Absorption spectroscopy Band structure Calculated optical properties Calculations Chemical bonds Crystal structure Density functional theory Density of states Electrical and optical properties Electronic structure Electronic structure and optical properties Energy gap Exchange-correlation potential First-principles First-principles calculation Gallium compounds Generalized gradient approximations Light absorption Optical absorption P-type doping Pure β-Ga2O3 Red Shift Refractive index Sr-doped β-Ga2O3 Strontium Structural properties Theoretical calculations Tools Visible light absorption