X. Marie

Robust optical emission polarization in MoS2 monolayers through selective valley excitation

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90% polarized emission at 4 K is observed with 40% polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T indicating robust, selective valley excitation.

Giant enhancement of the optical second-harmonic emission of WSe(2) monolayers by laser excitation at exciton resonances.

Monolayers (MLs) of MoS2 and WSe2 are 2D semiconductors with strong, direct optical transitions that are governed by tightly Coulomb bound eletron-hole pairs (excitons). The optoelectronic properties of these transition metal dichalcogenides are directly related to the inherent crystal inversion symmetry breaking. It allows for efficient second harmonic generation (SHG) and is at the origin of chiral optical selections rules, which enable efficient optical initialization of electrons in specific K-valleys in momentum space. Here we demonstrate how these unique non-linear and linear optical properties can be combined to efficiently prepare exciton valley coherence and polarization through resonant pumping of an excited exciton state. In particular a new approach to coherent alignment of excitons following two-photon excitation is demonstrated. We observe a clear deviation of the excited exciton spectrum from the standard Rydberg series via resonances in SHG spectroscopy and two- and one-photon absorption. The clear identification of the 2s and 2p exciton excited states combined with first principle calculations including strong anti-screening effects allows us to determine an exciton binding energy of the order of 600 meV in ML WSe2.

Valley dynamics probed through charged and neutral exciton emission in monolayer WSe 2

Optical interband transitions in monolayer transition metal dichalcogenides such as WSe2 and MoS2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K valley in momentum space. Here we probe the valley dynamics in monolayer WSe2 by monitoring the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4 ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of ≈20%, for which a typical valley polarization decay time of the order of 1 ns can be inferred.

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.

Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers

We have experimentally studied the pump-probe Kerr rotation dynamics in WSe2 monolayers. This yields a direct measurement of the exciton valley depolarization time tau(nu). At T = 4 K, we find tau(nu) approximate to 6 ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases, with tau(nu) being as short as 1.5 ps at 125 K. The temperature dependence is well explained by the developed theory, taking into account the exchange interaction and fast exciton scattering time on the short-range potential.

Time-resolved photoluminescence in self-assembled InAs/GaAs quantum dots under strictly resonant excitation

We investigate the carrier dynamics in self-assembled InAs/GaAs quantum dots under strictly resonant excitation of the ground state. The spectral selectivity of the resonant excitation allows us to study the physical properties of a class of dots characterized by an energy distribution comparable to the excitation laser spectrum. We detect no Stokes shift of the photoluminescence (PL) line. The PL decay time yields a straightforward determination of the radiative recombination time.

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS

Exciton radiative lifetime in transition metal dichalcogenide monolayers

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Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor

. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and

Splitting between Bright and Dark Excitons in Transition Metal Dichalcogenide Monolayers

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