observation of excited biexciton states in cucl quantum dots : control of the quantum dot energy by...
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Observation of Excited Biexciton States in CuCl Quantum Dots : Control of the Quantum Dot Energy by
a Photon
Itoh Lab.
Hiroaki SAWADA
Michio IKEZAWA and Yasuaki MASUMOTOJpn. J. Appl. Phys. Vol. 36 (1997) pp. 4191-4193
(references)
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Contents
• Introduction
Exciton and biexciton
Quantum-dot
• Experiments
• Summary
• My work
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Exciton and biexciton
exciton
biexciton
An electron-hole pair combined by the Coulomb force
electron
holeDiscrete energy levels like those of the hydrogen atom
A bound two exciton state Exciton: 励起子Biexciton: 励起子分子
The distance of electron-hole is called the bohr radius (aB).
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Quantum dot
• A quantum dot is a nanometer-sized semiconductor. It consists of 103~106 atoms.
• Quantum effects appear due to three dimensionally confined excitons.
• The energy levels are discrete.
E
E
E
DO
SD
OS
DO
SD
OS
bulk Quantum well
Quantum dotQuantum wire
bulk
we
llw
ired
ot
E
DOS (density of state): 状態密度
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Quantum confinement effect
Weak-confinement Strong-confinement
Center-of-mass motion is confined. Motions of electron and hole are confined individually.
aB a ≪ aB a≫
electron
hole
aB : Bohr radius
a:dot radius
Consider the effect on an exciton in a spherical dot.
lowest stateexcited state
lowest stateexcited state
lowest stateexcited state
2a2a
CdS QD and CdSe QD etc.
CuCl QD in NaCl matrix etc.
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Quantum size effect
sizeenergy
Weak-confinement Strong-confinement
The energy of exciton in quantum dot
2
*
2
2)(
rM
GErE xg
xg G
r
e
rErE 248.0786.1
2)(
22
*
2
Eg: the band gap energyGx: the bonding energy of excitonme: the mass of electron:
mh: the mass of holer: the quantum dot radiusaex: the exciton radius
ex*
he
a-rr
mmM
he m
1
m
11
)1(
Higher energy than excitons in bulkThe energy depends on dot size.
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Purpose
weak-confinement case
CuCl quantum dots in NaCl
Subpicosecond pump probe spectroscopy
CdS and CdSe quantum dots
Biexciton
To date strong-confinement case
This report
nonlinear optical responses
EXX’2EXEXX
EX
G
pump
G: ground stateEX: exciton stateEXX: biexciton state2EX: two exciton EXX’:excited biexciton state
CuCl
NaCltwo excitonRydberg 1s state
J=0EXX: biexciton state
J=2EXX’:excited biexciton state
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Experimental setup
optical delay
Ti sapphire laser
Ti sapphire regenerative amplifier
Pulse duration: 300fs Repetition rate: 1kHzEnergy: 200μJ/pulse
apparatus
Pulse duration: 1.2psFWHM: 1.7meV
cryostat
CCD
Probe beam White continuum
Pump beam
CuCl QDs in NaCl
T = 77KSHG crystal
Pure water
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The absorption spectra
(a) The absorption spectrum
The absorption spectrum at 10 ps after excitation
EXX’
2EX
EXX
EX
G
Absorption change spectra with various excitation energies
(b)
The absorption saturationat the excitation energy
>
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Excitation energy dependence
The slope : 2.0
:the transition from the exciton state to excited biexciton
Crossing at the exciton energy of bulk CuCl (3.218 eV)
EXX’
2EX
EXX
EX
G
The excited biexciton state come from the spatial confinement
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Temporal evolution of absorption change peaks
Excitation photon energy : 3.269 eV
the creation of biexciton by two excitons
:
fast decay component
main decay component
: 480 ps
: 480 pssame
exciton radiative lifetime
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Application
This unique property New optical devices
For example
ON OFF
Optical switch
ON/OFF control with a single photon
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Summary
•The transient absorption change of CuCl quantum dots embedded in a NaCl crystal was measured by means of the pump-probe method.
•Strong induced absorption was observed at higher energy side of the exciton resonance.
•It is attributed to the transition from the exciton to the excited biexciton state and it depends on the excitation photon energy.
•The result of the temporal evolution of the transient absorption supported the identification of the excited biexciton states.
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About my work
Pump-probe spectroscopy with tunable picosecond pulse laser
CuCl quantum dots in NaCl (avarage dot radius ~4nm )tranverse Bridgman method
PL spectrum
Temporal profile of transient absorption
Transient absorption spectrum
excited biexcitontwo excitonsbiexicton
ground
excited exciton
pump pump
pump
probe
probe
the excited states of excitons and biexcitons
1s state
2p state
(1s,2p) state
(1s,1s) state
exciton biexciton EX’: excited exciton
exciton
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Previous work
0 100 200 300 400 500 600 7000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07Probe 250meV (4.9m)
experiment fitting curve fast decay ~56ps slow decay ~490psO
.D.
Time (ps)100 200 300 400 500 600
0.00
0.05
0.10
0.15
biexciton exciton
O.D
.
Photon Energy (meV)
Temporal profile of transient absorption Transient absorption spectrum
Infrared transient absorption of CuCl quantum dots has two decay component. Exciton and biexciton, respectively.
S/N ratio was not sufficient.There is not enough number of experimental data.
problems
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Experimental setup
OPA
optical delay
MCT
AD Converter
Monochro-mator
signal
Boxcar
Pump pulseFHG355nm
~2 ps, 1 kHz
Probe pulseDFG 2.6μm ~ 8μm
Photo diode
Chopper 500Hz
CuCl QDs in NaCl
T = 70K
OPA
Boxcar
PC
cryostat
Long Pass Filter
ND Filter
・ Detection of the signal shot by shot by a box car integrators
・ Improvement of spectral resolution
( closely shifting the wavelength of probe beam )
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Photo luminescence spectrum
Excitation energy 3.492eV (355nm)
Excitation Intensity 1.41mJ/cm2
Many excitons and biexcitons are generated in quantum dots.
3.15 3.20 3.25 3.30
PL Inte
nsity
Photon Energy (eV)
M
Exciton
:PL of exciton
:PL of biexciton
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Temporal profile of transient absorption
Probe photon energy of 309 meV (4000 nm)
-200 0 200 400 600 800
0.00
0.05
0.10
0.15
0.20
Δα
l
Delay Time (ps)
experiment fitting curve fast decay 50 ps slow decay 800 ps
Two decay components
Fast decay : 50 ps
Slow decay : 800 ps
Biexciton
Exciton
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Transient absorption spectrum
•biexciton : 340 meV
•exciton : 320 meV
Transition energy for the biexciton is higher than that for the single exciton.
150 200 250 300 350 400 450 5000.000
Δα
l (a
.u.)
Photon Energy (meV)
biexciton exciton Absorption peak energies
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Summary
•We measured the infrared transient absorption of CuCl quantum dots embedded in a NaCl crystal.
•The absorption peaks are derived from the transition of Rydberg 1s-2p state for exciton and the transition to the excited-states of biexciton composing of 1s and 2p excitons for biexciton.
•Transition energy of biexciton is higher than that of the exciton.
In the near future
•The dependence of dot size of the excited states of the biexcitons and excitons will be presented under the size-selective excitation by two photon excitation.