Why NLO properties exist in noncentrosymmetric materials?

Answer 1

Nonlinear optical property requires the asymmetrical unit of molecules. This can bring the large second hyperpolarizability. So, noncentrosymmetric space group indicate that the compound have no symmetry operations including mirror plane and glind plane. We can determine whether the compounds are NLO active from their space groups.

Answer 2

Noncentrosymmetric materials lack inversion symmetry, leading to broken symmetry in their electronic and atomic configurations. This asymmetry allows for the generation of nonlinear optical (NLO) effects, such as second harmonic generation and electro-optic modulation, due to the non-zero macroscopic polarization that emerges in response to an external electric field. These NLO properties arise from the unique arrangement of atoms and electron distributions in noncentrosymmetric materials, enabling them to exhibit enhanced optical functionalities compared to centrosymmetric materials.