Phase-sensitive terahertz spectroscopy with BWOs in reflection mode

We report on a method, which allows accurate measurements of complex reflection coefficient of a solid at frequencies 1 to 50 cm-1 (30 GHz - 1.5 THz). Backward-wave oscillators (BWOs) are used as sources of monochromatic coherent radiation, tunable in frequency. The amplitude of the complex reflection (the reflectivity) is measured in a standard way, while the phase shift, introduced by the reflection from the sample surface, is measured using a polarizing Michelson interferometer. This method is particular useful for not-transparent samples (such as metals, superconductors, etc.), where phase-sensitive transmission measurements are not possible.
The method requires no Kramers-Kronig transformation in order to extract the sample's electrodynamic properties (such as complex dielectric function or complex conductivity). Another area of application of this method is the study of magnetic materials with complex dynamic permeability different from unity at the measurement frequencies (for example, metamaterials). Measuring both, the phase-sensitive transmission and the phase-sensitive reflection, would allow a straight-forward model-independent determination of the dielectric permittivity and magnetic permeability of such materials.