1. Generation and detection of ultrabroadband terahertz radiation
Y. C. Shen, et al., Appl. Phys. Lett. 85, 164 (2004)
Itoh laboratory Taisuke Katashima

2. Contents ・Introduction of THz wave ・Previous experiments ・Experiments
・Applications of THz wave
・Generation and detection of THz wave
・Previous experiments
・Experiments
・Experimental results
・Summary

3. What is a THz wave ? THz region:0.1~10THz 300µm ・Wavenumber 33cm-1
Microwaves
Visible
X-ray
γ-ray
Frequency(Hz)
103
106
109
1012
1015
1018
1021
zetta
killo
mega
giga
tera
peta
exa
Example
Radio
Radar
Optical
Medical
Astrophysics
・Frequency: 1THz=1012Hz
・Wavelength: 1THz
300µm
・Wavenumber
33cm-1

4. Applications of THz ・THz spectroscopy
・THz Time-domain Spectroscopy (TDS)
・THz imazing
・3D-rendered imaging
・Imaging of teeth, cancerous tissue and flames ・・・ etc.
・Security screening

5. We can get the amplitude and phase of electromagnetic wave
THz -TDS
splitter
mirror
We can get the amplitude and phase of electromagnetic wave
at the same time!!

6. The principle of THz-TDS
THz wave
At x=d
Refractive index
Sample x d
・Φはx=0の時の位相である。
Refractive index:
Extinction cofficient:

7. THz-imaging Imaging of teeth Security screening
Imaging of teeth
Security screening
THz imaging is safer than X-ray imaging

8. Generation and detection of THz wave
Two major elements to generate THz wave with using fs laser
・photoconductive antenna (PC antenna)
・nonlinear crystal(NL crystal)

9. Generation: Photoconductive antenna
fs laser pulse
THz wave
Photoconductive antenna
fs laser pulse is focused on the gap
Gap
Generation of carriers
Transient current
Electrode
Emission of THz wave

10. Detection:Photoconductive antenna
THz wave
fs laser pulse Ã
preamplifier
①fs laser pulse is focused on the gap
Lock-in amp
②Generation of carriers PC
③Carriers are accelerated by THz electric field
Carriers movement
Current signal of ETHz(t)

11. Experiments: previous study
Y. C. Shen, et al Appl., Phys. Lett. 83, (2003)
Generation: LT-GaAs photoconductive antenna
Detection: 20-µm-thick ZnTe crystal using electro-optic sampling
(EO sampling)
Frequency components over 30THz were observed!!

12. Experimental results:previous study
(1)Fourier-transform amplitude spectrum
・Over 30THz were observed!!
Low frequency components　were not observed!!
The dips at 5.2THz and the peaks at 6.2THz are TO and LO phonon resonance in ZnTe, respectively.
(TO =Transverse-optical, LO =Longitudinal-optical )
This spectral distribution up to 8THz is not ideal for spectroscopy.

13. （This complicated measurement of THz signals in previous study）
Purpose of this study
・Generate and detect ultrabroadband THz wave
To reduce the phonon resonances of ZnTe crystal
（This complicated measurement of THz signals in previous study）
Using LT-GaAs PC emitter and receiver instead of PC emitter and ZnTe receiver
・Apply this system for spectroscopy
Sample: Maltose pellet
Measuring method: detecting transmitted light

14. Experiments ・LT-GaAs PC antenna ・Samples Gap: 400μm Gap
0.53-mm-thick GaAs substrate
1.0μm-thick LT-GaAs layer
・Samples
Electrode
・Mixed maltose polycrystalline power with polyethylene powder pellet
(in a mass ratio 1:10)
・Thickness 1.3mm

15. Experimental set up à PC Lock-in amp sample delay stage
Parabolic mirror
Lock-in amp
PC antenna
PC antenna Ã
sample PC
Pump beam:400mW
Probe beam:30mW
Beam splitter
delay stage
fs pulsed Ti-sapphire
・15fs duration
: Vacuum-tight box
purged with dry nitrogen gas
・Repetition 76MHz
・Center wavelength 790nm

16. Experimental results ・Low frequency components were observed.
(a) Temporal THz wave form
They can’t be observed in previous study.
・Distinct dips at 5.2THz and peaks at 6.2THz were reduced!!
Distinct dips at 8.0THz and peaks at 8.7THz
are TO and LO phonon resonance in GaAs.
We can observe smooth spectral distribution up to 8THz!!

17. Experimental results (spectroscopy)
・Conventional THz-TDS・・・frequency range 0.3~3.0THz
・This system・・・・・・・・・・・frequency range 0.3~8.0THz
pure polyethylene
14 vibrational modes are observed
at room temperature
maltose/polyethylene
Corresponding to inter-
and intra-molecular interactions
maltose
Fourier-transform amplitude spectrum of pure polyethylene and maltose/polyethylene

18. Summary ・PC-generation/PC-detection scheme
leads to smooth spectral distribution up to 8THz.
・PC-detection provides about 8 times better signal-to-noise-ratio
than EO-detection up to frequencies in excess of 8THz.
PC-generation/PC-detection scheme is an ideal system
for THz-TDS in the frequency range THz.

19. Comparison with THz-TDS and other spectroscopy
Fourier-transform infrared spectroscopy:5vibrational modes
Raman spectroscopy:14vibrational modes
Neutron inelastic scattering spectroscopy:all vibrational modes can be observed
in principal
THz-TDS: 19 vibrational modes of cytidine were observed
in the frequency range 1-20THz

20. mary

21. Generation:Optical rectification of NL crystal
second-order non-linear optical effects
fs laser pulse:
THz wave
fs laser pulse:
Non-linear crystal
①fs laser pulses falls on the NL-crystal
②Nonlinear polarization generates
＊ω3= ω1-ω2
③Electromagnetic wave ω3 is emitted

22. Detection: Optical rectification of EO crystal
λ/4 plate
EO crystal
THz wave
:preamplifier
photo diode
beam splitter
:lock-in amp
:delay stage
:sampling pulse (fs-laser pulse) PC
・Electric field of THz induces anisotropy of refraction index in a crystal
sampling pulse proves change in intensity

23. Abstract
(b)
・Recent years terahertz science and technology have developed
・THz technology
Longitudinal-optical (LO)
phonon resonance in ZnTe

24. Experimental results:previous study
(1)Temporal THz waveform
Full width at half maxima is 40fs
It’s the shortest THz pulse
with using PC emitter
(2)Fourier-transform amplitude spectrum
Over 30THz are observed!!
・The dips at 5.2THz
TO phonon resonance in ZnTe
(TO =Transverse-optical)
・The peaks at 6.2THz
LO phonon resonance in ZnTe
(LO =Longitudinal-optical )

25. 3D-renderd image(two plastic cylinders)
Refractive index of each cross sectional slice
Optical image
Surface-rendered image

26. ・Carrier lifetime Emitter:0.1ps Reciever:10ps ・Electrodes
Emitter:NiCr/Au
Reciever:Ti/Pd/Au