Abstract To enable high-precision, non-destructive measurement of dielectric properties of millimeter-scale integrated circuit (IC) materials in the terahertz regime, this paper proposes a free-space method for complex permittivity extraction based on a dual-lens pair focusing system. Addressing the limitations of traditional free-space setups, such as beam divergence, energy loss, and insufficient spatial resolution, this study develops a spot-size-optimized quasi-optical system model based on a dual-lens pair architecture. The model enables precise control of component positioning along the optical path, resulting in significant reduction of the beam spot size and improved measurement accuracy. Under the optimized configuration, the ratio of depth of focus to sample thickness exceeds the reference threshold, thereby satisfying the plane-wave approximation and ensuring the validity of the algorithm model. Employing an optimized quasi-optical configuration, we conducted S-parameter measurements on large-sized fused silica and millimeter-sized gallium arsenide (GaAs) specimens within the 325–500 GHz band, extracting complex permittivity values. The dielectric characterization results showed less than 1% error for both materials when compared with reference data. The results validate the enhanced system's capability for precise characterization of both large-sized materials and millimeter-scale samples, overcoming dimensional limitations inherent to conventional free-space measurement systems while improving measurement accuracy. This study provides a viable approach for terahertz dielectric characterization and offers both theoretical and experimental insights into the co-optimization of beam focusing and dielectric algorithms.
Wang et al. (Tue,) studied this question.