would be the square of this, or 6.25 percent at 600 GHz, because the attenuation would affect the illuminating radiation from either the environment or the active illuminator.

It has also become commonplace to ascribe to millimeter-wavelength/terahertz systems the ability to image objects through walls and buildings. Figure 2-5 shows measurements made by OSU of the transmission through common building materials of signals from 100 GHz to 600 GHz. Some measurements made with millimeter-wave imagers at 95 GHz have been published, but these are through drywall or dry plywood which, as can be seen in Figure 2-5, have little effect. For materials of a structural nature such as oak or pine, the attenuation is severe.

FIGURE 2-5 Measurements made with millimeter-wavelength/terahertz imaging systems of transmission through various building materials. SOURCE: Data courtesy of Ohio State University.

Terahertz time domain spectroscopy (TTDS) is a new technique that has offered the promise of detection and identification of concealed explosives. The system uses a short-pulse laser and a pair of specially designed transducers as transmitters and receivers. By gating these transducers with ultrafast optical pulses, one can generate subpicosecond bursts of electromagnetic radiation, and subsequently detect them with high signal to noise using gated detectors. These transients consist of only one or two cycles of the electromagnetic field, and they consequently span a very broad bandwidth. Bandwidths extending from 100 GHz to 2 or 3 THz are routine, although the power generated is concentrated more in the lower frequencies of the emission band. Although the average intensity of the radiation is quite low, the high spatial coherence produces a brightness that exceeds that of conventional thermal sources. The gated detection is