sangam chatterjee tac-lsum, 6.10.2013

Exciton interstate dynamics 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• 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…

crys

tal

bands4

3

2

n=1

ener

gy

atom

levels “atom-like dispersion”

exci

ton

+-

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

time0

Introduction: Optical vs. THz Spectroscopy

6

Eg

Valence Band – “p-type”

k

E

THz Photon

Conduction Band – “s-type”

Detection of Excitons:Optical pump + THz probe

•  

7

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)

Reminder: Driven 2-Level-System (I)

• Linear absorption of a 2LS:

E0

Reminder : Driven 2-Level-System (II)

• Rabi-Splitting, Mollow triplet

E0

Reminder : Autler-Townes Effect

• Rabi-Splitting, Autler-Townes effect,

E0

Messages & Questions (I)

E0 E0

Inter-level Coherences:Optical-Pump – THz-Four-Wave Mixing

15

t

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

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 OFF THz ON

Energy E-E1s (meV)

Tim

e (p

s)

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 scatteringbreaks THz-dipolesymmetry

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

Resonant 2s Optical Excitation

34

FEL off FEL on

Energy (meV)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, 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

Messages & Questions (III)

• Excitonic Autler-Townes Effect• Multi-THz-Photon Ionization• Coulomb-Mediated Coupling of

2s and 2p Exitons• Shelving of Excitons

(no carrier loss)

• Interlevel Transitions:homogeneously Broadened

• Interlevel Interband Coherence• Dephasing and Decay by

Acoustic Phonons

37

38

2s

pum

p1

s pu

mp

FEL off FEL on

Resonant 1s Optical Excitation

39

FEL off FEL on

Energy (meV)Energy (meV)

Nonresonant Excitatation, Long Time Delay

40

THz-Power-Dependence: 2s Optical Pumping

41

THz-Power-Dependence

2s optical pumping 1s optical pumping

42

Optical-Pump – THz-Four-Wave Mixing

43

ΔtTHz-THz(ps) 2000-100 100

TH

z In

ten

sit

y (

arb

. U

nit

s)

Optical-Pump – THz-Four-Wave Mixing

44

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

ΔtTHz-THz(ps) 2000-100 100

TH

z In

ten

sit

y (

arb

. U

nit

s)

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

fluence46

Experimental Setup

• Source: 1kHz Ti.Sapphire Amplifier120fs, 1mJ, 800nm pulses

• Coaxial geometry for NIR & THz

Strong Single-Cycle THz-PulsesSample: Narrow-Linewidth MQW

THz pulse

1s hh exciton @ 4K30x Ga0.96In0.04As, 8nm

Experiment: THz-Induced Bleaching

Experiment: Time Evolution

Experiment: Density Dependence

Experiment – Theory Comparison

Experiment Theory

Weak Optical + Strong THz Excitation (IV)

• B. Ewers et al, Phys. Rev. B 85, 075307 (2012)

Introduction: THz Experiments in Semiconductors

54

0 500 1000 1500 2000-100

-50

0

50

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2s

Diff

ere

ntia

l PL

Inte

nsi

ty

time / ps

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

d e m o d e m o d e m o d e m o d e m o

0 500 1000 1500 20000

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L In

tens

ity /

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ts

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with THz without THz

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L In

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with THz without THz

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ts

time / ps

with THz without THz

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L In

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with THz without THz