University of Tokushima	

Osaka University	

THz time-domain spectroscopy①	 F.T.

Temporal waveform	 
Spectral resolution=inverse of time window	 

Fourier spectra	 
Spectral accuracy= Precision of time delay	 Marking of frequency scale is basedon mechanical movement of stage	

Pump-probe experiment•Trade-off between spectral
resolution and measurement time•Spectral accuracy depends onpositioning precision of stage	 University of Tokushima	

Osaka University	

②	 
Asynchronous-optical-sampling THz-TDS(AOS-TH-TDS)

Overlap timing between THz andprobe pulse is automatically shiftedevery pulse	 ref) T. Yasui, Appl. Phys. Lett. 87, 061101 (2005).

f1

Δ=f1 -f2

THzdetector

f2Oscilloscope

No need for mechanical stage
No limitation for size of timewindow

Time scale of ps THz pulse islinearly expanded to µs order	

High spectral resolution and accuracy can beachieved depending on laser stability	 University of Tokushima	

Osaka University	

Conversion of frequency scale byASOPS-THz-TDS	
µs ASOPS pulse	
ps THz pulse	
f1×T
ΔfASOPS(down-scaling)	

ΔfTHz spectrum	

f1This factor is fluctuatedby timing jitter of twoF.T.	
lasers.
€RF ASOPS spectrum	f1Δf(～THz region)

fTHz

€

Calibration(up-scaling)	

fASOPS③	 University of Tokushima	

Osaka University	

Influence of timing jitter in absorption spectroscopy	
④	 
Conventional THz-TDS

integration	

spectral resolution＝inverse of time window	
Presenttalk

ASOPS-THz-TDS

integration	

spectral resolution≥inverse of time window	

Evaluate actual resolution and accuracy in ASOPSTHz-TDS using low-pressure gas spectroscopy

University of Tokushima	

Osaka University	

Experimental setup⑤	 

λc=1550nm, Δτ=50fs, Pavg =100mW,
f1=56.124MHz, f2= 56.124MHz+5Hz, Δ= f2-f1= 5Hz, Timing jitter<300fs

Dipole-shapedDipole-shaped

Scan rate = 5Hz

Osaka University	

Singlesweep(200ms)

200sweeps(100s)

University of Tokushima	

Basic performance⑥	 

Osaka University	

University of Tokushima	

Low-pressure gas cell	

L=300m, φ=20mm
Min. Pressure=0.04kPa⑦	 

University of Tokushima	

Osaka University	Absorption lines of water vapor	
Amplitude (a.u.)	

Amplitude (a.u.)

250x10⑧	 -6

110→101

200150100211→202

50
0
0.0

0.5

1.0

1.5

2.0

2.5

Frequency (THz)	
Frequency [THz]Transition	

Absolute frequency(NASA database)	

Pressure broadeningat 1 kPa1)	

110→101211→2020.556 936 0 THz
0.752 033 2 THz	

30.1 MHz	29.7 MHz	1) T. Seta, et al., J. Quantitative Spectroscopy & Radiative Transfer 109, 144-150 (2008).

Osaka University	

University of Tokushima	

Evaluation of spectral resolution 	
Time window =17.9 ns (= one pulse period)
Inverse of time window = 56 MHz

Actual spectral resolution ≥ inverse of time window
Decline of spectral resolution by timing jitter is small	⑨	 

University of Tokushima	

Osaka University	

Frequency deviation from NASA database	
ASOPS-THz-TDS	

Deviation
1.6 GHz	

6000
4000
2000
00.550

0.555

0.560

0.565

Transmittance	NASA database (110→101)
Transmittance (a.u.)Transmittance (a.u.)

Amplitude (a.u.)	

Conventional THz-TDS
8000

0.570

1.2

NASA database (110→101)

1.00.8

Deviation
39 MHz	

0.6
0.4
0.550

0.555

Deviation
1.1 GHz	

4000
3000
2000
1000
00.740

0.745

0.750
Frequency (THz)

0.755

0.760

Transmittance	NASA database (211→202)
5000

0.560

0.565

0.570

Frequency (THz)

Transmittance (a.u.)Transmittance (a.u.)

Amplitude (a.u.)	

Frequency (THz)6000⑩	 1.2

NASA database (211→202)

1.1
1.0
0.9
0.8

Deviation
29 MHz	

0.7
0.6
0.740

0.745

0.750
Frequency (THz)

0.755

0.760

University of Tokushima	Osaka University	

Spectroscopy of acetonitrile gas	-6

Gas pressure
0.04 kPa

1210

Interstellar molecule	

864

Important in the fieldof radio astronomy	

200.0

0.5

1.0

1.5

2.0

Frequency (THz)

1.2Transmittance (a.u.)

Amplitude (a.u.)

14x10

⑪	 

1.0

Groups of absorption lines
periodically appearedat an interval of 20 GHz

2.5

0.8
0.6
0.4
0.2
0.0
0.60

20GHz	
0.62

0.64

0.66

Frequency (THz)

0.68

0.70

University of Tokushima	

Osaka University	

Hyperfine structure of absorption lines
in acetonitrile molecules	
1.0Transmittance

0.80.60.4
0.2
0.0
0.64200.64250.6430Frequency (THz)

0.6435⑫	 University of Tokushima	

Osaka University	

Evaluation of spectral accuracy
using absorption lines of acetonitrile 	
Experimental results[THz]
0.642 675 924 00.642 827 458 80.642 973 381 20.643 080 016 70.643 158 590 40.643 231 551 60.643 276 450 8NASA database	 
[THz]
0.642 670 849 70.642 829 579 30.642 963 997 80.643 074 051 00.643 159 694 20.643 220 892 80.643 269 866 7

Mean accuracy = 10-5⑬	 

Deviation[MHz]5.07432.12059.38345.96571.103810.65886.5841

University of Tokushima	

Osaka University	Discussion on spectral accuracy	

⑭	 

Achieved accuracy=10-5 >> Rb atomic clock=10-12

f1Fluctuated by laser instability
fTHz= fASOPSΔfFrequency
fluctuation	

f1 €several 10s

ΔfmHz

Mean
frequency	

Instability	

56 MHz10-12	5 Hz	

10-5	Limit the spectral accuracy

University of Tokushima	

Osaka University	

⑮	 

Future plan: Analysis of atmospheric gas molecules	 Many absorption line of molecular gases are concentrated in THz region	 -2	Log (1/T)	-6	-10	-14	0	

1	Frequency (THz)	

2	3	To identify each molecular gases correctly,
high accuracy, high resolution, and broadband are required.	 

Osaka University	

University of Tokushima	

Conclusions	⑮	 (1) Evaluation of actual spectral resolution andaccuracy in ASOPS-THz-TDS using lowpressure gas spectroscopy• resolution = 72.9 MHz, accuracy = 10-5	
(2) Decline of spectral resolution caused by timing
jitter is small.
(3) Spectroscopy of acetonitrile molecular gas• periodical groups of absorption lines at every20 GHz• hyperfine structure of individual absorption lines