## 1 Introduction
The expression for the change in bias current of the GDD due to incident MMW/THz radiation is given by [20]:  
\[\Delta I(t)=\frac{G\cdot q^{2}\cdot V\cdot n}{V_{i}\cdot m}\cdot\left(\frac{ \tau}{\tau_{i}}\right)\cdot\eta_{0}\cdot P_{D}\cdot\left(\frac{V}{V^{2}+w^{2}} \right)\cdot\left(1-\ e^{\frac{-t}{\tau}}\right) \tag{1}\]  
where \(G\approx\frac{exp(2\cdot\eta_{i}\cdot t_{d})}{2\cdot\eta_{i}\cdot t_{d}}\) is designated as internal signal amplification (calculations of \(G\) indicate that it can be on the order of a million), \(q\) is the electron charge, \(V\) is average electron velocity, \(n\) is the electron density, \(V_{i}\) is gas ionization potential, \(m\) is the electron mass, \(\tau\) is time response to create current changes, \(\tau_{i}\) is the time between ionization collisions of electrons with gas atoms, \(\eta_{0}\) is free space wave impedance, \(P_{D}\) is the incident MMW/THz  
Figure 1: Current-voltage (I-V) characteristics of direct current (DC) electrical discharges. radiation power density on the detector, \(v\) is the electron-neutral atom elastic collision frequency, \(\omega\) is the electromagnetic radiation frequency, \(t_{d}\) is the average electron drift time to the anode, and \(v_{i}\) is ionization collision frequency. The dominant MMW/THz detection mechanism inside the plasma was found to be enhanced cascade ionization rather than diffusion current [4; 20; 21; 22]. The enhanced cascade ionization mechanism increases the current while the diffusion current mechanism decreases it. The response time of the plasma inside the GDD is on the order of picoseconds. However, the electronic response time is limited by GDD's electrode parasitic impedance and the electronic circuit to about 1 \(\upmu\)s [16]. According to the glow discharge theory, the light emission intensity is directly linked to the detection of the MMW/THz radiation. GDD illumination is originated from the recombination between free electrons and positive ions which occur mostly around the negative glow area [20], which is adjacent to the cathode. This area is where the DC electric field and the total electron energy are at a maximum. Furthermore, when the frequency of the MMW radiation is equal to the inelastic collision rate, maximum response of the GDD is obtained (see Equation (1)) [20]. The radiation affects mostly the area around the cathode dark space, which is adjacent to the cathode. A new detection mechanism for MMW/THz radiation detection using GDD, named up-conversion method, was suggested [17]. The up-conversion method (optical detection) refers to the detection of changes in the emitted light from the GDD due to incident MMW/THz radiation, rather than the changes in its electrical current (electronic detection). In the up-conversion method, a basic photodiode or any basic light sensor is required in order to measure the changes in the GDD emitted light. The advantages of the up-conversion method are: (1) a very fast response time which is limited by the light sensor response, (2) a lower internal noise compared to the electric detection [17], and (3) it is very inexpensive. The spectral emission of typical neon lamp and its energy levels are shown in Figure 2[23]. Figure 2a shows a spectral measurement of a typical neon lamp carried out by ISF 125M, Bruker Inc. The measurement accuracy of the radiated lines is better than 1 GHz [23]. The radiated energy levels of Figure 2b are in excellent agreement with the experimental results of Figure 2a. In preliminary research aiming to investigate the influence of MMW radiation on the GDD emitted light spectrum, it was discovered that the strongest influence of the MMW/THz on the GDD emitted light spectrum is located in the NIR around 800 nm-1000 nm (see solid blue line in Figure 3). Furthermore, almost no influence of the MMW/THz around 500 nm-600 nm was noticeable.