Far infrared laser spectroscopy of surface (Fano) waves for the determination of the optical constants of solids

It is very difficult to measure the optical constants of solids by reflection and transmission whenever the real part of the permittivity becomes negative. However, if this permittivity is sufficiently small, surface waves may form in a sample. These may be excited and their dispersion can be determined experimentally. The dispersion can then be used to calculate the permittivity. A method for far infrared laser measurements is described and the limits of its validity are given.