Far-infrared ellipsometry using synchrotron radiation

We discuss ellipsometric measurements using synchrotron radiation and estimate the errors that are due to focusing light onto the sample. Our results lead to the conclusion that ellipsometry is a brightness experiment and can benefit considerably from the use of synchrotron radiation in the mid and far infrared. We report implementation of a synchrotron far-infrared ellipsometer at the U4IR beam line at the National Synchrotron Light Source, Brookhaven National Laboratory, covering the $50–700-{\mathrm{cm}}^{-1}$ wave-number range. To illustrate the capability of our setup we have determined the complex dielectric function of the ionic insulator LiF and of the heavily doped semiconductor GaAs $(n\approx 8\times {10}^{18}\hspace{0.5em}{\mathrm{cm}}^{-3})$. We also report direct measurements of the optical interplane conductivity of single crystalline La₂CuO_4+δ and a twinned YBa₂Cu₃O₇ crystal. From these data, dynamic atomic effective charges have been determined for ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$.