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Micromachine technique creates terahertz sensor

Posted: 14 Aug 2002 ?? ?Print Version ?Bookmark and Share

Keywords:photonic-crystal? silicon micromachined? optical component? sensor? compound-semiconductor electronic detector?

Photonic-crystal technology is being eyed by a European research consortium as a new route to a single-chip terahertz sensor. Silicon micromachined structures that reject photons in certain terahertz bands have been demonstrated in the project, which aims to use the effect to fashion optical components for terahertz radiation.

The initial work came out of a collaborative project managed by the European Space Agency. Now a new initiative called Star Tiger, which is being directed out of England's Rutherford Appleton Laboratory, will attempt to fashion sensor arrays using the silicon photonic-bandgap material. The program is designed to accelerate the emergence of practical, low-cost terahertz sensors by tapping leading experts in materials and radiation research.

Terahertz radiation, which occupies a region of the spectrum between infrared and radar, has recently become interesting for a variety of applications including medical-imaging projects, airport security systems, astronomy, and earth-viewing satellites. A number of laboratories - including Lucent Technologies Bell Labs, Rensselaer Polytechnic Institute, Leeds University in the U.K., and Delft University in the Netherlands - have been working on schemes to generate and detect terahertz radiation.

Expensive sources

Current terahertz sources and detectors are expensive and bulky due to laser-based sources and exotic compound-semiconductor electronic detectors. For medical imaging, both a source and detector are required. For satellite or astronomical observations, the object to be observed is the source of the terahertz radiation, but detection is harder due to the weaker signal.

Progress is being made in reducing the complexity of terahertz equipment. The field started with large free-electron lasers as sources and helium-cooled detectors, but subsequent research has turned up more efficient methods. For example, a femtosecond laser pulse striking a nonlinear optical element will generate terahertz electromagnetic radiation.

Terahertz-sensing equipment made the headlines after the 9/11 terrorist attacks in the U.S., since the radiation is ideal for detecting knives or guns hidden in clothing. Unfortunately, the technique is too effective since it also reveals the body in detail, raising privacy concerns. However, its ability to look inside the body to image soft tissue would be a boon to medical diagnostics.

As in other areas of photonic-sensor design, materials that act as good radiation reflectors are essential for gathering faint signals and concentrating them on detectors. Silicon micromachined structures can act as mirrors that totally reflect terahertz radiation, which tends to penetrate most materials.

Array construction

In the Star Tiger project an array of silicon bars is stacked so that each layer is at right angles to the previous layer. The regular array has a periodic structure in which the alternation of the refractive index of silicon and air generate interference patterns that totally block the transmission of radiation at a specific optical "bandgap."

A pixel sensor on an array was constructed with a micromachined funnel made of the periodic structure, which gathers a larger area of radiation and focuses it on detecting electronics. Behind the electronic layer is a flat silicon periodic layer that acts as a mirror, so that the captured radiation does not escape through the back of the imager.

The technological challenge is to build complex silicon structures at dimensions comparable with the wavelength of the terahertz radiation, which ranges from 1mm to 105m. The first demonstrations of photonic-bandgap materials were done at much longer wavelengths, allowing periodic structures to be assembled from macroscopic components.

The researchers have been able to fashion a single-pixel structure using a stacking technique. A silicon wafer is thinned and parallel grooves are carved in both sides, creating a double array of parallel bars. A number of these can then be stacked to create a periodic silicon structure.

The Star Tiger researchers expect that 32-pixel sensors on a single chip might be feasible. They plan to use beam-steering techniques to scan an imaging area, effectively multiplying the number of pixels in a terahertz image.

- Chappell Brown

EE Times

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