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Phonon coupling to exciton complexes in single quantum dots

Phonon coupling to exciton complexes in single quantum dots. D. Dufåker a , K. F. Karlsson a , V. Dimastrodonato b , L. Mereni b , P. O. Holtz a , B. E. Sernelius a , and E. Pelucchi b. a IFM Semiconductor materials, Linköping University, Sweden

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Phonon coupling to exciton complexes in single quantum dots

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  1. Phonon coupling to exciton complexes insingle quantum dots D. Dufåkera, K. F. Karlssona, V. Dimastrodonatob, L. Merenib, P. O. Holtza, B. E. Serneliusa , and E. Pelucchib a IFM Semiconductor materials, Linköping University, Sweden b Tyndall National Institute, University College Cork, Ireland The 11th edition of the international conference PLMCN:Physics of Light-Matter Coupling in Nanostructures Cuernavaca (Mexico), 12-16 April, 2010

  2. Introduction to Pyramidal QDs Introduction to LO-phonon coupling Experimental results Interpretation/Computational results Conclusions Outline

  3. InGaAs QDs in AlGaAs barriers MOCVD InGaAs QD GaAs AlGaAs Barrier A. Hartmann PRL 84 5648(2000) Pyramidal QDs Patterned GaAs substrate (111)B • self-limiting profile • growth anisotropy • capilarity effects • alloy segregation G. Biasiol et al., PRL 81, 2962 (1998);Phys. Rev. B 65, 205306 (2002)

  4. Simplified model AlGaAs VQWR1 4 % InGaAs QD 15 % Surrounding AlGaAs Barrier 20-30 % 1Q. Zhu el al., Nano Lett. 6 1036 (2006) Pyramidal QDs AlGaAs layer 30 % Al InGaAs layer 15 % In

  5. 2X C3v X Vac • Designed with excited electron levels (x4)p (x2)s (x2)s Pyramidal QDs • Efficient light extraction >120 kcnts/sec • Site-controlled, isolated QDs • C3v-symmetry – emitters of entangled photons1 1R. Singh et al., PRL 103 063601 (2009); K. F. Karlsson el al., PRB Accepted (R) (2010); A. Schliwa et al., PRB 80 161307R (2009); A. Mohan et al., Nature Phot. 2 (2010)

  6. Pyramidal QDs • Control of charge population by excitation conditions1 QD2 Normalized PL Intensity 1A. Hartmann PRL 84 5648(2000)

  7. Top view Side view Side view Charge density Charge distribution Gray:Quantum dot profile Red: Hole probability density (10% of max) Blue:Electron probablity density (10% of max) LO-phonon coupling Coupling of LO-phonons with excitons is electric (Fröhlich) The total coupling is given by the difference between the couplings ofelectrons and holes An exciton formed by an electron-hole pair is a neutral entitiy Equal probability density function of electrons and holes  vanishing coupling In real systems: electrons and holes have different charge distribution

  8. 0-phonon 1-phonon ħLO ħLO ħLO ħLO 2-phonon Energy 0-phonon Emission spectrum 1-phonon 2-phonon Energy LO-phonon coupling Excitation spectrum T = 0 K No spectral linewidth Dispersion less phonon branch Huang-Rhys parameter S

  9. Ensemble measurements InAs/GaAs QDs S ~ 0.015 R. Heitz et al., PRL 83 4654 (1999) Single CdSe/ZnCdSe QD (X, 2X) S ~0.035, 0.032 F. Gindele et al., PRB 60 2157R (1999) Single InAs/GaAs QDs, PL-excitation spectroscopy P. Hawrylak et al., PRL 85 389 (2000) LO-phonon coupling

  10. Extra charge? PL-excitation spectroscopy InAs/GaAs QDs PRL 85 389 (2000) LO-phonon coupling Spherical GaAs microcrystallities (r>11 nm) S enhanced from 0.001 to 0.01 by an extra charge Nomura & Kobayashi PRB 45 1305 (1992)

  11. Experimental results QD1 X X Direct emission T=4K Phonon replicas (1st order) X X2 X X2 X+ 2X 1000

  12. Experimental results QD1 • Replica of X+ significantly weaker than X and X- • Replica of X- similar strength as replica of X • LO-phonon energy 36.40.1 meV • Larger spectral linewidth of replicas

  13. Experimental results 17 QDs Measured Huang-Rhys Parameter

  14. Computations Excitonic ground states computed self-consistently by 88 band kp theory in Hartree approximation Strain induced deformation potentials simulated by continuum elastic theory

  15. Huang-Rhys parameters S1000 Computations X 2X X X+ Charge density (e/nm3) Real space maps

  16. X+ X Side Top Interpretation Coulomb interactions induces changes in the charge distribution; different exciton complexes have different charge distributions Repulsion  Delocalization Attraction  Localization J. J. Finley et al., PRB 70 201308R (2004)

  17. Integrated diagonal phonon scattering matrix elements relative X Computations X 2X X X+  Charge density (e/nm3)  initial • Strong phonon coupling for an exciton comples does not imply strong phonon replicas.

  18. GaAs-like LO-phonon energy in AlGaAs VQWR (4% Al) ħLO= 36.4 meV 04%: E -0.2 meV Interpretation Measured LO-phonon energy: 36.40.1 meV (GaAs bulk: ~36.6 meV) Surrounding barrier (20-30% Al) ħLO= 35.0-35.5 meV

  19. Interpretation Spectral linewidth Bulk-like LO-phonon dispersion  broadening < 50 eV GaAs LO-phonon lifetime  broadening ~ 70 eV1 • Composition variations and alloys disorder2 1M. Canonico PRL 88 215502 (2002) 2B. Jusserand PRB 24 7194 (1981)

  20. Comparison of phonon replicas of charged and neutral exciton complexes. S = 0.001 – 0.004 Extra positive charge may result in strongly reducedphonon replicas due to the heavier mass of the hole X+ X Coulomb induced charge cancellation of an electron- hole pair X+: Strongest LO-phonon scattering matrix element and simultaneously the weakest phonon replicas Adiabatic independent-phonon model yield values of the Huang-Rhys parameter in agreement withexperiments Conclusions

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