Controlling the electromagnetic waves that lie in the region between light waves and millimeter waves    What kind of technology is it? Our goal is to develop technologies for generating, modulating, transmitting, and detecting terahertz (THz) electromagnetic waves, which lie in the frequency region between 0.1 and 10 THz. So far, the electromagnetic waves in this region have been little exploited, and no powerful terahertz signal generators or sensitive detectors have been developed. By exploiting advances in ultra-fast optoelectronic technology, which is expanding the capacity of the communication channels in fiber-optic networks, we have made great progress in developing a compact portable terahertz wave generator / detector.      What are the features? Figure 1 shows the relationship between frequency and wavelength as regards electromagnetic waves. Terahertz waves occupy the region between light waves and millimeter waves, and share some of the useful properties of both, namely the high spatial resolution of light waves and the ability of millimeter waves to penetrate a wide variety of non-metallic materials. Many molecules also have characteristic absorption lines that can be used for material identification.        What can it do? A compact portable terahertz electromagnetic wave generator / detector offers us a way of detecting things that are hidden from direct observation. For example, this capability might be used at disaster sites to determine if victims are present, to verify the state and extent of the disaster, and to detect from a remote distance whether toxic gas is present. Such a system might also be used for security checking purposes at airports and train stations, and for detecting impurities in food.