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

We demonstrate a hybrid nanocomposite combining mesoporous silica, p ${\text{SiO}}_2$ , as a host medium and guest lithium niobate ${\text{LiNbO}}_3$ 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 ${\text{LiNbO}}_3$ nanocrystals within the p ${\text{SiO}}_2$ nanochannels, indicating their random texture morphology. Annealing at high temperatures ( 950 $_{}^{\circ }$ C) during calcination also leads to partial crystallization of the p ${\text{SiO}}_2$ matrix with the formation of trigonal $\alpha$ - ${\text{SiO}}_2$ nanocrystals. The Raman microscopy analysis of the p ${\text{SiO}}_2$ : ${\text{LiNbO}}_3$ nanocomposite reveals three structural crystalline phases, $\alpha -{\text{SiO}}_2$ , ${\text{LiNbO}}_3$ and a mixed phase which involves the $\alpha$ - ${\text{SiO}}_2$ phase of host membrane and ${\text{LiNbO}}_3$ nanocrystals embedded into the membrane. The finite size of the ${\text{LiNbO}}_3$ 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 ${\text{SiO}}_2$ : ${\text{LiNbO}}_3$ composite, consistent with the randomly oriented textural morphology of the deposited ${\text{LiNbO}}_3$ nanocrystals. The contribution of the guest ${\text{LiNbO}}_3$ nanocrystals to the second harmonic generation effect was found to be strongly dominant compared to the partially crystallized host p ${\text{SiO}}_2$ matrix. The nanocomposite p ${\text{SiO}}_2$ : ${\text{LiNbO}}_3$ 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 ${\text{SiO}}_2$ : ${\text{LiNbO}}_3$ nanocomposite membranes expands the prospects for their photonic and nonlinear optical applications.