Optical spectroscopy studies on FeTe_1-xSe_x and A_xFe_2-ySe₂ (A=K, Rb, Cs)：A brief overview

In this short overview, we summarize the optical spectroscopy studies on iron selenide superconducting systems FeTe_1-xSe_x and A_xFe_2-ySe₂. We elaborate that optical spectroscopy measurements yield fruitful information about the band structure evolution across the AFM phase transition temperature, the electronic correlation effect, the superconducting pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.