In this thesis the optical properties of $AgGaS_2:Co^{2+}$ and the substitutional site of $Co^{2+}$ ions in $AgGaS_2:Co^{2+}$ single crystal were studied. $AgGaS_2$ single crystal and Co-doped ones were synthesized by melt growth technique and chemical transport reaction. The lattice constants of Co-doped crystals were almost the same as those of undoped one. The analysis of the absorption spectra shows that the $Co^{2+}$ ions are located at the Ga sites($T_d$ symmetry) not Ag sites ($D_{2d}$ symmetry). From the energy of transitions of $Co^{2+}$ in $AgGaS_2$, the crystal field parameter Dq, Racah parameter B, and spin-orbit constants $λ_2, λ_3$ were obtained to be $359 cm^{-1}, 605 cm^{-1}$, and $182 cm^{-1}, 113 cm^{-1}$, respectively. In Raman spectra all the Raman modes were observed to soften in the Co-doped crystals, and that also implies that the $Co^{2+}$ ions occupy the Ga sites.
