The amplitudes of terahertz radiation are measured for a series of GaAs surface intrinsic-n<sup>+</sup> (SIN<sup>+</sup>) structures with
various built-in surface electric fields as the bias. As the surface field is lower than the so-called "critical electric field"
related with the energy difference between the &Ggr; to L valley of the semiconductor, the amplitude is proportional to the
product of the surface field and the number of photo-excited carriers. As the intensity of surface field exceeds the critical
field, the THz amplitude is independent of the surface field but proportional the number of the photo-excited carriers.
Our study proposed two optimal conditions for an SIN<sup>+</sup> structure to serve as a THz emitter: the width of its intrinsic layer
is nearly equal to the penetration depth of the pump beam, and the intensity of built-in electric field is nearly equal to the
critical electric field. Notably, the critical field determined from the THz amplitude under various electric fields provides
one way to estimate the &Ggr; to L valley splitting in semiconductors.
A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.