High frequency resonant techniques for studying the complex electrodynamic response in solids

An overview is given on resonant cavity techniques utilized to study the electrodynamical properties of solids in the microwave and millimeter wave spectral range. The method combines three major advantages over other techniques: it is best developed for frequencies from 3 to 300 GHz, a spectral range hardly accessible by any other method; as a resonant technique it reaches a high sensitivity; and finally it allows for measuring both components of the complex response. Various configurations of microwave and millimeter wave cavities are presented and the advantages of different designs are discussed. A number of examples are presented in which the resonant cavity technique was successfully employed to investigate low energy excitations of solids.