Pulse propagation in optically controlled microstrip MIS slow wave structures

The RF-propagation characteristics of the Metal-Insulator-Semiconductor (MIS) type microstrip line, such as effective index (or slowing factor) and attenuation, show large variation with optically generated carrier concentration and frequency. For a typical MIS line on silicon substrate of dielectric constant 11.8 and thickness 190μm with an insulating (SiO₂) layer of thickness 0.3μm and dielectric constant 4.5, the effective index varies from 3.43 under no illumination to 53 under large illumination. Hence, the propagation of narrow pulses through such lines forms an interesting study. We studied the effect of illumination on the propagation of a Gaussian pulse of unit amplitude through such a 10mm line. The propagation delay can be varied from 0.1ns to a maximum value of 1.6ns by controlled optical illumination. Dispersion and attenuation result in pulse distortion and reduced amplitude with propagation. We also obtained analytical expressions for pulse delay and amplitude of a Gaussian pulse modulated on a carrier of frequency 1GHz for propagation in such a line where both effective index and attenuation are frequency dependant. The analysis concludes that a true time delay can be obtained at microwave frequencies by optical control of a MIS line on silicon.