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Optical SpectroscopyOptical Spectroscopyonon
Semiconductor NanostructuresSemiconductor Nanostructures
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OutlineOutline
BackgroundBackground
99 Low dimensional nanostructuresLow dimensional nanostructures
From bulk to quantum dotsFrom bulk to quantum dots
99 Optical spectroscopy techniquesOptical spectroscopy techniques
ResultsResults
99 Optical spectroscopy onOptical spectroscopy on Quantum wiresQuantum wires
Quantum dotsQuantum dots
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Low dimensional nanostructuresLow dimensional nanostructures
EEGG
3D
DOS
3D
DOS
2D
DOS
2D
DOS
1D
DO
S
1D
DO
S
0D
DO
S
0D
DO
S
EEGG
EEGG EEGG
EnergyEnergy EnergyEnergy
EnergyEnergyEnergyEnergy
EE11 EE22 EE33
EE1111 EE1212 EE1313 EE111111 EE112112 EE113113
(a)(a) (b)(b)
(c)(c) (d)(d)
tz
tz
ty
tz
tytx
EEGG
3D
DOS
3D
DOS
2D
DOS
2D
DOS
1D
DO
S
1D
DO
S
0D
DO
S
0D
DO
S
EEGG
EEGG EEGG
EnergyEnergy EnergyEnergy
EnergyEnergyEnergyEnergy
EE11 EE22 EE33
EE1111 EE1212 EE1313 EE111111 EE112112 EE113113
(a)(a) (b)(b)
(c)(c)
(d)(d)
tz
tz
ty
tz
ty
tz
tytx
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Example: QWRExample: QWR
Return to selfReturn to self--
limited profilelimited profile
GaGa--rich VQWrich VQW
AlGaAs barrierAlGaAs barrier
GaAs QWRGaAs QWR
20 nm20 nm
(a)(a) (b)(b)
zz
xx
TT((xx))
cbcb
vbvb
Electron 1DElectron 1D
subbandssubbands
Hole 1DHole 1D
subbandssubbands
HighHigh
bandgapbandgap
LowLow
bandgapbandgap
Return to selfReturn to self--
limited profilelimited profile
GaGa--rich VQWrich VQW
AlGaAs barrierAlGaAs barrier
GaAs QWRGaAs QWR
20 nm20 nm
Return to selfReturn to self--
limited profilelimited profile
GaGa--rich VQWrich VQW
AlGaAs barrierAlGaAs barrier
GaAs QWRGaAs QWR
20 nm20 nm
Return to selfReturn to self--
limited profilelimited profile
GaGa--rich VQWrich VQW
AlGaAs barrierAlGaAs barrier
GaAs QWRGaAs QWR
20 nm20 nm
(a)(a) (b)(b)
zz
xx
TT((xx))
cbcb
vbvb
Electron 1DElectron 1D
subbandssubbands
Hole 1DHole 1D
subbandssubbands
HighHigh
bandgapbandgap
LowLow
bandgapbandgap
zz
xx
TT((xx))
cbcb
vbvb
Electron 1DElectron 1D
subbandssubbands
Hole 1DHole 1D
subbandssubbands
HighHigh
bandgapbandgap
LowLow
bandgapbandgap
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Example: QDsExample: QDs
S-K Growth Pyramidal QDs
22 mm22 mm
QD
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Optical spectroscopy techniquesOptical spectroscopy techniques
Barrier continuumBarrier continuum
CBCB
VBVB
Non
radiative
Non
radiative
hh outouthh inin
SampleSample
Exciting lightExciting light
(a)(a) (b)(b)
MicroscopeMicroscope
objectiveobjective
(c)(c)
SNOMSNOM
tiptip(d)(d)
ElectronElectron
beambeam
SampleSample
Exciting lightExciting light
(a)(a) (b)(b)
MicroscopeMicroscope
objectiveobjective
(c)(c)
SNOMSNOM
tiptip(d)(d)
ElectronElectron
beambeam
10-100m ~1m
~100nm
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PL / PLEPL / PLE
energy
PL
JDOS
det
de
t
PLE
PL: Spectroscopy of occupied levels
PLE: Spectroscopy of unocuppied levels ( ~ absorption)
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Photoluminescence setupPhotoluminescence setup
DetDetSingleSingle
spectrometerspectrometer
CCD
CCD
samplesample
HeHe--flow cryostatflow cryostat
MicroscopeMicroscopeobjectiveobjective
LaserLaser TemperatureTemperaturecontrollercontroller
CutCut--offoff
filterfilter
Spatial filterSpatial filterJY HR 460JY HR 460
DetDetSingleSingle
spectrometerspectrometer
CCD
CCD
samplesample
HeHe--flow cryostatflow cryostat
MicroscopeMicroscopeobjectiveobjective
LaserLaser TemperatureTemperaturecontrollercontroller
CutCut--offoff
filterfilter
Spatial filterSpatial filterJY HR 460JY HR 460
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Optical spectroscopy onOptical spectroscopy onQWRsQWRs
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Photoluminescence on QWRsPhotoluminescence on QWRs
Polarisation anisotropy in
absorption spectrumSignature of 1D confinement
Black arrows:
6-band kp theoretical model
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Carrier localisation in QWRsCarrier localisation in QWRs
X localisation on a ~m length scale:Chain of QDs
Monolayer fluctuation of confinementpotential along QWR axis
REALITY
DREAM
Fully delocalised 1D states
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Optical spectroscopy on QDsOptical spectroscopy on QDs
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Single QD spectroscopySingle QD spectroscopy
Inhomogeneous broadening in QD ensembles
cb
energy
30-80 meV
10-50meV
1-5meV
Electronic fine structure onlyvisible if single QDs are probed!
lu
minescence
vb
SK QD P id l QD
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SK QDs vs. Pyramidal QDsSK QDs vs. Pyramidal QDs
QD
Typical micro-photoluminescence experiment
~1m laser spot
1 m
energy energy
Al shadow mask
QT d i l QD
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Towards single QD spectroscopyTowards single QD spectroscopy
1.20 1.25 1.30 1.35 1.40
PL energy (eV)
P
Lintensity
(arb.units
)
= 130 eV1 QD
WLEnsemble
3 10 QDs5
Exci tation power 0.1 mWT=4.2 K
QD l iQD l t i t t
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QD electronic structureQD electronic structure
Artificial atomElectronic propertiescan be tailored
Quantum dot2D Harmonic oscillator model
energy
emission
XX 2XX2
Xchange
A few meV
Ch d i SCh d t t i S K QDK QD
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Charged states in SCharged states in S--K QDsK QDs
Energy (meV)
Ch d t t i id l QDCh d t t i id l QD
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Charged states in pyramidal QDsCharged states in pyramidal QDs
1.56 1.565 1.57 1.575 1.58 1.585photon energy [eV]
PL
intensity
[a.u.]
X 10
30 pW
600 nW
2.5 nW
1.56 1.565 1.57 1.575 1.58 1.585
Energy (eV)
PL
inten
sity
X
X
X2X2
2X
SS
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SummarySummary
99 Optical spectroscopy techniquesOptical spectroscopy techniques
99 Optical properties of QWRsOptical properties of QWRs
Polarisation anisotropyPolarisation anisotropy
Exciton localisationExciton localisation
99 Optical properties of QDsOptical properties of QDs
MultiMulti--charged excitonscharged excitons
QDs as artificial atomsQDs as artificial atoms
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