100 research outputs found
Electromechanical coupling in free-standing AlGaN/GaN planar structures
The strain and electric fields present in free-standing AlGaN/GaN slabs are
examined theoretically within the framework of fully-coupled continuum elastic
and dielectric models. Simultaneous solutions for the electric field and strain
components are obtained by minimizing the electric enthalpy. We apply
constraints appropriate to pseudomorphic semiconductor epitaxial layers and
obtain closed-form analytic expressions that take into account the wurtzite
crystal anisotropy. It is shown that in the absence of free charges, the
calculated strain and electric fields are substantially differently from those
obtained using the standard model without electromechanical coupling. It is
also shown, however, that when a two-dimensional electron gas is present at the
AlGaN/GaN interface, a condition that is the basis for heterojunction
field-effect transistors, the electromechanical coupling is screened and the
decoupled model is once again a good approximation. Specific cases of these
calculations corresponding to transistor and superlattice structures are
discussed.Comment: revte
Excitons in InGaAs Quantum Dots without Electron Wetting Layer States
The Stranski-Krastanov (SK) growth-mode facilitates the self-assembly of
quantum dots (QDs) using lattice-mismatched semiconductors, for instance InAs
and GaAs. SK QDs are defect-free and can be embedded in heterostructures and
nano-engineered devices. InAs QDs are excellent photon emitters: QD-excitons,
electron-hole bound pairs, are exploited as emitters of high quality single
photons for quantum communication. One significant drawback of the SK-mode is
the wetting layer (WL). The WL results in a continuum rather close in energy to
the QD-confined-states. The WL-states lead to unwanted scattering and dephasing
processes of QD-excitons. Here, we report that a slight modification to the
SK-growth-protocol of InAs on GaAs -- we add a monolayer of AlAs following InAs
QD formation -- results in a radical change to the QD-excitons. Extensive
characterisation demonstrates that this additional layer eliminates the
WL-continuum for electrons enabling the creation of highly charged excitons
where up to six electrons occupy the same QD. Single QDs grown with this
protocol exhibit optical linewidths matching those of the very best SK QDs
making them an attractive alternative to standard InGaAs QDs
Correlations between Optical Properties and Voronoi-Cell Area of Quantum Dots
A semiconductor quantum dot (QD) can generate highly indistinguishable
single-photons at a high rate. For application in quantum communication and
integration in hybrid systems, control of the QD optical properties is
essential. Understanding the connection between the optical properties of a QD
and the growth process is therefore important. Here, we show for GaAs QDs,
grown by infilling droplet-etched nano-holes, that the emission wavelength, the
neutral-to-charged exciton splitting, and the diamagnetic shift are strongly
correlated with the capture zone-area, an important concept from nucleation
theory. We show that the capture-zone model applies to the growth of this
system even in the limit of a low QD-density in which atoms diffuse over
m-distances. The strong correlations between the various QD parameters
facilitate preselection of QDs for applications with specific requirements on
the QD properties; they also suggest that a spectrally narrowed QD distribution
will result if QD growth on a regular lattice can be achieved
Quantum optics with near lifetime-limited quantum-dot transitions in a nanophotonic waveguide
Establishing a highly efficient photon-emitter interface where the intrinsic
linewidth broadening is limited solely by spontaneous emission is a key step in
quantum optics. It opens a pathway to coherent light-matter interaction for,
e.g., the generation of highly indistinguishable photons, few-photon optical
nonlinearities, and photon-emitter quantum gates. However, residual broadening
mechanisms are ubiquitous and need to be combated. For solid-state emitters
charge and nuclear spin noise is of importance and the influence of photonic
nanostructures on the broadening has not been clarified. We present near
lifetime-limited linewidths for quantum dots embedded in nanophotonic
waveguides through a resonant transmission experiment. It is found that the
scattering of single photons from the quantum dot can be obtained with an
extinction of , which is limited by the coupling of the quantum
dot to the nanostructure rather than the linewidth broadening. This is obtained
by embedding the quantum dot in an electrically-contacted nanophotonic
membrane. A clear pathway to obtaining even larger single-photon extinction is
laid out, i.e., the approach enables a fully deterministic and coherent
photon-emitter interface in the solid state that is operated at optical
frequencies.Comment: 27 pages, 7 figure
Energy levels in polarization superlattices: a comparison of continuum strain models
A theoretical model for the energy levels in polarization superlattices is
presented. The model includes the effect of strain on the local
polarization-induced electric fields and the subsequent effect on the energy
levels. Two continuum strain models are contrasted. One is the standard strain
model derived from Hooke's law that is typically used to calculate energy
levels in polarization superlattices and quantum wells. The other is a
fully-coupled strain model derived from the thermodynamic equation of state for
piezoelectric materials. The latter is more complete and applicable to strongly
piezoelectric materials where corrections to the standard model are
significant. The underlying theory has been applied to AlGaN/GaN superlattices
and quantum wells. It is found that the fully-coupled strain model yields very
different electric fields from the standard model. The calculated intersubband
transition energies are shifted by approximately 5 -- 19 meV, depending on the
structure. Thus from a device standpoint, the effect of applying the
fully-coupled model produces a very measurable shift in the peak wavelength.
This result has implications for the design of AlGaN/GaN optical switches.Comment: Revtex
Large-Range Frequency Tuning of a Narrow-Linewidth Quantum Emitter
A hybrid system of a semiconductor quantum dot single photon source and a
rubidium quantum memory represents a promising architecture for future photonic
quantum repeaters. One of the key challenges lies in matching the emission
frequency of quantum dots with the transition frequency of rubidium atoms while
preserving the relevant emission properties. Here, we demonstrate the
bidirectional frequency-tuning of the emission from a narrow-linewidth
(close-to-transform-limited) quantum dot. The frequency tuning is based on a
piezoelectric strain-amplification device, which can apply significant stress
to thick bulk samples. The induced strain shifts the emission frequency of the
quantum dot over a total range of , about three orders of
magnitude larger than its linewidth. Throughout the whole tuning process, both
the spectral properties of the quantum dot and its single-photon emission
characteristics are preserved. Our results show that external stress can be
used as a promising tool for reversible frequency tuning of high-quality
quantum dots and pave the wave towards the realisation of a quantum dot --
rubidium atoms interface for quantum networking.Comment: 6 pages, 3 figure
Low-Noise GaAs Quantum Dots for Quantum Photonics
Quantum dots are both excellent single-photon sources and hosts for single
spins. This combination enables the deterministic generation of Raman-photons
-- bandwidth-matched to an atomic quantum-memory -- and the generation of
photon cluster states, a resource in quantum communication and
measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched
in frequency to a rubidium-based photon memory, and have potentially improved
electron spin coherence compared to the widely used InGaAs quantum dots.
However, their charge stability and optical linewidths are typically much worse
than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an
---diode specially designed for low-temperature operation. We
demonstrate ultra-low noise behaviour: charge control via Coulomb blockade,
close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity
optical electron-spin initialisation and long electron-spin lifetimes for these
quantum dots. Our work establishes a materials platform for low-noise quantum
photonics close to the red part of the spectrum.Comment: (19 pages, 12 figures, 1 table
Spin-photon interface and spin-controlled photon switching in a nanobeam waveguide
Access to the electron spin is at the heart of many protocols for integrated
and distributed quantum-information processing [1-4]. For instance, interfacing
the spin-state of an electron and a photon can be utilized to perform quantum
gates between photons [2,5] or to entangle remote spin states [6-9].
Ultimately, a quantum network of entangled spins constitutes a new paradigm in
quantum optics [1]. Towards this goal, an integrated spin-photon interface
would be a major leap forward. Here we demonstrate an efficient and optically
programmable interface between the spin of an electron in a quantum dot and
photons in a nanophotonic waveguide. The spin can be deterministically prepared
with a fidelity of 96\%. Subsequently the system is used to implement a
"single-spin photonic switch", where the spin state of the electron directs the
flow of photons through the waveguide. The spin-photon interface may enable
on-chip photon-photon gates [2], single-photon transistors [10], and efficient
photonic cluster state generation [11]
A bright and fast source of coherent single photons
A single photon source is a key enabling technology in device-independent
quantum communication, quantum simulation for instance boson sampling, linear
optics-based and measurement-based quantum computing. These applications
involve many photons and therefore place stringent requirements on the
efficiency of single photon creation. The scaling on efficiency is an
exponential function of the number of photons. Schemes taking full advantage of
quantum superpositions also depend sensitively on the coherence of the photons,
i.e. their indistinguishability. It is therefore crucial to maintain the
coherence over long strings of photons. Here, we report a single photon source
with an especially high system efficiency: a photon is created on-demand at the
output of the final optical fibre with a probability of 57%. The coherence of
the photons is very high and is maintained over a stream consisting of
thousands of photons; the repetition rate is in the GHz regime. We break with
the established semiconductor paradigms, such as micropillars, photonic crystal
cavities and waveguides. Instead, we employ gated quantum dots in an open,
tunable microcavity. The gating ensures low-noise operation; the tunability
compensates for the lack of control in quantum dot position and emission
frequency; the output is very well-matched to a single-mode fibre. An increase
in efficiency over the state-of-the-art by more than a factor of two, as
reported here, will result in an enormous decrease in run-times, by a factor of
for 20 photons.Comment: Main text: 5 pages (including 4 figures), Supplementary Information:
8 pages (including 7 figures
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