Exciton interstate dynamics in photoexcited quantum wells
manipulated by strong narrow-band THz pulses Sangam Chatterjee
TAC-LSUM, 6.10.2013
Slide 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
Slide 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
Slide 4
Introduction: From Atoms to Solids to Excitons crystal bands 4
3 2 n=1 energy atom levelsatom-like dispersion exciton + -
Slide 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
Slide 6
Introduction: Optical vs. THz Spectroscopy 6 EgEg Valence Band
p-type k E THz Photon Conduction Band s-type
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)
Slide 11
Reminder: Driven 2-Level-System (I) Linear absorption of a 2LS:
E 0
Slide 12
Reminder : Driven 2-Level-System (II) Rabi-Splitting, Mollow
triplet E 0
Slide 13
Reminder : Autler-Townes Effect Rabi-Splitting, Autler-Townes
effect, E 0
Slide 14
Messages & Questions (I) E 0 E 0
Slide 15
Inter-level Coherences: Optical-Pump THz-Four-Wave Mixing 15 to
Ge:Ga Detector
Slide 16
Optical-Pump THz-Probe 1s exciton lifetime: ~330ps (THz)
Similar to 1s TRPL lifetime 16
Slide 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
Slide 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
Slide 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:
118m 19
Slide 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:
118m
Slide 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: 40eV Quantum beat of 2s-2p
Splitting confirmed independently by magneto-PL
Slide 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: 40eV Quantum beat of 2s-2p
Splitting confirmed independently by magneto-PL
Slide 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: 40eV Quantum beat of 2s-2p
Splitting confirmed independently by magneto-PL
Slide 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: 40eV Quantum beat of 2s-2p
Splitting confirmed independently by magneto-PL
Slide 25
Next: Population Manipulation Optical & THz Pumping, TRPL
Detection 25 Resonant with 1s Resonant with 1s 2p FEL
Slide 26
Population Control: Optical & THz Pumping, TRPL Detection
26 THz OFFTHz ON Energy E-E 1s (meV) Time (ps) 0 1
Slide 27
Population Control: Optical & THz Pumping, TRPL Detection
27
Slide 28
Population Control: Optical & THz Pumping, TRPL Detection
28
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
Slide 31
Back to Population Manipulation: Optical & THz Pumping,
TRPL Detection 31 Resonant with 1s Resonant with 1s 2p FEL
Slide 32
Back to Population Manipulation: Optical & THz Pumping,
TRPL Detection 32 Resonant with 2s Resonant with 1s 2p FEL
Slide 33
33 Resonant 2s Optical Excitation
Slide 34
34 FEL offFEL on Energy (meV)
Slide 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
Slide 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
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
Slide 37
Messages & Questions (III) 37
Slide 38
38 2s pump 1s pump FEL offFEL on
Slide 39
Resonant 1s Optical Excitation 39 FEL offFEL on Energy
(meV)
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
Slide 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