Exploring solids with far infrared radiation

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Abstract
The development of refined experimental techniques in far infrared spectroscopy has resulted in the application of studies in this spectral region to a wide range of solid-state problems. Experimental improvements include the widespread use of interferometric or Fourier techniques, better detectors, and the introduction of far infrared lasers. These are briefly discussed as a prelude to a selection of recent experimental results reflecting various solid-state properties. The vibrational modes of pure and doped crystals have received considerable study, and here attention is focused on the properties of low frequency impurity resonance modes and the tunnelling states of off-centre impurity ions. Developments in the field of resonance in ordered magnetic materials are described, followed by examples of measurements on superconductors. The last two sections deal with various electronic properties of solids which have been probed in the far infrared: the energy levels of impurities and defects, and cyclotron resonance of charge carriers.