Isotropically Dyed Single Crystals Produced via Gel-Incorporation

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 gel-networks, 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.