Strong second harmonic generation in LiInX₂ (X=Se, Te) chalcopyrite crystals as explored by first-principles methods

Complex first-order linear optical dispersion and complex second-order non-linear optical dispersion of LiInX₂ (X = Se, Te) chalcopyrite single crystals were calculated in the framework of the density functional theory using the local density approximation (LDA), general gradient approximation (GGA) and modified Becke-Johnson potential The performed calculations show that these materials posses direct energy band gap (-Γ) of about 1.58 eV (LDA), 1.85 eV (GGA) and 2.04 eV (mBJ) for LiInSe₂ and 1.76 eV (LDA), 1.92 eV (GGA) and 2.40 eV (mBJ) for LiInTe₂. Comparison with the experimental data unambiguously indicates that the mBJ succeeds by bringing the calculated gaps in close agreement with the measured ones (2.05 eV -LiInSe₂ and 2.41 eV -LiInTe₂). It has been found that both compounds exhibit negative uniaxial anisotropy and positive birefringence at the static limit. In addition, LiInTe₂ exhibits a higher second harmonic generation (SHG) signal than LiInSe₂. Moreover, LiInSe₂ (LiInTe₂) exhibits a SHG signal of about seven (nine) times greater than the corresponding experimental value for the well known KTiOPO₄ (KTP) single crystals.