Functional assembly and modification of nonlinear optical fundamental motifs in phosphates

Phosphates have emerged as a subject of considerable interest as prospective ultraviolet (UV, λ < 400 nm) nonlinear optical (NLO) materials, given their large UV bandgaps and strong NLO effects. However, the optical birefringence of most phosphates is typically insufficient for achieving birefringent phase matching in the UV and particularly in the deep-UV (λ < 200 nm) spectral regions, as the fundamental motifs have not been effectively combined to yield the desired anisotropic functionality. This review article presents a comprehensive overview of the methods employed to improve the birefringent properties of phosphates, with a particular focus on functional motif assembly and modification. Moreover, it provides an exhaustive analysis of the principles and examples of NLO materials engineering, offering invaluable insights into the arrangement and combination of phosphate and coordinating oxide motifs for the development of highly promising UV NLO materials. We commence by delineating the overarching design and assembly strategies for phosphate NLO materials engineering, with an emphasis on the role of polar polyhedral motifs in associating birefringence control and the role of polar chemical bonds in modulating the formation of anisotropic phosphate motifs. We then proceed to examine the specifics of structural assembly and modification through two distinct materials engineering schemes. These include the assembly of polar motifs for the enlargement of birefringence and the modification of polar bonds for the enhancement of anisotropy. Finally, we present an outlook on the inherent challenges associated with the design of NLO structures and the exploration of materials in phosphates. This review study offers a systematic summary of the structure-property correlation and prospective opportunities for the realization of UV and deep-UV NLO performance in functional phosphate material systems.