Osaka University	

MC5

Terahertz frequency-domain spectroscopy
referring to as terahertz frequency comb
(THz comb spectroscopy)

T.	 Yasui,	 Y.	 Kabetani,	 S.	 Yokoyama	 and	 Tsutomu	 Araki	 Grad.	 Sch.	 Engg.	 Sci.,	 Osaka	 Univ.,	 JapanOTST2007

Osaka University	

THz time-domain spectroscopy (THz-TDS)
F.T.

Temporal waveform
Resolution = inverse of temporal window

Fourier spectrum
Accuracy = precision of time delay

Spectral scaling based on
moving of mechanical stage

Stage-scanning THz-TDS(1) Trade-off between spectral
resolution and measurement time(2) Spectral accuracy depends on
positioning precision of stage

Osaka University	THz frequency comb
Nobel prize2005 in Physics•Spectroscopy•Freq. standard•Distance meter

Attractive features for high-precision THz spectroscopy
Accurate, stable, broadband selectivity, high spectral purity,
exact multiplication, and absolute frequency calibration

Frequency ruler
in THz region!

Osaka University	Present talkTHz frequency comb forhigh-precision THz spectroscopy(1) Generation of accurate THz comb
Use of ML-frequency-stabilized femtosecond laser
and photoconductive antenna for THz generation(2) Accurate reading of frequency scale of THz comb

Multi-frequency-heterodyning photoconductive detection
Ref. T.Yasui et al. “Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for
high-accuracy, high-resolution terahertz spectroscopy” APL 88, 241104 (2006)

Osaka University	

Laser source for THz comb spectroscopy

Asynchronous lock(260.000kHz)

100th-harmoniccomponent

External reference

f1=81,834,630,000Hz, f2=81,834,630,100HzΔ = f1-f2 = 100Hz

Osaka University	

Frequency stability of laser sourceRb freq. standard

Osaka University	

Multi-frequency-heterodyning photoconductive detection
Laser source

THz comb

f1f1

Pump laser (f1)
Stabilization

PCA emitter

Photocarrier comb

THz frequency

f2Rb freq. standard(Accuracy=10-11, stability=10-11)

Stabilization

Probe laser (f2)

PCA detector
f2

Δ	

3Δ	

2Δ	

Freq. downscaling (FDS)
factor =f1/Δ

Δ
4*106V/A@1MHzBWFDS =MFH PC detection

f1

RF comb= 1.221977 ×10-9
FDS Stability≈ δΔ = 2.5 ×10−70

Δ	

2Δ	

3Δ	

... .. . . . ... . .. .. .. . ... .

Replica of THz comb

nΔ	Radio frequencyOsaka University	Comparison of amplitude spectrum（Bowtie-PCA emitter and Bowtie-PCA detector）	

Measurement time=10sec
RBW=10kHzRe-scaling(×818346.30)

00.1

0.4
0.2
0.3
Frequency [THz]Stage-scanning
THz-TDS
Amplitude (a.u.)

Amplitude (a.u.)

THz comb spectroscopy

Measurement time=5min

0.5

THz amplitude spectrum is measured correctly！	

Osaka University	

Direct observation of THz comb mode
Mode-locked freq.
f1=81,834,630Hz
Measurement time
=0.5sec

f1

f1

f1

f1

RBW=10Hz

accuracy≒2.5*10-7（from stability of FDS）	

THz frequency scale with accuracy of 2.5*10-7
and resolution of 81.8MHz

Osaka University	

Expansion of spectral range
Modifications(1) THz detector⇒dipole PCA(2) FDS factor⇒16,366,926,000@Δ=5Hz(3) Current preamplifier⇒×5*107V/A@100kHzBW

Re-scaling

Spectral range is over 1THz !

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Summary
Comparison with conventional THz-TDS
Time delay

Unnecessary

Conventional
THz-TDS
Mechanical stage

FFT

UnnecessaryComputer

Measurement time

10sec5minSpectral resolution

Amplitude spectrum

81.8MHz
2.5*10-7(depend on FDSstability)
Possible

9GHz10-2(depend on positioning
precision of stage)
Possible

Phase spectrum

Impossible

PossibleTHz comb
spectroscopy

Spectral accuracy