252 research outputs found
Extraction of the beta-factor for single quantum dots coupled to a photonic crystal waveguide
We present measurements of the beta-factor, describing the coupling
efficiency of light emitted by single InAs/GaAs semiconductor quantum dots into
a photonic crystal waveguide mode. The beta-factor is evaluated by means of
time-resolved frequency-dependent photoluminescence spectroscopy. The emission
wavelength of single quantum dots is temperature tuned across the band edge of
a photonic crystal waveguide and the spontaneous emission rate is recorded.
Decay rates up to 5.7 ns^(-1), corresponding to a Purcell factor of 5.2, are
measured and beta-factors up to 85% are extracted. These results prove the
potential of photonic crystal waveguides in the realization of on-chip
single-photon sources.Comment: 3 pages, 3 figure
Spectral broadening and shaping of nanosecond pulses: towards shaping of single photons from quantum emitters
We experimentally demonstrate spectral broadening and shaping of
exponentially-decaying nanosecond pulses via nonlinear mixing with a
phase-modulated pump in a periodically-poled lithium niobate (PPLN) waveguide.
A strong, 1550~nm pulse is imprinted with a temporal phase and used to
upconvert a weak 980 nm pulse to 600 nm while simultaneously broadening the
spectrum to that of a Lorentzian pulse up to 10 times shorter. While the
current experimental demonstration is for spectral shaping, we also provide a
numerical study showing the feasibility of subsequent spectral phase correction
to achieve temporal compression and re-shaping of a 1~ns mono-exponentially
decaying pulse to a 250 ps Lorentzian, which would constitute a complete
spectro-temporal waveform shaping protocol. This method, which uses quantum
frequency conversion in PPLN with >100:1 signal-to-noise ratio, is compatible
with single photon states of light.Comment: 4 pages, 4 figure
Bright single photon emission from a quantum dot in a circular Bragg grating microcavity
Bright single photon emission from single quantum dots in suspended circular
Bragg grating microcavities is demonstrated. This geometry has been designed to
achieve efficient (> 50 %) single photon extraction into a near-Gaussian shaped
far-field pattern, modest (~10x) Purcell enhancement of the radiative rate, and
a spectral bandwidth of a few nanometers. Measurements of fabricated devices
show progress towards these goals, with collection efficiencies as high as ~10%
demonstrated with moderate spectral bandwidth and rate enhancement. Photon
correlation measurements are performed under above-bandgap excitation (pump
wavelength = 780 nm to 820 nm) and confirm the single photon character of the
collected emission. While the measured sources are all antibunched and
dominantly composed of single photons, the multi-photon probability varies
significantly. Devices exhibiting tradeoffs between collection efficiency,
Purcell enhancement, and multi-photon probability are explored and the results
are interpreted with the help of finite-difference time-domain simulations.
Below-bandgap excitation resonant with higher states of the quantum dot and/or
cavity (pump wavelength = 860 nm to 900 nm) shows a near-complete suppression
of multi-photon events and may circumvent some of the aforementioned tradeoffs.Comment: 11 pages, 12 figure
Optical sensing with Anderson-localised light
We show that fabrication imperfections in silicon nitride photonic crystal
waveguides can be used as a resource to efficiently confine light in the
Anderson-localised regime and add functionalities to photonic devices. Our
results prove that disorder-induced localisation of light can be utilised to
realise an alternative class of high-quality optical sensors operating at room
temperature. We measure wavelength shifts of optical resonances as large as
15.2 nm, more than 100 times the spectral linewidth of 0.15\,nm, for a
refractive index change of about 0.38. By studying the temperature dependence
of the optical properties of the system, we report wavelength shifts of up to
about 2 nm and increases of more than a factor 2 in the quality factor of the
cavity resonances, when going from room to cryogenic temperatures. Such a
device can allow simultaneous sensing of both local contaminants and
temperature variations, monitored by tens of optical resonances spontaneously
appearing along a single photonic crystal waveguide. Our findings demonstrate
the potential of Anderson-localised light in photonic crystals for scalable and
efficient optical sensors operating in the visible and near-infrared range of
wavelengths.Comment: 10 pages, 3 figure
Metallic nanorings for broadband, enhanced extraction of light from solid-state emitters
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.Comment: 10 pages, 3 figure
Simple Dynamics for Plurality Consensus
We study a \emph{Plurality-Consensus} process in which each of anonymous
agents of a communication network initially supports an opinion (a color chosen
from a finite set ). Then, in every (synchronous) round, each agent can
revise his color according to the opinions currently held by a random sample of
his neighbors. It is assumed that the initial color configuration exhibits a
sufficiently large \emph{bias} towards a fixed plurality color, that is,
the number of nodes supporting the plurality color exceeds the number of nodes
supporting any other color by additional nodes. The goal is having the
process to converge to the \emph{stable} configuration in which all nodes
support the initial plurality. We consider a basic model in which the network
is a clique and the update rule (called here the \emph{3-majority dynamics}) of
the process is the following: each agent looks at the colors of three random
neighbors and then applies the majority rule (breaking ties uniformly).
We prove that the process converges in time with high probability, provided that .
We then prove that our upper bound above is tight as long as . This fact implies an exponential time-gap between the
plurality-consensus process and the \emph{median} process studied by Doerr et
al. in [ACM SPAA'11].
A natural question is whether looking at more (than three) random neighbors
can significantly speed up the process. We provide a negative answer to this
question: In particular, we show that samples of polylogarithmic size can speed
up the process by a polylogarithmic factor only.Comment: Preprint of journal versio
Photovoltaic Probe of Cavity Polaritons in a Quantum Cascade Structure
The strong coupling between an intersubband excitation in a quantum cascade
structure and a photonic mode of a planar microcavity has been detected by
angle-resolved photovoltaic measurements. A typical anticrossing behavior, with
a vacuum-field Rabi splitting of 16 meV at 78K, has been measured, for an
intersubband transition at 163 meV. These results show that the strong coupling
regime between photons and intersubband excitations can be engineered in a
quantum cascade opto-electronic device. They also demonstrate the possibility
to perform angle-resolved mid-infrared photodetection and to develop active
devices based on intersubband cavity polaritons.Comment: submitted to Applied Physics Letter
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