Spin dynamics and exchange interaction in semiconductor quantum dots

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1 Spin dynamics and exchange interaction in semiconductor quantum dots Thierry AMAND Laboratoire de Nanophysique, Magnétisme et Opto-électronique de Toulouse (FRE LPMC UPS-INSA-CNRS n 686) Spin-electronics and Spin-optoelectronics in Semiconductors Bad Honnef, -3 November 003

2 Outline From magneto-electronics electronics to spintronics Spin relaxation in semiconductor nanostructures Intrinsic quantum dots (QD): X 0 exchange fine structure and pseudo-spin spin relaxation quenching. Negatively charged QDs : X spin relaxation quenching, spin memory of resident electrons with short optical pulses Multicharged exciton : X, X 3... Conclusion, perspectives

3 Spintronics in semiconductor quantum nanostructures Magnetoelectronics : based on GMR, TMR in metallic ferromagnetic (nano)structures devices : magnetic positioning sensors, reading heads, MRAMs Spintronics : Use spin degree of freedom in addition of charge in SC based devices spin magneto-optoelectronic devices : Spin effect Transistor Spin-LED, Spin-VCSEL Single electron spin memories in Quantum dots Entangled photon pair sources Spin-based quantum gates (C-NOT, Spin rotation )

4 Spintronics in semiconductor quantum nanostructures The problems to deal with : Spin injection into semiconductors - Well known optical pumping techniques (rely on spin-orbit interaction) - From a spin aligner : DMS under magnetic field (CdMnTe, II Mn VI...) ferromagnetic SC (GaMnAs, ) Ferromagnetic metal (Fe, Co) (see poster B. Liu, H. Carrère et al. ) or half metal (Fe O 3 ) Control of spin relaxation mechanism Crystal orientation, localisation of electron wave function Control of spin precession Magnetic field, electric field, g-tensor engineering Spin transport Spin diffusion length

5 Outline From magneto-electronics electronics to spintronics Spin relaxation in semiconductor nanostructures Intrinsic quantum dots (QD): X 0 exchange fine structure and pseudo-spin spin relaxation quenching. Negatively charged QDs : X spin relaxation quenching, spin memory of resident electrons with short optical pulses Multicharged exciton : X, X 3... Conclusion, perspectives

6 Spin interaction mechanisms Spin orbit coupling : Elliot-Yafet (1963) Spin orbit induced spin flip during momentum scattering τ ~ s τ k D Yakonov-Perel (1971) crystal with inversion asymmetry Electron-hole exchange interaction : Bir-Aronov-Pikus (1975) p-doped material Electron-nuclear hyperfine interaction : Overhauser (1958), Lampel (1968) electrons localised on donors S = ( ΩBIA + ΩSIA) S τ <<1 H e h exc = Ω k τ 1 s ~ Ω τ k H e h SR H=Aˆ. I Sˆ + H τ e h LR 1 s ~ π e h H exc N hole 1 τ s ~ AI ( I + 1) / V loc

7 Spin relaxation mechanisms EA, DP and BAP spin relaxation mechanisms closely linked to E(k) 3D Electron spin relaxation times : τ s,e ~ ns range (10 K) D : Single particle processes : spin-flip D Yakonov and Kachorovskii (1986) : electron spin relaxation in QW (~ 100 ps) suppression in [110] GaAs/AlGaAs QW (Y. Ohno et al., 1999) τ s,e ~ 0 300K Ferreira-Bastard (1991) : Hole spin relaxation in quantum wells (QW) (10ps-1ns) quenching of heavy-hole spin relaxation at small k (X. Marie et al. 1999) Two particles process : spin flip-flop M. Z. Maialle, E. Andrada et Silva, Sham et al. (1993) (MAS) : Electron-hole exchange interaction in D exciton : s X LT X, k τ s,x ~ 10 ps τ 1, ~ Ω τ

8 Optical orientation in quantum wells σ + Oz AlGaAs/GaAs/AlGaAs s z -1/> 1/> σ + σ j z 3/> -3/> Electrons (CB) HH (VB) k = D EXCITONS (HH) J z =j z +s z J=1 Circular excitation 1 = 3/, 1/ 1 = 3/,1/ J= + σ + σ 0 k = 0 X = Linear excitation ( ) Y = σ X σ Y ( 1 1 ) i 0 0

9 Exciton spin relaxation in QW GaAs/AlGaAs quantum wells Circular (σ + ) excitation 3x10 8 cm - 4x10 9 6x10 9 RESONANT EXCITATION : T s1 ~ ps S. Bar-Ad et al., PRL 68 (199) A. Vinattieri et al., PRB 50 (1994) T. Amand et al., PRL 78 (1997) Linear excitation 10 9 cm - 5x10 9 x TIME (ps) T s ~ 0-50 ps R.Worsley et al., PRL 76 (1996) X. Marie et al., PRL (1997) Fast exciton spin relaxation due to internal (P circ,,mas) or mutual exciton exchange (P lin ) Spin relaxation in QW- microcavities : See poster by P. Renucci et al.

10 Spin dynamics in Quantum Dots Discrete energy levels Slowing down of the spin relaxation? localised electrons in bulk SC : mutual e-e anisotropic exchange (DM) * R. I. Dzhioev et al., Soviet Phys. JETP 56 (198) Electron bound to * J. M. Kikkawa et al. Phys. Lett. 7, 1341 (1998) donors in GaAs : * K. Kavokin PRB 64 (001) τ SA ~ 100 ns! Single QD cw spectroscopy: * D. Gammon et al., Science 73 (1996) GaAs QD * Kuther et al., PRB 58 (1998) InGaAs QD Time-resolved spectroscopy in QD s * M. Chamarro et al., J. Lum. (1996) * D. Awschalom et al., PRB 59 (1999) CdSe QD * Gotoh et al. Appl. Phys. Lett. 7, 1341 (1998) Theory * A. Khaetski, Y. Nazarov, PRB 61,(000) * Merkulov et al., PRB 65 (00)

11 Electronic states in quantum dots J.P. Mc Caffey et al., JAP 88, 7(000) GaAs barrier (3D states) Wetting layer (D states) Conduction band D(E) Discrete energy levels Excited levels CB ground state c p x, y c s s hh hh p x, y Valence band E s, p : carrier envelope orbital state

12 Single QD spectroscopy substrate back contact tunnel barrier InAs rings blocking barrier AlAs/GaAs gate charge tuneable structures V g 1 Fermi energy V g 1,90 X 0 X 1- X - X 3-1,85 Energy (ev) 1,80 1,75 R.J. Warburton, et al., Nature 405, 96 (000) 1,70 WL charged -0,6-0,4-0, 0,0 0, Gate Voltage (V g )

13 Single QD spectroscopy : charged exciton states cw spectroscopy using confocal microscope + sub-micron mask B. Urbaszek, et al., Phys. Rev. Lett. 90, (003). Fine structure splitting PL (counts) Determination of the relevant interparticle exchange energies (without B) 0 1,10 1,11 1,1 1,13 0 X V X - x 0 1,1 1,13 1,14 X 1- X V X 1- x V 1,13 1,14 1,15 Energy (ev)

14 Outline From magneto-electronics electronics to spintronics Spin relaxation in semiconductor nanostructures Intrinsic quantum dots (QD): X 0 exchange fine structure and pseudo-spin spin relaxation quenching. Negatively charged QDs : X spin relaxation quenching, spin memory of resident electrons with short optical pulses Multicharged exciton : X, X 3... Conclusion, perspectives

15 InAs/GaAs Quantum Dots Self-organized QD Anisotropic exchange interaction 1 X>=( 1>+ -1>)/ // [110] Y>=( 1>- -1>)/ // [1-10] GaAs Exciton fine structure 150 µev Single dot cw spectroscopy Bayer et al., PRL 8 (1999)

16 Exciton fine structure in quantum dots D d 1, + 1, 1, 1 C v X (110) 1 Y (110 ) 1 s c s v ± s c s v X 0 0 << 1, +,, electron-hole exchange parameters : 0 : dark/bright splitting 1 : interfaces/shape asymmetry {(1,1,0), (1,-1,0)} R. I. Dzhioev et al., JETP Lett., 65, 804 (1997); C. Gourdon et al. Phys. Rev. B 46, 4644 (199)

17 Exciton fine structure in quantum dots : electron-hole exchange effective Spin Hamiltonian : K. V. Kavokin, Phys. Stat. Sol. 00 Hˆ exc e h = = 0 0 j j Z Z s s Z Z ( j s j s ) + ( j s + j s ) X ( ) j s + j s + ( j s + j s ) + X Y Y + X + X + Y Y s : electron spin j : heavy hole pseudo-spin states (neglect light holes) ± 1 3 hole 0 : short range contribution ~ 500 µev 1 : short + long range contributions (couple bright excitons, J=1) ~ µev : short range contribution (couple dark excitons, J=) ~ 0-50 µev

18 Pseudo-spin spin relaxation quenching of the X 0 ground state Resonant photo-generation of linear excitons : X 0 Time resolved PL spectroscopy σ x P L 1 I I = X Y I + I 0.75 No measurable decay τ s1 > 0 ns! X Y s c s v + Strong linear polarization s c s v Intensity (arb. units) σ X, T=10 K Time (ps) I X 0.4 I Y 0. Linear Polarization Neither the electron, nor the hole spin relax on the exciton time scale M. Paillard et al, PRL 86 (001)

19 X 0 PL linear polarisation dynamics/temperature Strong temperature dependence for T > 30 K Activation energy E a ~ 30 mev 1 Linear Polarization 0.1 Time decay 10K 30K 40K 50K 60K 70K 80K /kT Time (ps) Temperature dependence on spin relaxation in QD : see poster M. Sénès, X. Marie et al.

20 Mechanism for linear exciton spin relaxation Two (InAs) LO phonon quasi-resonant nd order process Yields an activation energy of ~ 30 mev at low temperature ( s (S c, c p hh,s hh ) ) 1 T N S LO N = e LO ( N Ω kt LO LO 1 + 1) 1 1 Ω kt T S e LO,T 0 Bimberg and Grundmann PRB (1999) E. Tsitsishvili et al. Phys. Rev. B 66 (00)

21 Exciton Spin coherence quantum beats in QD s self-assembled InAs/GaAs ensemble of small QD s σ + circularly-polarized laser excitation creates 1 the quantum superposition : ψ = ( X + i Y ) Intensity 1400 period = 140 ps P = 40 mw ω exc = ev ω det = 1.30 ev τ decay = 900 ps τ s,inhom = 75 ps I + exp I - exp Fit I + Fit I Time (ps) Pc exp Fit Pc 0,6 0,3 0,0-0,3-0,6-0,9 Circular Polarisation Y X g 1 σ + 1 ~ 30 µev Damping : spin dephasing due to inhomogeneous broadening ω LO Time resolved PL spectroscopy : M. Sénès et al. (004)

22 Exciton states under longitudinal magnetic field QD EXCITON EIGENSTATES : B z = 0 B z 0 +1> X>=( 1>+ 1>)/ Y>=( 1> 1>)/i 1 Linear σ X excitation Linearly-polarized PL Ω z = ( g + g ) µ B >> e Ωz Circular σ + excitation Circularly-polarized PL h B 1> z 1

23 Spin dynamics in longitudinal magnetic field PL circular polarisation dynamics σ + circularly-polarized laser resonant excitation M. Paillard et al, PRL 86 (001) T=1.7 K σ + σ + B =.5T σ σ + 1,0 0, ,0 Circular Polarization (quasi-circular eigenstates) No decay regardless of the magnetic field value up to 5 T The electron and hole spins are totally frozen at low temperature

24 Spin Dynamics under magnetic field Pseudo-spin formalism for QD exciton : S = 1/ (R.I. Dzhioev et al., Phys. of Solid State 40, 790 (1998)) X> et Y> S x =1/ et 1/ P Lin = S x +1> et -1> S z =1/ et 1/ P C = S z Z z S Z (0) B z H = Pump σ + σ x S Z (0) S X (0) ( ωσ + Ω σ ) x Inhomogeneous system z z 1 =ω S z (t) = ω S (t) S x (t) X σ x(z) : Pauli matrixes S ( x+ωz z) S t Ω. S(0) S( ) Ω Ω average

25 Orientation - alignment conversion Circular polarisation 1,0 0,8 0,6 0,4 0, σ + exc. P C P C ~ Ω ω + ~ ω laser ~ ' 0, ,5 0,4 0,3 0, 0,1 P C P Lin laser ~ ω ' ~ ~ ω Ω z ~ + Ω z ~ ~ + ω + Ω z 0, z σ X exc. Linear polarisation 1,0 0,8 0,6 0,4 0, σ X exc. Magnetic field (T) ~ ω laser. ~ ~ ~ ' ω + ω + Ω z P Lin P Lin 0, ,5 0,4 0,3 0, 0,1 P Lin P C laser ~ ~ ω. Ω ~ ω ' z ~ ~ + ω + Ω z σ + exc. 0, ~ ω ω ~ Ω Ω ( ) average ( ) average g e + g hh ~3 M.Bayer et al., Phys. Rev. Lett. 8, 1748 (1999) 1/ 1 10 µev

26 Circular/Linear polarisation under longitudinal magnetic field Resonant excitation of : σ + s c s v or 1 s c s v + σ x (elliptical eigenstates) s c s v X 0 Polarisation decay time ( ps ) σ + σ + σ x σ x P exc = 1 mw λ exc = 1100 nm B = 0.4 T Tem perature (K) X 0 Circular polarisation more robust against temperature than linear polarisation Temperature dependence on spin relaxation in QD : see poster M. Sénès, X. Marie et al.

27 Outline From magneto-electronics electronics to spintronics Spin relaxation in semiconductor nanostructures Intrinsic quantum dots (QD): X 0 exchange fine structure and pseudo-spin spin relaxation quenching. Negatively charged QDs : X spin relaxation quenching, spin memory of resident electrons with short optical pulses Multicharged exciton : X, X 3... Conclusion, perspectives

28 The X complex : QD trion cw non resonant excitation s c s v s c s v X or sc s v s c s v X - PL (counts) ,1 1,13 1,14 X V X 1- x 0 1,13 1,14 1,15 Energy (ev) (ev) The trion ground state doublet is not split by anisotropy (Kramers theorem)

29 N-doped self-assembled InAs/GaAs QDs ensemble time resolved PL, non resonant excitation PL Intensity. (arb. units) p c v x -p x First excited state 00 ps 600 ps 1000 ps 1400 ps 1800 ps s c -s v Ground State nm 40 nm InAs Si InAs Si GaAs W avelength (nm) - observation of far-ir intraband absorption around 43 mev - observation of luminescence in strictly resonant excitation ~1 doping electron/qd

30 Spin dynamics of trion states X in doped QD Resonant σ + excitation Photo-generation of : s c s v P c σ + 1 I I + + I I 0.7 No measurable decay T s1 > 0 ns! = + s c s v Strong circular polarization s c s v X - Intensity (arb. units) I - T=10 K Time (ps) I + 1 0,1 0,01 M. Sénès et al. 6th ICPS, Edinburg (00) Circular Polarization Hole spin stability in the ground state trion

31 Circular polarisation dynamics in charged QD Non resonant σ + excitation in the wetting layer (1.44 ev) : Intensity (arb. units) I + I - T= 10 K Time (ps) Two regimes : - Fast initial sign reversal (~10 ps) - slow evolution of the negative polarization down to -50% (~ 1 ns) - power dependent effect in cw S. Cortez et al., M. Sénès et al. Phys. Rev. Lett. 89, (00) 0, 0,0-0, -0,4-0,6 Circular polarisation Intensity (arb. units) I Time (ps) I - 0,3 0, 0,1 0,0-0,1-0, Circular polarisation

32 Circular polarisation dynamics in charged QD Simplified theoretical interpretation (without fine structure of excited trions) : We suppose that : the hole spin is completely depolarized before its capture in the dots we photo-inject only a single electron-hole pair per dots ( low excitation intensity). At t = 0, there are 4 possible configurations for the dots: Bright Dark «Unpolarized trions» Exchange Fast energy relaxation (~1ps) 3/ -3/ Pauli blocking 3/ -3/

33 Circular polarisation dynamics in charged QD Evolution of bright trion : AEI σ ++ 3/ - 3/ σ Fast simultaneous electron/hole spin flip ( ~10 ps ) due to electron-hole exchange interaction 1 After a fast energy relaxation, the trion is stable negative circular polarisation emitted Evolution of dark trion : SOI dark - 3/ - 3/ σ Slow single particle spin flip ( τ S 1 ns ) of the P c electron (Spin-Orbit Interaction). Same final state as bright trions, σ - emitted Experimentally, we don t observe any effect due to the dark state splitting

34 Circular polarisation dynamics in charged QD resident electron : or t = 0 + AEI σ + dark σ + σ Emission circular polarization : P = +33 % 3/ -3/ 3/ -3/ time t > τ flip -flop σ σ + σ dark P = 33 % -3/ -3/ 3/ -3/ t > τ e spin-flip σ σ + σ σ P = 50 % -3/ 3/ -3/ -3/

35 Circular polarisation dynamics in charged QD n 1 : S 0 s v (σ + PL) n : S 0 s v (σ - PL) Unpolarised trions n 3 : T -1 s v (σ + PL) n 4 : T -1 s v (σ - PL) bright trion dark trion dn dt 1 dn dt dn dt 3 dn dt 4 n = τ 1 r n = τ n = τ r 3 r n = τ ( n 4 sf n + τ n τ + n 1 3 P circ = ( n1 + n3) + 3 ff 3 ff n + τ ) n n 4 sf Circular polarisation 0,4 τ r =700 ps WL σ + excitation 10K τ ff =60 ps 0, τ sf =800 ps 0,0-0, -0,4-0,6-0,8 experiment model Time (ps)

36 H Iˆ Excited Trion X fine structure ( ˆj Iˆ ~ ˆj Iˆ ) + ( ˆj Iˆ + ˆj Iˆ ) + η ~ ˆj Kˆ + η ~ ( ˆj Kˆ ˆj Kˆ ) + η ~ ( ˆj Kˆ ˆj Kˆ ) ex 3 ~ ˆ e e h e e jziˆ ~,, = + 0 z + 1 x x y y x x y y 0 z z 1 x x y y x x + 4 I ˆ = s + s ˆ1 ˆ Kˆ = s s ˆ1 ˆ ~ = 1 s_ p ( s i s i + ) i x _ ~ η = 1 s_ p ( s i s i ) i x _ y i=0,1, y Symmetric QD : S * 0 s v or The isotropic e-h exchange splits the electron triplet Asymmetric QD : s c,p c,s v e e 5 ~ ~ mev 0 0 v T 1 s,t + 1 v T 0 s or v T 1 s,t + 1 s s v v (bright) (dark) The anisotropic e-h exchange couples optically active triplet states with excited singlet s c,s c,s v E 45meV s p S 0 s v or

37 Circular polarisation dynamics in charged QD Intra-dot σ + circular excitation (d v -d c ) λ exc =9478 A λ det =10850 A Intensity (arb.units) σ + σ + σ σ + τ r =700ps τ ff =0 ps τ sf =800 ps 1,0 0,8 0,6 0,4 0, 0,0 Circular polarisation Time (ps) Fast (~0 ps) depolarisation, no negative circular polarisation

38 Circular polarisation dynamics in charged QD Intra-dot σ + circular excitation (d v -d c ) photogeneration of X t = 0 AEI Emission circular polarization : σ + σ + P = +100 % time 3/ 3/ t > τ flip -flop σ σ + P = 0-3/ 3/ Experimentally, τ flip-flop ~ 0 ps Hole spin stable during intra-dot relaxation

39 Spin memory in charged QD - to write and read the spin of a resident electron in N-doped self- assembled quantum dot using non resonant polarized light excitation - spin state non limited by the radiative lifetime or Wetting layer «gap» 1.4 ev σ+ Ground state luminescence 1.15 ev

40 Spin memory in charged QD Circular polarisation induced by the pump pulse onto the luminescence of a linearly-polarised probe Delay Pump-induced circular polarisation (%) Pumpσ + - Pump pulse σ + Probe σ X - Probe pulse σ x, delay >> τ rad - Circular polarisation of the time-integrated luminescence associated to the probe pulse Spin relaxation time of resident electron ~ 15 ns S. Cortez et al., M. Sénès et al. Phys. Rev. Lett. 89 (00)

41 Spin dynamics of trion states X in doped QDs Temperature dependence of the negative circular polarisation Non resonant σ + excitation in the wetting layer Circular Polarisation 0,4 0, 0,0-0, -0,4-0,6 10 K 30 K 50 K 70 K 90 K 0,4 0, 0,0-0, -0,4-0,6-0, ,8 Time (ps)

42 Outline From magneto-electronics electronics to spintronics Spin relaxation in semiconductor nanostructures Intrinsic quantum dots (QD): X 0 exchange fine structure and pseudo-spin spin relaxation quenching. Negatively charged QDs : X spin relaxation quenching, spin memory of resident electrons with short optical pulses Multicharged exciton : X, X 3... Conclusion, perspectives

43 X - single dot spectroscopy S 0 * T ±1 T 0 0 * final state :, T ±1,T I(T ±1 ) = I(T 0 ) S0 0 T +1 T 0 S 0 * s c -p xc e-e exchange: (ee) 7 mev s v -p xc e-h exchange: X eh 0 = _ Broad Singlet final state : τ relax (S 0 * S 0 ) p x s 0. 1meV /Γ ~ 1 ps B. Urbaszek et al. Phys. Rev. Lett. 90, (003).

44 X 3- single dot spectroscopy PL (counts) a p c x 0 1,10 1,11 1,1 1,13 E < X ee X 3- E s c s v p c y X 3- Energy (ev) weakly asymmetric QDs : -0.14V S=1 S=3/ S=1/ or Fine structure splitting : ( ) eh eh X + X 0. mev 0 = px _ s p y _ s ( ) eh eh eh X p s + X p _ s X p _ s ( X ) X eh px_ s y _ x 3 x different value for if 1 or electrons in the p-shell ( X ) B. Urbaszek et al. Phys. Rev. Lett. 90, (003).

45 Spin dynamics of trion states X 3 in doped QDs Intra-dot σ + circular excitation (p v -p c ) Intensity (a.u.) σ + σ + σ σ Time (ps) 70 K 30 K 10 K X 3- spin relaxation 10 K P circ = 0% Temperature activation : E a ~ 10 mev PSSN symposium, Kyoto (003) : Intensity (a.u.) Intensity (a.u.) 1000 WL σ + circular excitation σ + σ + σ σ + λ exc = 860 nm λ det = 1045 nm P = 50 mw T = 10 K Time (ps) λ exc = 860 nm λ det = 1045 nm P = 50 mw T = 90 K Time (ps) counter polarised PL (σ ) 1,0 0,5 0,0-0,5 0,4 0, 0,0-0, -0,4 Circular Polarization

46 Spin dynamics of trion states X 3 in doped QDs strongly asymmetric QDs : b E > X ee X 3- p c x p c y or S=0 S=1/ S=-1/ Single emission line, p s broadened by fast relaxation (~1 ps) c x c

47 Spin dynamics of trion states X 3 in doped QDs Intra-dot σ + circular excitation (p v -p c ) p xc resident electron : or AEI Emission circular polarization : t = 0 σ + σ + σ + P = +100 % time 3/ 3/ 3/ t > τ flip -flop σ σ + σ + P = + 33 % -3/ 3/ 3/ Activation energy E a E(p yc ) - E(p xc ) for spin relaxation

48 Spin dynamics of trion states X 3 in doped QDs WL excite σ + AEI t = 0 P = 0 σ + σ + σ + σ σ σ 3/ 3/ 3/ SOI -3/ -3/ -3/ t > τ flip -flop P = 33 % σ σ + σ + σ σ σ -3/ 3/ 3/ -3/ -3/ -3/ t > τ e spin-flip P = 33 % σ σ + σ + σ σ σ -3/ 3/ 3/ -3/ -3/ -3/

49 CW / Time resolved PL spectroscopy X : τ 1 flip flop X ~ τ S 1 * px _ s s _ s 1 1 px _ s px _ s s _ s _ e _ e ( ) / S τ * ~ 1 S _ S ps p s e x e s _ 0 p x s _ s 0 s _ s 1 : X singlet/triplet splitting (~ 5-7 mev) : X triplet fine structure (~ 0. mev) : X biexciton fine structure (~ 0.6 mev) { } : X 0 X, Y fine structure (~ µev) measured (cw-pl, transmission) p x _ s px _ s px _ s 1 : scaling 1 0? s _ s τ s _ s flip-flop ~ 100 ps 1 X 3 : 0 s _ s p 1 y _ s : Replace 1(0) by 1(0) τ flip flop 3 > τ flip flop p s p X X x _ p y τ 3 flip flop X e x e e _ e

50 Spin dynamics in QD s : conclusion, perspectives Comparison of cw-single dot and TRPL on QD ensembles exchange interaction plays a major role, spin-orbit interaction strongly reduced X 0 : - exciton ground state : anisotropic e-h exchange, fine structure - τ s,x ~ 0 ns, T 30 K, quenching of spin relaxation processes X : interest : manipulate localised spins with optical pulses - ground state trion : e-h exchange cancellation stable hole τ s,hh ~ 0 10 K - excited trion : e-h exchange efficient in anisotropic QD s τ ff ~ ps hole spin is stable in intra-dot relaxation - resident electron spin memory effect : τ s,e K long term spin decoherence : hyperfine interaction with nuclear spin? anisotropic exchange between localised electron? X n :- fine structure due to e-e exchange in final state (n > 1) - general trend : incomplete conduction electron orbital strong e-h exchange, τ ff all orbitals doublet filled e-h exchange cancelled

51 Conclusion, perspectives Time resolved experiments on single QD with time resolution < 0 ps Charge tuneable structure : X 0, X, X n Excitation with shaped optical pulses (chirped pulses ) light induced spin-spin coupling (C. Piermarocchi et al. Phys. Rev. Lett. 00) Other QD systems : Si QD? (QW : τ se ~ 1-10 µs), but optics?

52 Acknowledgements : Co-workers in Toulouse : Quantum Opto-electronic group, LNMO : X. Marie M. Sénès B. Urbaszek P. Renucci H. Carrère E. Vanelle Collaborations : S. Cortez, O. Krebs, P. Voisin, LPN (Marcoussis) R. Ferreira, G. Bastard, LPMC de l ENS (Paris) J.M. Gérard, CEA (Grenoble) R. J. Warburton, Heriot Watt University (Edimburg) B. D. Gerardot, P. M. Petroff, Univ. of California (Santa Barbara) H. Kalt, E. Tsitsishvili, (Karlsruhe) K. Kavokin, V. Kalevich, V. Ustinov, Ioffe Institute (St Petersburg)

Influence of hyperfine interaction on optical orientation in self-assembled InAs/GaAs quantum dots

Influence of hyperfine interaction on optical orientation in self-assembled InAs/GaAs quantum dots O. Krebs, B. Eble (PhD), S. Laurent (PhD), K. Kowalik (PhD) A. Kudelski, A. Lemaître, and P. Voisin Laboratoire