Discussion on appropriate evaluation methods for low absorbers in the case of terahertz spectroscopy

We have investigated methods for the correct evaluation of low absorbers in the terahertz frequency. We theoretically and experimentally optimized sample conditions, measurement methods, and optical systems necessary to correctly evaluate the low absorption coefficients of quartz crystal and polymers. Experiments using a narrow terahertz collimated beam with a Gaussian-type intensity distribution allowed us to estimate absorption coefficients for each sample that were lower than the previous literature values. This method can serve as a guideline for the correct measurement using THz-TDS.