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.