Optical quantum control in semiconductors nano-systems Carlo Piermarocchi Department of Physics and Astronomy Michigan State University, East Lansing, Michigan Support by Colloquium at Oakland University March 17 th 2005

I.Spins in semiconductors (Guillermo Quinteiro) II.Atoms in organic quantum wires (Michael Katkov) III.Currents in quantum rings (Yuriy Pershin, Mark Dykman) Systems

Control of spins by light Part I

Quantum control of two donors D + s 1 s 2 Neutral donors GaAs:Si Control Hamiltonian

Optical RKKY Conduction band Valence band E Gap (GaAs) Si Itinerant excitons mediate the interaction C. Piermarocchi,P.Chen,L.J.Sham,G.D.Steel, Phys. Rev. Lett. (2002)

m* e =0.07m, m* h =0.5m,  =300 Å Quantum Well Exponential decay of the interaction

Beyond ORKKY Can we have anti-ferromagnetic coupling? What is the effect of multiple scattering? What if the exciton is bound to the impurity? Beyond second order in the exciton-spin coupling C. Piermarocchi and G. F. Quinteiro, Phys. Rev B (2004)

We seek a solution in terms of T matrix equation. Solution for the 2 spins using Solution for spin A + exciton TATA Solution for spin B + exciton TBTB A  B 

Analytical effective H of two localized spins: Effective magnetic field : Heisenberg coupling:

Spin-spin coupling ORKKY 2 Si in GaAs R=2a B (~20 nm) Bonding Anti-bonding 1 Ry * =5 meV

R-dependence Free excitons Long range Excitons bound donors. Short range

Rare earth impurities Yb 3+ in InP Long decoherence for spin Coupling with exciton by s-f exchange Deep impurities

R dependence InP:Yb Deep confinement Triplet channel Singlet channel

Experiments Excitons bound to single Te pairs in ZnSe. Deep isoelectronic (non magnetic) Average separation between pairs: 1 micron Single-impurity pair spectroscopy A. Muller, P. Bianucci, C. Piermarocchi, M. Fornari, I. C. Robin, R. André and C. K. Shih (submitted, 2004).

Light-spin thermodynamics ZnSe:Mn Can we induce a PM/FM transition using coherent light? J Fernandez-Rossier, C Piermarocchi, P Chen, LJ Sham, and AH MacDonald, Phys. Rev. Lett. (2004)

Light-induced ferromagnetism Mean Field approach

Conclusions (I) Light can induce spin-spin interaction in doped semiconductors. Strength and sign of the interaction are controllable. Light-induced phase transitions.

Control of atoms in organic quantum wires Part II

Polymer chain under strong non-resonant ac field Coherent optical polarization coupled to phonons Force on the “light-dressed” atoms Control of local lattice deformations Coherent control of atomic chains

C C C C C C R R R R C C C C C C R R R R … POLYDIACETYLENE Excitons localized in the unit cells EgEg unun u n+1 B † n+1 B n u n-1

HAMILTONIAN Su-Schrieffer-Heeger for excitons Intensity of the field and laser energy are control parameters Control Hamiltonian

Light-dressed ground state Optical detuning Optical polarization ENERGY

Nonlinear equation for the polarization Nonlinear attractive interaction: polarization self trapping due to phonon coupling Nonlinear repulsive interaction: saturation effects External field: Determines the total polarization in the field

Polarization Self-trapping  = t 0  = 2  t 0  = 5  t 0

 = t 0  = 2  t 0  = 5  t 0 Force

With light Without light Lattice deformation  = 2  t 0

Conclusions (II) Lattice deformation induced by the light Soliton-like solutions with a characteristic saturation The force acting on the lattice can be finely controlled through the field parameters Katkov/Piermarocchi cond-mat/

Control of currents in quantum rings PART III

Quantum Rings A. Lorke, R. J. Luyken, A. O. Govorov, and J. P. Kotthaus, Phys. Rev. Lett. 84, 2223 (2000). Self-assembled InAs quantum rings on GaAs surface, R ≈10nm A. Fuhrer, S. Luscher, T. Ihn, T. Heinzel, K. Ensslin, W. Wegscheider, and M. Bichler, Nature. 413, 822 (2001). Quantum ring fabricated on AlGaAs-GaAs heterostructures

Circularly polarized light controls currents in a quantum ring Polarized radiation … Transition from the ground state to an excited state characterized by a strong current

. Excitation dynamics Excitation pulse sequence Evolution of level population in a 3-electron quantum ring Liouville equation

Current in the ring Pulse sequence Relaxation mechanisms: photon emission, t r ~ 0.1 ms. phonon emission, t r ~ 10 ns. For GaAs quantum ring of R = 10 nm, N =11 B 0 ≈ 3 mT. Continuous wave excitation

Conclusions (III) Trains of circularly-polarized pulses can control the angular momentum of N electrons in a ring High angular momentum gives strong localized current Externally-controlled source of local magnetic field for single-spin quantum logic Pershin/Piermarocchi cond-mat/