Strong Second Harmonic Generation in a Tungsten Bronze Oxide by Enhancing Local Structural Distortion

To discover the nonlinear optical
(NLO) materials with strong second
harmonic generation (SHG), the design of NLO-active molecular units
with large polarization is considered as a common strategy. Herein,
we propose that the local structural distortion induced with vacancies,
apart from the NLO-active units, can be employed to improve the NLO
effect in solids as well. Accordingly, a new tungsten bronze (TB)
oxide, Pb2(Pb0.15Li0.7□0.15)­Nb5O15 (□ representing vacancies),
is successfully designed and prepared, which exhibits a strong SHG
response of 39 times that of KH2PO4. The detailed
analysis reveals that the local structural distortions enhanced by
the vacancies in PLN strengthen the local dipole moments of neighboring
NbO6 octahedra, and thus significantly prompt the SHG effect.
Moreover, a series of new TB compounds with large NLO effects are
discovered by this molecular design strategy, which are perspectives
for new NLO materials synthesis.