This presentation will begin with a general discussion on the current landscape of THz research and development. In particular, the state-of-the-art of MHz-through-THz material characterization will be briefly reviewed. Subsequently, two groups of free-space THz measurement techniques will be described for accurate material characterization and parametric extraction in connection with frequency independent optical paths. Quasi-optical mirror and lens systems are studied and developed over THz range to achieve precision complex permittivity measurements of dielectric substrates, films, and materials. To achieve a wide plane wave zone for the center of four-parabolic-mirror systems, two corrugated horns are designed and fabricated for the measurement systems. The Gaussicity of the corrugated horn is larger than 97.4%. For the proposed multiple reflection model and direct wave model, a set of closed-form expressions of loss tangent are derived from transmission parameters of the measurement systems. The resolution and uncertainty of loss tangent are examined according to the working frequency, the thickness of wafer or substrate, the real part of relative permittivity, and the transmission measurement uncertainty. The complex permittivity of Rogers/Duroid series PCB substrates, which are commonly used at microwave frequencies, and silicon wafers are measured over THz ranges. Co-sponsored by: STARaCOM (Montreal) Speaker(s): Ke Wu, Agenda: 10:45 AM – 10:55 AM: Opening the workshop, Mansour Naslcheraghi, Chair of YP Montreal 10:55 AM – 11:05 AM: Workshop and Speaker introduction by chair of Terahertz Days workshop Series, Dr. Gunes Karabulut Kurt 11:05 AM – 12:05 PM: Talk by Prof. Ke Wu 12:05 PM – 12:45 PM: Q & A session Montreal, Quebec, Canada, Virtual: https://events.vtools.ieee.org/m/303374