Which metals are suitable to make structures in GHz and THz applications? In GHz applications, Cu is mainly used to make structures and in THz applications Al or Au is used to make structure. Is there a special reason to use these metals in particular wavelengths?
 A: It is because of the resistivity of the material. It has two units, ohm-m and seconds. The seconds unit tells us the amount of time the electrical signal takes to reach the other point. Basically, if I apply an electric field on $\rm Cu,$ you may be knowing that inside a conductor, no electric field is present, but that only happens after the electrons displace to create an equal and opposite electric field thereby making the net field inside zero. This time is in orders of $10^{-18}$ seconds and for $\rm GHz$ range, a feedback cycle of maximum $10^{-9}$ can be established, thus the $\rm GHz$ range for $\rm Cu.$ The resistivity in seconds of Au is also in $10^{-18}$ but, with a higher magnitude, around $9\times 10^{-18}~\mathrm s.$ Thus, is more suitable for $\mathrm{THz}$ applications. Check out some data sheets online for detailed study and accurate figures. 
A: As far as I know Al is only used in the electroforming process for waveguides, horns and other "plumbing."  You machine the aluminum on the outside, then coat it with copper. The aluminum is eaten away by acid and you end up with copper.  Gold is only used to coat the copper to keep it from oxidizing. Electron discharge milling directly on copper is also done.
