Download Besombes - International Conference on Quantum Dots (QD 2012)

Optical control of the spin of individual magnetic atoms
L. Besombes1, C. Le Gall1, A. Brunetti1, C.L. Cao1, 2, H. Boukari1, J. Fernandez-Rossier2,3
1
Institut Néel, CNRS, 25 av. des Martyrs, 38042 Grenoble, France.
2
Departamento de Fisica, Universidad de Alicante, San Vicente del Raspeig, 03690 Spain.
3
International Iberian Nanotechnology Laboratory, Av. Jose Veiga, 4715-330 Braga, Portugal.
E-mail: [email protected]
The decrease of the structure size in semiconductor electronics and magnetic information storage
devices has dramatically reduced the number of atoms necessary to process and store one bit of
information: An individual magnetic atom would represent the ultimate size limit for storing and
processing information. With semiconductor quantum dots (QDs) doped with Mn atoms, the probing of
a single atomic spin in a solid state environment become possible using optical techniques. The state of a
photon emitted or absorbed by a II-VI semiconductor QD containing a Mn atom is directly related to the
spin state of the magnetic atom (S=5/2): the exciton acts as an effective magnetic field along the QD
growth axis that lifts up the degeneracy between the six (2S+1) Mn spins states. Depending on the Mn
spin orientation, the recombination of an injected exciton emits a photon with a given energy and
polarization. We already demonstrated that a Mn atom embedded in a QD may act as an optically
addressable spin based memory [1]. The next steps would be to perform a fast coherent control of an
individual atom and to tune the coupling between such ultimate memories: a carrier mediated interaction
between two Mn spins inserted in the same QD is one step towards these challenging goals. For a QD
containing two Mn atoms, the optical spectra are in general quite complex (see figure), nevertheless each
collective state of the two Mn spins can be optically addressed.
Left: AFM image of a CdTe layer deposited on a ZnTe
substrate and schematic view of a QD containing an
individual Mn atom and an exciton. Right:
Photoluminescence (PL) spectra of a CdTe QD
containing 1 (X, 1Mn) or 2 (X, 2Mn) Mn atoms. The
PL intensity maps present the emission of a QD
containing 2 Mn atoms versus the power (bottom
panel) of a single mode laser on resonance with the
high energy state |Sz1=+5/2;Sz2=+5/2;J=+1> (Szi and
J are the angular momentum of the Mn atom i and
exciton respectively) and the emission of the same QD
when a single mode laser is scanned around this high
energy state E1 (top panel). One observes a power
dependent and detuning dependent splitting of the low
energy state |Sz1=+5/2;Sz2=+5/2;J=-1> showing that
the Mn spins state |Sz1=+5/2;Sz2=+5/2> is dressed by
the resonant laser field.
We have shown that the resonant optical injection of spin polarized carriers can be used to initialize
these localized spins [2]. We then demonstrated that under a strong resonant optical field, the energy of
any spin state of one or two Mn atoms can be independently tuned using the optical Stark effect [3]. The
strong coupling with a resonant laser field mixes the states associated with a given Mn spin orientation
in the presence (X, Mn) or absence (Mn alone) of the exciton. At the resonance, hybrid matter-field
states are created. As the laser detuning increases, the optically active transitions asymptotically
approach the original excitonic transitions, where the remaining energy offset is the optical Stark shift
(PL intensity map in the figure). In the ground state of the QD (one or two Mn without exciton), the laser
induced shift modifies the Mn fine and hyperfine structure. This optically controlled energy shifts affects
the spin dynamics of the magnetic atoms. It will be used in future experiments for a coherent
manipulation of the coupled electronic and nuclear spins of individual Mn atoms.
References:
[1] Phys. Rev. Lett. 102, 127402 (2009). [2] Phys. Rev. B 81, 245315 (2010). [3] Phys. Rev. Lett. 107, 057401
(2011).