Structure-Property Relationships in Non-Linear Optical Crystals II

Mid-IR second-order NLO crystal is indispensable in the frequency conversion applications in the mid-IR region. Compared with DUV and UV/Vis/near-IR NLO crystals, practical mid-IR NLO crystals are relatively rare, and many of them are still at the stage of laboratory research. This chapter reviews the recent progress on the mid-IR NLO crystals, which mainly includes growing the classical mid-IR NLO crystals into large high-quality ones or into quasi-phase-matching structures that are suitable for the laser devices by various growth methods and exploring new potential mid-IR NLO crystals by introducing new design and synthesis strategies. Recent mid-IR NLO crystals can be divided into four categories, i.e., classical binary and ternary metal pnictides and chalcogenides, quaternary metal chalcogenides, binary and ternary metal halides, and different-bond-type hybrid compounds that contain at least two types of obviously different chemical bonds in the crystal structures. Metal pnictides and chalcogenides have got much attention on growing large crystals. Different-bond-type hybrid is a new family of mid-IR NLO materials, and many of them were found in the last decade. In metal halide system, both progress in growing large crystals and discovering new ones have been made.

In this minireview, we overview the recent advances and perspectives in the developments of the infrared second-order nonlinear optical materials. The traditional semiconductors are discussed first including the problems encountered such as the facility of large second-order nonlinearity but difficulty in practical materials for laser applications. We then focus our special interest on the area of the transition-metal polynuclear cluster compounds which is a great promising area for developing new-generation infrared second-order nonlinear optical materials and molecule-scaled photoelectronic devices. We present in detail the computational studies on the microscopic mechanism of second-order nonlinear optical response and the structure–property relationship insight of these metal cluster compounds.

Although tungstates posses lower coefficient of thermal conductivity, tungstates doped with active ions have higher quanta efficiency of fluorescence owing to their higher doping concentration of active ions resulted from the higher covalence of WO ₄ ²⁻ units. Therefore, they are favorable for the medium of low power laser when doped with active ions. On the other hand, they have higher stimulated Raman scattering (SRS) plus in a general way. Therefore, tungstates doped with active ions can serve as a SRS self-frequency conversion multifunctions laser medium. After wide surveys of known research on the growth, crystal structure, and properties including optical and spectra characteristics and laser property, this chapter reviews the recent advances in the development of KGd(WO₄)₂ and SrWO₄ Raman and SRS self-frequency conversion laser crystal. The SRS self-frequency conversion laser technology was dealt with. As a result, the Raman and self-Raman laser outputs with high efficiency at ~1,180 nm wavelength and its frequency-doubling laser outputs at ~590 nm wavelength have been achieved.

This review mainly highlights recent research progress on the exploration syntheses, crystal structures, and nonlinear optical properties of multinary chalcogenides. Some examples show that slight radius change (Ga³⁺ vs. In³⁺) leads to different packing patterns of the same asymmetric units that eventually result in NLO properties with different origins. Besides, combination of two types of asymmetric units, SbQ _x polyhedron and TrQ₄ tetrahedron, not only gives rise to a rich structural chemistry but also yields many NCS compounds with excellent IR NLO performance. Additionally, the studies of the new mid-IR NLO crystal BaGa₄S₇ developed by a Bridgman–Stockbarger technique and a series new NLO chalcogenides based on TrQ₄ and TtQ₄ tetrahedra are also included.