THz-TDS Time-Trace Analysis for the Extraction of Material and Metamaterial Parameters

We report on a method to fit time-trace data from a terahertz time-domain-spectroscopy system enabling the extraction of physical parameters from a material or metamaterial. To accomplish this, we developed a Python-based open-source software called Fit@TDS that functions on a personal computer. This software includes commonly used methods where the refractive index is extracted from frequency-domain data. This method has limitations when the signal is too noisy or when an absorption peak saturates the spectrum. Thus, the software also includes a new method where the refractive indices are directly fitted from the time trace. The idea is to model a material or a metamaterial through parametric physical models (Drude-Lorentz model and time-domain coupled mode theory) and implement this in the propagation model to simulate the time trace. Then, an optimization algorithm is used to retrieve the parameters of the model corresponding to the studied material/metamaterial. In this paper, we explain the method and test it on fictitious samples to probe its feasibility and reliability. Finally, we used Fit@TDS on real samples of high-resistivity silicon, lactose, and gold metasurface on quartz to show the capacity of the method.