In search of inorganic nonlinear optical materials for second harmonic generation

Abstract

In a search for inorganic oxide materials showing second-order nonlinear optical (NLO) susceptibility, we investigated several borates, silicates, and a phosphate containing trans-connected MO₆ octahedral chains or MO₅ square pyramids, where M=d⁰: Ti(IV), Nb(V), or Ta(V). Our investigations identified two new NLO structures: batisite, Na₂Ba(TiO)₂Si₄O₁₂, containing trans-connected TiO₆ octahedral chains, and fresnoite, Ba₂TiOSi₂O₇, containing square-pyramidal TiO₅. Investigation of two other materials containing square-pyramidal TiO₅, viz., Cs₂TiOP₂O₇ and Na₄Ti₂Si₈O₂₂·4H₂O, revealed that isolated TiO₅ square pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of TiO₅ units to produce-Ti-O-Ti-O-chains with alternating long and short Ti-O distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite.