Revised metrology for enhanced accuracy in complex optical constant determination by THz-time-domain spectrometry

THz time-domain spectrometry (TDS) probes the complex polarization response of materials. Various analytical procedures are applied by many to extract the associated material optical constants. This has commonly been done by iteratively varying material parameters in order to achieve a match between experiment and a theoretical transfer function (TF). The poly root behavior of a TF is emphasized for measurements with reflections in the time domain. This study provides a comprehensive analysis of the influence of the initial guesses on the accuracy of extracted material parameters. In addition, various ways of representing multiple reflections inside the sample (a Fabry-Perot-like effect) are compared and their contribution to the uncertainty of material parameters is analyzed. Experimental evidence is provided where appropriate to support theoretical statements. Furthermore, this paper offers a basis for data comparison between different THz-TDS systems in transmission mode. Finally, a clear distinction is made between a commonly used basic analysis and an enhanced one, in terms of associated uncertainties in determination of the real and imaginary parts of the complex refractive index.