University of Tokushima	

Osaka University	

①	

Sensitive measurement of water content
in dry material using low-frequency
terahertz time-domain spectroscopy
system equipped with micro-structured
photoconductive antennas
Takeshi Yasui1,2 and Tsutomu Araki2
1Inst. Tech. Sci., Univ. Tokushima, Japan
2Grad. Sch. Engg. Sci., Osaka Univ., Japan

1st EOS Topical Meeting on Micro- and Nano-Optoelectronic
Systems @ BremenUniversity of Tokushima	

Osaka University	

What is THz electromagnetic pulse ?

②	University of Tokushima	

Osaka University	Introduction③	University of Tokushima	

Osaka University	

Experimental setupΔτ=80fs, λc=808nm,
P=300mW, f=87 MHz

Major modification
Use of bow-tie shape antenna as a THz emitter and detector
to achieve sensitivity in the low-frequency THz region less than 0.3 THz④	University of Tokushima	

Osaka University	

Basic performance
Water content measurement

Fourier
Transform

THz Time-Domain Spectroscopy(THz-TDS)⑤	University of Tokushima	

Osaka University	

Transmittance spectrum of pure water⑥	0.07THz

THz wave over 0.1 THz is disappeared by broadband,strong absorption of water.
THz absorption at 0.07 THz is used for quantification of water content.

University of Tokushima	

Osaka University	Relationship between THz absorption
and amount of waterGood correlation between water thickness and THz absorption at 0.07 THz.
Amount of water content is calibrated from THz absorption
using this relationship.

⑦	

University of Tokushima	

Osaka University	

Application (1)⑧	Drying progress of wet tissue paper

To evaluate performance of THz-TDS for water content
measurement
Comparison with an weight method using an electronic balanceUniversity of Tokushima	

Osaka University	

Drying progress of wet tissue paper
Temporal change⑨	Correlation

Good correlation between the two methods indicates good
performance of water content measurement using THz-TDS.

University of Tokushima	

Osaka University	

Application (2)Water content in adhesive for wood

⑩	Osaka University	

University of Tokushima	

Water content in adhesive for wood (1)THz absorption spectra

2Absorption (a.u.)

Sample⑪	

Before hardening
10

After hardening
00.07THz

0.5

Frequency (THz)
Difference of THz absorption spectra before andafter hardening of the adhesive is obvious.

1University of Tokushima	

Osaka University	

Water content in adhesive for wood (2)⑫	Absorption at 0.07 THz (a.u.)	

Temporal change of THz absorption
1	0.8	

Evaporation of water
115mg/cm2

0.6	

0.4	0.2	0	0	Forced heating	with hair drier	Completion of	hardening	10	 20	 30	 40	 50	 60	 70	 80	Time (min)	

Gradual decrease of THz absorption indicates slow evaporation of water,andhence slow hardening of adhesive.
Temporal change of THz absorption is stopped at 70 min, which implies
completion of adhesive hardening

University of Tokushima	

Osaka University	

Application (3)Water content in instant coffee powder⑬	University of Tokushima	

Osaka University	

Water content in instant coffee powder (1)⑭	THz transmittance spectra
Empty glass bottle strongly
absorbs THz wave > 0.1 THz
because of water in the glass.
THz wave < 0.1THz is not
significantly absorbed andscattered by coffee powder.

Low-frequency THz-TDS
is available for watercontent measurement.

Osaka University	

University of Tokushima	

Water content in instant coffee powder (2)⑮	Water content in the bottled coffee powder increased by forced humidifying

THz transmittance spectra of bottled coffee powder
25

Amplitude (a.u.)

Before forced humidifying
20

Water content=7.5mg/cm3
Detection limit=1.0mg/cm3

15
10After forced humidifying for3.5h (>90%RH)

5000.07THz

0.1

Frequency (THz)

0.2

University of Tokushima	

Osaka University	

Summary⑯	

(1) Sensitive measurement of water content in dried materials
using low-frequency THz-TDS(2) Basic performance>High correlation with a electronic-balance weight method>Detection limit = 1mg/cm3 at coffee powder measurement(3) Application>Hardening monitoring of adhesive for wood
>Quality control of instant coffee powder in a glass bottle
Acknowledgements
Industrial Technology Research Grant Program 2004 from NEDO of Japan
the Nestle Science Promotion Committee