Herrmann, M.M.HerrmannPlatte, F.F.PlatteNalpantidis, K.K.NalpantidisBeigang, R.R.BeigangHeise, H.M.H.M.Heise2022-03-042022-03-042012https://publica.fraunhofer.de/handle/publica/22861910.1016/j.vibspec.2012.01.017Absorption and reflection spectra have successfully been used for substance identification, which is also applicable to the THz spectral range. For optical spectroscopy, the Kramers-Kronig transformation (KKT) is a powerful tool to determine the complex refractive index ñ (with k, n absorption and refractive indices, respectively) from either an absorption or reflection measurement. By terahertz time-domain spectroscopy (THz-TDS) materials are probed with short pulses of radiation. The detection is sensitive to the sample on both the amplitude and the phase of the electrical field. Thus, THz-TDS spectroscopy can provide more information than conventional Fourier-transform spectroscopy, by which a power spectrum is measured. In the case of transmission measurements, for instance, formulae exist by which the frequency-dependent complex refractive index is directly calculated from the time-dependent electrical field (waveform) without the necessity of a KKT. In the case of reflection experiments, a comparable computation is possible for the frequency-dependent phase angle; here a KKT can help to recover essential parameters. We present a combination of KKT and TDS methods for the calculation of optical constants in the THz regime.en621540journal article