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

We demonstrate a hybrid nanocomposite combining mesoporous silica, p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> , as a host medium and guest lithium niobate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocrystals within the p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> nanochannels, indicating their random texture morphology. Annealing at high temperatures ( 950 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> </mml:math> C) during calcination also leads to partial crystallization of the p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> matrix with the formation of trigonal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> nanocrystals. The Raman microscopy analysis of the p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> : <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocomposite reveals three structural crystalline phases, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi> <mml:mo>-</mml:mo> <mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> and a mixed phase which involves the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> phase of host membrane and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocrystals embedded into the membrane. The finite size of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> : <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> composite, consistent with the randomly oriented textural morphology of the deposited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocrystals. The contribution of the guest <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocrystals to the second harmonic generation effect was found to be strongly dominant compared to the partially crystallized host p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> matrix. The nanocomposite p <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> : <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> membrane, set in the 90 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn></mml:msub> </mml:math> : <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>LiNbO</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:math> nanocomposite membranes expands the prospects for their photonic and nonlinear optical applications.