210 research outputs found
Quantum dot-cavity strong-coupling regime measured through coherent reflection spectroscopy in a very high-Q micropillar
We report on the coherent reflection spectroscopy of a high-quality factor
micropillar, in the strong coupling regime with a single InGaAs annealed
quantum dot. The absolute reflectivity measurement is used to study the
characteristics of our device at low and high excitation power. The strong
coupling is obtained with a g=16 \mueV coupling strength in a 7.3\mum diameter
micropillar, with a cavity spectral width kappa=20.5 \mueV (Q=65 000). The
factor of merit of the strong-coupling regime, 4g/kappa=3, is the current
state-of-the-art for a quantum dot-micropillar system
Nuclear spin physics in quantum dots: an optical investigation
The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a
semiconductor quantum dot offers a unique setting for the study of many-body
spin physics in the condensed matter. The dynamics of this system and its
coupling to electron spins is fundamentally different from its bulk
counter-part as well as that of atoms due to increased fluctuations that result
from reduced dimensions. In recent years, the interest in studying quantum dot
nuclear spin systems and their coupling to confined electron spins has been
fueled by its direct implication for possible applications of such systems in
quantum information processing as well as by the fascinating nonlinear
(quantum-)dynamics of the coupled electron-nuclear spin system. In this
article, we review experimental work performed over the last decades in
studying this mesoscopic,coupled electron-nuclear spin system and discuss how
optical addressing of electron spins can be exploited to manipulate and
read-out quantum dot nuclei. We discuss how such techniques have been applied
in quantum dots to efficiently establish a non-zero mean nuclear spin
polarization and, most recently, were used to reduce fluctuations of the
average quantum dot nuclear spin orientation. Both results in turn have
important implications for the preservation of electron spin coherence in
quantum dots, which we discuss. We conclude by speculating how this recently
gained understanding of the quantum dot nuclear spin system could in the future
enable experimental observation of quantum-mechanical signatures or possible
collective behavior of mesoscopic nuclear spin ensembles.Comment: 61 pages, 45 figures, updated reference list, corrected typographical
error
Dynamic nuclear polarization of a single charge-tunable InAs/GaAs quantum dot
We report on the dynamic nuclear polarization of a single charge-tunable
self-assembled InAs/GaAs quantum dot in a longitudinal magnetic field of
0.2T. The hyperfine interaction between the optically oriented electron
and nuclei spins leads to the polarization of the quantum dot nuclei measured
by the Overhauser-shift of the singly-charged excitons ( and ).
When going from to , we observe a reversal of this shift which
reflects the average electron spin optically written down in the quantum dot
either in the state or in the final state of recombination. We
discuss a theoretical model which indicates an efficient depolarization
mechanism for the nuclei limiting their polarization to ~10%.Comment: 4+ pages, 3 figure
Frequency cavity pulling induced by a single semiconductor quantum dot
We investigate the emission properties of a single semiconductor quantum dot
deterministically coupled to a confined optical mode in the weak coupling
regime. A strong pulling, broadening and narrowing of the cavity mode emission
is evidenced when changing the spectral detuning between the emitter and the
cavity. These features are theoretically accounted for by considering the
phonon assisted emission of the quantum dot transition. These observations
highlight a new situation for cavity quantum electrodynamics involving
spectrally broad emitters
Hyperfine interaction in InAs/GaAs self-assembled quantum dots : dynamical nuclear polarization versus spin relaxation
We report on the influence of hyperfine interaction on the optical
orientation of singly charged excitons X+ and X- in self-assembled InAs/GaAs
quantum dots. All measurements were carried out on individual quantum dots
studied by micro-photoluminescence at low temperature. We show that the
hyperfine interaction leads to an effective partial spin relaxation, under
50kHz modulated excitation polarization, which becomes however strongly
inhibited under steady optical pumping conditions because of dynamical nuclear
polarization. This optically created magnetic-like nuclear field can become
very strong (up to ~4 T) when it is generated in the direction opposite to a
longitudinally applied field, and exhibits then a bistability regime. This
effect is very well described by a theoretical model derived in a perturbative
approach, which reveals the key role played by the energy cost of an electron
spin flip in the total magnetic field. Eventually, we emphasize the
similarities and differences between X+ and X- trions with respect to the
hyperfine interaction, which turn out to be in perfect agreement with the
theoretical description.Comment: 10 pages, 5 figure
Tight-binding calculations of image charge effects in colloidal nanoscale platelets of CdSe
International audienceCdSe nanoplatelets show perfectly quantized thicknesses of few monolayers. They present a situation of extreme, yet well de ned quantum con nement. Due to large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hundred meVs
Speech Technologies for African Languages: Example of a Multilingual Calculator for Education
International audienceThis paper presents our achievements after 18 months of the ALFFA project dealing with African languages technologies. We focus on a multilingual calculator (Android app) that will be demonstrated during the Show and Tell session
Controlling the polarization eigenstate of a quantum dot exciton with light
We demonstrate optical control of the polarization eigenstates of a neutral
quantum dot exciton without any external fields. By varying the excitation
power of a circularly polarized laser in micro-photoluminescence experiments on
individual InGaAs quantum dots we control the magnitude and direction of an
effective internal magnetic field created via optical pumping of nuclear spins.
The adjustable nuclear magnetic field allows us to tune the linear and circular
polarization degree of the neutral exciton emission. The quantum dot can thus
act as a tunable light polarization converter.Comment: 5 pages, 3 figure
Wall crossings for double Hurwitz numbers
AbstractDouble Hurwitz numbers count covers of P1 by genus g curves with assigned ramification profiles over 0 and â, and simple ramification over a fixed branch divisor. Goulden, Jackson and Vakil have shown double Hurwitz numbers are piecewise polynomial in the orders of ramification (Goulden et al., 2005) [10], and Shadrin, Shapiro and Vainshtein have determined the chamber structure and wall crossing formulas for g=0 (Shadrin et al., 2008) [15]. This paper gives a unified approach to these results and strengthens them in several ways â the most important being the extension of the results of Shadrin et al. (2008) [15] to arbitrary genus.The main tool is the authorsÊŒ previous work (Cavalieri et al., 2010) [6] expressing double Hurwitz number as a sum over certain labeled graphs. We identify the labels of the graphs with lattice points in the chambers of certain hyperplane arrangements, which give rise to piecewise polynomial functions. Our understanding of the wall crossing for these functions builds on the work of Varchenko (1987) [17], and could have broader applications
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