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)

Contents

• Introduction

Exciton and biexciton

Quantum-dot

• Experiments

• Summary

• My work

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).

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

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.

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.

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

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

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

>

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

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

Application

This unique property New optical devices

For example

ON OFF

Optical switch

ON/OFF control with a single photon

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.

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

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

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 ）

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

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

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

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.