Abstract: Infrared spectroscopy, as a bulk-sensitive technique, plays an important role in the study of unconventional superconducting materials. In this article, I will introduce the fundamentals of Fourier transform infrared spectroscopy, the in situ gold evaporation technique used to accurately measure the absolute reflectivity of solid materials, the methods and models we used for optical data analysis, and how we can use infrared spectroscopy to investigate the important and fascinating quantum phenomena in iron-based superconductors, such as the superconducting gaps, electronic pairing symmetry, spin-density-wave gaps, non-Fermi liquid behavior, lattice vibrations, as well as the relation and interaction between different orders and excitations.

Key words: iron-based superconductor; infrared spectroscopy; superconducting gap; spin density wave; non Fermi liquid; phonon