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

We demonstrate a hybrid nanocomposite combining mesoporous silica, p $$ , as a host medium and guest lithium niobate $$ 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 $$ nanocrystals within the p $$ nanochannels, indicating their random texture morphology. Annealing at high temperatures ( 950 $$ C) during calcination also leads to partial crystallization of the p $$ matrix with the formation of trigonal $$ - $$ nanocrystals. The Raman microscopy analysis of the p $$ : $$ nanocomposite reveals three structural crystalline phases, $$ , $$ and a mixed phase which involves the $$ - $$ phase of host membrane and $$ nanocrystals embedded into the membrane. The finite size of the $$ 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 $$ : $$ composite, consistent with the randomly oriented textural morphology of the deposited $$ nanocrystals. The contribution of the guest $$ nanocrystals to the second harmonic generation effect was found to be strongly dominant compared to the partially crystallized host p $$ matrix. The nanocomposite p $$ : $$ membrane, set in the 90 $$ 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 $$ : $$ nanocomposite membranes expands the prospects for their photonic and nonlinear optical applications.