Coherent broadband continuous-wave terahertz spectroscopy on solid-state samples

Measuring the complex dielectric function ε(ω)=ε1+iε2 of solid-state samples in the terahertz frequency range with high spectral resolution remains difficult. Using a continuous-wave terahertz spectrometer based on photomixing in the frequency range from 60 GHz to 1.8 THz, we obtain the most precise data of ε(ω) reported to date for the well-studied example of α-lactose monohydrate. We are able to determine both ε1 and ε2 due to coherent detection and show that the results are Kramers-Kronig consistent. Our analysis is based on scanning an interference pattern in frequency and relies on the high spectral resolution in the MHz range. This enables us to avoid mechanically moving parts such as a delay stage. Moreover, we show that the optical data can be used to determine both ε(ω) and the sample thickness d independently.