MARCH 24--Terahertz imaging is finding applications in security scanners. It can detect metallic objects beneath clothing and potentially detect explosives or biological weapons by deriving high-resolution 3-D tomographic imaging with terahertz waves without the need to rotate the sample or surround it with a detector array. A single-projection imaging technique, outlined by S. Wang and X. C. Zhang from Rensselaer Polytechnic Institute (www.rpi.edu) in Applied Physics Letters 8, p. 1821, uses broadband terahertz pulses and a binary lens. It could prove useful for imaging objects that are large or located in a difficult location. Experiments carried out in the 0.5- to 2-THz frequency range suggest that the technique can capture image slices with a depth resolution and image-plane resolution of a few millimeters.
The method relies on the focusing properties of a binary lens, which is a Fresnel zone plate that uses a series of concentric ring structures to focus light. Unlike an ordinary refractive lens, the focal length of a binary lens exhibits a linear wavelength dependence. Using the lens with a multiple-frequency lighting source and a CCD camera can produce tomographic image slices of a 3-D object.
In experiments, the Rensselaer team used a 30-mm-diameter silicon binary lens with a focal length of 2.5 cm at 1 THz. Wang and Zhang generated images of a stack of three acrylic plastic rectangles (60 x 45 x 2 mm), each containing a different shaped aperture. The masks were placed at 3, 7, and 14 cm from the lens, and images were captured on a CCD camera at frequencies of 0.75, 1.24, and 1.57 THz.
At each frequency, the binary lens imaged a pattern corresponding to a certain depth, while images from other patterns remained blurred. The resolution in the image plane is about 1 mm, which is limited by diffraction.
Although the binary method is easy to implement and requires less computer processing power than other tomographic imaging techniques, it does have a shortcoming. Unlike other terahertz-imaging methods it does yield spectroscopic information about the sample.
