1. Exciton interstate 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, Sybine Zybell, 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) 2

3. Outline: Introduction: Excitons, Autler-Townes Effect & Ionization 1s-2p Interstate Coherence and 2s-2p Quantum Beats Population Control: Rabi-Flopping of 1s-2s Excitons Conclusions & Outlook 3

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

5. 5 Time-Resolved Spectroscopy on Semiconductors Different physical phenomena  different time scales  Optical Excitation  Polarization and dephasing  Carier injection and cooling  Formation many-body excitations time 0

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

7. Detection of Excitons: Optical pump + THz probe 7

8. THz Absorption Identifies Excitons and Plasma 8

9. Polarization Manipulation: Weak Optical + Strong THz Excitation 9

10. 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)

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

12. Reminder : Driven 2-Level-System (II) Rabi-Splitting, Mollow triplet E 0

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

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

15. Inter-level Coherences: Optical-Pump – THz-Four-Wave Mixing 15  to Ge:Ga Detector

16. Optical-Pump – THz-Probe 1s exciton lifetime: ~330ps (THz) Similar to 1s TRPL lifetime 16

17. Optical-Pump – THz-Pump – THz-Probe 1s exciton lifetime: ~330ps (THz) Similar to 1s TRPL lifetime Opt. Pump – THz Pump – THz Probe Interference pattern “Shelving“ at later times 17

18. Optical-Pump – THz-Pump – THz-Probe 1s exciton lifetime: ~330ps (THz) Similar to 1s TRPL lifetime Opt. Pump – THz Pump – THz Probe Interference pattern “Shelving“ at later times 18

19. Optical-Pump – THz-Pump – THz-Probe 1s exciton lifetime: ~330ps (THz) Similar to 1s TRPL lifetime Opt. Pump – THz Pump – THz Probe Interference pattern “Shelving“ at later times Oscillation period: 118µm 19

20. Optical-Pump – THz-Pump – THz-Probe 20 1s exciton lifetime: ~330ps (THz) Similar to 1s TRPL lifetime Opt. Pump – THz Pump – THz Probe Interference pattern “Shelving“ at later times Oscillation period: 118µm

21. Optical-Pump – THz-Four-Wave Mixing 21 Decay ~50ps Similar to 2s TRPL lifetime Mechanism: acoustic phonons Overlayed by slow oscillation Period: 100ps Energy: 40µeV Quantum beat of 2s-2p Splitting confirmed independently by magneto-PL

22. Optical-Pump – THz-Four-Wave Mixing 22 Decay ~50ps Similar to 2s TRPL lifetime Mechanism: acoustic phonons Overlayed by slow oscillation Period: 100ps Energy: 40µeV Quantum beat of 2s-2p Splitting confirmed independently by magneto-PL

23. Optical-Pump – THz-Four-Wave Mixing 23 Decay ~50ps Similar to 2s TRPL lifetime Mechanism: acoustic phonons Overlayed by slow oscillation Period: 100ps Energy: 40µeV Quantum beat of 2s-2p Splitting confirmed independently by magneto-PL

24. Optical-Pump – THz-Four-Wave Mixing 24 Decay ~50ps Similar to 2s TRPL lifetime Mechanism: acoustic phonons Overlayed by slow oscillation Period: 100ps Energy: 40µeV Quantum beat of 2s-2p Splitting confirmed independently by magneto-PL

25. Next: Population Manipulation – Optical & THz Pumping, TRPL Detection 25 Resonant with 1s Resonant with 1s – 2p FEL

26. Population Control: Optical & THz Pumping, TRPL Detection 26 THz OFFTHz ON Energy E-E 1s (meV) Time (ps) 0 1

27. Population Control: Optical & THz Pumping, TRPL Detection 27

28. Population Control: Optical & THz Pumping, TRPL Detection 28

29. Theory: Coulomb-Induced Symmetry Breaking 2-particle exciton correlation Exciton dynamics Conservation law Diffusive scattering Coulomb scattering breaks THz-dipole symmetry 29

30. Messages & Questions (II) Excitonic Autler-Townes Effect Multi-THz-Photon Ionization Coulomb-Mediated Coupling of 2s and 2p Exitons Shelving of Excitons (no carrier loss) THz-Power-dependence? 30

31. Back to Population Manipulation: Optical & THz Pumping, TRPL Detection 31 Resonant with 1s Resonant with 1s – 2p FEL

32. Back to Population Manipulation: Optical & THz Pumping, TRPL Detection 32 Resonant with 2s Resonant with 1s – 2p FEL

33. 33 Resonant 2s Optical Excitation

34. 34 FEL offFEL on Energy (meV)

35. THz-Power-Dependence Resonant 2s pumping: Nonmonotoneous PL strength Density-dependent period (Coulomb-induced change of dipole-matrix element) Resonant 1s pumping: Similar behaviour Less pronounced due to „unaffected“ 1s emission 35

36. Conclusions & Outlook Manipulation of exciton populations using strong THz pulses Unexpected strong 2s photoluminescence after 1s-2p excitation Evidence of Coulomb-mediated coupling of 2s and 2p levels „Shelfving“ of excitons in states with large angular momenta –Phys. Rev. Lett. 110, 137404 (2013) 1s-2p transition 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... 36

37. Messages & Questions (III) 37

38. 38 2s pump 1s pump FEL offFEL on

39. Resonant 1s Optical Excitation 39 FEL offFEL on Energy (meV)

40. Nonresonant Excitatation, Long Time Delay 40

41. THz-Power-Dependence: 2s Optical Pumping 41

42. THz-Power-Dependence 2s optical pumping1s optical pumping 42

43. Optical-Pump – THz-Four-Wave Mixing 43 Δt THz-THz (ps) 200 0 -100 100 THz Intensity (arb. Units)

44. Optical-Pump – THz-Four-Wave Mixing 44 Δt THz-THz (ps) 200 0 -100 100 THz Intensity (arb. Units)

45. 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 45

46. 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 46

47. Experimental Setup Source: 1kHz Ti.Sapphire Amplifier 120fs, 1mJ, 800nm pulses Coaxial geometry for NIR & THz

48. Strong Single-Cycle THz-Pulses Sample: Narrow-Linewidth MQW THz pulse 1s hh exciton @ 4K 30x Ga 0.96 In 0.04 As, 8nm

49. Experiment: THz-Induced Bleaching

50. Experiment: Time Evolution

51. Experiment: Density Dependence

52. Experiment – Theory Comparison ExperimentTheory

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

54. Introduction: THz Experiments in Semiconductors 54