## 2 Materials and Methods  
Figure 1 shows a schematic diagram of the THz detection system. The MMW/THz source is based on the MMW electron accelerator of Ariel University [16], emitting a 0.1 THz single pulse, 5 KW power, and 10 \(\upmu\)s width. The THz pulse was split by a power splitter. Transmitting a portion of the signal approximately 1000 times weaker than the input signal to the Schottky diode detector, i.e., for a 5 kW signal, roughly 5 W should reach the Schottky diode detector and the rest of the pulse power, transmitted by 2 mm tube to the free space. Two gold-coated parabolic mirrors were used to focus MMW/THz radiation on the GDD cross-section between electrodes. For an effective focus, it was necessary to simulate the MMW/THz radiation in the free space with portable source stuck to the tube that produced THz pulses; then, the GDD was placed in the focal point. A commercial green neon indicator lamp (from International Light Technologies, model N521) was used in these experiments as a GDD. Changes in GDD light were caused by the MMW/THz pulse incident on the GDD lamp, monitored by an optical detector and an electronic detector.  
Figure 1: Schematic diagram for 0.1 THz pulse detection.  
For an efficient comparison between the upconversion method of the GDD detectors and the Schottky diode detector for MMW/THz pulse detection, the MMW/THz pulse was detected by three methods: optical GDD detector, electronic GDD detector, and Schottky diode detector. The incident THz pulse increased gas ionization, which caused the current through the GDD to change. The optical detector for GDD detected the change in GDD illumination by a large-area balance detector (Thorlabs, model PDB210) that was available. The electronic detector method for the GDD consisted of a resistor, capacitor, amplifier (that could be employed to remove the noise and improve the output signal), and a 100 VDC power supply to break down plasma intergas. The used Schottky diode detector was a Millitech DXP-10 RPFW0 to utilize Schottky barrier zero-bias diodes. The maximal safe input to the detector is 16 dBm or 40 mW, explaining why a Schottky diode detector receives approximately 30 dB less than the signal power. The used oscilloscope was DSOX3104A from Keysight, with a bandwidth of 1 GHz and a max sample rate of 5 gigasamples (GSa)/s.  
In order to compare the frequency response of the Schottky diode and GDD detectors, the frequency chirp in the MMW/THz pulse was analyzed (Figure 2). So, a 0.11 THz pulse similar to the MMW/THz pulse of the accelerator was generated, which was performed by a local oscillator. Then, the MMW/THz pulse was mixed with a local oscillator signal. The difference between MMW/THz pulse and local oscillator signal was monitored by the oscilloscope.  
An H-plane T junction was used in the setup in order to split the sampled signal for simultaneous power and frequency measurements (see Figure 2). A continuous wave signal from the sweep oscillator was arbitrarily chosen to be 16.667 GHz. This was then transformed by the multiplier into a local oscillator signal of 100 GHz. To measure and record the detected signal, the output signal of the mixer and the three detectors, they were directly connected to an oscilloscope.  
Figure 2: Schematic of frequency chirp measurements.