Detection and correction of the misplacement error in THz Spectroscopy   by application of singly subtractive Kramers-Kronig relations
  In THz reflection spectroscopy the complex permittivity of an opaque medium is determined on the basis of the amplitude and of the phase of the reflected wave. There is usually a problem of phase error due to misplacement of the reference sample. Such experimental error brings inconsistency between phase and amplitude invoked by the causality principle. We propose a rigorous method to solve this relevant experimental problem by using an optimization method based upon singly subtractive Kramers-Kronig relations. The applicability of the method is demonstrated for measured data on an n-type undoped (100) InAs wafer in the spectral range from 0.5 up to 2.5 THz. 
Valerio Lucarini, Yusuke Ino, Kai-Erik Peiponen and Makoto   Kuwata-Gonokami