1. ICABU 2013, Nov. 11-13, 2013, Daejeon, Korea
Time Resolved THz Spectroscopy and its Applications
Jaehun Park
Pohang Accelerator Laboratory Pohang University of Science and Technology
fs-THz
XFEL
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2. Outline Introduction THz Light sources  By femtosecond laser pulses
 Accelerator-based
Femtosecond THz Program at PAL
 Coherent Transition Radiation (CTR)
 Electro-Optic Sampling , etc.
THz Experiments
THz Future
Summary & Application
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3. THz wave 0.1 THz 10 ps 3000 m 3.3 cm-1 0.41meV
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0.1 THz 10 ps m cm meV
1 THz ps m cm meV
10 THz 0.1 ps m cm meV

4. Why is THz radiation important?
Energy range overlaps with
- band gaps of superconductors
- excitation energy for protein folding
- phonon energies
- Resonant with many vibrational and rotational modes in molecules
Imaging: complementary to X-ray imaging without the problems of ionizing radiation
Security and short range communications
Nonlinear optical experiments
Accelerator diagnostics

5. THz Light sources Table Top THz pulses  Photoconductive antenna
 Optical rectification: up to mJ
 Plasma
Accelerator based THz pulses : up to a few 100s of mJ/pulse or ~50 MV/cm-1
 Coherent Transition Radiation(CTR), >100 mJ, < 5 THz
 Synchrotron radiation
 Free Electron Laser
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6. fs-THz Program at PAL
 Purpose: Nonlinear and Ultrafast science using intense fs-THz radiation and laser beams.
 Construction:
 Commissioning and user service: 2010-
 Radiation Specifications
 Wavelength : 100 – 1000 m (0.3–3 THz)
 Energy/pulse : up to 10 J (LINAC Based),
~200 pJ (fs-Laser Amplifier Based)
 Pulse duration :< 200 fs
 Type:
1. Electron Linac based Coherent THz Radiation Source
 75 MeV / 100 fs electron beam
 Coherent Transition Radiation (CTR)
2. fs-Laser Amplifier Based THz source: Optical rectification
 Beamtime: 70% for Users, 30 % for Managers

7. Coherent Transition Radiation
OTR
Electron Beam q
OTR Radiator
45o
Transition radiation occurs when an electron crosses the boundary between two different media. For a relativistic electron (b º 1) incident on a perfect conductor, the number of photons emitted per solid angle and wavelength range is:
Intensity is 0 on axis, peaks at ~1/ .
Coherent radiation emission:
dWN /dw = N2 dW1 /dw | f (w)|2
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E= 50 MeV ->  = >  = 10 mrad
Charge
[nC]
Radiation energy [μJ]
0.2
3.5
0.5 22 1 88
Intensity Distribution

8. fs THz Laboratory LINAC OPA Laser Room Laboratory Lead blocks RF gun
QD1
AC2
QD2
QT1
Ti foil
EO Sampling
Tripler
Chicane 1
Chicane 2
PSD
OTR
CTR
THz radiation
266 nm, 1 kHz
LINAC
Stretcher QD
~1.4 mJ
~1 cm diameter
Analog PID
controller
Analog PID
controller
OPA
Pulse Compressor
Spectrometer
Regen. Amp.
Oscillator
Regen. Amplifier
3W, 120 fs, 1KHz
Laser Room
Laboratory

9. THz Energy Measurement
Linac
Lead blocks
Accelerating
Column
Quadruple
Magnet
RF gun
Al foil
Chicane
electron bunch
compressor
THz radiation
PSD
266 nm, >250 uJ
1 kHz, > 2 ps
Pyrocam III
(THz beam imaging detector)
50 MeV
~0.3 nC
10 Hz
~5 μJ 9

10. Single-shot Electro-Optic Detection
Shen et al. PRL 99, , 2007

11. Bunch Length Measurement

12. THz TDS(Time domain spectroscopy)
Not time resolved measurement
The measurements are made in the time domain.
Fourier transformed into the frequency domain.
- The far-IR frequency-dependent absorption coefficient α(ω) and refractive index n(ω), permittivity , susceptibility (t), etc.
(a) Real and (b) imaginary parts of the complex dielectric constants of liquid H2O, D2O, and H2 18O at 296 K. The absolute values of the dielectric constants of liquid D2O and H2 18O are shifted by 2 and 4, respectively. (Yada et al. Chemical Physics Letters 473, 279, 2009)

13. Wild type bacteriorhodopsin
THz Spectra h
all-trans-retinal cis-retinal
Wild type bacteriorhodopsin
Reflectivity spectrum of strongly boron doped diamond film, PRL 97, , 2006
Representative ε, n spectra of nucleobases via THz-TDS, Phys. Med. Biol. 47, 3807, 2002

14. fs-THz Time Domain Results
Semi-clathrate hydrates
Cluster
Kang et al. CPL 587, 14, 2013
Ahn et. al. RSC Advances 3, 1055, 2013
Koh et. al. RSC Advances 3, 8857, 2013

15. Wound Response in Mouse Skin
Kim et. al. submitted
Top: A schematic of the procedures for in vivo exposure and analysis of the effects of fs-THz radiation.
Bottom left: Experimental scheme. THz is generated via optical rectification method. Laser spec.: 150 fs pulse width, 1 kHz, 800 nm; THz spec: 0.26 nJ/pulse, 1 kHz, THz E field and spectrum is shown in the figure.
Bottom right: Delayed wound healing associated with fs-THz radiation. Gross and microscopic time course photographs of skin wounds following
treatment with sham or THz radiation. Upper panels: photographs are size-compensated and shown as binary images. Bottom panels: dotted lines
indicate the margins between re-epithelialized skin and the original wound site. An arrow denotes strongly stained parts of scab spread in wounds.
Kim et. al. Sci. Rep. 3, 2296, 2013

16. Phase Transition of Lipid bilayer
Temperature dependence of absorption coefficient (α) of de-ionized water (solid square), lipid solution (empty square), and lipid film (solid circle) at 1 THz frequency.
Choi et. al. JCP 137, , 2012

17. Pump- Probe Experiment

18. TRTS(Time resolved THz Spectroscopy)
Time resolved measurement
Need pump and probe beam
Probe the collective low-frequency solvent response in a liquid
Determine the transient photoconductivity in materials
The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene.
The measured change in the real part of the complex amplitude transmission (gray) and the theoretical fit (black) for pump pulse energy of 14.8 nJ
(George et al., Nano Lett., 8, 4248, 2008)

19. Pump dependent dynamics of nanowires
Si1-xGex NWs
Manuscript in preparation

20. Ultrafast Optical-Pump Terahertz-Probe Spectroscopy
of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene
Figure 1. Measured terahertz pulses transmitted through the epitaxial graphene sample B without (gray) and with (black, scaled) an optical pump pulse preceding the peak of the terahertz pulse by 1 ps.
Figure 2. (a) The measured change in the real part of the complex amplitude transmission (gray) and the theoretical fit (black) for pump pulse energy of 14.8 nJ (sample B).
George et al., Nano Lett., 8, 4248, 2008

21. Influence of the Electron-Cation Interaction on Electron Mobility in Dye-Sensitized ZnO and TiO2 Nanocrystals: A Study Using Ultrafast Terahertz Spectroscopy
Fig. (a) Evolution of transient THz conductivity (normalized to unity). The lines serve only to guide the eye. (b) Transient absorption of ZnTPP-Ipa/ZnO probed at 655 nm (symbols).
Nemec et al., PRL, 104, , 2010
Electron Mobility and Injection Dynamics in Mesoporous ZnO, SnO2, and TiO2 Films Used in Dye-Sensitized Solar Cells
Fig. Early and later time photoconductivity dynamics in nanoporous ZnO, TiO2, and SnO2 films sensitized with Z907 dye.
Tiwana et al., ACS Nano, 5, 5158, 2011

22. What can we do with intense fs-THz pulse?
Resonant and nonresonant control over matter and light by intense terahertz
Scheme for controlling matter using strong THz transients.
Resonant THz control over ionic lattice dynamics and molecular rotation.
Kampfrath et al., Nature Photon. 7, 680, 2013

23. What should we do with intense fs-THz pulse?
Intense terahertz pulse induced exciton generation in carbon nanotubes
Fig. Results for the near-IR-pump ( = 1.55 eV) ((a)–(c)) and THz-pump experiments ( ∼4 meV) ((d)–(f)).
Watanabe et al., Opt. Exp., 19, 1533, 2011

24. Soft Modes in Ferroelectrics & Perovskites (PbTiO3)
National Synchrotron Light Source

25. Future Plan-UV to THz
Widely tunable UV, visible, IR output from Optical Parametric Amplifiers (OPAs)
Mid-IR from high-energy OPA
Terahertz (THz)
- ZnTe
- LiNbO3

26. Future Plan-Synchronization
Fundamental Laser
Laser based THz
OPA Laser
LINAC based THz

27. fs-THz Beamline LINAC OPA Laser Room Laboratory Lead blocks RF gun AC1
QD1
AC2
QD2
QT1
Ti foil
EO Sampling
Tripler
Chicane 1
Chicane 2
PSD
OTR
CTR
THz radiation
266 nm, 1 kHz
LINAC
Regen. Amp. QD
~1.4 mJ
~1 cm diameter
Analog PID
controller
Analog PID
controller
OPA
Pulse Compressor
Spectrometer
Regen. Amp.
Oscillator
Regen. Amplifier
3W, 120 fs, 1KHz
Laser Room
Laboratory

28. THz & X-ray synch. via Time of Flight
J. Phys. B: At. Mol. Opt. Phys. 44 (2011)

29. Undulator Terahertz Radiation
E – field:
● Up to 10 THz
- Study phonon modes of materials
- Study intermolecular hydrogen bonding in solutions.
BaFe1.85Co0.15As2
Avigo et al., J. Phys.: Condens. Matter 25, , 2013
Khoury et al., Terahertz Science and Technology 3, 183, 2010

30. Summary & Applications
● Ultrafast dynamics
- electronic excitations
- THz driven magnetic dynamics
● Study Nonlinear properties of materials
- vibrational modes (local phonon modes)
- electronic modes
● Drive atoms in their local potential wells and probe motion as a function of time
● Medical applications
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31. Thank You for Your Attention!
Acknowledgements
Funding Support:
● Korean Ministry of Education, Science and Technology.
● National Research Foundation of Korea Grant funded by the Korean Government (grant code: )
● WCI-KAERI
Collaborators:
Dr. Heung-Sik Kang(PAL)
Mr. Junho Ko(Postech)
Mr. Seonghoon Jung(PAL)
Prof. Taiha Joo(Postech)
Thank You for Your Attention! 31