Advances in Solid-State Nonlinear Optical Materials: From Fundamentals to Applications

Nonlinear optical (NLO) materials are an interesting class of compounds that have found application in many aspects of our ordinary lives. Solid-state NLO materials can be classified into organic, inorganic, and hybrid. This brief review article has the aim of introducing the readers to the fundamental principles of nonlinear optics while focusing on the inorganic solid-state NLO materials with an emphasis on their structural peculiarities, synthetic methods, and applications. The materials will be grouped by their intended spectral application: mid- and far-IR (chalcogenides, halides, and pnictides), near-IR and visible ranges (KDP, KTP and iodates), and UV and deep-UV (borates, carbonates, nitrates, and phosphates). Among the compounds that are discussed, several classes are highlighted: promising IR compounds with wide band gaps (E_g > 3.5 eV) with second harmonic generation (SHG) coefficients comparable to industry standard AgGaS₂ (d_ij ≥ 1 × AGS) suitable for mid-IR NLO applications, NLO compounds that can meet commercial requirements in the vis-NIR and UV regions, and deep-UV materials with even wider band gaps (E_g > 6.2 eV) and SHG coefficients surpassing those of traditional KH2PO4 crystals (d_ij - 1 × KDP). Several prevailing synthetic methods (solid-state, flux, and hydrothermal) are discussed, and different types of characterization techniques are presented. Finally, applications, ranging from lasers and quantum and neuromorphic computing to "uses beyond optics"in battery electrodes and ionic conductors are discussed. Although brief, we hope this review will provide valuable insights into the fast-expanding field of solid-state NLO materials.