Nonlinear active materials: An illustration of controllable phase matchability

For a crystal to exhibit nonlinear optical (NLO) activity such as second-harmonic generation (SHG), it must belong to a noncentrosymmetric (NCS) space group. Moreover, for these nonlinear optical (NLO) materials to be suitable for practical uses, the synthesized crystals should be phase-matchable (PM). Previous synthetic research into SHG-active crystals has centered on (i) how to create NCS compounds and/or (ii) how to obtain NCS compounds with high SHG efficiencies. With these tactics, one can synthesize a material with a high SHG efficiency, but the material could be unusable if the material was nonphase-matchable (non-PM). To probe the origin of phase matchability of NCS structures, we present two new chemically similar hybrid compounds within one composition space: (I) [Hdpa]₂NbOF₅·2H₂O and (II) HdpaNbOF₄ (dpa = 2,2′-dipyridylamine). Both compounds are NCS and chemically similar, but (I) is non-PM while (II) is PM. Our results indicate - consistent with organic crystallography - the arrangement of the organic molecule within hybrid materials dictates whether the material is PM or non-PM.