Transcript Ultrafast Spectroscopy of Quantum Dots (QDs) Ulrike Woggon

Ultrafast Spectroscopy of Quantum Dots (QDs)
Ulrike Woggon
FB Physik, Universität Dortmund
With thanks to:
M.V. Artemyev, P. Borri, W. Langbein, B. Möller, S. Schneider
calculations:
samples:
Fruitful cooperations:
R. Wannemacher, Leipzig,
D. Bimberg and coworkers, Berlin
D. Hommel and coworkers, Bremen
A. Forchel and coworkers, Würzburg
Experimentelle Physik IIb
Outline:
• 1.
Types of QDs and Techniques of Ultrafast Spectroscopy
Outline:
• 2. Application Aspects: Dynamics of Amplification in QD-Lasers
Monitoring of high-frequency optical operation in semiconductor
nanostructures by
ULTRAFAST SPECTROSCOPY
predicted advantages of QD-lasers:
•
•
•
•
low threshold current density
high characteristic temperature
high differential gain
large spectral tunability, from NIR to UV
D. Bimberg and coworkers, TU Berlin
Outline:
• 3. Fundamental aspects:
Semiconductor QDs as artificial atoms
Monitoring of the „discrete-level“ - structure of semiconductor
nanostructures by
ULTRAFAST SPECTROSCOPY
L.Banyai, S.W. Koch, Semiconductor Quantum Dots
Part 1: Types of QDs and Techniques...
Quantum Dots:
Nanocrystals and epitaxially grown Islands
Precipitation of spherical
nanocrystals in colloidal solution
or glass, polymer etc. matrix
Lattice-mismatch induced
island growth
CdSe QDs emitting in the visible (nanocrystals)
CdSe QDs
R = 1.5 nm
T = 300 K
1.5
1.0
0.5
0
500
550
600
650
Wavelength (nm)
5 nm
2.2
2
CdSe QDs
R = 2 nm
T = 300 K
0.4
0.2
500
550
600
650
Wavelength (nm)
700
1.8
PL Intensity (arb. units)
Optical Density
2.4
CdSe
in glass
1.8
PL Intensity (arb. units)
Optical Density
2.4
Photon Energy (eV)
2.2
2
700
InGaAs self-assembled islands emitting in the NIR
Calculated confined eh-pair energies
for InAs assuming pyramidal shape
D. Gerthsen et al., Karlsruhe
Grundmann, Bimberg et al., TU Berlin
Part 1: Types of QDs and Techniques...
Femtosecond Heterodyne Technique
w2
w2 probe
w1
signal
2w2-w1 four-wave
mixing (FWM)
Dt
probe
pump
waveguide
Ti:Sa + OPO, 80 fs ... 2 ps
Femtosecond Ultrafast Spectroscopy
Usually:
J. Shah, Ultrafast Spectroscopy
Femtosecond Heterodyne FWM- and PP-Spectroscopy
Usually:
Here:
150fs
76MHz
AOM1
laser
79MHz
delay
AOM2
80MHz
reference
beam
AOM-
76MHz
Acousto-Optical Modulator
FWM
probe
pump
pump
beam
sample
3MHz
Idet erefesignal
e= electric field
4MHz
_
2MHz
HF-Lock-in
delay
+
Part 2: Applied aspects: QD-laser...
Gain Dynamics in Quantum Dots
InAs/InGaAs QDs
3 x stack,
20nm GaAs barrier
Gain Dynamics of InGaAs QDs
ASE (a.u.)
65meV
ES
GS
+
p-GaAs
p-AlGaAs
GaAs
GaAs
n-AlGaAs
ridge waveguide
5x500mm,
3 stacked QD layers
areal dot density
~2x1010cm-2
n-GaAs
_
optical density ~ 1.5
(a~30cm-1)
20 mA
0.5 mA
laser
1.1
1.2
1.3
Energy (eV)
InGaAs QDs
1.4
Ground State Emission (GS):
1070nm @ 25K, 1170nm @ 300K
Carrier injection
electrically (0...20 mA)
1.5
Sample from TU Berlin,
Prof. Bimberg
Gain Dynamics of InGaAs QDs
Pump-induced
gain change in a
heterodyne pumpprobe experiment
at maximum gain
(20 mA) and
without electrical
injection (0 mA)
Gain recovery in
< 100 fs at 300 K !
P. Borri et al., J. Sel. Topics Q. El. 6, p. 544 (2000); Appl. Phys. Lett. 76, p.1380 (2000).
T=6K
(1)
R~2.5 nm
I0=5 mJ/cm2
(3)
2
(2)
(4)
..
.
..
I0
0
2.0
2.1
2.2
Photon Energy (eV)
pairs
1pe1ph 1pe1ph
1se1sh 2se1sh
1se1sh 1se1sh
0.03 I0
1.9
CdSe nanocrystals
in glass matrix
R ~ 2.5 nm
two
ad
PL Intensity (arb. units)
Gain Dynamics of CdSe QDs
2.3
(4)
2.4
(1),(2)
Ground State Emission (1): 605 nm @ 6K
Woggon et al., Phys. Rev. B 54, 17681 (1996), J. Lum. 70, 269 (1996).
..
..
1pe1ph
1se2sh
1se1sh
(3)
one
pair
Gain Dynamics of CdSe QDs
Excitonic and biexcitonic
contributions to optical gain
Gain recovery time
spectrally varying, <1...100ps
Optics Lett. 21, 1043 (1996).
Gain Dynamics of CdSe QDs
Gain spectrum inhomogeneously broadened:
Spectral hole burning in gain spectrum with two fs-pump and one fs-probe beam
Spectral hole width
of a single
gain process
~20 meV
Intrinsic limit
of gain recovery
below 100 fs !
Chem. Phys. 210, 71 (1996)
Part 2: Applied aspects: QD-laser...
Quantum dots as active media in
optical microcavities
CdSe QDs linked to microspheres
5 mm
Picture: M.V.Artemyev, I. Nabiev
„Dot - in - a - Dot“ - Structure
CdSe
nanodot
TM 136
l=619.22nm
R=2.77mm
R=2.5mm
R=2.2 nm
Glass
microsphere
R=3.1 mm
Artemyev et al., APL 78, p.1032 (2001),
Nano Lett. 1, 309 (2001).
Cavity Modes of a CdSe-doped Microsphere
WGM
RPD = 2.5 mm
TM, l=36, n=1
TM, l=36, n=2
Nano Lett. 1, p. 309 (2001), Appl. Phys. Lett. 80, p.3253 (2002)
RQD = 2.5 nm
Optical Pumping of a CdSe-doped Microsphere
RPD ~ 15 mm
cw-Ar laser, 488 nm
Excitation spot size
40 mm2
T = 300 K
520 nm < lem < 640 nm
10 mW
CdSe nanocrystals
(not on microsphere)
14 mW
See also:
Artemyev, Woggon et al.
Nano Letters 1, 309 (2001)
Part 3: Fundamental aspects: Artficial atoms...
Rabi Oscillations in Quantum Dots
Bloch-sphere:
population oscillation
Rabi-Oscillations in Atoms
Simple model: two coupled oscillators
wR
..
.
|e>
|g>
|3>
|2>
|1>
|0>
photon field
atom states
..
.
|e>
|g>
|3>
|2>
|1>
|0>
Rabi frequency
 
m  E0
wR =

Two-level system in resonance with photon field
Eb
E0 = w0 = Eb - Ea
Ea
E0 : electromagn. field vector
w0: transition energy
m : transition dipole moment
Rabi Oscillations versus Pulse Area
Here pulsed excitation !
Pulse area: time-integrated Rabi frequency
(~ input field intensity)
t =1ps (DE
=0.75meV)
0
HWHM


 m  E
0
 =
dt


4
Population oscillation
blue = -1
red = +1
Initial conditions:
for t << -t0 in ground state
No dephasing!
3
detuning (meV)
(meV)
detuning
Occupation probability
of the ground (excited)
state
2
1
0
00
22
4
6
4pulse
6
area ()
pulse area ()
88
10
10
Effect of Dephasing T2 on Rabi oscillations
The effect of a damping g =1/T2 of polarization:
g=0
1.0
g/wR=1/9
2
|b|
w=w0
0.5
g/wR=1
0.0
0
2
wRt/2
4
6
Population flopping over many periods is possible in systems with
long dephasing times and large transition dipole moments: g / wR<<1.
TI FWM field amplitude (a.u.)
Dephasing time T2 of InGaAs quantum dots
From 300K to 100K the FWM
decay is dominated by a
short dephasing time < 1ps
500ps
Below T=10 K a slow dephasing time
> 500 ps is observed (suppression of
LO-phonon scattering!)
InGaAs - QDs
T=10K
-1 0 1 2 3
200
Delay time (ps)
Is the observed dephasing time
T2 large enough to observe
population flopping, i.e.
Rabioscillation in QDs ?????
400
P. Borri et al., Phys. Rev. Lett. 87, 157401 (2001)
Rabi Oscillations in InGaAs Quantum Dots
Experiment
T = 10 K
Rabi oscillation:
two oscillation maxima
can be clearly distinguished
Intensity (a.u.)
 a sharpened distribution of
the spectral intensity
improves the visibility of the
oscillations.
DT field amplitude (a.u.)
Use of spectrally shaped
ps-pulses
1.146
Borri et al., Phys. Rev. B (Rapid Comm.), in press
0
1.149 1.152
Energy(eV)
4
8
Pulse area (a.u.)
Distribution in Transition Dipole Moments m
DT

T

 dw E

probe
w)
2



2
TE
0
r )dr  Pm )dm 

0
Pm )m 2 =

1
e
 2


DT/T (arb. units)
))  )
a w, w , m,  E pump , m f w dw

 m  m0 )2
2 2
0
0.15
0.2
in average
m = 35 D
 = 20%
/m0=0.25
T2  
0

0
2
4
6
Pulse area ()
T2=1.5ps
0
2
4
6
Pulse area ()
Borri et al.,
Phys. Rev. B
(Rapid Comm.),
in press
Part 3: Fundamental aspects: Artficial atoms...
Quantum Beats in Quantum Dots
|2>
|1>
|0>
DE
DE can be derived from beat period
FWM-Intensity (log. units)
Discrete Level-System
DE = 25 meV
Dtbeat= 168 fs
0
500
1000
Delay Time (fs)
Exciton-Biexciton Quantum Beats in QDs
uncorr.
electron
and hole
|1e,1h>
|x>
Coulomb interaction
|G>
|G>
|G>
Quantum Beats between
two optical transitions:
|G>
|x>
|xx>
|x> with EX
|xx> with EXX
EX - EXX = Ebin (biexciton binding)
exciton |x>
biexciton |xx>
formation
|2x>
|xx>
|x+>
Ebin
 
 
|G>
|x_>
Exciton-Biexciton Quantum Beats in QDs
FWM Intensity (log. u.)
FWM (log.)
Determination of biexciton binding
energy in CdSe/ZnSe QDs by
femtosecond quantum beat
spectroscopy
20meV
Ex
Exx
2.45
2.5
E (eV)
B=208 fs
0
500
1000
Delay Time t12 (fs)
Biexciton binding
energy DE = 21 meV
Exx
1500
Gindele, Woggon et al., Phys. Rev. B 60, p. 8773 (1999).
2mm
Summary
CdSe QDs in microspheres

InGaAs QDs in waveguides
Types of Quantum Dots and Techniques of Ultrafast Spectroscopy

Application Aspects: Dynamics of Amplification in
Quantum Dot Lasers

Fundamental aspects: Semiconductor Quantum Dots
as Artficial Atoms