C-axis optical properties of high T-c cuprates

A review is given of the experimental status of the interlayer coupling ene rgy in the cuprates. A second c-ads plasmon is identified in the double lay er compound Y123 for various dopings. The anomalous transport properties al ong the c-direction and in the planar directions are compared to model calc ulations based on strongly anisotropic scattering. An excellent description of the optical data at optimal doping is obtained if an anomalously large anisotropy of the scattering rate between cold spots and hot spots is assum ed. This raises questions as to the physical meaning of these parameters.