A double-layer scheme for extrapolating terahertz complex refractivity from reflectance of aerocraft materials

The complex refractive indexes of aero craft materials are seldom investigated. In this paper, we fabricate two aero craft materials: a white painted aluminum plate and a coated window glass, and measure the reflectance spectra in the region of 3–50 THz. Different from the acquainted metal materials, the reflectance spectra of the two materials have apparent peaks and valleys, which is known as the spectral fingerprint. With the preliminary analysis by using Lorentz-Drude/Lorentz dispersion oscillator models and Kramers-Kronig (KK) algorithm, we find that the errors of the extracted complex refractivity from the dispersion oscillator models are apparent at the abrupt regions, and the accuracy of the real refractive index n and extinction coefficient κ from KK algorithm differs dramatically. To reduce the inevitable errors, we design a double-layer extraction scheme to reestablish the oscillator models based on the KK relation. Compared with the inversed data from KK relation and dispersion oscillator models, the procedure assures the accuracy of the real refractive index and extinction coefficient concurrently, the error of n/κ values at the abrupt region is also improved significantly. The method is reliable and convenient to obtain the precise broadband refractive index from spectra with apparent spectral fingerprints. And the obtained complex refractivity can support the further THz scattering analysis for aero craft targets.