Measurement of surface roughness effects on conductivity in the terahertz regime with a high-Q quasi optical resonator

Summary form only given. Successful design and engineering of sources and components in the terahertz (THz) regime benefits from good characterization of material properties. Previous research reports have shown that calculations of material parameters that are valid at radio frequencies are no longer accurate at THz frequencies. A high quality factor (Q ~ 4.2>;<;10⁵) quasi-optical (QO) hemispherical resonator operating at 400 GHz and 650 GHz (with capability for operation at 1 THz) has been designed and implemented for the measurement of electronic properties of conductors as well as low-loss dielectrics. This apparatus is the first QO resonator to achieve Q >; 4χ10⁵ at near-THz frequencies greater than 400 GHz. It is also the first open resonator designed to determine effective conductivity at these frequencies. The measurements of metallic samples with controlled, nano-scale textures will provide insight into how surface roughness alters effective conductivity when the topographical features are on the order of the skin depth. Preliminary results with mechanically polished samples indicate a roughly linear relationship between average roughness, and effective conductivity. The smoothest sample, with an average roughness of 5.5 nanometers, or 4.5% of the skin depth at 400 GHz, has an effective conductivity that is 28% lower than the DC value. In addition to mechanically lapped specimens, additional samples with controlled, microfabricated grating-like structures will be examined with the resonator apparatus. These samples provide geometries that can be modeled with theoretical calculations for comparison purposes. This paper will discuss the techniques that enabled high-Q operation at 400 GHz and 650 GHz. It will also compare the measured relationship between surface roughness and effective conductivity with theoretical predictions.