium flux in both QD layers. Time-resolved photoluminescence (PL) intensity suggested that the radiative lifetime of their exciton emission is 1.5~1.6 ns. The second-order correlation function of g (2)(0) < 0.5 which demonstrates a pure single-photon emission.

Experimental Demonstration of a Hybrid-Quantum-Emitter Producing Individual Entangled Photon Pairs in the Telecom Band.

Quantum emitters generating individual entangled photon pairs (IEPP) have significant fundamental advantages over schemes that suffer from multiple photon emission, or schemes that require post-selection techniques or the use of photon-number discriminating detectors. Quantum dots embedded within nanowires (QD-NWs) represent one of the most promising candidate for quantum emitters that provide a high collection efficiency of photons. However, a quantum emitter that generates IEPP in the telecom band is still an issue demanding a prompt solution. Here, we demonstrate in principle that IEPPs in the telecom band can be created by combining a single QD-NW and a nonlinear crystal waveguide. The QD-NW system serves as the single photon source, and the emitted visible single photons are split into IEPPs at approximately 1.55 µm through the process of spontaneous parametric down conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide. The compatibility of the QD-PPLN interface is the determinant factor in constructing this novel hybrid-quantum-emitter (HQE). Benefiting from the desirable optical properties of QD-NWs and the extremely high nonlinear conversion efficiency of PPLN waveguides, we successfully generate IEPPs in the telecom band with the polarization degree of freedom. The entanglement of the generated photon pairs is confirmed by the entanglement witness. Our experiment paves the way to producing HQEs inheriting the advantages of multiple systems.

Storage of multiple single-photon pulses emitted from a quantum dot in a solid-state quantum memory.

Quantum repeaters are critical components for distributing entanglement over long distances in presence of unavoidable optical losses during transmission. Stimulated by the Duan-Lukin-Cirac-Zoller protocol, many improved quantum repeater protocols based on quantum memories have been proposed, which commonly focus on the entanglement-distribution rate. Among these protocols, the elimination of multiple photons (or multiple photon-pairs) and the use of multimode quantum memory are demonstrated to have the ability to greatly improve the entanglement-distribution rate. Here, we demonstrate the storage of deterministic single photons emitted from a quantum dot in a polarization-maintaining solid-state quantum memory; in addition, multi-temporal-mode memory with 1, 20 and 100 narrow single-photon pulses is also demonstrated. Multi-photons are eliminated, and only one photon at most is contained in each pulse. Moreover, the solid-state properties of both sub-systems make this configuration more stable and easier to be scalable. Our work will be helpful in the construction of efficient quantum repeaters based on all-solid-state devices.

In situ probing and integration of single self-assembled quantum dots-in-nanowires for quantum photonics.

The realization of fiber-output single photon sources is necessary for quantum photonics. Here we present in situ probing and integration of single self-assembled quantum dots (QDs)-in-nanowires. Single self-assembled AlGaAs QDs were synthesized in GaAs/AlGaAs core-shell nanowires by molecular beam epitaxy and characterized by optical excitation in both micro-PL and fiber-integrating set-up. Cascaded biexciton-exciton emission with a saturation signal of 1000 counts per second at nitrogen temperature is achieved through the fiber-integrating setup, which makes single mode fibers an ideal candidate for single photons sources and paves the way for the realization of 'all fiber' devices. Numerical calculations were carried out to illustrate the collection efficiency and polarized photoluminescence characteristics. Extraction efficiencies as high as 70% over a broadband emission are reported and increase by a factor of about seven in comparison with air extraction, due to the larger refractive index of the fiber core.

Morphological engineering of self-assembled nanostructures at nanoscale on faceted GaAs nanowires by droplet epitaxy.

Fabrication of advanced artificial nanomaterials is a long-term pursuit to fulfill the promises of nanomaterials and it is of utter importance to manipulate materials at nanoscale to meet urgent demands of nanostructures with designed properties. Herein, we demonstrate the morphological tailoring of self-assembled nanostructures on faceted GaAs nanowires (NWs). The NWs are deposited on different kinds of substrates. Triangular and hexagonal prism morphologies are obtained, and their corresponding {110} sidewalls act as platforms for the nucleation of gallium droplets (GDs). We demonstrate that the morphologies of the nanostructures depend not only on the annealing conditions but also on the morphologies of the NWs' sidewalls. Here, we achieve morphological engineering in the form of novel quantum dots (QDs), 'square' quantum rings (QRs), 'rectangular' QRs, 3D QRs, crescent-shaped QRs, and nano-antidots. The evolution mechanisms for the peculiar morphologies of both NWs and nanostructures are modeled and discussed in detail. This work shows the potential of combining nano-structural engineering with NWs to achieve multifunctional properties and applications.

Self-assembled quantum dot structures in a hexagonal nanowire for quantum photonics.

Two types of quantum nanostructures based on self-assembled GaAs quantumdots embedded into GaAs/AlGaAs hexagonal nanowire systems are reported, opening a new avenue to the fabrication of highly efficient single-photon sources, as well as the design of novel quantum optics experiments and robust quantum optoelectronic devices operating at higher temperature, which are required for practical quantum photonics applications.

Single InAs quantum dot grown at the junction of branched gold-free GaAs nanowire.

We report a new type of single InAs quantum dot (QD) embedded at the junction of gold-free branched GaAs/AlGaAs nanowire (NW) grown on silicon substrate. The photoluminescence intensity of such QD is ~20 times stronger than that from randomly distributed QD grown on the facet of straight NW. Sharp excitonic emission is observed at 4.2 K with a line width of 101 µeV and a vanishing two-photon emission probability of g(2)(0) = 0.031(2). This new nanostructure may open new ways for designing novel quantum optoelectronic devices.

In situ accurate control of 2D-3D transition parameters for growth of low-density InAs/GaAs self-assembled quantum dots.

A method to improve the growth repeatability of low-density InAs/GaAs self-assembled quantum dots by molecular beam epitaxy is reported. A sacrificed InAs layer was deposited firstly to determine in situ the accurate parameters of two- to three-dimensional transitions by observation of reflection high-energy electron diffraction patterns, and then the InAs layer annealed immediately before the growth of the low-density InAs quantum dots (QDs). It is confirmed by micro-photoluminescence that control repeatability of low-density QD growth is improved averagely to about 80% which is much higher than that of the QD samples without using a sacrificed InAs layer.

Photoluminescence study of low density InAs quantum clusters grown by molecular beam epitaxy.

We report a systematic optical spectroscopy study of low density InAs quantum clusters (QCs) grown by molecular beam epitaxy. The photoluminescence (PL) spectra show emission features of a wetting layer (WL) which contains hybridized quantum well states. The low-energy tail of the QCs' PL profile is actually an ensemble of some sharp lines, originating from the emission of different exciton states (e.g. X, X*, XX*) in a single quasi-three-dimensional (Q3D) cluster as detailed in the micro-PL spectra. The temperature dependence of PL spectra indicates photocarrier distribution and transport in the QC-WL system. Furthermore, this small InAs Q3D cluster is integrated with a distributed Bragg reflector structure, and using optical excitation creates a single photon source with the second-order correlation function of g((2))(0) = 0.31 at 16 K.