## 1 Introduction  
The lack of rigid, inexpensive, and easy-to-use room temperature MMW/THz sensors and pixels for focal plane arrays (FPAs) makes it difficult to develop some applications, especially those that require MMW/THz imaging systems. Currently, one of the main interests of MMW technology is the development of low-cost, fast, highly sensitive, compact, room-temperature sensors. This will allow applications in medicine, communications, homeland security, material science, and space technology [1, 2]. The common MMW/THz detectors are semiconductor MMW detectors, Schottky diodes, Golay cells, pyroelectric detectors, bolometers, and microbolometers [3, 4]. Semiconductor MMW detectors have high sensitivity and can operate at room temperature [1]. However, they are extremely sensitive to electrostatic discharge (ESD), very expensive and they will be affected and get damaged during instances of incidence by high MMW power. For example, the zero-biased detector (ZBD) WR10 ZBD of Virginia Diodes Inc. has a damage input power of 1 mW [5]. Schottky diodes have a fast response time and lower noise-equivalent power (NEP). However, they are not used for high-power THz pulses, as they are less sensitive at higher frequencies of the MMW/THz band, and require much more local oscillator power [6]. Previous studies had proved that weakly ionized plasma (WIP) in a glow discharge neon lamp can be more sensitive to MMW/THz radiations and has a shorter response time when compared to that of a Schottky diode [7]. In addition, GDDs can be used to detect higher power MMW radiation with both higher and faster response [8]. Hence, we used glow discharge detectors (GDD) as MMW/THz detectors. This will also help to reduce the complexity and cost of the detection system. The cost of individual MMW detectors can play a key role in MMW focal plane array imaging with large numbers of detectors. Fortunately, these GDD devices can run for only about 50 cents each. The GDD is orders of magnitude less expensive--50 cents compared to the price of other MMW detectors, which is at least hundreds of dollars. The glow discharge detector (GDD) was found to be one of the typical MMW/THz radiation detectors [3; 4; 6; 7; 8; 9; 10; 11; 12] and pixels in focal plane arrays [13; 14; 15; 16; 17; 18] but is orders of magnitude less expensive compared to other known MMW/THz sensors [1]. This is particularly important for detector arrays as in imaging. Two detection methods of the GDD were investigated: the first is the electrical current method [3; 4; 6; 7; 8; 9; 10; 11], in which the changes in the bias current of the GDD due to incident MMW/THz radiation were measured. The second is the optical up-conversion method [12; 18], in which the changes in the emitted light of the GDD due to the incident MMW/THz radiation were measured using a standard optical sensor. The GDD is a common plasma indicator lamp which can be used as an MMW/THz detector when connected to DC voltage and operating in the abnormal glow regime of the current-voltage (I-V) characteristics as can be seen in Figure 1[19]. Breakdown of the gas occurs at voltage \(V_{E}\) after the Townsend regime. The current in the plasma increases, entering the normal glow domain (point E to point F on the curve of Figure 1). Further increase of the current will bring us to the abnormal glow regime (point G to H on the curve of Figure 1) where the detection of MMW/THz radiation is taking place. The GDD's detection mechanism is based on a slight change in the bias current between the two electrodes of the lamp due to the electric field of the MMW/THz electromagnetic radiation being incident on the GDD.