Optical control of an individual spin

People : L. Besombes, H. Boukari, T. Clement, D. Ferrand, H. Mariette
Former members : Y. Leger, L. Maingault, J. Bernos
Contact : lucien.besombes@grenoble.cnrs.fr  
Our group has active research activities in optical and magnetic interactions in semiconductor quantum structures and spin dynamics in condensed matter systems ("spintronics”). Our work focuses on the physics of newly-developed diluted magnetic semiconductors systems, such as quantum dots doped with a controlled number of magnetic atoms. Quantum dots containing a single magnetic atom (Mn) and a tuneable density of carriers have been recently realized. The aim of our work is to study by optical methods the fundamental mechanism controlling the relaxation and the decoherence of a single spin (carrier or magnetic atom) in a quantum dot. We also investigate different possibilities of spin manipulation to realize the initialization, the coherent control and the optical readout of a single spin in an individual quantum dot. The coherent control of quantum objects such as individual spins represent an actual topic of interest due to both fundamental interest and possible application for the implementation of quantum information processing in the solid state. Our main experimental techniques include low temperature spatially and time-resolved magneto-optical spectroscopy to reveal the spin structure and the spin dynamics in these quantum dots systems.

Recent results
Optical probing of spin fluctuations of a single paramagnetic atom
We have analyzed the photoluminescence intermittency generated by a single paramagnetic spin localized in an individual semiconductor quantum dot. In such system, the spin orientation is converted to photon energy and polarization. The statistics of the photons emitted by an individual QD reflects the quantum fluctuations of the localized spin. Photon correlation measurements reveal unique signatures of these fluctuations. We have shown that a rate equation model based on a partial thermalization of the exciton-Mn complex can quantitatively describe the observed statistics allowing a measurement of the spin dynamics of an individual localized magnetic atom at zero magnetic field. The data suggest the existence a fast spin relaxation channel arising from a spin-exchange with individual free carriers surrounding the quantum dot.
To learn more about this see : PHYSICAL REVIEW B 78, 125324 (2008)
Valence-band mixing in neutral, charged, and Mn-doped self-assembled quantum dots
We analyzed the optical emission of single II-VI quantum dots containing 0 or 1 magnetic atom (Mn) and a controlled number of carriers (0, ±1 electron). The emission of these quantum dots presents a large degree of linear polarization. This linear polarization is attributed to a valence-band mixing and we shown that in nonmagnetic quantum dots combining both shape anisotropy and an anisotropic in-plane strain distribution, the linear polarization direction of the exciton are controlled by interplay between valence-band mixing and electron-hole exchange interaction. Similarly, under strong transverse magnetic field, the direction of the linearly polarized emission of the charged excitons is simultaneously controlled by the valence-band mixing and the direction of the magnetic field. In quantum dots containing a Mn atom, the valence-band mixing allows simultaneous hole-Mn spin flips coupling bright and dark excitons. These spin flips are responsible for linearly polarized transitions in the emission of the charged excitons at zero magnetic field.
To learn more about this see : PHYSICAL REVIEW B 76, 045331 (2007)
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 has been 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 d 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. 
To learn more about this see : PHYSICAL REVIEW B 75, 205313 (2007)
Electrical control of a single Mn atom in a quantum dot
We recently realized reversible electrical control of the magnetic properties of a single Mn atom in an individual quantum dot. Our device permits us to prepare the dot in states with three different electric charges, 0, +1e, and -1e which result in dramatically different spin properties, as revealed by photoluminescence. Whereas in the neutral configuration the quantum dot is paramagnetic, the electron-doped dot spin states are spin rotationally invariant and the hole-doped dot spins states are quantized along the growth direction. 
To learn more about this see : PHYSICAL REVIEW LETTER 97, 107401 (2006)
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 behaviour in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-Mn 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).
To learn more about this see : PHYSICAL REVIEW LETTER 95, 047403 (2005)
Hole spin anisotropy in single Mn-doped quantum dots
The anisotropy in the exchange interaction between a single Mn atom and a single exciton confined in a quantum dot has been revealed experimentally. In a transverse magnetic field we directly observed the orientation of the magnetic ion spin along the resultant direction of the external magnetic field and the hole exchange field. With an increasing transverse magnetic field, this orientation progressively cancels the exchange interaction with the hole and at a high field the fine structure is mainly controlled by the electron-Mn coupling. At intermediate fields, we observed emission replicas caused by multiple spin flips within the Zeeman split ground state of a single Mn. All these features are well modelled by the magnetic field dependence of the stationary states of a single Mn spin in the exchange field of a heavy-hole exciton. 
To learn more about this see : PHYSICAL REVIEW B 72, 241309(R) (2005)
Carrier-induced spin splitting of an individual magnetic atom embedded in a quantum dot
The influence of the number of confined carriers on the spin splitting of a single Mn atom embedded in a semiconductor quantum dot has been observed. Investigating both the biexciton and the exciton transitions in the same Mn-doped quantum dot, we analyzed the impact of the Mn-exciton exchange interaction on the fine structure of the quantum dot emission. A single electron-hole pair is enough to induce a spontaneous splitting of the exciton-Mn system. The injection of a second electron-hole pair cancels the exchange interaction with the Mn ion and the Mn spin splitting is significantly reduced. A detailed analysis of the biexciton-Mn transitions reveals that the carriers’ orbital wave functions are perturbed by the interaction with the magnetic ion.     
To learn more about this see : PHYSICAL REVIEW B 71, 161307(R) (2005)
Probing the spin state of a single magnetic atom in an individual quantum dot
 The magnetic state of a single magnetic atom (Mn) embedded in an individual quantum dot has been optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn (S =5/2) has been analyzed in detail. The exciton-Mn exchange interaction shifts the energy of the exciton depending on the Mn spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements revealed that the emission intensities in both circular polarizations are controlled by the Mn spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photo-created carriers. 
To learn more about this see : PHYSICAL REVIEW LETTER 93, 207403 (2004)
Optical spin orientation of a single manganese atom in a quantum dot
A high degree of spin polarization is achieved for a single Mn atom localized in a quantum dot using quasi-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots.
To learn more about this see : arXiv:0811.2165v1
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