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![Page 1: Ultrafast Spectroscopy of Quantum Dots (QDs) Experimentelle Physik IIb FB Physik, Universität Dortmund Ulrike Woggon With thanks to: M.V. Artemyev, P.](https://reader035.fdocuments.net/reader035/viewer/2022062422/56649f225503460f94c3b5ee/html5/thumbnails/1.jpg)
Ultrafast Spectroscopy of Quantum Dots (QDs)
Experimentelle Physik IIb
FB Physik, Universität Dortmund
Ulrike Woggon
With thanks to: M.V. Artemyev, P. Borri, W. Langbein, B. Möller, S. Schneider
Fruitful cooperations: calculations: R. Wannemacher, Leipzig, samples:D. Bimberg and coworkers, Berlin
D. Hommel and coworkers, Bremen A. Forchel and coworkers, Würzburg
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• 1. Types of QDs and Techniques of Ultrafast Spectroscopy
Outline:
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• 2. Application Aspects: Dynamics of Amplification in QD-Lasers
Outline:
D. Bimberg and coworkers, TU Berlin
predicted advantages of QD-lasers:
• low threshold current density • high characteristic temperature• high differential gain • large spectral tunability, from NIR to UV
Monitoring of high-frequency optical operation in semiconductor nanostructures by
ULTRAFAST SPECTROSCOPY
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• 3. Fundamental aspects: Semiconductor QDs as artificial atoms
Outline:
L.Banyai, S.W. Koch, Semiconductor Quantum Dots
Monitoring of the „discrete-level“ - structure of semiconductor nanostructures by
ULTRAFAST SPECTROSCOPY
size energy
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Quantum Dots: Nanocrystals and epitaxially grown Islands
Part 1: Types of QDs and Techniques...
Lattice-mismatch inducedisland growth
Precipitation of spherical nanocrystals in colloidal solutionor glass, polymer etc. matrix
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CdSe QDs emitting in the visible (nanocrystals)
500 550 600 650 7000
0.5
1.0
1.5
CdSe QDsR = 1.5 nmT = 300 K
PL In
tensity (a
rb. u
nits)
Optic
al D
ensi
ty
Wavelength (nm)
2.4 2.2 2 1.8
Photon Energy (eV)
500 550 600 650 700
0.2
0.4CdSe QDsR = 2 nmT = 300 K
Opt
ical
Den
sity
Wavelength (nm)
PL Intensity (arb. units)
2.4 2.2 2 1.8
CdSe in glass
5 nm
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InGaAs self-assembled islands emitting in the NIR
Grundmann, Bimberg et al., TU Berlin
D. Gerthsen et al., Karlsruhe
Calculated confined eh-pair energiesfor InAs assuming pyramidal shape
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Femtosecond Heterodyne Technique
2 probe
22-1 four-wavemixing (FWM)
waveguidepumpprobe
12 signal
t
Part 1: Types of QDs and Techniques...
Ti:Sa + OPO, 80 fs ... 2 ps
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Femtosecond Ultrafast Spectroscopy
Usually:
J. Shah, Ultrafast Spectroscopy
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Femtosecond Heterodyne FWM- and PP-Spectroscopy
Usually:
Here:
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AOM1
AOM2
pump beam
probebeam
+
_
HF-Lock-in
delay
delay
sample
reference beam 76MHz
laser
4MHz3MHz
probe pumpFWM
2MHz
79MHz
80MHz
Idet refsignal
electric field
150fs76MHz
AOM-Acousto-Optical Modulator
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Gain Dynamics in Quantum Dots
InAs/InGaAs QDs3 x stack,20nm GaAs barrier
Part 2: Applied aspects: QD-laser...
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Gain Dynamics of InGaAs QDs
1.1 1.2 1.3 1.4 1.5
+
_
n-GaAs
n-AlGaAs GaAs GaAs
p-AlGaAsp-GaAs
InGaAs QDs
ES
GS
laser
65meV
A
SE
(a.
u.)
Energy (eV)
Ground State Emission (GS): 1070nm @ 25K, 1170nm @ 300K
Sample from TU Berlin,Prof. Bimberg
ridge waveguide 5x500m, 3 stacked QD layers
areal dot density ~2x1010cm-2
optical density ~ 1.5 (~30cm-1)
Carrier injectionelectrically (0...20 mA)0.5 mA
20 mA
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Gain Dynamics of InGaAs QDs
P. Borri et al., J. Sel. Topics Q. El. 6, p. 544 (2000); Appl. Phys. Lett. 76, p.1380 (2000).
Pump-inducedgain change in a heterodyne pump-probe experiment at maximum gain(20 mA) and without electricalinjection (0 mA)
Gain recovery in< 100 fs at 300 K !
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Gain Dynamics of CdSe QDs
Ground State Emission (1): 605 nm @ 6K
CdSe nanocrystalsin glass matrixR ~ 2.5 nm
1.9 2.0 2.1 2.2 2.3 2.4
PL
Inte
nsity
(ar
b. u
nits
)
Photon Energy (eV)
0.03 I0
I0
(4)
(3)(2)
(1)
0
2
d
T=6KR~2.5 nmI0=5 mJ/cm
2
Woggon et al., Phys. Rev. B 54, 17681 (1996), J. Lum. 70, 269 (1996).
...
.....
onepair
twopairs
1se1sh
1se2sh
1pe1ph...
1se1sh 1se1sh
1se1sh 2se1sh
1pe1ph 1pe1ph...
(1),(2)
(4)
(3)
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Gain Dynamics of CdSe QDs
Optics Lett. 21, 1043 (1996).
Gain recovery time spectrally varying, <1...100ps
Excitonic and biexcitonic contributions to optical gain
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Gain Dynamics of CdSe QDs
Chem. Phys. 210, 71 (1996)
Gain spectrum inhomogeneously broadened:Spectral hole burning in gain spectrum with two fs-pump and one fs-probe beam
Spectral hole widthof a single
gain process ~20 meV
Intrinsic limitof gain recoverybelow 100 fs !
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Quantum dots as active media in optical microcavities
5 m
CdSe QDs linked to microspheres
Picture: M.V.Artemyev, I. Nabiev
Part 2: Applied aspects: QD-laser...
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„Dot - in - a - Dot“ - Structure
=619.22nm
R=2.77m
CdSenanodot
136 TM
R=2.5m
R=2.2 nmGlassmicrosphere
R=3.1 m
Artemyev et al., APL 78, p.1032 (2001), Nano Lett. 1, 309 (2001).
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Cavity Modes of a CdSe-doped Microsphere
RPD = 2.5 m RQD = 2.5 nm
Nano Lett. 1, p. 309 (2001), Appl. Phys. Lett. 80, p.3253 (2002)
WGM
TM, =36, n=1
TM, =36, n=2
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Optical Pumping of a CdSe-doped Microsphere
RPD ~ 15 m
Excitation spot size40 m2
cw-Ar laser, 488 nm
10 mW
14 mW
T = 300 K520 nm < em < 640 nm
CdSe nanocrystals(not on microsphere)
See also: Artemyev, Woggon et al. Nano Letters 1, 309 (2001)
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Rabi Oscillations in Quantum Dots
Part 3: Fundamental aspects: Artficial atoms...
Bloch-sphere:population oscillation
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Rabi-Oscillations in Atoms
Simple model: two coupled oscillators
R
|g>|e>
|g>|e>
|3>|2>|1>|0>
|3>|2>|1>|0>
photon field
0
RE
Rabi frequency
Ea
Eb
atom states
Two-level system in resonance with photon field
E0 == Eb - Ea
: transition dipole moment
: transition energy
E0 : electromagn. field vector
...
...
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Rabi Oscillations versus Pulse Area
Pulse area: time-integrated Rabi frequency
Here pulsed excitation !
dtE
0
0 2 4 6 8 100
1
2
3
4
t0=1ps (E
HWHM=0.75meV)
pulse area ()
detu
ning
(m
eV)
Population oscillationblue = -1red = +1
No dephasing!
Initial conditions:for t << -t0 in ground state
0 2pulse area (
4 6 8 10
detu
ning
(m
eV)
Occupation probability of the ground (excited) state
(~ input field intensity)
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Effect of Dephasing T2 on Rabi oscillations
The effect of a damping =1/T2 of polarization:
0 2 4 60.0
0.5
1.0
R=1
R=1/9
|b|2
Rt/2
=0
Population flopping over many periods is possible in systems with long dephasing times and large transition dipole moments: / R<<1.
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Dephasing time T2 of InGaAs quantum dots
P. Borri et al., Phys. Rev. Lett. 87, 157401 (2001)
From 300K to 100K the FWMdecay is dominated by a short dephasing time < 1ps
Below T=10 K a slow dephasing time > 500 ps is observed (suppression of LO-phonon scattering!)
-1 0 1 2 3 200 400
500ps
T=10K
TI F
WM
fiel
d am
plitu
de (
a.u.
)
Delay time (ps)
Is the observed dephasing time T2 large enough to observe population flopping, i.e. Rabioscillation in QDs ?????
InGaAs - QDs
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Rabi Oscillations in InGaAs Quantum Dots
Use of spectrally shaped ps-pulses
a sharpened distribution of the spectral intensity improves the visibility of the oscillations.
Rabi oscillation: two oscillation maxima can be clearly distinguished
0 4 8
1.146 1.149 1.152
T = 10 K
DT
fiel
d am
plitu
de (
a.u.
)
Pulse area (a.u.)
Inte
nsity
(a.
u.)
Energy(eV)
Experiment
Borri et al., Phys. Rev. B (Rapid Comm.), in press
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Distribution in Transition Dipole Moments
2
20
22
2
1
eP
dfEdPdrrEdT
TpumpTEprobe ,,,,
0
22
0
0 2 4 60
T2
=0.25
0.2
0.15
0
T/T
(ar
b. u
nits
)
Pulse area ()0 2 4 6
T2=1.5ps
Pulse area ()
in average = 35 D = 20%
Borri et al., Phys. Rev. B (Rapid Comm.), in press
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Quantum Beats in Quantum Dots
0 500 1000
tbeat
= 168 fs
E = 25 meV
Delay Time (fs)
FW
M-I
nten
sity
(lo
g. u
nits
)
Part 3: Fundamental aspects: Artficial atoms...
E
|0>
|1>|2>
Discrete Level-System E can be derived from beat period
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Exciton-Biexciton Quantum Beats in QDs
|2x>|xx> Ebin
|x+> |x_>
|G>
|G>
|x> |xx>
|G>
|1e,1h>
Quantum Beats betweentwo optical transitions:
|G> |x> with EX
|x> |xx> with EXX
EX - EXX = Ebin (biexciton binding)
|G>
uncorr.electronand hole Coulomb interaction
exciton |x>biexciton |xx>formation
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Exciton-Biexciton Quantum Beats in QDs
0 500 1000 1500
ExxFW
M In
tens
ity (
log.
u.)
B=208 fs
Delay Time t12 (fs)
2.45 2.5
FW
M (
log.
)
E (eV)
Exx Ex
20meV
Gindele, Woggon et al., Phys. Rev. B 60, p. 8773 (1999).
Determination of biexciton bindingenergy in CdSe/ZnSe QDs byfemtosecond quantum beat spectroscopy
Biexciton bindingenergy E = 21 meV
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CdSe QDs in microspheres
InGaAs QDs in waveguides
2m
Summary
Fundamental aspects: Semiconductor Quantum Dots as Artficial Atoms
Application Aspects: Dynamics of Amplification in Quantum Dot Lasers
Types of Quantum Dots and Techniques of Ultrafast Spectroscopy