Mirrors play key role in terahertz communications

Lauren I. Rugani

The potential of terahertz technology in communications applications is contingent upon the development of essential devices — such as sources and detectors — and supplementary components — such as filters, modulators and mirrors. Because of the high atmospheric and free-space path losses of terahertz radiation, communications systems will require high-directivity antennas. However, in practical applications, direct beam paths are often blocked by people or objects, creating the need for beam redirection via a mirror.

To optimize its function, a terahertz mirror should be highly reflective within a system’s frequency range for a wide range of incident angles, according to researchers at Technische Universität Braunschweig in Germany and at Rice University in Houston, who have developed such a device. The mirror, when strategically placed on walls inside a room, reflects beams from a terahertz transmitter. This allows for the signal to reach previously blocked areas.

The mirrors developed by the team consist of four 63-μm-thick layers of high-resistivity silicon sandwiched between five 150-μm-thick layers of polypropylene. While silicon reduces the physical flexibility of the mirror, the researchers believe that polymer layers with enhanced refractive indices will solve this problem.

Using a fiber-coupled terahertz time-domain spectrometer, the researchers measured transmission and reflection spectra for both s- and p-polarized waves. For normal incidence and a frequency range from 0.247 to 0.388 THz, the mirror reflected at least 95 percent of the incident power. The resulting 5 percent loss of radiation is far lower than that of most common building materials and, therefore, beneficial for communications.

As the angle of incidence increases, the frequency range of high reflectivity, called a reflection band, experiences a blueshift. If the width of the reflection band is sufficiently large compared with the magnitude of the blueshift, a band can be identified for both polarizations in which the mirror is highly reflective for all incident angles. The researchers have found this omnidirectional band to be between 0.319 and 0.375 THz, thus demonstrating the ability of the mirror to improve terahertz communications systems.

Applied Physics Letters, May 15, 2006, 202905.