Mid-Infrared Plasmonic Platform based on Heavily Doped Epitaxial Ge-on-Si: Retrieving the Optical Constants of Thin Ge Epilayers

The n-type Ge-on-Si epitaxial material platform enables a novel paradigm for plasmonics in the mid-infrared, prompting the future development of lab-on-a-chip and subwavelength vibrational spectroscopic sensors. In order to exploit this material, through proper electrodynamic design, it is mandatory to retrieve the dielectric constants of the thin Ge epilayers with high precision due to the difference from bulk Ge crystals. Here we discuss the procedure we have employed to extract the real and imaginary part of the dielectric constants from normal incidence reflectance measurements, by combining the standard multilayer fitting procedure based on the Drude model with Kramers-Kronig transformations of absolute reflectance data in the zero-transmission range of the thin film.