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Sol-Gel Principles

Sol gel deposition is a low cost and versatile approach for the realization of glass and glass ceramic waveguides systems. This method exploits chemical reactions of a main precursor, generally an organometallic compound in alcoholic solutions. From an historical point of view, even if the synthesis of the first silica gel dates back to 1846, sol-gel chemistry has been investigated extensively only since the mid-1970s, when sol-gel reactions were shown to produce a variety of inorganic networks that can be formed from metal alkoxide solution. Here we will focus on the metal-organic route with metal alkoxides in organic solvent, which offers unique opportunities for the synthesis of optical materials in different forms such as fine powders, monosize nanoparticles, aerogels, xerogels and waveguides, as represented in following diagram.
The sol-gel chemical process is self-described in the definition of a sol, a gel, and a summary of the processes in which the sol evolves into a gel.

The main reactions involved in the sol-gel chemistry are based on hydrolysis and condensation of metal alkoxides, which can be outlined as shown as follows:

In the first step the hydrolysis reaction, thanks to a nucleophilic attack of water to the metalloid atom, allows the substitution of the alkoxide groups (OR) with the hydroxyl groups (OH). In the second step the condensation reaction brings to the constitution of the amorphous network M-O-M with the elimination of water and alcohol. Due to the fact that the hydrolysis kinetic in a neutral environment is very slow, generally it is preferable to make the reaction happen in acid or basic catalysis. Because of the presence of a lot of chemical reactions and several involved compounds, the thermodynamic and kinetic description of the process results to be very complex; the main parameters that affect the reactions are the water-to-alkoxide ratio, the type and amount of catalyst, the type of organic groups attached on M (metallic) atom, and the solvent. All these conditions should be considered in order to create a stable suspension necessary for the realization of efficient optical devices.
Other aspects concern the form of the final material: the precursor sol, in fact, can be either deposited on a planar substrate to form a film (waveguide), e.g. by dip-coating or spin- coating, or can be cast into a suitable container with the desired shape (xerogel), or it can be used to coat substrates with nonplanar shape. A limitation to the film deposition is the time length of the process: due to the risk of stress cracking, that is a result of the reduction of volume of the deposited layer during the drying process, generally it is impossible to deposit films with thickness greater than 200 nm in a single step. Several deposition/heating cycles are therefore necessary to achieve films with a thickness suitable for an optical waveguide. Viscosity of the solutions, deposition parameters (dipping rate for dip-coating process and rotation speed for spin coating one) and post-deposition heat treatment conditions are crucial aspects that have to be calibrated for each material.
Silica-based WGs are mostly produced by using alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS); the acid nature of the catalyst permits to obtain a material with high uniformity, low porous volume and high density, fundamental properties for the development of photonic devices. A complementary approach uses organic-inorganic hybrids materials (e.g. hydrogen silsesquioxane, 3-glycidoxypropyl-trimethoxysilane) as starting reagents; in this case the removal of the organic part occurs using heat treatment (HT) process at high temperature. In all materials obtained by sol-gel route the HT process is critical for the densification of the structures: HT, in fact, permits not only the stiffening of the network, but also the removal of hydroxyl groups (OH-) that are one of the main causes of absorption in the near infrared and of the luminescence quenching for rare earth ions, especially Erbium ions. Finally, playing with HT it is possible to pass from an amorphous matrix to a glass-ceramic system, usually exhibiting better characteristics.

G.C. Righini, A. Chiappini, "Glass optical waveguides: a review of fabrication techniques", Optical Engineering 53 (2014) pp. 071819-1/14 (Mar 14, 2014), ISSN: 1560-2303, doi: 10.1117/1.OE.53.7.071819.