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The last research frontier in high-frequency electronics is in the terahertz (or submillimeter-wave) region, between microwaves and the infrared (i.e., 0.3–15 THz). While the terahertz frequency region offers many technical advantages (e.g., wider bandwidth, improved spatial resolution, compactness), the solid state electronics capability within that frequency region has been very limited from a basic signal source and systems perspective (i.e., < milliwatts). This limited development is mainly due to two fundamental factors. First, extremely challenging engineering problems exist in this region where component size is on the order of λ. Second, applications of this shorter-wavelength microwave region have been restricted, so far, to a few specialized fields (e.g., molecular spectroscopy). On the lower-frequency side, electronic devices reach an upper frequency limit of several hundred gigahertz due to transient times and parasitic RC time constants. On the higher-frequency side, photonic devices such as interband laser diodes can only be used at approximately 10 THz. The other important component of a system working in the far-infrared region (FIR) regime, besides the FIR source, is the FIR detector. In the field of FIR detectors, specifically those based on resonant tunneling in quantum well heterostructures, a great deal of research has been conducted and some promising results have been published. Today, increasingly more important applications of terahertz technology are rapidly emerging that are relevant to civilian and military applications. For example, at frequencies above 300 GHz, the strong absorption of electromagnetic energy by atmospheric molecules makes any communication link impossible to achieve. On the other hand, this same fundamental interaction mechanism allows terahertz electronics to be a very promising tool for the identification and interrogation of chemical and biological (CB) agents. Recent developments in microwave remote-sensing techniques and submillimeter-wave heterodyne radiometric systems have led to the use of limb sounders to study the upper atmosphere of Earth [1]. The Antarctic ozone hole discovered in 1985 [2], and its effect in shielding life from solar ultraviolet radiation, shows that it is necessary to monitor the upper atmosphere in order to detect the change of the atmospheric ozone layer, which is being depleted systematically by pollution [3]. There is an urgent need to include local oscillators and detectors for radiometers at 2.5 THz in new satellites, for more accurate monitoring of ozone depletion and tropospheric chemistry in general.
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