Mid-infrared time-domain ellipsometry: Application to Nb-doped SrTiO3

We present a method for determining the dielectric function of opaque materials precisely and reproducibly in the frequency range from 8 to 30 THz and higher. Our approach is based on measuring the polarization- and phase-resolved THz electrical transients reflected by the sample. This mid-infrared time-domain ellipsometry is applied to pure and Nb-doped strontium titanate SrTiO3, which allows us to infer the longitudinal and transverse optical phonon frequencies and the free-carrier plasma frequency as a function of the charge carrier concentration. We extract and discuss the value of the effective mass of the charge carriers.