Optical thick films, few microns in thickness, are currently obtained without cracks, using hybrid silica gels (DEDMS-TEOS) as matrices and significant results have been achieved in the areas of solid state lasers, nonlinear optical materials and photochromics. A variety of dyes (rhodamine, coumarin) doped in sol-gel matrices have demonstrated laser action. Their properties are comparable to those of dyes in solution and their stability appears to be significantly greater than in organic polymeric matrices. The photo stability of dye-doped Ormosils have greatly surpassed the laser lifetimes obtained with polymer hosts. Organic dyes are just embedded within sol-gel matrices giving rise to class I hybrid compounds. Optical properties can be significantly improved when the organic dye ”R” is grafted onto the silica backbone via trifunctional R-Si_(OR‘)3 precursors. This increases solubility of organic dyes, prevents their aggregation and favors the random distribution of active species within the sol-gel matrix. A very nice example was provided by C. Sanchez et al. for the synthesis of optical materials exhibiting second order non linear optical properties.

The best currently known optical medium of low optical losses is silica glass, but the non linear optical properties (NLO) of organic chromophores are frequently much higher than those of inorganic compounds. The sol-gel synthesis of hybrid silica based NLO materials is therefore quiet promising. HoweverIt is well known that second harmonic generation can not be obtained in Centro symmetric materials. Therefore most studies deal with third order effects which do not require any specific orientation of organic species within the sol-gel matrix. For Χ(2) materials, the molecular orientation of organic chormophores has to be performed via the poling process by applying an external electrical field. This could be done around room temperature before the gel matrix is fully densified. Subsequent heat treatment causes the pores to shrink and locks the molecules in the poled orientation. The thermal relaxation of NLO (Non -Linear Optical properties) molecules  back to random orientation is rather fast and Χ(2) properties may vanish quite rapidly. Second harmonic generation was first demonstrated in doped sol-gel films by using a chromophore group bonded to a functionalized silicon alkoxide such as N-(3-triethoxysilyl)-2,4 dinitrophenyl amine (TSDP).  The NLO molecule is then grafted onto the oxide network rather than simply embedded within the sol-gel matrices. Sol-gel matrices were synthesized from a mixture of TSDP, HSiCH3(OEt)2 and Zr(OPr)4. Chemically bonded TSDP choromophores can be introduced in higher concentration, up to 80% in weight, without crystallization. (Χ)2 measurements on thermally cured and poled films containing TSDP give initial values in the range 2.5 pm/V. These values do not decrease too much upon aging indicating significant stabilization of the induced polar order. Better results can be obtained by using organic chromophore with higher nonlinearities, increasing the glass transition temperature of hybrid matrix or by  anchoring the chromophore at both ends.   Organically doped sol-gel glasses also find applications as chemical sensors when organic dyes are trapped within porous silica matrices. Small molecules can then diffuse through the open porosity of the xerogel and react with encapsulated dyes. pH sensitive silica films have been made via the incorporation of fluorescent molecules exhibit several colored protonated forms and optical spectrum changes with pH.  One of the main advantage is possibility to make sol-gel coatings at the surface of optical fibers. Fiber optic chemical sensors (optrodes) employ an intermediate reagent which responds optically to the analyte.