Plasma Characterization Using a Silicon-Based Terahertz Frequency Comb Radiator

In this letter, temporal evolution of plasma density is characterized in the time-domain using a low-power THz frequency-comb radiator in the 130-nm SiGe BiCMOS process. The frequency-comb radiator covers from 10 s of GHz up to 1.1 THz where the spacing between tones is programmable and can be set by the frequency of an external trigger signal to as high as 10.5 GHz. Due to the complex permittivity of plasma, electromagnetic waves experience a phase and amplitude change as they pass through the plasma. The THz-pulse radiator used in this letter is locked to a low-phase noise external source. Hence, the phase-noise of the radiated tones is extremely low, which enables detecting small phase changes. In this measurement, phase and amplitude changes of THz waves are captured from 333 to 495 GHz for different plasma parameters by varying gas pressure, plasma pulse rate, and plasma excitation power.