Phase Matching of Infrared Nonlinear Optical Salt-Inclusion Chalcogenides Achieved by the Modulation of the Fractal Dimension of Anionic Motifs

To address the challenge of nonphase matching in many infrared (IR) nonlinear optical (NLO) materials, despite having substantial NLO coefficients, we present a new strategy for realizing phase matching (PM) in IR NLO materials by manipulating the fractal dimension (FD) of covalent motifs. As a representative demonstration, we select a salt-inclusion chalcogenide system and validate its efficacy. Consequently, we successfully synthesized two new chalcogenides, denoted as K2[K4Cl][Ga5Sn7S24] (1) and [Na4I][Ga9S15] (2). The latter exhibits PM, while the former does not, primarily attributed to their markedly different FD values of the electron localization function map, resulting in distinct optical anisotropy. Compounds 1 and 2 demonstrate enhanced IR NLO efficiency, measuring 0.25 and 0.70 times that of AgGaS2 at 1910 nm, respectively. Notably, compound 2 features a wide band gap of 3.10 eV and a substantial-high laser-induced damage threshold (LIDT) of 93.15 MW cm–2@1064 nm (9.6 × AgGaS2). This work illuminates the exploration of new PM IR NLO materials derived from nonphase matching counterparts.