Nonlinear optical (NLO) crystals can efficiently convert the frequency of common solid-state laser with fixed or limited wavelengths. Since the first observation of second-harmonic generation (SHG) phenomenon in the early 1960s, they have played an increasing role in the fields of laser spectra, precision measurements, and quantum information, as well as industrial laser processing, and so on. However, it is still challenging to rationally design and synthesize new NLO crystals.

This special issue contains 10 papers, which includes 2 short communications, 2 articles, and 6 reviews. We thank a lot for the authors’ great contribution to this special issue. A brief summary of all 10 accepted papers is provided as follows. 

Firstly, the short communication “From Cs₈Sb₄Nb₅O₅F₃₅ to Cs₆Sb₄Mo₃O₅F₂₆: the first non-centrosymmetric fluoroantimonite with d⁰ transition metal”, Fang Kong, Jiang-Gao Mao, and co-authors report two novel d⁰ transition metal fluoroantimonites, and discuss the structural transformation between them. This paper indicates that d⁰ transition metal fluoroantimonites may be a promising NLO material system.

Another short communication “Polymorphism of LiCdBO₃: crystal structures, phase transitions and optical characterizations” reports two new polymorphic phases of LiCdBO₃, referred as m- and t-LiCdBO₃ according to their crystallization temperatures. Qian Wu, Mingjun Xia, and co-authors found an interesting temperature-dependent phase transition with a non-centrosymmetric-to-centrosymmetric symmetry breaking, which results in a sharp transition from SHG-on state to the SHG-off state for LiCdBO₃. 

In the article “Deep-ultraviolet nonlinear-optical crystal BaNa₂[PO₃(OH)]₂ with large birefringence and strong second-harmonic-generation response”, Yi-Gang Chen, Xian-Ming Zhang and co-authors report a new metal phosphate, BaNa₂[PO₃(OH)]₂, which realizes an excellent balance between short UV absorption edge less than 190 nm, strong SHG response with the magnitude of ~1.2 × benchmark KH₂PO₄, and the large birefringence with the experimental value of 0.064@590 ± 3 nm. These results imply that BaNa₂[PO₃(OH)]₂ is a very promising DUV NLO crystal.

The article “AIO₃∙H₆TeO₆ (A = NH₄, Rb): two telluric acid and iodate co-crystalline compounds

with second harmonic generation response” reports two interesting isostructural co-crystalline compounds. Hongming Liu and co-authors found that both compounds show moderate phase-matching SHG responses and large optical band gaps of > 4.25 eV.

In the review titled “Recent progress on sulfide infrared nonlinear optical materials with large SHG response and wide band gap”, Junjie Li and co-authors highlight the progress of high-performance sulfide IR NLO materials with desired balance between strong SHG response (≥ 1.0 × AgGaS₂) and wide band gap (≥ 3.0 eV) since 2018. They also point out the potential opportunities and challenges in IR NLO materials. 

The review “Recent advances in F-containing iodate nonlinear optical materials” summarizes the research progress of F-containing iodates in the past ten years and addresses the effect of structure-directing properties of oxide-fluoride anions on the crystal structures of F-containing iodates. Hongwei Yu and co-authors analyze the performance differences of F-containing iodates in different systems, and provide useful suggestions on how to achieve desirable balance between large SHG efficiency and large band gap in one crystal. 

The review “Evaluating refractive index and birefringence of nonlinear optical crystals: classical methods and new developments”, Lingyun Dong, Ying Wang and co-authors summarize the effective methods for the measurements of refractive index and birefringence. This review would shed useful insights on how to realize angle phase-matching conditions for NLO crystals. 

The review “Compounds consisting of coplanar π-conjugated B₃O₆-typed structures: an emerging source of ultraviolet nonlinear optical materials” by Min Luo and co-authors describe the research advances of NLO materials with π-conjugated B₃O₆-typed groups, and summarize the crystal structures, synthetic methods, optical performances, as well as the relationships between structures and properties. Notably, B₃O₆ is the basic functional group of the so-called “China brand” crystal β-BaB₂O₄. This review may accelerate the development of the next-generation UV NLO materials. 

The review “Recent advances on the synthesis of Sb(III)-based inorganic ultraviolet nonlinear optical materials” presents a short review of the reported Sb(III)-based UV-NLO materials. Guohong Zou and co-authors give an in-depth overview on how to solve the difficult problems including oxidation and hydrolysis in the synthesis process of these materials. This review offers some synthetic concepts for synthesizing the subsequent class of high-performance UV NLO materials.

In the review of “Accurate design and synthesis of nonlinear optical crystals employing the KBe₂BO₃F₂ as structural template”, Yaoguo Shen, Sangen Zhao and co-author present their successful examples on the accurate design and synthesis of high-performance NLO materials by employing the KBe₂BO₃F₂ as structural template. In addition, they propose a new concept so-called quality factor to comprehensively evaluate the overall performance of new UV NLO materials.