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Abstract:
Mesoporous silica films (MSFs) are very promising candidates for the use as low refractive index supports in the future optical integrated circuits and devices.
MSFs have been fabricated by dip-coating in a template-modified sol-gel process. In this process, evaporation induced self-assembly leads to the formation of an ordered pore structure. A strong dependence of the formed structure on the processing conditions, especially humidity, has been revealed allowing an appreciable structure tuning. The possibility of the thickness tuning by drawing velocity was shown in examples with normal coating and with the coating with a tilting movement. Two types of films, denoted as A and B, with different optical properties have been found in dependence on the humidity during the synthesis. The synthesis field has been explored in order to locate optically perfect A-type films.
Both film types have been characterized by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). From these results, the different structures for the two film types have been determined. A-type films have a stable wormlike structure, whereas for B-type films a novel sustained lamellar structure has been found. A formation mechanism for the two structure types has been proposed.
An ultra-low refractive index of n = 1.18 ± 0.01 has been measured for A-type films by angle-dependent interferometry and its stability has been examined. These films are crack-free, optically clear and transparent. They are mechanically and thermally stable, with a good adherence on diverse supports up to the thickness of about 1 μm. The optical scattering measured with diffuse reflectance spectroscopy was very low and the film surface measured with AFM was extremely smooth.
Because of these extraordinary properties, A-type films have been used as low-n supports to realize 2D photonic crystal waveguides based on polymers. The light guiding ability of PMMA waveguides deposited on MSF was proven.
MSFs are also a suitable support for the deposition of the ferroelectric material PZT leading to suppressed crack formation in the PZT films. Besides fabricated PZT films, first attempts for PZT structuring have been carried out.
