Terahertz remote-sensing applications require high sensitivity detectors and high-power terahertz sources. The reason for this is that terahertz signals can be quickly attenuated by water molecules present in the target, as well as in the environment between the source and the target. Of the many terahertz source technologies available, the one based on the optical parametric generation technique seems to be the most promising, as it is portable and can produce a relatively high-power terahertz beam. At present we are developing a terahertz source based on an optical parametric technique. We have used a Nd:YAG Q-switched laser as the pump source, and a LiNbO3 crystal as the optical parametric medium. With LiNbO3 our generator could produce a terahertz beam over the frequency range of 100 GHz through 3 THz. The output power was highly dependent on the properties of the material. For terahertz detection we have used a Si-bolometer or an electro-optic (EO) detector, which was specifically developed to detect CW terahertz signals. In addition to the EO sensor, we are presently developing a new detector based on a quantum-dot structure, whose noise equivalent power (NEP) is expected to be about 10-21 W/(Hz)1/2. This is several orders of magnitude better than the sensitivity of our bolometer (10-13 W/(Hz)1/2) at 4.2 K, or our EO detector (10-12 W/(Hz)1/2) at room temperature.
We review the current status of terahertz technologies, especially terahertz beam generators and detectors, which include those being developed in our laboratory. Of the many promising terahertz technologies, only a few may currently be suitable for military applications in the field, which require a high-power terahertz output and a high sensitivity in detection. In addition to these requirements, terahertz imaging for military applications demands focal-plane-arrayed detectors. Presently we are developing a terahertz source based on a modified difference-frequency technique, in which we employ an optical parametric method. We used a Nd:YAG Q-switched laser as a pump source, and LiNbO3 and GaSe crystals as the optical parametric medium. The phase matching condition is achieved by means of our new electro-optical tuning technique. Our preliminary experiments indicate that the new tuning technique enhances the terahertz-beam output power. Although the output power is currently unstable, our source can produce terahertz peak power as high as a few watts. For terahertz detection we use a Si-bolometer or an electro-optic (EO) detector. In addition to these sensors we are presently developing a new detector based on a quantum dot structure. The sensitivity of the quantum dot detector is expected to be about 10-21 W/(Hz)1/2 in terms of the noise equivalent power (NEP); this is orders of magnitude better than the sensitivity of our bolometer (10-13 W/(Hz)1/2) at 4.2 K or our EO detector (10-12 W/(Hz)1/2) at room temperature.