1. Ultrabroadband terahertz generation using DAST single crystal
APPLIED PHYSICS LETTERS 97,
I.Katayama, R.Akai, M.Bito, H.Shimosato, K.Miyamoto, H,Ito and M.Ashida
Ashida lab.
Takuya Iwasa

2. Contents Introduction Motivation Experimental setup
Results & discussion
Summary
My work

3. THz Spectroscopy System Time Domain Spectroscopy（THz-TDS）
THz-TDS uses short pulses of broadband THz radiation.
The transmitted THz electric field is measured coherently, which provides both high sensitivity and time-resolved phase information. t
THz wave
Probe laser E
Time delay
THz-TDS pump probe system
Principle of THz-TDS

4. Electromagnetic wave
Wavelength
30mm mm μm μm μm nm
Submilimeter wave
Far infrared
Near infrared
Micro wave
optical
10GHz THz THz THz THz PHz
Frequency
broader
THz region
THz radiation is from 1011 to 1013Hz approximately.
If we generate broader radiation, we can measure from THz region to optical region in continuity.

5. THz emitter A 400-μm-thick DAST crystal was used as nonlinear crystal.
Difference frequency generation
A 400-μm-thick DAST crystal was used as nonlinear crystal.
A DAST crystal is an organic compound crystal and has large nonlinear optical coefficient.(非線形光学係数）
Nonlinear crystal
fs laser
THz wave
Fourier transform
Intensity
Intensity ω2 ω1 ω３
frequency
frequency
DAST crystal

6. THz detector Photoconductive antenna Current meter A
μ: mobility of electron
N: the number of carrier fs
laser
THz
wave

7. Motivation
To generate the terahertz wave using a DAST crystal by considering the refractive index dispersion.
To detect the THz wave generated by a DAST crystal using monochromator and THz-TDS.

8. Experimental setup
(Spectral Phase Interferometry for Direct Electric-field Reconstruction)

9. BaF2 (a positive dispersion medium)
pulse image
Positive chirping
Short wavelength
Positive dispersion medium
Long wavelength
Time lag
Group velocity dispersion (郡速度分散)

10. Results & discussion
The data of laser phase spectra observed using SPIDER.
BaF2 thickness
―　0mm
―　2mm
―　4mm
0mm-thick BaF2
2mm-thick BaF2
0mm-thick BaF2
Long wavelength
Short wavelength
Negative chirping
Positive chirping

11. Results & discussion
BaF2 thickness
―　0mm
―　2mm
―　4mm
The best of these data is the one of negative chirping.

12. Experimental setup Chirp mirror Ti:Sapphire laser BS Chopper
Pump pulse
BaF2 plate
DAST
400mm
Delay stage
Band-pass filter
PC antenna
Probe pulse
I/V amp
lock-in amp PC

13. Results & discussion
In this graph, very high frequency part is present.

14. Results & discussion
BaF2 thickness
―　0mm
―　2mm
―　4mm
The Fourier transform of the electric waveform. (dotted line)

15. Results & discussion DAST The energy is absorbed a lot. 0mm-thick BaF2
(Large positive dispersion)
Short wavelength
Short wavelength
Long wavelength
Long wavelength
The region of generating
High-frequency THz wave
The energy is absorbed a lot.

16. Summary
In the DAST crystal, the group velocity dispersion is important for the generation of the ultrahigh-frequency terahertz wave.
A waveform is detected by using photoconductive antenna.

17. My work
I try to measure a wave form by using this optical system when there is a sample in front of PC antenna.

19. Monochromator Concave mirror Diffraction grating Slit Slit
HgCdTe and InAs Detector
THz pulse

21. SPIDER法 (Spectral Phase Interferometry for Direct Electric-field Reconstruction)
Sum frequency
w (t)
I (t) W W
I (w) w
Intense strongly-chirped pulse from Ti: sapphire amplifier
Nonlinear Crystal t
I (t)
Interferogram
Replica2
Replica1
Pulse to be measured and its duplication
I (w)
SPIDER interferogram w