Synthesis, nanocrystalline morphology, lattice dynamics and nonlinear optics of mesoporous SiO2&LiNbO3 nanocomposite

Abstract

Abstract We demonstrate a hybrid nanocomposite combining mesoporous silica, p $$\hbox {SiO}_2$$ SiO 2 , as a host medium and guest lithium niobate $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals embedded into tubular silica nanochannels by calcination of the precursor mixed solution of lithium and niobium salts. High-resolution transmission electron microscopy, X-ray diffraction and Raman scattering techniques reveal trigonal $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals within the p $$\hbox {SiO}_2$$ SiO 2 nanochannels, indicating their random texture morphology. Annealing at high temperatures ( 950 $$^{\circ }$$ ∘ C) during calcination also leads to partial crystallization of the p $$\hbox {SiO}_2$$ SiO 2 matrix with the formation of trigonal $$\alpha$$ α - $$\hbox {SiO}_2$$ SiO 2 nanocrystals. The Raman microscopy analysis of the p $$\hbox {SiO}_2$$ SiO 2 : $$\hbox {LiNbO}_3$$ LiNbO 3 nanocomposite reveals three structural crystalline phases, $$\alpha -\hbox {SiO}_2$$ α - SiO 2 , $$\hbox {LiNbO}_3$$ LiNbO 3 and a mixed phase which involves the $$\alpha$$ α - $$\hbox {SiO}_2$$ SiO 2 phase of host membrane and $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals embedded into the membrane. The finite size of the $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals results in specific features of the LO-TO phonon frequency splitting, which are investigated by Raman microscopy. In the transmission geometry, the second harmonic generation emission exhibits no Maker fringes and is characterized by a broad angular diagram of diffusely scattered light. The second harmonic generation response is independent of the polarization direction of the incident pump light, thus indicating a spatial isotropy of the nonlinear optical conversion in the p $$\hbox {SiO}_2$$ SiO 2 : $$\hbox {LiNbO}_3$$ LiNbO 3 composite, consistent with the randomly oriented textural morphology of the deposited $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals. The contribution of the guest $$\hbox {LiNbO}_3$$ LiNbO 3 nanocrystals to the second harmonic generation effect was found to be strongly dominant compared to the partially crystallized host p $$\hbox {SiO}_2$$ SiO 2 matrix. The nanocomposite p $$\hbox {SiO}_2$$ SiO 2 : $$\hbox {LiNbO}_3$$ LiNbO 3 membrane, set in the 90 $$^\circ$$ ∘ nonlinear optical geometry, shows unusually high diffusely transmitted second harmonic generation light (back-reflected emission), apparently supported by internal light reflection from the tubular nanochannel network. Despite the fundamental interest, the revealed anomalous back-reflected second harmonic generation emission from p $$\hbox {SiO}_2$$ SiO 2 : $$\hbox {LiNbO}_3$$ LiNbO 3 nanocomposite membranes expands the prospects for their photonic and nonlinear optical applications.