Osaka University

17A-2

Real-time Two-dimensional
Terahertz Tomography
Takeshi Yasui, Takashi Yasuda, Ken-ichi Sawanaka,
Emmanuel Abraham and Tsutomu Araki

Graduate School of Engineering Science,
Osaka University, Japan

Osaka UniversityColleague

Prof. Araki(Osaka Univ.)

Dr. Abraham(Bordeaux Univ.)

Mr. Yasuda(Osaka Univ.)

Mr. Sawanaka(Osaka Univ.)

Financial Support
Strategic Information and Communications R&D Promotion Programmefrom the Ministry of Internal Affairs and Communications (MIC) of Japan
Industrial Technology Research Grant Program 2004 from the New Energy andIndustrial Technology Development Organization (NEDO) of Japan	Osaka UniversityOutline(1) Background(2) Single-shot measurement of THz temporal
waveform(3) Real-time 2D spatiotemporal THz imaging
for THz tomography(4) THz tomographic movie of paint film
(5) Real-time THz-TDS imaging(6) Summary

Osaka University(1) BackgroundOsaka University

THz tomography
Nondestructive inspection
alternative to “invasive” X-ray and “contact”
ultrasound
Floppy Disk

D.M.Mittleman et al. Opt. Lett. , Vol.22, p904(1997)

Pharmaceutical coating

A. J. Fitzgerald et al. J. Pharmaceutical Sci. ,Vol.94, pp. 177 (2005).

Osaka University

THz paintmeter (1)Multi-layer paint film

~painting thickness~
Paint-off film

thickness precision = 4µm, thickness resolution=40µm, transverse resolution = 1.7mm
T.Yasui et.al., “A terahertz paintmeter for non-contact monitoring of thickness and drying
progress in paint film,” Appl.Opt., Vol. 44 (32), pp. 6489-6850 (2005)

Osaka University

THz paintmeter (2)THz absorption spectra@THz-TDS at transmittance configuration~drying degree~

Temporal change of THz echo
signal before and after painting(Sample thickness = 900µm)

Absorption (a.u.)

Spectral change accompanying
wet-to-dry transformationT.Yasui et.al., “A terahertz paintmeter for
non-contact monitoring of thickness anddrying progress in paint film,” Appl.Opt.,
Vol. 44 (32), pp. 6489-6850 (2005)

Attenuation,
delay anddistortion ofTHz echo

Osaka University

Conventional THz tomography(Point-to-point measurement)
2D tomographic image
Mechanical scanning stages
・time delay
・sample position

Long acquisition time
・A few minutes@temporal waveform
・Several hours @2D image

Limited to stationary objects

Motivation

Osaka University

To extend THz tomography to moving objects,
real-time image acquisition is strongly required !

Omit any mechanical stages
for time delay and sample position!

Single-shot measurement of THz temporal waveform
and its 1D transverse imaging

Osaka University(2) Single-shot measurement ofTHz temporal waveformOsaka University

2D Free-Space EO-Sampling (2D-FSEOS)
Collinear configurationof THz beam and probe beam
2D-FSEOS

Transverse profile of THz beam
EO Pockels effectTransverse profile of probe beam

Real-time 2D THz transmittance image

For 2D THz tomography
Mechanical time-delay scanning is still required!
Long measurement timeOsaka University

Single-shot measurement of THz temporal waveformbased on 2D Free-Space EO-Sampling (2D-FSEOS)
Principle

Method(1)Time-to-wavelength conversion

Chirped probe pulse
Multi-channel spectrometer(2)

Time-to-space conversion(3)ref) Z. Jiang et al, APL, Vol.73, pp. 1945 (1998).

ref) Z. Jiang et al, APL, Vol.24, pp. 1245 (1999).

Time-to-space conversionref) J. Shan et al, OL, Vol.25, pp. 426 (2000).

Long probe pulse
Streak camera

Non-collinear 2D-FSEOS
1D photodiode array

Because of simple configuration without any expensive apparatuses,
the method (3) is suitable for single-shot measurement and its 1Dtransverse imaging (real-time 2D spatiotemporal THz imaging).

Osaka University

Non-collinear 2D-FSEOSref) J. Shan et al, OL, Vol.25, pp. 426(2000)

Non-collinear configurationof THz beam and probe beam

Wavefronts in two beams are noncollinearly overlapped in EO crystal.

Osaka University

Propagation of THz and probe beams in EO crystal
Two different wavefronts of THz
beam overlap with a wavefront ofprobe beam on different transverse
position of probe beam.

vTHzcosθvTHz

vopt=vTHzcosθ	vopt

Overlapping of each wavefrontkeeps the same transverse position
during propagation in EO crystal.

ΔhTime delay Δτ is converted
into spatial distribution Δh.
Δh tan θΔτ =c

Osaka University

Non-collinear 2D-FSEOSref) J. Shan et al, OL, Vol.25, pp. 426(2000)

EO time-to-space conversion
modulates spatial distribution ofprobe beam depending on
temporal waveform of THz pulse.

Temporal waveform can be measured
as a position signal at real time.

Osaka University

Time window and maximum probing depth
Timewindowd ⋅ tanθ
ΔT =cMaximumProbing depthcΔT
MPD =2n gd: diameter of probe pulse
θ: crossed angle of two beams
in airc: velocity of light in vacuum
ng: group refractive index

Osaka University(3) Real-time 2D spatiotemporal THz
imaging for THz tomography

Osaka University

Experimental setup of real-time 2D THz tomography
100fs, 800nm, 1kHz
1mJ/pulse(4mm thickness)

2D detector(temporal axis)

Line focusingof THz beamon sample(spatial axis)

2D spatiotemporal THz image
for X-Z cross-section image of sample(2D THz tomography)

Thermoelectriccooling
640×480 pixels
10 frame/sec

Osaka UniversityPerformance of present systemOsaka UniversityFeatures(1) Slight modification of usual 2D-FSEOS•Collinear→Non-collinear•Line focusing of THz beam on sample(2) Single-shot measurement•Frame rate up to a laser repetition rate is possible•Suppress effect of pulse-to-pulse fluctuation in amplified fs pulse•Improve signal-to-noise ratio(3) Applicable to moving objects•On-line products (e.g., industrial products on moving stage)•Human bodyOsaka University

2D spatiotemporal THz movieof metal mirror surface (9ps×5mm)(Subtracted CCD image from BG noise image)500
Electric field (a.u.)

10 frame/sec
θ=28º, d=5mm

3D distribution-200

Temporal waveform
0.5 ps
SNR=80Time increment = 14 fs/pixel

Osaka University(4) THz tomographic movie ofpaint film

Osaka University

Real-time 2D THz tomography of moving paint film
~ in-process THz paintmeter for moving object~・Sample : half-paint film on metal plate(thickness=175µm@contact-type thickness meter)・Continuous moving of a paint sample(moving speed = 5 mm/s)

Osaka UniversityTHz tomographic movie ofmoving half-paint film
10 frame/sec
θ =20º, d=5mmPosition(mm)5Snapshot

Unpainted area

Blurryedge

Boundary00Time(ps)6

Painted area

162 ± 21 µm@ 1~4 mmng=2.14First demonstration of real-time THz tomography of movingobject
Boundaryデータをとる？	 

Osaka University

Real-time 2D THz tomography of wet paint film
~ in-process THz paintmeter for monitoring of drying progress ~・Sample : quick-drying paint film on Al plate

Paint film

Painting!

Paint film

Drying & Measurement!

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THz tomographic movie of drying progress
Quick movie (20min)

Temporal change of THz echo

6ps

5mm

PaintingsurfaceGeometrical
thickness

Painting/plateboundary
Opticalthickness

Geometrical and opticalshrinking of paint film throughwet-to-dry transformation

Osaka University(5) Real-time THz-TDS imaging

Osaka University

Experimental setup of real-time THz-TDS imaging

Thermoelectric cooling
640×480 pixels
10 frame/sec

Osaka University

2D spatiotemporal THz image (15ps*19mm)
for real-time THz-TDS imaging
without sample
10frame/sec

Fouriertransform

THz-TDS line image

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Real-time THz-TDS line image
Amplitude image

without sample@ 100ms

Phase image

Image size: 1THz × 19mm
Frequency resolution = 67GHz @ time window = 15 ps
Maximum frequency range = 43 THz @ time increment = 23 fs/pixel

Osaka UniversitySummary
Real-time 2D THz tomography for moving object

Moving half-paint film

2D spatiotemporal THz image

Drying progress in wet paint