In this thesis the optical properties of $CuGaS_2$ and $CuGaS_2$ : $Mn^{2+}$ single crystals were studied. $CuGaS_2$ and $CuGaS_2$ : $Mn^{2+}$ single crystals were grown by melt-growth method and chemical transport reaction. The Mn impurity did not change the lattice constants of $CuGaS_2$ appreciably. But the optical absorption spectra showed the Mn impurity effect. The optical bandgap was increased by the effect of Mn impurity (2.430eV→2.458eV at room temperature, 2.465eV→2.525eV at 5K). The transition peak from $^6A_1$ to $^4T_1$ was also observed at 2.511eV.
