Waveguide Has Been Developed for Terahertz Waves

Anne L. Fischer

Terahertz waves, or T-rays, fall between microwave and infrared radiation on the electromagnetic spectrum. Some metals and other electrical conductors are opaque to T-rays; however, as with x-rays, the radiation can penetrate vinyl, paper, plastic and glass, but, unlike x-rays, it is not hazardous. Also, it can reveal not only the shapes of objects, but also their chemical makeup.

Using T-rays for gathering spectroscopic information is useful, but it is not always a simple process. Other inspection technologies are compatible with waveguides, with which one can direct the energy. Terahertz waves were not, until scientists at Rice University in Houston developed a suitable waveguiding technology, enabling various applications. "There's a lot you can do in the terahertz range, but it's easier to do with a waveguide," said lead researcher Daniel M. Mittleman.

The terahertz waveguide beamsplitter works by dividing the propagating mode equally into two parts. The detector is shown at the end of one of the two arms.

Previously, terahertz waveguides borrowed from other technologies that were better-suited for either higher or lower frequencies, which then were scaled to size for the particular terahertz wavelength. For example, conventional metal waveguides for microwave radiation could be used, but metal isn't a perfect conductor. Although that effect is minimal with microwaves, the mechanism for attenuation of T-rays becomes very important, Mittleman said.

He described the university's terahertz waveguide as a "dipstick sensor" because it's exposed to whatever medium into which the guide is inserted. The energy travels down a wire, hits a mirror and sends back the spectroscopic information. Mittleman explained that the unique waveguide is like optical fiber for lasers in that both allow for energy to be directed where it's needed, even in hard-to-reach places. The waveguide also is similar to coaxial cable in the sense that there's a central metal wire that confines the waves near the surface.

"The difference is that, with coaxial, there's a metal shield, and we removed that outer shield to reduce ohmic losses and eliminate group velocity dispersion," he said.

The researchers have used different metals and have determined that the type doesn't matter much; all that is required is high conductivity. They've explored a range of diameters from 0.9 to about 6 mm and have found that size doesn't make much difference either.

In application, input coupling is a scattering process in which a free-space terahertz plane wave is scattered off a crossed metal structure. Some of the scattered radiation mode matches with the guided mode, which generates a propagating guided mode. "This is not an efficient way to do things, but it is enough to demonstrate the principle," Mittleman said.

Output coupling is simpler. The guided mode is allowed to propagate off the end of the wire into free space to a detector.

To demonstrate how the waveguide structure works with terahertz radiation, the researchers used it in an endoscope. Adding a waveguide to the terahertz tool kit could put the technology to work in places it has never been before, such as in package inspection, quality control and trace gas sensing. There is also talk of using T-rays for free-space communications. And it has found use in medical imaging, such as in the detection of skin cancer.

The researchers will explore the physics of propagation of the terahertz waves along the wires. Mittleman noted that there is a residual loss that they want to understand and control. They also plan to work on input coupling from waveguide to mode, which will involve a dramatic increase in the coupling efficiency.