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Overview:
 One
major research interest of Xi-Cheng Zhang, Terahertz (THz)
Research Center director, is THz wave imaging. The attraction
of THz imaging lies in its ability to produce phase-sensitive
spectroscopic images, which holds the potential for target
identification or "functional imaging." THz systems
are ideal for imaging dry dielectric substances including
paper, plastics and ceramics. These materials are relatively
non-absorbing in this frequency range, yet different materials
may be easily discriminated on the basis of their refractive
index, which is extracted from the THz phase information.
Many such materials are opaque at optical frequencies and
provide very low contrast for X-rays. THz imaging systems
may therefore find important niche applications in security
screening and manufacturing quality control.
Technical Description:
Zhang is exploring one of the most
advantageous applications of free-space electro-optic detection:
two-dimensional (2-D) terahertz wave imaging where THz radiation
is imaged on a <110> oriented ZnTe crystal. An optical
readout beam with a diameter larger than that of the THz beam
probes the electric field distribution within the crystal
via the Pockels effect. The 2-D THz field distribution in
the sensor crystal is converted into a 2-D optical intensity
distribution after the readout beam passes through a crossed
polarizer (analyzer). The optical image is then recorded using
a linear diode array or a digital CCD camera. The benefit
of a system like this is its capability for noninvasive imaging
of moving objects, turbulent flows, or explosions.
Electro-optic imaging makes it possible
to see THz wave images of electric fields, diseased tissue,
the chemical composition of plants, and much more that is
undetectable by other imaging systems. The real-time monitoring
of the THz field supports real-time diagnostic techniques.
T-ray tomographic imaging – including
computed tomography (T-ray CT), diffractive tomography, and
tomography with binary lenses – is a new tomographic
imaging modality being explored by Zhang. It allows pulsed
terahertz radiation to probe the dielectric properties of
three-dimensional (3D) structures and provides sectional images
of objects in an analogous manner to conventional computed
tomography techniques such as X-ray CT. T-ray CT systems directly
measure the transmitted amplitude and phase of broadband THz
pulses at multiple projection angles. This allows a wealth
of information to be extracted from the target object including
both its 3D structure and its frequency-dependent far-infrared
optical properties.
Contact Information:
Xi-Cheng Zhang
Director, Center for Terahertz Research
Professor, Department of Physics, Applied Physics and Astronomy
Professor, Department of Electrical, Computer and System Engineering
Erik Jonsson Chair Professor
of Science
Rensselaer Polytechnic Institute
110 8th Street
Troy, NY 12180-3590 USA
(518) 276-3079
zhangxc@rpi.edu
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