Propagation characteristics of terahertz electrical signals on micro-strip lines made of optically transparent conductors

We investigated the propagation properties of terahertz (THz) electrical pulses in micro-strip lines (MSLs) in which the signal line or the ground plane is made of indium-tin-oxide (ITO), which is a widely used transparent conductor. Finite-difference time-domain calculations showed that, in cases where ITO is used as the ground plane, an amplitude attenuation of as low as alpha similar to 0.35 mm(-1) (3 dB/mm) can be realized at 1 THz. It was also found that the use of the dc value for conductivity for ITO results in a slightly larger value for the attenuation. In experiments, although the conductivity of the ITO film was not sufficiently high, clear picosecond electrical pulses were observed after 2 mm of propagation with nearly the same waveform and an amplitude only 30% smaller than those in metal-based MSLs. These results suggest that ITO can be used as a ground plane conductor in millimeter-scale MSLs in the THz range.