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Sangam Chatterjee TAC-LSUM, 6.10.2013

Exciton i nterstate d ynamics in photoexcited quantum wells manipulated by strong narrow-band THz pulses. Sangam Chatterjee TAC-LSUM, 6.10.2013. Acknowledgements. Experiment: Nils Rosemann, Alexey Chernikov , Benjamin Ewers, Martin Koch

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Sangam Chatterjee TAC-LSUM, 6.10.2013

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  1. Excitoninterstate dynamics in photoexcited quantum wells manipulated by strong narrow-band THz pulses Sangam Chatterjee TAC-LSUM, 6.10.2013

  2. Acknowledgements Experiment: • Nils Rosemann, Alexey Chernikov, Benjamin Ewers, Martin Koch • Martin Teich, SybineZybell, Harald Schneider, Manfred Helm (Helmholtz-Zentrum Dresden-Rossendorf) • William D. Rice, Junichiro Kono (Rice University) Theory: • Andrea Klettke, Lukas Schneebeli, Mackillo Kira, Stephan W. Koch Samples: • Galina Khitrova, Hyatt M. Gibbs (The University of Arizona) • Stephan Schartner, Gottfried Strasser (TU Wien)

  3. Outline: • Introduction: Excitons, Autler-Townes Effect& Ionization • 1s-2p InterstateCoherenceand 2s-2p Quantum Beats • Population Control: Rabi-Floppingof1s-2s Excitons • Conclusions & Outlook

  4. Introduction: From Atoms to Solids to Excitons… + atom exciton crystal - levels bands “atom-like dispersion” 4 3 2 n=1 energy

  5. time 0 Time-Resolved Spectroscopy on Semiconductors • Different physicalphenomena different time scales  Optical ExcitationPolarizationanddephasingCarierinjectionandcooling Formation many-body excitations

  6. E Eg k Valence Band – “p-type” Introduction: Optical vs. THz Spectroscopy Conduction Band – “s-type” THz Photon

  7. Detection of Excitons:Optical pump + THz probe

  8. Polarization Manipulation:Weak Optical + Strong THz Excitation

  9. Polarization Manipulation:Weak Optical + Strong THz Excitation (II) • M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010) • B. Ewers et al, Phys. Rev. B 85, 075307 (2012)

  10. Reminder: Driven 2-Level-System (I) • Linear absorption of a 2LS: E 0

  11. Reminder : Driven 2-Level-System (II) • Rabi-Splitting, Mollowtriplet E 0

  12. Reminder : Autler-Townes Effect • Rabi-Splitting, Autler-Townes effect, E 0

  13. Messages & Questions (I) E E 0 0

  14. Inter-level Coherences:Optical-Pump – THz-Four-Wave Mixing to Ge:Ga Detector t

  15. Optical-Pump – THz-Probe 1s excitonlifetime: • ~330ps (THz) • Similarto 1sTRPL lifetime

  16. Optical-Pump – THz-Pump – THz-Probe 1s excitonlifetime: • ~330ps (THz) • Similarto 1sTRPL lifetime Opt. Pump – THz Pump – THzProbe • Interferencepattern • “Shelving“ atlatertimes

  17. Optical-Pump – THz-Pump – THz-Probe 1s excitonlifetime: • ~330ps (THz) • Similarto 1sTRPL lifetime Opt. Pump – THz Pump – THz Probe • Interferencepattern • “Shelving“ atlatertimes

  18. Optical-Pump – THz-Pump – THz-Probe 1s excitonlifetime: • ~330ps (THz) • Similarto 1sTRPL lifetime Opt. Pump – THz Pump – THz Probe • Interferencepattern • “Shelving“ atlatertimes • Oscillationperiod: 118µm

  19. Optical-Pump – THz-Pump – THz-Probe 1s excitonlifetime: • ~330ps (THz) • Similarto 1s TRPL lifetime Opt. Pump – THz Pump – THz Probe • Interferencepattern • “Shelving“ atlatertimes • Oscillationperiod: 118µm

  20. Optical-Pump – THz-Four-Wave Mixing Decay ~50ps • Similarto 2sTRPL lifetime • Mechanism: acousticphonons Overlayedbyslowoscillation • Period: 100ps • Energy: 40µeV • Quantum beatof 2s-2p Splitting confirmedindependentlybymagneto-PL

  21. Next: Population Manipulation – Optical & THzPumping, TRPL Detection Resonant with 1s – 2p Resonant with 1s FEL

  22. Population Control:Optical & THz Pumping, TRPL Detection THz OFF THz ON 1 Time (ps) 0 Energy E-E1s (meV)

  23. Population Control:Optical & THz Pumping, TRPL Detection

  24. Population Control:Optical & THz Pumping, TRPL Detection

  25. Theory:Coulomb-Induced Symmetry Breaking • 2-particle exciton correlation • Exciton dynamics • Conservation law Coulomb scattering breaks THz-dipole symmetry Diffusive scattering

  26. Messages & Questions (II) • Excitonic Autler-Townes Effect • Multi-THz-Photon Ionization • Coulomb-MediatedCouplingof 2sand 2pExitons • ShelvingofExcitons(nocarrierloss) THz-Power-dependence?

  27. Back to Population Manipulation:Optical & THz Pumping, TRPL Detection Resonant with 1s – 2p Resonant with 1s FEL

  28. Back to Population Manipulation:Optical & THz Pumping, TRPL Detection Resonant with 1s – 2p Resonant with 2s FEL

  29. Resonant 2s Optical Excitation

  30. Resonant 2s Optical Excitation FEL off FEL on Energy (meV) Energy (meV)

  31. THz-Power-Dependence Resonant 2spumping: • Nonmonotoneous PL strength • Density-dependentperiod(Coulomb-inducedchangeofdipole-matrix element) Resonant 1spumping: • Similarbehaviour • Lesspronounced due to „unaffected“ 1s emission

  32. Conclusions & Outlook Manipulation ofexcitonpopulationsusing strong THzpulses • Unexpected strong 2s photoluminescence after 1s-2pexcitation • Evidenceof Coulomb-mediated coupling of 2s and 2plevels • „Shelfving“ ofexcitons in stateswith large angular momenta • Phys. Rev. Lett.110, 137404 (2013) • 1s-2ptransition predominantly homogeneously broadened • 1s-2p coherence times several 100’s of ps • Observation of quantum beat corresponding to 2s-2p separation • Secondary emission shows Rabi-Flopping between 1s and 2s Outlook: Coherent State Control, THz Gain...

  33. Messages & Questions (III) • Excitonic Autler-Townes Effect • Multi-THz-Photon Ionization • Coulomb-MediatedCouplingof2sand 2pExitons • Shelving of Excitons(no carrier loss) • Interlevel Transitions:homogeneouslyBroadened • Interlevel Interband Coherence • DephasingandDecaybyAcousticPhonons

  34. FEL on FEL off 1s pump 2s pump

  35. Resonant 1s Optical Excitation FEL off FEL on Energy (meV) Energy (meV)

  36. Nonresonant Excitatation, Long Time Delay

  37. THz-Power-Dependence: 2s Optical Pumping

  38. THz-Power-Dependence 2s optical pumping 1s optical pumping

  39. Optical-Pump – THz-Four-Wave Mixing THz Intensity (arb. Units) 0 100 -100 200 ΔtTHz-THz(ps)

  40. Optical-Pump – THz-Four-Wave Mixing THz Intensity (arb. Units) 0 100 -100 200 ΔtTHz-THz(ps)

  41. Outline: • Introduction: Excitons… • Interband Polarization Control: • Excitonic Autler-Townes Effect and THz-Field ionization • Population Control: Coupling of 1s-2s excitons • Excitation: Optical-Pump – THz-Pump • Detection: Time-Resolved Photoluminescence • 1s-2p Interstate Coherence and 2s-2p Quantum Beats • Opt.-Pump - THz-Probe / Opt.-Pump - THz-Pump - THz-Probe • Degenerate Four-Wave Mixing • Conclusions & Outlook

  42. Show 1s-2s- coupling • Explain showing theory • Wonder about coherence of this transition • Shown FWM • FWM lifetime roughly equal to 2s PL lifetim • 1s-2p homogeneously broadened (PP-lifetime ~ FWM lifetime • Mechanism for both is AC phonon coupling • Beat show signatures of 2s-2p by beating • Energy separation independently veryfied by magnetp-PL bhattacharya et al. In press • If coherent can we see rabi oscillations • Yes, we can in P Intensity of 1s and 2s pl varies with changing THz fluence

  43. Experimental Setup • Source: 1kHz Ti.Sapphire Amplifier120fs, 1mJ, 800nm pulses • Coaxial geometry for NIR & THz

  44. Strong Single-Cycle THz-PulsesSample: Narrow-Linewidth MQW 1s hhexciton @ 4K 30x Ga0.96In0.04As, 8nm THz pulse

  45. Experiment: THz-Induced Bleaching

  46. Experiment: Time Evolution

  47. Experiment: Density Dependence

  48. Experiment – Theory Comparison Experiment Theory

  49. Weak Optical + Strong THz Excitation (IV) • B. Ewers et al, Phys. Rev. B 85, 075307 (2012)

  50. Introduction: THz Experiments in Semiconductors

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