Adjacent In-Plane Air Referencing for Quantitative Complex Refractive-Index Mapping in Reflection THz Imaging

Quantitative terahertz (THz) reflection imaging requires a complex-field reference with a stable phase relationship to the specimen. A metal reflector measured in place of the sample, or an axially displaced window interface, can introduce systematic errors in tightly focused imaging. Here, an empty chamber on a quartz window serves as an adjacent in-plane air reference for three liquid-filled chambers. At each pixel, the quartz–sample field is divided by the quartz–air field recorded in the same scan and focal plane, and the known quartz–air Fresnel coefficient converts this ratio into the complex refractive index. For water and sucrose solutions at 30 and 60 wt%, the reflection-derived refractive index, absorption coefficient, and permittivity agreed with transmission measurements over 0.5–1.5 THz, with root-mean-square relative differences of 2.4–3.4%, 5.9–8.9%, and 5.0–9.2%, respectively. At 1.0 THz, the permittivity loss term decreased from 2.37 for water to 1.96 and 1.21 for the 30 and 60 wt% solutions, whereas the real permittivity remained between 3.95 and 4.44. In a double-Debye fit, the slow-relaxation contribution at 60 wt% was below the dilution estimate. Maps of these quantities were obtained from a single reflection image, and the surrounding bare quartz–air area served as an internal check of the reference.