Nonlinear optical (NLO) materials, known for their strong second harmonic generation (SHG), have garnered significant interest attributed to their wide-ranging applications [1-2]. While the conventional approach to enhance SHG involves the design of NLO-active molecular units, recent studies have explored an innovative approach by incorporating vacancies and local structural distortions in solid-state materials, further improving the NLO effect.

This presentation aims to highlight recent advancements in the design and synthesis of tungsten bronze (TB) oxides with enhanced NLO properties. It investigates the influence of vacancies-induced local structural distortions on the SHG response and presents a molecular design strategy for the development of novel NLO materials [3]. The talk covers the synthesis, structures, and characterizations of a series of novel TB oxides, along with the introduction of transition metal-doped tungsten bronze oxides that exhibit unique structures and exceptionally large SHG responses [4].

Additionally, a polar tetragonal TB oxide with a reversible phase transition and an extraordinary SHG intensity is presented [5]. The incorporation of vacancies in TB oxide structures results in local structural distortions that strengthen the dipole moments of neighboring octahedra, significantly enhancing the SHG response. The observed colossal SHG intensity and phase-matchable behavior in the TB oxides underscore the effectiveness of the proposed molecular design strategy. Furthermore, the vacancies-induced structural distortions hold promise for the development of NLO materials with superior performance.