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

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

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

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

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

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

Detection of Excitons: Optical pump + THz probe 7

THz Absorption Identifies Excitons and Plasma 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, (2010) B. Ewers et al, Phys. Rev. B 85, (2012)

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

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

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

Messages & Questions (I) E 0 E 0

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

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

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

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

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

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

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

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

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

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

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

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

Population Control: Optical & THz Pumping, TRPL Detection 27

Population Control: Optical & THz Pumping, TRPL Detection 28

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

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

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

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

33 Resonant 2s Optical Excitation

34 FEL offFEL on Energy (meV)

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

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, (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

Messages & Questions (III) 37

38 2s pump 1s pump FEL offFEL on

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

Nonresonant Excitatation, Long Time Delay 40

THz-Power-Dependence: 2s Optical Pumping 41

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

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

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

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

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

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

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

Experiment: THz-Induced Bleaching

Experiment: Time Evolution

Experiment: Density Dependence

Experiment – Theory Comparison ExperimentTheory

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

Introduction: THz Experiments in Semiconductors 54