Anisotropic optical properties of GeS investigated by optical absorption and photoreflectance

We present a comprehensive study of bulk GeS optical properties and their anisotropy, by investigation of the fundamental band gap of the material and the energetically higher direct transitions. Optical absorption and photoreflectance measurements allowed us to establish the indirect gap of GeS at E0 = 1.56 eV at room temperature, and closely-lying direct transitions at E1 = 1.59 eV and E2 = 1.64 eV. Polarization-resolved measurements revealed that E1 is polarized along the armchair crystallographic direction and E2 is polarized along the zigzag direction. Finally, the experimental results are discussed in terms of first-principles calculations, which allowed us to assign all the optically active transitions in the Z–G–Y region of the Brillouin zone