Submillimeter Quasioptical Spectroscopy of Multilayer Conducting and Superconducting Systems

We demonstrate the efficiency of using interference effects in multilayer structures for a quantitative determination of submillimeter (submm) electrodynamic characteristics of materials using quasioptical spectroscopy based on backward-wave oscillators (BWOs). We present the results of studying fundamental physical properties of the conducting and superconducting objects of several types. Strong absorption in iron-based superconductors at “sub-gap” frequencies has been detected. In the temperature behavior of submm conductivity in LaNiO3/LaAlO3 heterostructures we found a metal–insulator phase transition that is observed in the case of thin (two-unit-cell) LaNiO3 layers and is absent in the case of thicker (four-unit-cell) LaNiO3 layers. In Ge/Si heterostructures with Ge quantum dots (QDs) we discovered the submm conductivity that is strongly enhanced compared with the conductivity of the structures containing the same amount of germanium, but not organized into a QD array.