  Iron chalcogenides superconductors, such as Fe(Te,Se) have recently garnered
significant attention due to their simple crystal structure with a relatively
easy synthesis process, high-temperature superconductivity, intrinsic
topological band structure, and an unconventional pairing of superconductivity
with ferromagnetism. Here, we report the complex in-plane refractive index
measurement of nanometer-thin Fe(Te,Se) flake exfoliated from a single crystal
FeTe$_{\text{0.6}}$Se$_{\text{0.4}}$ for photon wavelengths from 450 to 1100 nm
over a temperature range from 4 K to 295 K. The results were obtained by
employing a two-Drude model for the dielectric function of Fe(Te,Se), a
multiband superconductor, and fitting the absolute optical reflection spectra
using the transfer matrix method. A high extinction coefficient in the visible
to near-infrared range makes nanometer-thin Fe(Te,Se) flakes a promising
material for photodetection applications.
