We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4 % and a photon indistinguishability of up to 94 ± 1 % by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.
We report the growth of high-quality GaN films on large-size graphene films for visible light-emitting diodes (LEDs). The graphene films were synthesized by chemical vapor deposition and then transferred onto amorphous silica (SiO 2 ) substrates that do not have an epitaxial relationship with GaN. Before growing the high-quality GaN thin films, ZnO nanowalls were grown on the graphene films as an intermediate layer. The structural and optical characteristics of the GaN films were investigated, and the films exhibited stimulated emission even at room temperature, a highly c-axis-oriented crystal structure, and a preferred in-plane orientation. Visible LEDs that emitted strong electroluminescence under room illumination were fabricated using the GaN thin films.
Microstructural defects in GaN thin films grown on graphene produced via chemical vapor deposition have been investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD analysis reveals the preferred orientations of the GaN films. We further examined the microstructural defects such as grain boundaries and threading dislocations present in the films using TEM. Plan-view TEM analysis showed presence of both high- and low-angle grain boundaries and the threading dislocations mostly bound to those grain boundaries. Moreover, the characteristics and behavior of the threading dislocations were also investigated using cross-section TEM analysis.
Coherent generation of indistinguishable single photons is crucial for many quantum communication and processing protocols. Solid-state realizations of two-level atomic transitions or three-level spin-Λ systems offer significant advantages over their atomic counterparts for this purpose, albeit decoherence can arise due to environmental couplings. One popular approach to mitigate dephasing is to operate in the weak-excitation limit, where excited-state population is minimal and coherently scattered photons dominate over incoherent emission. Here we probe the coherence of photons produced using two-level and spin-Λ solid-state systems. We observe that the coupling of the atomiclike transitions to the vibronic transitions of the crystal lattice is independent of the driving strength, even for detuned excitation using the spin-Λ configuration. We apply a polaron master equation to capture the non-Markovian dynamics of the vibrational manifolds. These results provide insight into the fundamental limitations to photon coherence from solid-state quantum emitters. arXiv:1904.05103v3 [cond-mat.mes-hall]
We report on the increased extraction of light emitted by solid-state sources embedded within high refractive index materials. This is achieved by making use of a local lensing effect by sub-micron metallic rings deposited on the sample surface and centered around single emitters. We show enhancements in the intensity of the light emitted by InAs/GaAs single quantum dot lines into free space as high as a factor 20. Such a device is intrinsically broadband and therefore compatible with any kind of solid-state light source. We foresee the fabrication of metallic rings via scalable techniques, like nano-imprint, and their implementation to improve the emission of classical and quantum light from solid-state sources. Furthermore, while increasing the brightness of the devices, the metallic rings can also act as top contacts for the local application of electric fields for carrier injection or wavelength tuning.PACS numbers: 42.82. Bq, 78.55.Cr, 78.60.Lc, 78.67.Hc a) Electronic mail: o.trojak@soton.ac.uk b) Electronic mail: l.sapienza@soton.ac.uk; www.quantum.soton.ac.uk 1 Extracting light into free space is one of the challenges to face when dealing with solidstate emitters embedded within high-index materials. At the air interface total internal reflection can trap most of the light within the higher index material, thus preventing efficient light extraction, that can be as low as a few percent. Such an issue needs to be faced when dealing with emitters like light-emitting diodes 1 and lasers 2 based, for instance, on quantum wells or quantum dots. In the same way as for classical light emitters, extraction efficiency has been the focus of intensive research when dealing with intrinsically dimmer sources like single-photon emitters for fundamental science 3,4 and quantum information technology applications 5 . Amongst solid-state quantum light sources, molecular beam epitaxial quantum dots (QDs) are of particular interest as they are directly grown on a semiconductor chip (thus allowing easy integration within optical circuits), they can have lifetime-limited emission lines 6 and can emit pure and indistinguishable single photons 7 .Several approaches have been followed to increase the extraction efficiency of light emitted by QDs into free space. For instance, optical cavities embedding single emitters have been fabricated to channel the emitted light into specific optical modes. Examples are micropillars, based on distributed Bragg reflectors 7 , nanowires 8 and circular grating cavities on suspended membranes 9,10 . Such optical cavities require the coupling of the emission from a source into a specific optical mode that the emitter needs to be resonant with. Fabrication processes can require multilayer growth (for micropillars) and deep etching (for micropillars and nanowires) or a calibrated etch (for circular grating cavities). High aspect ratio devices or suspended membranes also require non-trivial fabrication processes if one wants to include electrical tuning or injection to improve the device performance...
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