レーザーを用いた赤外時間分解分光レーザーを用いた赤外時間分解分光

半導体量子ドットに閉じ込められた励起子の半導体量子ドットに閉じ込められた励起子の励起状態の研究励起状態の研究

枝松 圭一

大阪大学大学院基礎工学研究科

分子研研究会

赤外放射光の現状と将来計画

2002年11月13日

大阪大学大学院基礎工学研究科山中一克宮島顕祐伊藤正

大阪府立大学大学院工学研究科魚住孝幸萱沼洋輔

共同研究者共同研究者

1. Introduction• Excited states of excitons confined in semiconductor

quantum dots• CuCl quantum dots

2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems

3.Results• Excited-state absorption of confined excitons in CuCl

quantum dots• Excited-state absorption of confined biexcitons

4.Summary

OutlineOutline

3D: Bulk crystal

2D: Quantum well

1D: Quantum wire

0D: Quantum dot (QD)(or nanocrystal)

E

DO

S

Semiconductor quantum structureSemiconductor quantum structure

Photon Energy

Lum

ines

cenc

e In

tens

ity Excitation Laser

~2Γhomo

(b)

Photon Energy

Lum

ines

cenc

e In

tens

ity

Γinhomo

(a)

Photon Energy

Lum

ines

cenc

e In

tens

ity

Γhomo

(b)

Photon Energy

Lum

ines

cenc

e In

tens

ity

Γinhomo

(a)

1. Resonant size-selective excitation

Laser

2. micro-photoluminescence

SizeSize--selective observation of selective observation of QDsQDs

•Electron-hole individual confinement.(aB≫a)

•Exciton confinement.(aB≪a)

M

0 a r

∆E

∆EM a

M m mM

e h

=

= +

h2 2

2π

: translational mass

me

mh

2a

∆EC

∆EV

∆ ∆ ∆E E E

a

m m

C V

e h

= +

=

= +− − −

h2 2

1 1 1

2µπ

µµ:reduced mass

Quantum size effect in spherical Quantum size effect in spherical QDsQDs

Excitons in CuCl quantum dotsaB=0.7 nm (1S); Ry*=197 meVConfinement energy～ 10meV (a* = 4 nm)

⇒ Exciton confinement regime

Two quantum numbers of the confined exciton:

1s1p1d2s2p

1S

2S ,P3S ,P ,D

∞

2a B2a

1) Confinement oftranslational motion (nl)

2) Coulombic states (n’L)

Confined excitons inConfined excitons in CuCl QDsCuCl QDs

T. Uozumi, et al., Phys. Rev. B 59, 9826 (1999)

0 2 4 6 8 10 12 14 16 18

-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.4

n=2 (2S,np), (2P,ns)n=3 (3S,np), (3P,ns)

Ene

rgy

[E* R]

Radius [a *B ]

0

5

10

15

20

Ene

rgy

n=1 (1S,1s)

Energy levels of confined excitonsEnergy levels of confined excitons

(Infrared transient absorption)• Mimura et al., J. Lumin. 66&67, 401 (1996)• Yamanaka et al., J. Lumin. 76&77, 256 (1998)• Yamanaka et al., J. Lumin. 87&89, 312 (2000)• Itoh et al., Int. J. Mod. Phys. 15, 3569 (2001) • Miyajima et al., Phys. Stat. Sol. (b), in press.

(Two-photon excitation)• Edamatsu et al., Phys. Rev. B 59, 15868 (1999)

(Theoretical works)• Kayanuma, Phys.Rev. B 38, 9797 (1988)• Uozumi et al., Phys. Rev. B 59, 9826 (1999)• Uozumi et al., Phys. Rev. B 65, 165318 (2002)

ExcitedExcited--state absorption of excitons state absorption of excitons confined inconfined in CuClCuCl quantum dotsquantum dots

CuCl QDs (or nanocrystals) embedded in NaCl matrices

Dried NaClingot

CuCl flakes (~1mol%)

Quartz tube NaCl:Cu+ single

crystal

H2 gas (~0.3atm)

2cm/day

NaClpowder

Cl2gas

Melting (~820°C)

Annealing

500~600°C several hours

Small CuCl QDs(a few nm)

200~300°C

Large QDs(~10nm)

500~600°C

Sample preparationSample preparation

3.2 3.3PHOTON ENERGY(eV)

370380390WAVELENGTH(nm)

0

1

2

3

O.

D.

LUM

INES

CEN

CE

IN

TEN

SIT

Y (

a.u.

)

absorptionluminescence

77Ka*=3.7nm

Absorption and luminescence spectraAbsorption and luminescence spectraofof CuCl QDsCuCl QDs inin NaClNaCl matricesmatrices

Inhomogeneous broadening due to size distribution.｝

Probe light

Pump light

ｇ

1S

nP

3.2 3.3PHOTON ENERGY(eV)

370380390WAVELENGTH(nm)

0

1

2

3

O.

D.

pump

Transient absorption spectroscopy Transient absorption spectroscopy under sizeunder size--selective excitationselective excitation

1. Introduction• Excited states in semiconductor quantum dots• CuCl quantum dots

2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems

3.Results• Excited-state absorption of confined excitons in

CuCl quantum dots• Excited-state absorption of confined biexcitons

4.Summary

OutlineOutline

power unit

Xe lamp

monochro-mator(NaCl prism)

photo detectorCdHgTe

amp.

storageoscillo-scope

computer

pulse motor

PD

timingcircuit

YAG laser

Ti-sapphire

trigger

trigger

triggercryostat(77K)sample

SH

SH

•Pump light sourceSecond harmonic of Ti:Sa laser

•Tunability (SH)350∼ 470nm(2.64∼ 3.54eV)

•Pulse energy : ~1mJ•Pulse duration: ~15ns•Excitation power : ~70MW/cm2

•Line width <0.1nm(1meV)

•Probe light sourceXe flash lamp

•Pulse duration: ~8µs•1~8µm (160~1200meV)

Experimental setup (ns system)Experimental setup (ns system)

Probe Light

Pump Light

Mono-chromator

Cryostat

Sample

Detector(cooled MCT)

0 0.2 0.4TIME (µs)

22

23IN

TEN

SITY

(a.u

.) I0

I

pump

Observation of the infrared transient Observation of the infrared transient absorptionabsorption

-0.1 0 0.1 0.2 0.3TIME (µs)

0

0.05

0.1

∆O

.D.

τ f (≤ 1ns)

τ s (>> 100µs)

Fast decay component (τf): originates from the confined exciton (1S → 2P)

Excitation: 3.224 eV(a*~6 nm)

Probe: 246 meV(λ=5.03 µm)

Infrared transient absorption:Infrared transient absorption:temporal profiletemporal profile

R=15.0[a*B]

8.0[a*B]

Inte

nsity

[arb

. uni

ts]

3.0[a*B]

0 200 400 600

2.5[a*B]

Photon Energy [meV]200 400 600PHOTON ENERGY (meV)

345710WAVELENGTH (µm)

∆O

.D.

(A)

(B)

(C)

(D)

77K

a*≥10nm

a*=5.4nm

a*=1.9nm

a*=1.4nm

Experiment Theory (Uozumi and Kayanuma)

Transient absorption spectra Transient absorption spectra

T. Uozumi, et al., Phys. Rev. B 59, 9826 (1999)

0 2 4 6 8 10 12 14 16 18

-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.4

n=2 (2S,np), (2P,ns)n=3 (3S,np), (3P,ns)

Ene

rgy

[E* R]

Radius [a *B ]

0

5

10

15

20

Ene

rgy

n=1 (1S,1s)

Energy levels of confined excitonsEnergy levels of confined excitons

0 5 10EFFECTIVE RADIUS a* (nm)

200

400

TRA

NSI

TIO

N E

NER

GY

∆E

(meV

) (1pe, 1sh)(1se, 1ph)

theory

experiment

(2P,1s)

1S 1S →→ PP--like state transition energy:like state transition energy:comparison with theorycomparison with theory

1. Introduction• Excited states in semiconductor quantum dots• CuCl quantum dots

2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems

3.Results• Excited-state absorption of confined excitons in

CuCl quantum dots• Excited-state absorption of confined biexcitons

4.Summary

OutlineOutline

Motivation of theMotivation of the psps experimentexperiment

• Insufficient resolution (in energy and time) of the ns experiment ⇒ ambiguous attribution of the observed transient

absorption

• Excited-state absorption of confined biexcitons?

• Modification of the confined exciton states induced by strong IR pulses?

Ti:sapphire laser

regenerative amplifier

OPAOPA

monochro-

matorMCT

lock-in

amplifier

800 nm, ~2 ps, 1 kHz

Probe pulseIdler (2.1~2.9 µm) or DFM (3~10 µm)

~0.1µJ/pulse

Pump pulse4ωs(~384 nm）

8 µJ/pulse

chopper

CuCl QDs in NaCl

T = 70Ksample

cryostat

ND filter

Experimental setup (Experimental setup (psps system)system)

pump

IRTA

excited exciton

2 excitonbiexcitonIRTA

pump

excited biexciton

exciton

ground state

ExcitedExcited--state absorption of confined state absorption of confined biexcitonsbiexcitons ??

3.10 3.15 3.20 3.25 3.30 3.35

70K laser light (3.227eV)

M 3.18eV

PL

Inte

nsity

(a.u

.)

Photon Energy (eV)

PL spectrum: exhibiting biexciton PL spectrum: exhibiting biexciton luminescenceluminescence

• Excitation : Resonant to the confined excitons with the effective radius ~4.2 nm (3.227eV).

• PL from the biexcitons (M band) at ~3.18eV.

(Ex)

0 200 400 600 8000.00

0.02

0.04

0.06probe 4.89µm (0.25eV)

τ2~490ps

τ1~56ps

experiment calculation

∆O.D

.

Time (ps)

Two decay components

• Fast decayτ1 ~56±15ps⇒ Biexciton

• Slow decayτ2 ~490±290ps⇒ Exciton

Infrared transient absorption:Infrared transient absorption:psps decay profiledecay profile

fit

0.1 1 10

0.01

0.1

∝IEXC0.8

∝IEXC1.3

fast decay slow decay

∆O.D

.

Excitation Intensity IEXC (mJ/cm2)

• Fast decaysuperlinear⇒ Biexciton

• Slow decaylinear~sublinear⇒ Exciton

Infrared transient absorption:Infrared transient absorption:excitation power dependenceexcitation power dependence

100 200 300 400 500 6000.00

0.05

0.10

0.15

biexciton exciton

∆O.D

.

Photon Energy (meV)

100 200 300 400 500 600

a*=3.9 nm

Inte

nsity

(a.u

.)

Photon Energy (meV)

Experiment Theory (exciton)

Infrared transient absorption spectra:Infrared transient absorption spectra:exciton and biexciton componentsexciton and biexciton components

0 5 10EFFECTIVE RADIUS a* (nm)

200

400

TRA

NSI

TIO

N E

NER

GY

∆E

(meV

) (1pe, 1sh)(1se, 1ph)

theory (exciton)

experiment (exciton)

(2P,1s)

experiment (biexciton)

Transition energy:Transition energy:comparison with theorycomparison with theory

ground state

exciton

2 exciton

biexcitonexcited exciton

excited biexciton

pump

IRTA

IRTA

ExcitedExcited--state absorption of the state absorption of the confined biexcitonconfined biexciton

Excited biexcitons:one lowest exciton and one excited exciton.

In QDs, excited biexcitonsare stable because the two excitons are confined together in a QD.

We have investigated the infrared transient absorption spectra of CuCl QDs under size-selective excitation.

• Direct observation of the Rydberg 1S-2P transition of the confined exciton.

• The transition energy depends on the dot size, in agreement with the theoretical calculation.⇒ Deviation from the “exciton confinement” regime

• Finding of the excited-state absorption of the confined biexciton.

SummarySummary

赤外時間分解分光：放射光の利用赤外時間分解分光：放射光の利用

◎ エネルギー範囲： 遠赤外まで連続

○ エネルギー分解能： 分光器，強度に依存

○ 時間分解能： ns~ps？

△ ポンプ・プローブ分光： 可視～紫外の強度に依存

放射光とレーザー光源との組み合わせ（同期）