arXiv subjects

D. Lagarde

Publications and source records attributed to D. Lagarde.

31 records · Page 2Linked to original sources

arXiv2014

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$_2$. 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 $\mathbf k \cdot \mathbf p$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.

cond-mat.mes-hall↗

arXiv2014

Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2

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 4ps, 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 about 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.

cond-mat.mes-hall↗

arXiv2014

Carrier and polarization dynamics in monolayer MoS2

In monolayer MoS2 optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical valley initialization. In time resolved photoluminescence (PL) experiments we find that both the polarization and emission dynamics do not change from 4K to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature induced change in bandgap energy with the excitation laser energy an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.

cond-mat.mes-hall↗

arXiv2013

Fabrication of an InGaAs spin filter by implantation of paramagnetic centers

We report on the selective creation of spin filltering regions in non-magnetic InGaAs layers by implantation of Ga ions by Focused Ion Beam. We demonstrate by photoluminescence spectroscopy that spin dependent recombination (SDR) ratios as high as 240% can be achieved in the implanted areas. The optimum implantation conditions for the most efficient SDR is determined by the systematic analysis of different ion doses spanning four orders of magnitude. The application of a weak external magnetic field leads to a sizeable enhancement of the SDR ratio from the spin polarization of the nuclei surrounding the polarized implanted paramagnetic defects.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic field induced valence band mixing in [111] grown semiconductor quantum dots

We present a microscopic theory of the magnetic field induced mixing of heavy-hole states +/- 3/2 in GaAs droplet dots grown on (111)A substrates. The proposed theoretical model takes into account the striking dot shape with trigonal symmetry revealed in atomic force microscopy. Our calculations of the hole states are carried out within the Luttinger Hamiltonian formalism, supplemented with allowance for the triangularity of the confining potential. They are in quantitative agreement with the experimentally observed polarization selection rules, emission line intensities and energy splittings in both longitudinal and transverse magnetic fields for neutral and charged excitons in all measured single dots.

cond-mat.mes-hall↗

arXiv2012

Growth Direction Dependence of the Electron Spin Dynamics in {111} GaAs Quantum Wells

The electron spin dynamics is studied by time-resolved Kerr rotation in GaAs/AlGaAs quantum wells embedded in a negatively doped-intrinsic-positively doped structures grown on (111)A or (111)B-oriented substrates. In both cases the spin lifetimes are significantly increased by applying an external electric field but this field has to point along the growth direction for structures grown on (111)A and opposite to it for the ones grown on (111)B. This extended electron spin lifetime is the result of the suppression of the D'yakonov-Perel spin relaxation mechanism [Sov. Phys. Solid State 13, 3023 (1972)] due to the cancellation effect of the internal Dresselhaus term [Phys. Rev. 100, 580 (1955)] with the external electric field induced Rashba one [J. Phys. C 17, 6039 (1984)], both governing the conduction band spin-orbit splitting. These results demonstrate the key role played by the growth direction in the design of spintronic devices.

cond-mat.mes-hall↗

arXiv2011

Full Electrical Control of the Electron Spin Relaxation in GaAs Quantum Wells

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by timeresolved photoluminescence spectroscopy. By applying an external field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.

cond-mat.mtrl-sci↗

arXiv2011

Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots

In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly non-monotonous, sign changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of systems with C_3v point-group symmetry.

cond-mat.mes-hall↗

arXiv2011

Delay and distortion of slow light pulses by excitons in ZnO

Light pulses propagating through ZnO undergo distortions caused by both bound and free excitons. Numerous lines of bound excitons dissect the pulse and induce slowing of light around them, to the extend dependent on their nature. Exciton-polariton resonances determine the overall pulse delay and attenuation. The delay time of the higher-energy edge of a strongly curved light stripe approaches 1.6 ns at 3.374 eV with a 0.3 mm propagation length. Modelling the data of cw and time-of-flight spectroscopies has enabled us to determine the excitonic parameters, inherent for bulk ZnO. We reveal the restrictions on these parameters induced by the light attenuation, as well as a discrepancy between the parameters characterizing the surface and internal regions of the crystal.

cond-mat.mtrl-sci↗

arXiv2009

Relaxation and emission of Bragg-mode and cavity-mode polaritons in a ZnO microcavity at room temperature

The strong coupling regime in a ZnO microcavity is investigated through room temperature photoluminescence and reflectivity experiments. The simultaneous strong coupling of excitons to the cavity mode and the first Bragg mode is demonstrated at room temperature. The polariton relaxation is followed as a function of the excitation density. A relaxation bottleneck is evidenced in the Bragg-mode polariton branch. It is partly broken under strong excitation density, so that the emission from this branch dominates the one from cavity-mode polaritons.

cond-mat.other↗

arXiv2008

Exciton and hole spin dynamics in ZnO

The carrier spin dynamics in ZnO is investigated by time-resolved optical orientation experiments. We evidence a clear circular polarization of the donor-bound exciton luminescence in both ZnO epilayer and non-intentionally doped bulk ZnO. This allows us to measure the localized hole spin relaxation time. We find $τ^{s}_h$$\sim$350 ps at T=1.7 K in the ZnO epilayer. The strong energy and temperature dependences of the photoluminescence polarization dynamics are well explained by the fast free exciton spin relaxation time and the ionization of bound excitons.

cond-mat.other↗

arXiv2007

Room temperature Optical Orientation of Exciton Spin in cubic GaN/AlN quantum dots

The optical orientation of the exciton spin in an ensemble of self-organized cubic GaN/AlN quantum dots is studied by time-resolved photoluminescence. Under a polarized quasi-resonant excitation, the luminescence linear polarization exhibits no temporal decay, even at room temperature. This demonstrates the robustness of the exciton spin polarization in these cubic nitride nanostructures, with characteristic decay times longer than 10 ns.

cond-mat.other↗

arXiv2006

Spin-dependent electron dynamics and recombination in GaAs(1-x)N(x) alloys at room temperature

We report on both experimental and theoretical study of conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs(1-x)N(x) alloys with a small nitrogen content (x = 2.1, 2.7, 3.4%). It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40-45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ~150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in dynamical polarization of bound electrons. We have developed a nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers which describes the experimental dependencies, in particular, electron spin quantum beats observed in a transverse magnetic field.

cond-mat.mtrl-sci↗