Inclusion of dyes is efficient for regulating the performance of nonlinear optical (NLO) crystals. The commonly seen anisotropic dyeing of solution-grown crystals, promoted by specific dye−facet interactions, limits their optical applicability. Here, two classic dyes, methylene blue (MLB) and aniline blue (ALB), are each isotropic occluded within single crystals of the widely used NLO material, potassium dihydrogen phosphate (KDP). MLB and ALB, which only partially stain solution-grown KDP crystals, are incorporated isotropically into gel-grown crystals with unspoiled single-crystalline lattices. During the gel crystallization, dye molecules bind to the gelnetworks, and both become incorporated into the crystals, leading to efficient isotropic dyeing. Furthermore, enrichment of dye molecules from gels into crystals is achieved in the case of ALB because the ALB molecules bind strongly and accumulate onto the gel-networks. Hence, this work provides a generally applicable strategy to facilitate the isotropic dyeing of single crystals and further optimization of crystal properties.

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Dye doping can optimize the optical, thermal, and mechanical properties of crystal materials. It has been found that gel-incorporation into the crystals can effectively guide the dyeing process of gel-grown crystals, where the binding interaction between gel-networks and dye molecules plays a significant role. In this paper, Eosin B and Eosin Y dyes with relatively weak gel–dye interaction were taken into consideration for the first time. Both dyes failed to stain KDP single crystals in solutions. However, with uniform gel-incorporation, KDP crystals grown from silica gels occluded the eosin dyes anisotropically. When the crystal growth slowed and the gel could not be embedded, the dyes were also unable to stain the KDP crystals. The findings in this work reveal a synergetic mechanism in which the specific electrostatic attraction between certain KDP facets and dye anions is enhanced by gel-incorporation, since the dye desorption from KDP surfaces is restricted by the interconnected gel-networks that physically bound on growing crystals.