Electromagnetic radiation with wavelength between 10 μm and 1 cm-the key to solid state physics?

Summary form only given. The electromagnetic spectrum ranging from near IR to microwave frequency is of special importance to physics: the radiation energy corresponds to 100 meV to 0.1 meV, respectively, and covers most of the transition energies between orbital and spin states of quasi-free electrons in solids as well as collective excitations both of the electrons and the host lattice. A review of selected experimental methods including magneto spectroscopy is given emphasizing optical resonance spectroscopy in contrast to non-resonant methods