Determination of complex refractive index of thin metal films from terahertz time-domain spectroscopy

Thin metal films alone or incorporated in a multilayer stack structure with dielectric films are good candidates of terahertz absorbers necessary for achieving the maximum responsivity in microbolometer devices. However, the design and optimization of these absorber structures depend on the knowledge of the complex refractive index of metal films in the terahertz frequency range, which is not easy to measure or determine from experiment. This paper presents a novel method that allows fast and reliable extraction of the complex refractive index from terahertz time-domain spectroscopy. It starts with terahertz time-domain transmission measurements, followed by Fourier transforms to obtain the transmission spectrum in the frequency range of 0.1-3 THz, and finally an extraction process using the enhanced "on-the-downhill" algorithm. Some experimental examples are given, all of which show good agreement with theoretical calculations. This extraction method, combined with the scattering matrix model, can help design and optimize complicated absorber structures for terahertz microbolometers