THz Optoelectronics: Imaging

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