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Terahertz waves tackle tomography

21 Mar 2003

A simple method for performing 3D imaging with terahertz waves is revealed.

Scientists in the US have found a way to perform high-resolution 3D tomographic imaging with terahertz waves without the need to rotate the sample or surround it with a detector array.

Terahertz imaging is currently attracting much attention for use in security scanners. It can reveal the presence of metallic objects beneath clothing and potentially detect explosives or biological weapons.

The single-projection imaging technique, outlined by S Wang and X C Zhang from Rensselaer Polytechnic Institute, US, in Applied Physics Letters 82 1821, uses broadband terahertz pulses and a so-called binary lens. It could prove useful for imaging objects that are either very large or in an awkward 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 just a few millimetres.

The method relies on the novel 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. By using the lens in conjunction with a multiple-frequency source of illumination and a CCD camera it is possible to obtain tomographic image slices of a 3D object.

In their 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," reported Wang and Zhang. "We found that the resolution in the image plane is about 1 mm, which is limited by diffraction."

Although the binary method is potentially easy to implement and requires less computer processing power than other tomographic imaging techniques it does have a drawback. Unlike other terahertz imaging methods it does yield spectroscopic information about the sample.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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