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

• Linear and nonlinear optical properties of LiInX2 (X = Se, Te) were calculated.
• The investigated materials posses direct energy band gap (Γ–Γ).
• Both compounds exhibit negative uniaxial anisotropy and positive birefringence.
• The investigated materials exhibit strong SHG.

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.