  Far-infrared and terahertz spectroscopy of ferroelectric soft and central
modes in thin films on substrates is reviewed. In addition to classical
displacive proper ferroelectrics, also incipient and relaxor ferroelectrics and
multiferroics are discussed. Special attention is paid to differences between
the soft-mode behavior in thin films and bulk materials (ceramics and single
crystals) and their influence on the low-frequency permittivity. Particularly
the effects of the thin film strains and depolarizing electric fields of the
probing waves on the grain boundaries are emphasized. The soft-mode
spectroscopy is shown to be a very sensitive tool to reveal the thin film
quality.
