Design and Validation of Isomorphic Crystal Library for Nonlinear Optics and THz Wave Generation

Development of new organic crystals possessing large second-order optical nonlinearity is very challenging because of strong tendency of centrosymmetric dipole-dipole molecular assembly in crystals. This tendency makes it difficult to develop various analogous crystals that allow fine tuning of optical and physical properties to enhance the device performance. A design approach of an isomorphic crystal library consisting of 11 highly efficient nonlinear optical salt crystals is reported. Analyzing the so-called isomorphic tolerance space in previously reported mother crystals (PMnXQ chromophores, where PM denotes piperidin-4-ylmethanol electron donor, n corresponds to the substituted position of halogen (X) group on the quinolinium (Q) electron acceptor), various substituents are introduced into the PMnXQ crystals at different positions, considering their space-filling characteristics and interionic interaction ability. All 11 PMnXQ crystals exhibit an isomorphic (or pseudo-isomorphic) crystal structure, in which the cationic chromophores form a perfectly parallel assembly for maximizing the second-order nonlinear optical susceptibility. The optical, physical, and crystal characteristics of newly designed, synthesized, and grown isomorphic PMnXQ crystals show both similarities and differences. Excellent THz wave-generation performance is demonstrated in both kHz- and MHz-repetition optical pump systems with new PMnXQ crystals. Therefore, the design approach using isomorphic tolerance space is very attractive for developing diverse isomorphic analogous organic crystals.