Growth and optical properties of large-sized Co2+: ZnGa2O4 single crystal

• Large-sized Co2+-doped ZnGa2O4 single crystals (volume ~20 cm3) were successfully grown using the vertical gradient freeze (VGF) method, exhibiting high crystalline quality with a rocking curve FWHM of only 58 arcsec. • The Co2+ ions occupy tetrahedral Zn2+ sites in the spinel lattice, leading to strong absorption bands in the visible (550–670 nm) and near-infrared (1100–1700 nm) regions, attributed to 4A2(4F) → 4T1(4P) and 4A2(4F) → 4T1(4F) transitions, respectively. • The optical band gap of Co2+-doped ZnGa2O4 was measured to be 4.44 eV, confirming its ultra-wide bandgap semiconductor nature. • The ground state absorption cross section at the near-infrared wavelength was determined to be 3.07 × 10−19 cm2, which is comparatively large, indicating ZnGa2O4 as a promising saturable absorber for 1.5 µm passive Q-switched solid-state lasers.