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Terahertz multi-spectral focal plane imaging
Yan Zhang, Xinke Wang, Ye Cui, and Wenfeng Sun
Beijing Key Lab for Terahertz Spectroscopy and Imaging,
Key Laboratory of Terahertz Optoelectronics, Ministry of Education
Department of Physics, Capital Normal University, Beijing, 100037 China
yzhang@mail.cnu.edu.cn
ABSTRACT
Terahertz radiation is a special electromagnetic spectrum range sandwiched between the infrared ray and microwave.
With developing of super fast and super power laser technology, this special range has been explored extensively.
Among all terahertz technologies, terahertz imaging may be the first one can be used in the practical applications. In this
presentation, a novel terahertz multi-spectral focal plane imaging method is proposed. Using a CCD camera instead of
the single detector, this method can capture the time domain waveform of an object quickly. Using the fast Fourier
transform, both the amplitude and phase of the THz spectrum can be achieved. Based on this information, the layer
structure inside the object can be presented. The multi-spectral phase imaging technology has also been employed to get
high signal noise ration.
Keywords: Terahertz, focal-plane imaging, multi-spectral
1. INTRODUCTION
Terahertz (THz) radiation is usually defined as the electromagnetic wave with frequency from 0.1THz to 10THz, which
is sandwiched between the infrared ray and microwave in the electromagnetic spectrum. It was demonstrated that many
substances including explosives, illegal drugs, and biological molecules have special absorption spectrum in this range
[1-4]. These substances can be easily identified by using the absorption lines. THz imaging technology is expected to be
the first technology which can be used in the practical applications based on the THz spectrum technology. Many
technologies have been proposed to achieve multi-spectral THz images [5-8], thus the substances can be identified
two-dimensionally. However, most of these imaging methods were achieved using the raster scanning method by
which the THz beam is focused on the sample surface and the image is achieved point by point. Although the raster
scanning method can present higher signal-to-noise rate (SNR), it spends too much time to obtain the image. Therefore,
this method is limited for practical applications.
The THz focal-plane tomography imaging may be a good candidate for real applications [9-12]. In this approach, both
THz and probe beams are expended. The target is illuminated by the THz quasi-plane wave and the reflected THz beam
which brings the information of the object is detected by a lager size crystal. The expanded probe beam is overlapped by
the THz beam in the sensor crystal, so the information of the whole sample has been transferred to the probe beam. Then
the wave front of the probe beam is captured by a CCD camera. This method can get a two-dimensional (2D) THz image
of the sample in once measurement and the quality of the image can also be improved by a dynamic subtraction
technique
13
and balance detection technology. The experimental time can be effectively reduced since multi-points of the
MIPPR 2009: Multispectral Image Acquisition and Processing, edited by
Jayaram K. Udupa, Nong Sang, Laszlo G. Nyul, Hengqing Tong, Proc. of SPIE Vol. 7494
749408 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.833258
Proc. of SPIE Vol. 7494 749408-1
object are imaged simultaneously. By adjusting the delay line, the object can be imaged layer by layer. The tomographic
image of the object can be obtained. After Fourier transform of the waveform in time domain at each point, the spectrum
of each point can be obtained, thus this can be used to identify the sample [13].
In this presentation, a THz multi-spectral focal plane imaging technology is proposed. Images of a metallic cross
concealed behind a high resistivity silicon (Si) wafer are acquired by using a THz pulse focal plane tomography imaging
system. The layer structure inside the object can be clearly seen. Using the reflected pulses at each interface of the
sample, the thickness of each layer of the sample can also be accurately achieved.
2. EXPERIMENTAL SETUP
The optical system is schematically illustrated in Fig. 1. The laser used is a Spectra Physics Spitfire amplifier with 1 kHz
repetition rate, 50fs pulse duration, 920mW output power, and 10mm spot diameter. The THz wave is generated by a
2.5mm thick ZnTe crystal pumped by the femtosecond pulse. Then the THz wave is expanded to 40mm. The sample is
placed close to a golden plated metallic mirror and illuminated by the THz wave with 15
0
incident angle. A parabolic
mirror with 101.6mm focal length is used as an imaging lens. The object distance is 320mm and the image distance is
150mm. Therefore, the THz image obtained is a reduced inverted real image. The sensor crystal is 10*10*1mm
3
and the
probe beam has not been expanded. The probe beam co-propagates directly with the THz wave through the sensor
crystal. The information of the amplitude of the THz has been transferred to the polarization of the problem beam. After
a polarization analyzer, a CY-DB1300A CCD (1030 by 1300 pixels, ChongQing ChuangYu Optoelectronics Technology
Company) is used to capture the intensity distribution of the probe beam, which expresses the amplitude of the THz. By
scanning the delay line, the amplitude of the THz at different time can be obtained, and the time domain transient at each
point can be constructed. Only 1024 by 1024 pixels of the images are extracted and combined to 256 by 256 pixels for
simplifying data processing.
Laser
HWP
BS
Pump
Probe
Chopper
Delay line
ZnTe
PM1
(101.6mm)
Gold Plated
Metallic Mirror
THz pulse
PM2
(101.6mm)
ZnTe
P1
P2
L1(25mm)
ITO
L2
(100mm)
CCD
Sample
320mm
95mm
55mm
Fig. 1 Schematic of THz multi-spectral focus-plane imaging system.
Proc. of SPIE Vol. 7494 749408-2
3. RESULTS ANALYSIS
The first sample used is an aluminum cross concealed by a high resistivity Si wafer, as shown in the Fig. 2. The
thickness of the Si wafer is approximately 0.4mm and its diameter is 50mm. The cross is put between the Si wafer and
metallic mirror. The cross is 2.5mm width, 25mm length, and 0.3mm thick. The THz wave can easily pass through the Si
wafer but can not pass through the metallic cross. In order to calculating the thickness of each layer, we have also
measured the refractive index of each layer of the sample in the THz frequency range in advance. The medium between
the Si wafer and the cross and between the cross and the metallic mirror are air, the refractive index of these two layers is
1.0. The refractive index of the Si wafer is measured by a standard THz-TDS system. Its refractive index is
approximately 3.26 at 0.2 to 2.0 THz, which is consistent with the reported literature well [14].
Si
Metallic
Cross
25mm
2.5mm
Fig. 2 Sketch map of the sample, a metallic cross hidden by a 0.4mm high resistivity Si wafer.
The THz transients of the reference signal (averaged all over the pixels), reflected signals from each interface of the
sample, including the front surface and the back surface of the Si wafer, the cross, and the metallic mirror, are shown in
Fig. 3. Each transient is normalized for convenient observation. As shown in Fig. 3, the phase of the THz pulse reflected
from the front surface of the Si wafer does not change; however, the phase of the THz pulse reflected from the back
surface of the Si wafer has been changed
π
, because that the THz pulse propagates form the medium with high refractive
index to medium with low refractive index. Both the phases of reflected THz pulses from the cross and the metallic
mirror do not change because there is only one interface. The Si wafer is put at the front of the metallic mirror, so the
reflected pulses from the Si wafer are at ahead of the reference signal. Nevertheless, the reflected THz pulses from the
cross and the metallic mirror pass through the Si wafer twice, thus these pulses are behind of the reference signal. The
relative distances between these reflected pulses indicate the optical thickness of each layer of the sample, so these
thicknesses can be accurately calculated. Some small oscillations can also be found after the main peaks reflected by the
metallic mirror, this is caused by the multiple reflection between the Si wafer and metallic mirror. Using these transients,
the THz pulse reflective focal-plane tomography can be constructed. And the object can be imaged layer by layer. Two
dimensional (2D) transverse images of the object at three layers are shown in Fig. 4. Fig. 4 (a) presents the 2D image of
the Si wafer front surface, which is Gaussian distribution since the transverse mode of the incident THz wave is
Gaussian. Fig. 4 (b) presents the 2D image of the cross, which is a bright cross since the THz pulses are reflected at the
surface of the cross and pass though the air at other positions. Fig. 4 (c) presents the 2D image of the metallic mirror,
which is a dark cross since THz pulses cannot pass through the metallic cross. Therefore, it can be drawn that the interior
configuration of the sample can be acquired using the reflected THz pulses from each layer.
Proc. of SPIE Vol. 7494 749408-3
0 5 10 15 20 25 30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
The reflected pulse
from the mirror
Amplitude (a.u.)
Time Delay (ps)
(a)
(b)
(c)
The reflected pulse
from the cross
The reflected pulses
from the Si wafer
Fig. 3 Time domain waveforms of the signal (a) of the reference (average over all pixels), (b) reflected from the cross, and
(c) reflected from the metallic mirror.
X (pix e l s )
Y (pixels)
a
(a)
50 100 150 200 250
50
100
150
200
250
X (pix e l s )
Y (pixels)
(b)
50 100 150 200 250
50
100
150
200
250
X (pix e l s )
Y (pixels)
(c)
50 100 150 200 250
50
100
150
200
250
Fig. 4 2D transverse distribution images of (a) the Si wafer front surface, (b) the cross, and (c) the metallic mirror.
The longitudinal information of tomographic images can also be presented by the THz pulse reflective focal-plane
tomography. The spatial-temporal image of the sample in the y-t plane is drawn in Fig. 5. It is obtained by extracting the
200th row of each image recorded with different delay time. The first bright line around
3tps=
corresponds to the
THz pulses reflected from the front surface of the Si wafer. The dark line around
12tps
=
is the THz pulses reflected
from the back surfaces of the Si wafer. The bright lines around
18tps
=
and
20tps
=
are caused by the THz pulses
reflected from the cross and the metallic mirror. It should be noted that there are also reflected THz pulses from the
metallic mirror behind the cross. Because both the width of the cross and the wave length of the THz wave are in
millimeter range, the diffraction of THz wave may cause the appearance of these pulses. Furthermore, there are also
some horizontal fringes in Fig. 5, which is caused by the non-uniformity of the sensor crystal.
Using the reflected pulses in the Fig. 3 and Fig. 5, the thickness of each layer of the sample can be calculated. The
thickness can be expressed as:
Proc. of SPIE Vol. 7494 749408-4
2
tc
d
n
Δ
×
= (1)
where
d
is the thickness of each layer,
tΔ
is the time delay between two adjacent THz pulses,
c
is the speed of light
in vacuum,
n
is refractive index of the corresponding layer. The thicknesses of each layer in the sample are shown in the
Table 1.
The real thickness of the Si wafer is 0.4mm and the measured value is 0.39mm, the calculation error is only 2.5%. Since
the metallic cross reflects the THz wave totally, it is quite difficult to measure the thickness of the cross. However, the
distance between the cross and the metallic mirror is approximately equal to the thickness of the cross and can be used to
express the thickness of the cross. The real thickness of the cross is 0.3mm and the calculated distance between the cross
and metallic mirror is 0.34mm, the corresponding error is 13%. This is due to the cross is not so close to the metallic
mirror, so the measured value is a littler larger than the thickness of the cross. It can be drawn that the thickness of each
layer of the sample can be accurately measured by using the THz refractive pulse focal-plane imaging system.
Time Delay (ps)
Y (pixels)
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
50
100
150
200
250
Fig. 5 Spatial-temporal distribution of reflected THz pulses from sample. Each 2D image is extracted the two hundredth row
of pixels to make up of the image.
Table 1 Calculated result of the thickness of each layer of sample.
(
)
tpsΔ
n
()
dmm
Thickness of the Si wafer 8.48 3.26 0.39
Distance between the Si wafer and cross 5.61 1 0.84
Distance between the cross and metallic mirror 2.27 1 0.34
Proc. of SPIE Vol. 7494 749408-5
Fig. 6 THz images of the lens cover. Left, optical image. Middle, single spectrum image. Right, multi-spectral image.
The second sample used is a cover of Nikon camera. After getting the time domain transients, the Fourier transform is
used to get information in frequency domain. Both amplitude and phase at each frequency can be obtained. Using one
frequency, the image of the sample is shown in the middle of Fig. 6. The image is quite blurry, and the character of the
logo can not be distinguished. However, if more frequencies are used, for example, three frequencies, the quality of the
image can be obviously improved. As shown in the right of Fig. 6, the letters of the logo can be clearly seen.
4. CONCLUSION
In conclusion, a THz multi-spectral focal-plane imaging system is build up and the images of a layered sample are
achieved. The experimental results demonstrate that this novel technology can be used to tomographic image of the
sample with multiple layers. Meanwhile, the thickness of each layer of the sample can be accurately calculated using
reflected pulses from each interface. Multi-spectral imaging can present more information and get better images.
ACKNOWLEDGEMENTS
This work was supported by the National Basic Research Program of China (grants 2006CB302901 and
2007CB310408), the National Natural Science Foundation of China (NNSFC) (grants 10604042 and 10674038), the
Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction
of Beijing Municipality, and the Science and Technology Program of Beijing Educational Committee (grant
KM200810028008).
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