Measurable lower limit of thin film conductivity with parallel plate waveguide terahertz time domain spectroscopy

Parallel plate waveguide (PPWG) terahertz (THz) time domain spectroscopy (TDS) is a powerful tool to investigate the properties of thin and low conductive materials. In this Letter, we determine the lower limit of detection of the PPWG-THz-TDS approach. We provide a closed-form expression of the minimal measurable conductivity by the system. The experimental results of amorphous YBa₂Cu₃O_7-δ films indicate that the factor limiting the spectroscopic modality is the waveguide device misalignment. On the other hand, the expression of the minimal detectable conductivity provides a clear scheme of optimization by increasing the waveguide length and therefore enhancing the sensitivity of the system.