1. Ultrafast THz Spectroscopy and Nonlinear Optical Properties of Semiconductor Nanostructures
Zhen-Yu ZHAO
17 July 2008
Laboratoire Pierre Aigrain - Ecole Normale Supérieure, Paris
State Laboratory of Precise Spectroscopy - East China Normal University, Shanghai
For the practical reason, I work on 2 different subject both are ultrafast spectroscopy of semiconductor nanosturctures.
Both subjets concentrate on the ultrafast spectroscopy of semicondeuctor nanostructures.

2. Outline Section 1: Section 2:
Development of THz Time Domain Spectroscopy (THz-TDS)
Optical Rectification
Micro-Photoconductive Emitter
Application of THz - TDS
Gain Measurement of Quantum Cascade Laser (QCL)
Section 2:
Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Glasses
Fabrication
Characterization
Nonlinear Optical Measurement
In the section 1, I would like to present my work on the development and application of THz time domain spectroscopy. We tried 2 different THz emitter and compare their output performance respectively. Then, we use the photoconductive antenna to measure the gain of 2.9THz QCL.
In the section 2. I’d like to present the nonlinear optical properites of AgCl NCs doped tellurite glasses.

3. Section 1 Development & Application of THz Time Domain Spectroscopy
Now, let’s start with section 1.

4. Application THz spectroscopy : Semiconductor nanostructures
THz Radiation
Section 1
THz radiation is 10 to 12Hz on electromagenetic spectr which locate on the gap betwen infrared radiation and microwave.
For a long time, there is no proper source and detector in this electromagnetic region so that people call it THz gap.
Now, there is new technique -----THz-TDS one can obtain sub-ps pulse to investigate the materials’ physical properties without Kramer-Kronig transformation at a high signal-noise-ratio. It has become a powerful tool to study the semiconductor nanostructure.
1THz↔300μm↔1picosecond↔4.1meV↔10K
Application THz spectroscopy : Semiconductor nanostructures

5. THz Time domain Spectroscopy
Section 1
Ti : sapphire laser τ BS M2 M5 M1 M3 M4
Emitter
Electro-Optic Sampling S
ZnTe
λ/4 WP
Balanced photodiodes
Probe beam
Pump beam Δτ
The THz-TDS is base on a pump-probe optical setup.
The pump beam drive the THz emitter and the probe beam pulse sampling the THz waveform by change the time delay.
The probe beam turns to circular polarization and its 2 perpendicular components were equaly seperated by the wollastom prism into balance photodiode.
THz pulse change the polarization because of the Pockel’s effect so that the 2 component is not equal.
One can record the differece signal to describe the THz waveform.
Free Space Electro-Optic Sampling
THz emitter:
Optical Rectification
Photoconductive antenna

6. Optical Rectification
Section 1
Laser pulse,
Δτ :100fs, λ :800nm
ZnTe crystals
THz radiation Z
Lens
SHG & Transmitted beam
FFT
At first, we tried to use optical rectification method to optimize the THz output performance.
an appropriate crystal material is quickly electrically polarized at high optical intensities. This changing electrical polarization emits terahertz radiation.
However, there are other nonlinear optical process occurs when the intense femtosecond laser pass through the ZnTe crystal.

7. Optical Rectification
Section 1
Picarin Lens Z
Teflon
Bolometer Z
Filter
Lens
PMT
In the begining, we put the lens far from the ZnTe crystal and THz output increase.
The diffraction effect will cause the THz output saturation when the spot-size is equal to the THz wavelength.
Therefore, the THz Z-Hole must be originate from other nonlinear optical process Z
Lens
Photodiode

8. Optical Rectification
Section 1
Nonlinear Optical Processes
1. Optical Rectification
2. Second Harmonic Generation ħω
2ħω
Nonlinear Crystals
Nonlinear Crystals
3. Two Photon Absorption
4. Free Carrier Absorption
high state
Conduction band
In order to optimize the THz output of ZnTe crystal, we study the competition betwen optical rectification and other laser-induced nonlinear optical process. ħω
2ħω ħω
β: TPA coefficient
low state
Valence band

9. Optical Rectification
Section 1
[001] θ
Laser polarization
ZnTe
X.-C. Zhang et al. J. Opt. Soc. Am. B 18 : 823 (2001)
D.C. Hutchings and B.S. Wherrett, J. Opt. Mod. 41: 1141 (1994)

10. Optical Rectification
Section 1
Lens :f=4cm
ZnTe
Rotation
Laser beam
THz radiation
15mm
BBO
Two Color Experiments
15mm far from the focus so that there is no SHG

11. Interdigitated photoconductive antenna
Section 1 +
Laser pulse,
Δτ :100fs, λ :800nm
Hemisphere Si lens
Consider the competition between optical rectificationa and other nonlinear properties, we tried another THz emitter: photoconductive emitter.
The semiconductor changes abruptly from being an insulator into being a conductor. This conduction leads to a sudden electrical current across a biased antenna patterned on the semiconductor. This changing current emits terahertz radiation.
Conventional photoconductive antenna need hemi-sphere lens to collimating the THz output because of the diffraction limiting and need a larger bias voltage.
Dreyhaup et al prupose the interdigitated structure antenna.
We thanks Dr.Nathan Jukam to provide the antenna sample.
Conventional Photoconductive antenna —

12. - + Interdigitated photoconductive antenna
stripline gap1.5µm
500µm
Electrods
Opaques
A. Dreyhaupt et al. Appl. Phys. Lett. 86 : (2005)
A. Dreyhaupt et al. Opt. Lett. 31 :1546 (2006)
Nathan Jukam, UCSB

13. Interdigitated photoconductive emitter
Section 1
Bias dependence

14. Interdigitated photoconductive emitter
Section 1
Γ→L Intervalley scattering
F-L intervalley scattering: the F valley electrons were scattered into L valley when the bias field is large enough.
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)

15. Interdigitated photoconductive emitter
Section 1
Optimization by change the exciting intensity

16. Interdigitated photoconductive emitter
Section 1
Space Charge Screenings Effect
Low Optical Flux
High Optical Flux - -
Space charge screening + — + —
Bias Field
Coulomb Field

17. Interdigitated photoconductive emitter
Section 1
Temperature Dependence of THz emitter
J. S.Blakemore, J. Appl. Phys. 53: R123-R181 (1982)

18. Comparison of 2 THz emitters
Section 1
ZnTe 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

19. THz Quantum Cascade Laser
Section 1
Concept
Semiconductor Laser
Quantum Cascade Laser ħω
Why the casacade structure because of the wavelength and bandgap of materials
Interband transition
Inter-subband transition
Milestone
1970
1980
1990
2000
1971
1994
2002
2004
2006
Years
First Idea
First Bell Labs
THz QCL
2.9 THz QCL 77k
1.9 THz QCL 95k

20. THz Quantum Cascade Laser
Section 1
Bound to Continuum Active-injection Region of 2.9THz QCL
Cascade structure:

21. THz Quantum Cascade Laser
Section 1
Surface Plasmon Waveguide of 2.9 THz QCL
220µm
Active
Region
Metal 220µm
SI Substrate
12µm
(a)
Bottom
n+ layer
Metal
Contact
What is surface plasmon waveguide? Why this type of waveguide?
MPQ-Paris VII

22. THz Quantum Cascade Laser
Section 1
Characterization of 2.9 THz QCL V
QCL
Pyroelectric Detector A
THz Collimation
THz

23. THz Gain Measurement Zone Active Section 1 Ti : sapphire laser τ BS M2
ETHz
FSEOS S A B C D
Probe beam
Pump beam
Zone Active

24. THz Gain Measurement 2.9THz
Section 1
Amplified THz transmission by gain of quantum cascade laser
2.9THz
THz Gain at different injection current
THz Gain at different temperature

25. THz Gain Measurement Gain Clamping Section 1
Threshold increase with the temperature growing
The gain increase with the current when it below the threshold. However, the gain clampaged by the loss when the laser beyond the threshold.

26. Summary 1 Development of THz-TDS THz performance of ZnTe crystal.
Section 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

27. Section 2
Nonlinear Optical Properties of AgCl NCs doped Tellurite Glasses

28. Er+ Doped TeO2-Nb2O5-ZnO Glass
Introduction
Section 2
Photonic Glasses
Tellurite Glasses
Optical Switching
Optical limiting
Glass
Χ(3) ~ 1014esu
SiO2
2.4
46PbO–42Bi2O3–12Ga2O3 42
20Nb2O5–80TeO2 72
Er+ Doped TeO2-Nb2O5-ZnO Glass
------J. Lin et al. J. Non-Cryst. Solids 336 : 189–194 (2004)
------Y.Q. LI et al. J. Rare Earth 25 : 412 – 415 (2007)
Nanocrystals doped Tellurite Glasses

29. Fabrication Melting: 80TeO2:20Nb2O5 & 1%wt AgCl powder
Section 2
Melting:
80TeO2:20Nb2O5 & 1%wt AgCl powder
800°C / 15minutes.
Quenching:
Annealed at 300°C
Thermal treatment:
At 360°C
30min, 60min, 90min, 120min

30. Characterization Nanocrystals FESEM Image vs Termal treatment time
Section 2
Nanocrystals FESEM Image vs Termal treatment time
(a) 30min thermal treated
(b) 60min thermal treated

31. Characterization Nanocrystals FESEM Image vs Termal treatment time
Section 2
Nanocrystals FESEM Image vs Termal treatment time
(c) 90min thermal treated
(d) 120min thermal treated

32. Characterization Size distribution function vs thermal treatment time
Section 2
Size distribution function vs thermal treatment time
12nm / 30 min
17nm / 60 min
26nm / 90 min
35nm / 120 min

33. Characterization Cl- Ag+ Jahn-Teller effect : Lattice deformation
Section 2
Jahn-Teller effect : Lattice deformation
Cl-
Ag+
Reaction: 2Cl- →Cl2 + 2e- & 2Ag++2e-→2Ag
Cl- colour center
H. Vogelsang, Phys. Rev. B 61: (2000)

34. Characterization Urbach law: Eg Eg Samples Eg (eV) undoped 3.1 0 min
Section 2
Urbach law:
Trapped state
Samples
Eg (eV)
undoped
3.1
0 min
2.9
30 min
60 min
2.3
90 min
1.9
120 min
1.7 Eg Eg
Bandgap of glass matrix
Bandgap redshift of treated glass

35. Nonlinear Optical Properties
Section 2
lens
lens
Powermeter
50% BS
Attenutation
Sample
Powermeter

36. Nonlinear Optical Properties
Section 2
Lock-In PC
Ti :sapphire
Laser M1 M2 L S D Z
Open Aperture Z-scan
β: 1GW/cm ~~ 1.8 GW/cm

37. Nonlinear Optical Properties
Section 2
Lock-In M1 M2 M3 M4 M5 BS Δτ PC L S K1 K2
2K2-K1
2K1-K2 D
Ti :sapphire
Laser
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 14
90 min
1.9 11
120 min
1.7
9.5
1.6
DFWM experiment

38. Summary 2
Section 2
Nonlinear 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

39. Conclusion Section 1: Section 2:
Development of THz Time Domain Spectroscopy (THz-TDS)
Competition OR TPA FCA, Azimuthal dependence
Intervally scattering, Space charging screening, Electron mobility
Application of THz – TDS
Gain Measurement of 2.9THz QCL
Section 2:
Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Glasses
Fabrication
Optical limiting performance and Two-photon absorption
Enhancement of χ(3)

40. Acknowledgement THz group (LPA-ENS)
Advisor: Jérôme Tignon
Staff members: Sophie Hameau, Sukhdeep Dhillon et al.
Postdoc: Nathan Jukam;
Ph.D student: Dimitri Oustinov;
Master student: Julien Amijo, Geog Dürr;
Techniciens: Pascal Morfin, Phillipe Pace et al.
Collaborators: Carlo Sirtori et al.
Sun’s Group (East China Normal University)
Co-Advisor: Zhenrong Sun,
Staff members: Tianqing Jia, Xiaohua Yang et al.
Collaborators: Jian Lin, et al.