Design of gap structure used in photo conductive switches in CST

For an optically switched transmission line, can anyone suggest me for an optimum gap dimension or perhaps any specific design to start with, in order to construct a gap structure which is basically a Si switch, activates (conducts) during illumination and remains open in dark conditions (in CST). And also if someone has already been able to demonstrate this in CST, pls also let me know how to optimize the transmission line circuit? [The effect of light can be taken into consideration by changing the effective conductivity of the Si switch]