Nonlinear optical (NLO) crystals are key materials for laser science and technology as they can change the frequency of laser beam and modulate it in amplitude and phase through the process of second harmonic generation (SHG). No substantial theoretical progress on NLO material has been made since 1978.

The new approach to explore the structure-property relationship of materials is to use the concept of material genome or functional motif. However, a mature ansatz for revealing the origin of SHG phenomenon on the basis of general theoretical principles and such concepts of a material is still not available.

In a recent study published in Angew. Chem. Int. Ed., the research group led by Prof. DENG Shuiquan at Fujian Institute of Research on the Structure of Matter of Chinese Academy of Sciences developed a new atomic response theory on the origin of NLO material by inventing a universal partial response functional method.

Researchers investigated the origin of the unusual large SHG effect of the deep-ultraviolet NLO material LiCs₂PO₄ (LCPO) as an application by using the first principles calculations and theoretical model analysis.

The SHG response of LCPO was showed dominated by the metal-cation-centered groups CsO₆ and LiO₄, not by the nonmetal-cation-centered groups PO₄, as expected from the existing models and theories.

They then found that the occupied orbitals O-2p and Cs-5p as well as the unoccupied orbitals Cs-5d and Li-2p contribute largely to the SHG coefficients of LCPO.

The atomic response theory of NLO material is thought to be a major breakthrough since the development of the anionic group theory.

This study revealed the importance of the polarizable atomic orbitals of the occupied and the unoccupied states to the SHG response, and will benefit the understanding of the microscopic mechanism and the functional oriented material design of NLO materials.

The origin for the large SHG effect of LCPO is shown to be the metal-cation-centered groups CsO₆ and LiO₄ rather than the nonmetal-cation-centered groups PO₄. (Image by Prof. DENG’s Group)