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Exciton interstate dynamics in photoexcited quantum wells manipulated by strong narrow-band THz pulses Sangam Chatterjee TAC-LSUM, 6.10.2013

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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

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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

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Introduction: From Atoms to Solids to Excitons crystal bands 4 3 2 n=1 energy atom levelsatom-like dispersion exciton + -

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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

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

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Detection of Excitons: Optical pump + THz probe 7

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THz Absorption Identifies Excitons and Plasma 8

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Polarization Manipulation: Weak Optical + Strong THz Excitation 9

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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)

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

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

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

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Messages & Questions (I) E 0 E 0

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

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

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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

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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

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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: 118m 19

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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: 118m

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

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

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

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

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

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

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Population Control: Optical & THz Pumping, TRPL Detection 27

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Population Control: Optical & THz Pumping, TRPL Detection 28

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

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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

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

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

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33 Resonant 2s Optical Excitation

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34 FEL offFEL on Energy (meV)

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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

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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 100s 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

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Messages & Questions (III) 37

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38 2s pump 1s pump FEL offFEL on

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

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Nonresonant Excitatation, Long Time Delay 40

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THz-Power-Dependence: 2s Optical Pumping 41

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

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

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

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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

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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

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

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

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Experiment: THz-Induced Bleaching

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Experiment: Time Evolution

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Experiment: Density Dependence

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Experiment Theory Comparison ExperimentTheory

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

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Introduction: THz Experiments in Semiconductors 54

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