Metal chalcogenides: A rich source of nonlinear optical materials

Materials chemistry and the pursuit of new compounds through exploratory synthesis are having a strong impact in many technological fields. The field of nonlinear optics is directly impacted by the availability of enabling materials with high performance. Nonlinear optical (NLO) phenomena such as second harmonic and difference frequency generation (SHG and DFG, respectively) are effective at producing a coherent laser beam in difficult to reach frequency regions of the electromagnetic spectrum. Such regions include the infrared (IR), far-infrared, and terahertz frequencies. High performance NLO crystals are critical for applications utilizing these coherent light sources, and new materials are continuously sought for better conversion efficiency and performance. The class of metal chalcogenides is the most promising source of potential NLO materials with desirable properties particularly in the IR region where most classes of materials face various fundamental challenges. We review the recent developments in the discovery of several new high-performing chalcogenide NLO materials for the IR region of the spectrum. Among these, KPSe₆, NaAsSe₂, and Na₂Ge₂Se ₅ have been shown to exhibit some of the highest SHG coefficients (χ⁽²⁾) reported, namely, 150, 325, and 290 pm/V, respectively. We focus on their structural characteristics, optical transparency, and nonlinear optical properties. We also discuss a new concept to prepare strong NLO bulk glasses, fibers, and thin films without poling, which would be a promising solution to a main challenge in NLO applications. The impact of cutting-edge theoretical calculations in helping to move this field of materials science and chemistry forward is highlighted.