Theory of far infrared optics of ferroelectrics

We review a general fomalism for the calculation of both linear and nonline ar optical effects in ferroelectric materials from the Landau free energy c ombined with a Landau-Khalatnikov equation of motion. The first application is to the calculation of conventional nonlinear-optical tensors in the far infrared spectral region. We then show how the formalism can be extended t o give a general framework for the calculation of linear and nonlinear opti cal effects at surfaces and in thin films. It is shown that measurements of Linear reflectivity of a ferroelectric film should serve as a sensitive pr obe of the behaviour of the order parameter near the surfaces of the film.