Ultrafast THz Spectroscopy Ultrafast THz Spectroscopy and Nonlinear Optical and Nonlinear Optical Properties of Semiconductor Properties of Semiconductor NanostructuresNanostructuresZhen-Yu ZHAOZhen-Yu ZHAO

17 July 2008

Laboratoire Pierre Aigrain - Ecole Normale Supérieure, Paris State Laboratory of Precise Spectroscopy - East China Normal

University, Shanghai

1

OutlinOutlinee

Section 2: Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite

GlassesGlasses• FabricationFabrication• CharacterizationCharacterization• Nonlinear Optical MeasurementNonlinear Optical Measurement

Section 1: Development of THz Time Domain Spectroscopy (THz-TDS)Development of THz Time Domain Spectroscopy (THz-TDS) • Optical RectificationOptical Rectification• Micro-Photoconductive EmitterMicro-Photoconductive Emitter

Application of THz - TDSApplication of THz - TDS• Gain Measurement of Quantum Cascade Laser (QCL)Gain Measurement of Quantum Cascade Laser (QCL)

2

Section 1

Development & Application of Development & Application of THz Time Domain SpectroscopyTHz Time Domain Spectroscopy

3

THz THz RadiationRadiation

Section 1

1THz↔300μm↔1picosecond↔4.1meV↔10K

Application THz spectroscopy : Semiconductor nanostructuresApplication THz spectroscopy : Semiconductor nanostructures

http://www.thznetwork.org4

THz Time domain SpectroscopyTHz Time domain SpectroscopySection 1

Ti : sapphire laser

τ

BS

M2 M5

M1

M3 M4

Emitter

Electro-Optic Sampling

SZnTe

λ/4

WP

Balanced photodiodes

Probe beam

Pump beam

30 14

2THz

dn r E

Free Space Electro-Optic SamplingFree Space Electro-Optic Sampling

THz emitter:THz emitter:

Optical RectificationOptical Rectification

Photoconductive antennaPhotoconductive antenna

5

sinII-II 0

I

I

THz pulse

THz pulse

Δτ

Optical RectificationOptical Rectification

Laser pulse,Δτ :100fs, λ :800nm ZnTe

crystals

THz radiation

Z

Lens

SHG & Transmitted beam

2 IETHz

Section 1

6

-4 -2 0 2 4 6

(ps)

FFT

0 1 2 3 40.0

0.5

1.0

ET

Hz(

A.U

.)

THz

eVZnTeEg 35.2

Optical RectificationOptical RectificationSection 1

0,0

0,5

1,0

I THz (A

.U.)

0

1

2

3

I SH

G (m

W)

0 5 10 15 200,0

0,5

1,0

z (mm)

I Tra

ns (A

.U.)

7

Bolometer

PMT

Photodiode

Z

Z

Z

Teflon

Filter

Picarin Lens

Lens

Lens

8

Optical RectificationOptical RectificationSection 1

1. Optical Rectification 2. Second Harmonic Generation

3. Two Photon Absorption

4. Free Carrier Absorption

ħω 2ħω

ħω

ħω

Nonlinear Crystals Nonlinear Crystals

high state

low state

Conduction band

Valence band

2ħω

2Idz

dI

β: TPA coefficient

Nonlinear Optical ProcessesNonlinear Optical Processes

9

Optical RectificationOptical RectificationSection 1

0,0

0,5

1,00 60 120 180 240 300 360

0,0

0,5

1,0

0 60 120 180 240 300 3600,8

1,0

I TH

z (A

.U.)

I SH

G (

A.U

.)°

°

I Tra

ns (

A.U

.)

22 cos31sin f

f

21

θ

[001]

Laser polarization

ZnTe

X.-C. Zhang et al. J. Opt. Soc. Am. B 18 : 823 (2001)

fEdITHz40

214

D.C. Hutchings and B.S. Wherrett, J. Opt. Mod. 41: 1141 (1994)

Optical RectificationOptical Rectification

Lens :f=4cm

ZnTe

Rotation

Laser beam THz radiation

15mm

BBO

0 5 10 150,0

0,5

1,0

1,5 Z0

FCA

TPA

Expriment

z-2

TH

z in

tens

ity

z (mm)

-1211 cm 6)/ln(mm15 THzTHzTHz IIlz

Two Color ExperimentsTwo Color Experiments

Section 1

10

Interdigitated photoconductive Interdigitated photoconductive

antennaantenna Section 1

biasin

THz

UIJt

JE

11

Laser pulse,Δτ :100fs, λ :800nm

Hemisphere Si lens

+

—

eVGaAsEg 42.1

Conventional Photoconductive antenna

12

Interdigitated photoconductive Interdigitated photoconductive

antennaantenna

+-

Electrods

Opaques

A. Dreyhaupt et al. Appl. Phys. Lett. 86 :121114 (2005)A. Dreyhaupt et al. Opt. Lett. 31 :1546 (2006) Nathan Jukam, UCSB

stripline gap1.5µm

500µm

Interdigitated photoconductive Interdigitated photoconductive emitteremitter

Section 1

-1 0 1

ET

Hz (

A.U

.)

(fs)

Bias:20kv/cm Bias:40kv/cm Bias:60kv/cm Bias:80kv/cm Bias:160kv/cm

0 20 40 60 80 100 120 140 160 180

Bias field (kv/cm)

ET

Hz

(a.u

.)

THz intensity

0 1 2 3 40.0

0.5

1.0

ET

Hz(

a.u.

)

THz

Bias dependence

13

Interdigitated photoconductive Interdigitated photoconductive emitteremitter

Section 1

Γ→Γ→L Intervalley scatteringL Intervalley scattering

C. Ludwig and J. Kuhl, Appl. Phys. Lett. 69 (9), 1194 (1996)J.-H. Son, T. B. Norris, and J. F. Whitaker, J. Opt. Soc. Am. B 11, 2519 (1994)

14

Interdigitated photoconductive Interdigitated photoconductive emitteremitter

Optimization by change the exciting intensity

Section 1

15

0 1 2 3 4 50.0

0.5

1.0

E

TH

z (A

.U.)

THz

613 mW 215 mW 50 mW 10 mW

6 7 8 9-1,0

-0,5

0,0

0,5

1,0

ET

Hz (

A.U

.)

(ps)

10 mW 50 mW 215 mW 613 mW

Interdigitated photoconductive Interdigitated photoconductive emitteremitter

Space Charge Screenings Effect

Bias Field

Coulomb Field

Section 1

16

-

-

High Optical FluxLow Optical Flux

+ +— —

22p

ne

m

Interdigitated photoconductive Interdigitated photoconductive emitteremitter

Temperature Dependence of THz emitter

Section 1

17

dt

dneµEE lTHz

0 50 100 150 200 250 300

3,5

4,0

4,5

5,0

5,5

E

TH

z (A

.U.)

T(K)J. S.Blakemore, J. Appl. Phys. 53: R123-R181 (1982)

Section 1Comparison of 2 THz emittersComparison of 2 THz emittersZnTe Crystals Interdigitated Photoconductive

antenna

THz Amplitude 10V/cm 100V/cm

Central Frequency 2THz 1THz~~1.4THz

Bandwidth 0.5~~2.7THz 0~~3.5THz

S/N 500~1000 5000~~10000

18

0 1 2 3 40,0

0,5

1,0

ET

Hz (

A.U

.)

THz bandwidth

Antenna ZnTe

THz Quantum Cascade THz Quantum Cascade LaserLaser

Concept

1

2ħω

Interband transition Inter-subband transition

Semiconductor Laser Quantum Cascade Laser

Section 1

1970

First Idea

1980 1990 2000

First QCL @ Bell Labs THz QCL

Years1994 2002

Milestone

2.9 THz QCL 77k

2004

19

2006

1.9 THz QCL 95k

1971

THz Quantum Cascade THz Quantum Cascade LaserLaser

12 meV

300 200 100 0

50

100

Distance (nm)

17 meVMiniband

Miniband e-(1)

(2)

(1')

(g)

Section 1

Bound to Continuum Active-injection Region of 2.9THz QCL

20

21

THz Quantum Cascade THz Quantum Cascade LaserLaser

Section 1

Surface Plasmon Waveguide of 2.9 THz QCL

Active Region

Metal 220µm

SI Substrate

12µm

(a)

Bottomn+ layer

MetalContact

220µm

MPQ-Paris VII

THz Quantum Cascade THz Quantum Cascade LaserLaser

Section 1

V

QCL

Pyroelectric Detector

A

THz Collimation

THz

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,40,00

0,01

0,02

0,03

PT

Hz (

a.u.

)

I (A)

70K 65K 60K 55K 50K 40K 30K 20K 10K

Characterization of 2.9 THz QCL

22

THz Gain THz Gain MeasurementMeasurement

Ti : sapphire laser

τ

BS

M2 M5

M1

M3 M4

ETHz

FSEOS

SA B C

D

Probe beam

Pump beam

Zone Active

Section 1

23

THz Gain THz Gain MeasurementMeasurement

24 26 28 30 32 34 36 38 40

-40

-20

0

20

40

Ele

ctri

c fi

eld

(a.u

.)

Time (ps)

At threshold Below threshold

0 1 2 3 40,0

0,2

0,4

0,6

0,8

1,0

Frequency (THz)

Am

plitu

de (

a.u.

)

Amplified THz transmission by gain of quantum cascade laser

2.9THz

Section 1

24

THz Gain at different injection current THz Gain at different temperature

THz Gain THz Gain MeasurementMeasurement

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4

1,0

1,5

2,0

2,5

3,0

90K

80K

70K

60K

50K40K

10K

Gai

n Fi

eld

Current (A)

0 20 40 60 80 1000,2

0,4

0,6

0,8

1,0

1,2

Cur

rent

(A

)

Temperature (K)

Section 1

Gain Clamping

25

26

Section 1

Summary 1Summary 1

Development of THz-TDS

THz performance of ZnTe crystal.

THz output of interdigitated photoconductive antenna

Application of THz-TDS

First measurement of Gain of 2.9 THz Quantum Cascade Laser

Section 2

Nonlinear Optical Properties of Nonlinear Optical Properties of AgCl NCs doped Tellurite GlassesAgCl NCs doped Tellurite Glasses

27

IntroductionIntroduction Section 2

28

Glass Χ(3) ~ 1014esu

SiO2 2.4

46PbO–42Bi2O3–12Ga2O3 42

20Nb2O5–80TeO2 72

------J. Lin et al. J. Non-Cryst. Solids 336 : 189–194 (2004)

Photonic Glasses

------Y.Q. LI et al. J. Rare Earth 25 : 412 – 415 (2007)

Er+ Doped TeO2-Nb2O5-ZnO Glass

Optical Switching Optical limiting

Nanocrystals doped Tellurite Glasses

Tellurite Glasses

FabricatioFabricationn

• Melting: 80TeO2:20Nb2O5 & 1%wt AgCl powder

800°C / 15minutes.

• Quenching: Annealed at 300°C

• Thermal treatment: At 360°C 30min, 60min, 90min, 120min

Section 2

29

CharacterizatioCharacterizationn

(a) 30min thermal treated (b) 60min thermal treated

Section 2

30

Nanocrystals FESEM Image vs Termal treatment time

CharacterizationCharacterization

(c) 90min thermal treated (d) 120min thermal treated

Section 2

31

Nanocrystals FESEM Image vs Termal treatment time

CharacterizationCharacterization

0

5

10

15

Num

ber

(A.U

.)

0 10 20 30 40 50 600

5

10

15

Num

ber

(A.U

.)

grain-size (nm)grain-size (nm)

0 10 20 30 40 50 60

12nm / 30 min

Section 2

32

Size distribution function vs thermal treatment time

26nm / 90 min

17nm / 60 min

35nm / 120 min

CharacterizationCharacterization

450 500 5500

100

200

300

400

500

600

Flu

ore

scen

ce In

ten

sity

(a.

u.)

Wavelength (nm)

undoped 0 min 120min

Cl-

Ag+

Jahn-Teller effect : Lattice deformation

Cl- colour center

Reaction: 2Cl- →Cl2 + 2e- & 2Ag++2e-→2Ag

400 500 600 700 800 900 10000,0

0,2

0,4

0,6

0,8

1,0

Abso

rbance

(a.u

.)

Wavelength (nm)

base glass 0min 30min 60min 90min 120min

Section 2

33H. Vogelsang, Phys. Rev. B 61: 1847-1852 (2000)

CharacterizationCharacterization

Eg

gm EhKT

exp

Urbach law:

Samples Eg (eV)

undoped 3.1

0 min 2.9

30 min 2.9

60 min 2.3

90 min 1.9

120 min 1.7Bandgap of glass matrix

Bandgap redshift of treated glass

Eg

Trapped state

Section 2

34

35

Nonlinear Optical PropertiesNonlinear Optical Properties Section 2

0 15 30 45 600

2

4

6

8

10

120min

90min

60min

30min

0min

Tra

nsm

itte

d P

ow

er

(J)

Incident Power (J)

No dopant50% BS

lens lens

SampleAttenutation

Powermeter

Powermeter

Nonlinear Optical PropertiesNonlinear Optical Properties

Lock-In

PCTi :sapphireLaser

M1

M2

L L LS D

Z

Open Aperture Z-scan

Section 2

-7.5 -5.0 -2.5 0.0 2.5 5.0 7.5

0.85

0.90

0.95

1.00

T(z

)

Z (cm)

heating:120min heating:60min heating:0min Fitting curveβ: 1GW/cm ~~ 1.8 GW/cm

36

,2

320

gg

p

EF

En

EK

Nonlinear Optical PropertiesNonlinear Optical Properties Section 2

Lock-In

M1 M2

M3

M4

M5BS

Δτ

PC

L S

K1

K2

2K2-K1

2K1-K2 D

Ti :sapphireLaser

-400 -200 0 200 4000

10

20

30

DF

WM

sig

nal i

nten

sity

(A

.U.)

(fs)

glass 0min 30min 60min 90min 120min

Samples Eg (eV) n0 χ(3) (10-13esu) n2 (10-11esu)

undoped 3.1 2.0 7.2 1.2

0 min 2.9 2.1 7.3 1.3

30 min 2.6 2.2 7.8 1.4

60 min 2.3 2.2 14 2.3

90 min 1.9 2.2 11 1.9

120 min 1.7 2.2 9.5 1.6

DFWM experiment

37

38

Section 2SummarSummary 2y 2

Nonlinear optical properties of AgCl NCs doped tellurite glassNonlinear optical properties of AgCl NCs doped tellurite glass

•Samples were Prepared by Melt-Quenching and Thermal Treatment Methods

•Characterization with Microscopic and Spectroscopic Methods

•Nonlinear Optical Properties were Measured by Z-scan, Optical Limiting and DFWM

ConclusioConclusionn

39

Section 2: Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite GlassesNonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Glasses• Fabrication• Optical limiting performance and Two-photon absorption• Enhancement of χ(3)

Section 1: Development of THz Time Domain Spectroscopy (THz-TDS)Development of THz Time Domain Spectroscopy (THz-TDS) • Competition OR TPA FCA, Azimuthal dependence• Intervally scattering, Space charging screening, Electron mobility Application of THz – TDSApplication of THz – TDS• Gain Measurement of 2.9THz QCL

AcknowledgementAcknowledgement

THz group (LPA-ENS)THz group (LPA-ENS)Advisor:Advisor: Jérôme TignonJérôme TignonStaff members:Staff members: Sophie Hameau, Sukhdeep Dhillon et al. Sophie Hameau, Sukhdeep Dhillon et al. Postdoc:Postdoc: Nathan Jukam; Nathan Jukam; Ph.D student:Ph.D student: Dimitri Oustinov;Dimitri Oustinov;Master student:Master student: Julien Amijo, Geog Dürr; Julien Amijo, Geog Dürr; Techniciens:Techniciens: Pascal Morfin, Phillipe Pace et al.Pascal Morfin, Phillipe Pace et al.Collaborators:Collaborators: Carlo Sirtori et al.Carlo Sirtori et al.

Sun’s Group (East China Normal University)Sun’s Group (East China Normal University)Co-Advisor: Co-Advisor: Zhenrong Sun, Zhenrong Sun, Staff members: Staff members: Tianqing Jia, Xiaohua Yang et al.Tianqing Jia, Xiaohua Yang et al.Collaborators:Collaborators: Jian Lin, et al.Jian Lin, et al.