Generation and detection of ultrabroadband terahertz radiation

1
Generation and detection of ultrabroadband
terahertz radiation
Y. C. Shen, et al., Appl. Phys. Lett. 85, 164
(2004)
Itoh laboratory Taisuke Katashima
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Contents
Introduction of THz wave
Applications of THz wave
Generation and detection of THz wave
Previous experiments
Experiments
Experimental results
Summary
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What is a THz wave ?
THz region0.110THz
?-ray
X-ray
Visible
Microwaves
Frequency(Hz)
103
106
109
1018
1012
1015
1021
zetta
peta
exa
killo
mega
giga
tera
Example
Radio
Radar
Optical
Medical
Astrophysics
Frequency 1THz1012Hz
Wavelength 1THz
300µm
33cm-1
Wavenumber
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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
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THz -TDS
splitter
mirror
We can get the amplitude and phase of
electromagnetic wave at the same time!!
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The principle of THz-TDS
THz wave
At xd
Refractive index
Sample
x
d
0
Extinction cofficient
Refractive index
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THz-imaging
http//www.agri.tohoku.ac.jp/thz/jp/41_h15.htm
http//www.teraview.com/home_index.htm
Security screening
Imaging of teeth
THz imaging is safer than X-ray imaging
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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)
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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
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DetectionPhotoconductive antenna
THz wave
fs laser pulse
Ã
preamplifier
?fs laser pulse is focused on the gap
Lock-in amp
?Generation of carriers
PC
?Carriers are accelerated by THz electric field
Current signal of ETHz(t)
Carriers movement
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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!!
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Experimental resultsprevious 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.
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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
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Experiments
LT-GaAs PC antenna
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 110)
Thickness 1.3mm
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Experimental set up
Parabolic mirror
Lock-in amp
PC antenna
PC antenna
Ã
sample
PC
Pump beam400mW
Probe beam30mW
Beam splitter
delay stage
fs pulsed Ti-sapphire
15fs duration
Vacuum-tight box purged with dry nitrogen gas
Repetition 76MHz
Center wavelength 790nm
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Experimental results
Low frequency components were observed.
They cant 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!!
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Experimental results (spectroscopy)
Conventional THz-TDSfrequency range 0.33.0THz
This systemfrequency range 0.38.0THz
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
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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
0.3-8.0THz.
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Comparison with THz-TDS and other spectroscopy
Fourier-transform infrared spectroscopy5vibration
al modes
Raman spectroscopy14vibrational modes
Neutron inelastic scattering spectroscopyall
vibrational modes can be observed


in principal
THz-TDS 19 vibrational modes of cytidine were
observed in the frequency range 1-20THz
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mary
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GenerationOptical rectification of NL crystal
second-order non-linear optical effects
Optical rectification
fs laser pulse
THz wave
fs laser pulse
Non-linear crystal
?fs laser pulses falls on the NL-crystal
?Nonlinear polarization generates
?3 ?1-?2
?Electromagnetic wave ?3 is emitted
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Detection Optical rectification of EO crystal
?/4 plate
EO crystal
THz wave
preamplifier
photo diode
beam splitter
lock-in amp
delay stage
PC
sampling pulse (fs-laser pulse)
Electric field of THz induces anisotropy of
refraction index in a crystal
sampling pulse proves change in intensity
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Abstract
(b)
Recent years terahertz science and technology
have developed
THz technology
Longitudinal-optical (LO) phonon resonance in
ZnTe
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Experimental resultsprevious study
(1)Temporal THz waveform
Full width at half maxima is 40fs
Its 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 )
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3D-renderd image(two plastic cylinders)
Refractive index of each cross sectional slice
Optical image
Surface-rendered image
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Reciever10ps
Emitter0.1ps
Carrier lifetime
Electrodes
RecieverTi/Pd/Au
EmitterNiCr/Au