Laser-Based System Sees Through Walls

Michael D. Wheeler

Anyone who has read the comics knows about Superman's powers, from leaping tall buildings in a single bound to peering through walls with x-ray vision. But seeing through walls may no longer be the stuff of cartoons if a group from Rensselaer Polytechnic Institute has its way.
The researchers have developed a laser-based system that emits wavelengths that can pass through materials to detect the presence of asbestos and radiation contamination on the other side. Eventually, it could replace core sampling and laboratory testing, which are time-consuming and costly.
Last year, the US Department of Energy awarded George Xu, an environmental engineering professor, a three-year contract for nearly $600,000 to explore technology that could be used to examine buildings built in the 1940s and 1950s for the production of nuclear weapons.

Teaming with gamma rays
Xu's area of expertise is in gamma ray spectroscopy, which can determine the depth of radiation contamination. But to test for asbestos, he needed a second technology.
He collaborated with physics professor Xi-Cheng Zhang, who had worked for several years on developing a system that emitted and detected terahertz pulses. Terahertz wavelengths occupy that portion of the spectrum between 30 µm and the centimeter range.
The sensing system uses a fiber laser manufactured in Ann Arbor, Mich., by IMRA America Inc. that produces 17 mW at 1550 nm. The laser's femtosecond pulses pass through a series of nonlinear optics and are converted into terahertz waves. These "T-rays," as they are commonly called, pass into the sample through materials such as brick or concrete. The reflected terahertz beam reveals the spectral "fingerprint" of the sample under observation. From preliminary testing, Zhiping Jiang, an associate in Zhang's group, has discovered a way to identify types of asbestos by their absorption spectra.
The initial success has sparked interest from several companies in commercializing the technology, including Z Omega and Molecular OptoElectronics Corp. in Latham and Watervliet, N.Y., respectively. Besides evaluating the presence of asbestos, Zhang said there are potentially many uses for T-rays, from medical applications to locating plastic explosives.