Yoan Léger

Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).

Inserting one single Mn ion into a quantum dot

A method of growth to get one single Mn in self-assembled semiconductor quantum dot is presented. With a simple quantitative model, the appropriate low Mn density needed prior to the quantum dot nucleation is estimated. Such a low Mn concentration was reached by inserting a thin ZnTe spacer between a Zn1−xMnxTe buffer and the CdTe quantum dot layer. The control of Mn density is made by changing the thickness of the ZnTe spacer, with good reproducibility. Qualitative and quantitative comparisons of optical spectra for different samples assess the relevance of this growth method.

Ultrafast tristable spin memory of a coherent polariton gas

Non-linear interactions in coherent gases are not only at the origin of bright and dark solitons and superfluids; they also give rise to phenomena such as multistability, which hold great promise for the development of advanced photonic and spintronic devices. In particular, spinor multistability in strongly coupled semiconductor microcavities shows that the spin of hundreds of exciton-polaritons can be coherently controlled, opening the route to spin-optronic devices such as ultrafast spin memories, gates or even neuronal communication schemes. Here we demonstrate that switching between the stable spin states of a driven polariton gas can be controlled by ultrafast optical pulses. Although such a long-lived spin memory necessarily relies on strong and anisotropic spinor interactions within the coherent polariton gas, we also highlight the crucial role of non-linear losses and formation of a non-radiative particle reservoir for ultrafast spin switching.

Optical probing of spin fluctuations of a single paramagnetic Mn atom in a semiconductor quantum dot

L. Besombes, ∗ Y. Leger, J. Bernos, H. Boukari, H. Mariette, J.P. Poizat, J. Fernández-Rossier, and R. Aguado CEA-CNRS group ”Nanophysique et Semiconducteurs”, Institut Néel, CNRS & Université Joseph Fourier, 25 avenue des Martyrs, 38042 Grenoble, France Departamento de F́ısica Aplicada, Universidad de Alicante, San Vicente del Raspeig, 03690 Spain Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain Abstract We analyzed the photoluminescence intermittency generated by a single paramagnetic spin localized in an individual semiconductor quantum dot. The statistics of the photons emitted by the quantum dot reflect the quantum fluctuations of the localized spin interacting with the injected carriers. Photon correlation measurements which are reported here reveal unique signatures of these fluctuations. A phenomenological model is proposed to quantitatively describe these observations, allowing a measurement of the spin dynamics of an individual magnetic atom at zero magnetic field. These results demonstrate the existence of an efficient spin relaxation channel arising from a spin-exchange with individual carriers surrounding the quantum dot. A theoretical description of a spin-flip mechanism involving spin exchange with surrounding carriers gives relaxation times in good agreement with the measured dynamics.

From Single Particle to Superfluid Excitations in a Dissipative Polariton Gas

Using an angle-resolved heterodyne four-wave-mixing technique, we probe the low momentum excitation spectrum of a coherent polariton gas. The experimental results are well captured by the Bogoliubov transformation which describes the transition from single particle excitations of a normal fluid to soundlike excitations of a superfluid. In a dense coherent polariton gas, we find all the characteristics of a Bogoliubov transformation, i.e., the positive and negative energy branch with respect to the polariton gas energy at rest, soundlike shapes for the excitations dispersion, intensity, and linewidth ratio between the two branches in agreement with the theory. The influence of the nonequilibrium character of the polariton gas is shown by a careful analysis of its dispersion.

Fine structure of exciton excited levels in a quantum dot with a magnetic ion

The fine structure of excited excitonic states in a quantum dot with an embedded magnetic ion is studied theoretically and experimentally. The developed theory takes into account the Coulomb interaction between charged carriers, the anisotropic long-range electron-hole exchange interaction in the zero-dimensional exciton, and the exchange interaction of the electron and the hole with the

Spin properties of charged Mn-doped quantum dota)

d

Enhanced Second-Order Nonlinearity for THz Generation by Resonant Interaction of Exciton-Polariton Rabi Oscillations with Optical Phonons

electrons of a Mn ion inserted inside the dot. Depending on the relation between the quantum dot anisotropy and the exciton-Mn coupling, the photoluminescence excitation spectrum has a qualitatively different behavior. It provides a deep insight into the spin structure of the excited excitonic states.

Probing the Spin State of a Single Magnetic Ion in an Individual Quantum Dot

The optical properties of individual quantum dots doped with a single Mn atom and charged with a single carrier are analyzed. The emission of the neutral, negatively and positively charged excitons coupled with a single magnetic atom (Mn) are observed in the same individual quantum dot. The spectrum of the charged excitons in interaction with the Mn atom shows a rich pattern attributed to a strong anisotropy of the hole-Mn exchange interaction slightly perturbed by a small valence-band mixing. The anisotropy in the exchange interaction between a single magnetic atom and a single hole is revealed by comparing the emission of a charged Mn-doped quantum dot in longitudinal and transverse magnetic field.

Electrical control of a single Mn atom in a quantum dot.

Katharina Rojan,1, 2 Yoan Léger,3 Giovanna Morigi,2 Maxime Richard,4 and Anna Minguzzi1 Université Grenoble-Alpes, CNRS, Laboratoire de Physique et Modélisation des Milieux Condensés, F-38000 Grenoble, France Theoretische Physik, Universität des Saarlandes, D-66123 Saarbrücken, Germany FOTON Laboratory, CNRS, INSA, F-35708 Rennes, France Université Grenoble-Alpes, CNRS, Institut Néel, F-38000 Grenoble, France (Dated: June 13, 2017)Semiconductor microcavities in the strong-coupling regime exhibit an energy scale in the terahertz (THz) frequency range, which is fixed by the Rabi splitting between the upper and lower exciton-polariton states. While this range can be tuned by several orders of magnitude using different excitonic media, the transition between both polaritonic states is dipole forbidden. In this work, we show that, in cadmium telluride microcavities, the Rabi-oscillation-driven THz radiation is actually active without the need for any change in the microcavity design. This feature results from the unique resonance condition which is achieved between the Rabi splitting and the phonon-polariton states and leads to a giant enhancement of the second-order nonlinearity.