**4. Conclusions**

In the work presented, a novel method of deposition of optical coatings with a gradient of refractive index, using the RF PECVD technology, is introduced. Compared to the existing literature reports, this work is unique in the sense that it presents a successful endeavor to obtain a complex "optical system with a gradient of refractive index by means of the plasma-chemical technique, using a single organometallic precursor and a variable oxygen-to-nitrogen ratio in the working atmosphere. The unquestioned advantage of this work is an elimination, as precursor compounds, of explosive silane and corrosive silicon halogenides and their replacement with safe and easy to handle liquid organosilicon compounds. Two organosilicon precursors, namely TMDSN and HMDSN, were used as precursor compounds. As the reaction atmosphere, a mixture of oxygen and nitrogen was applied. An application of pure components resulted in a formation of SiOC films of low refractive index in the case of oxygen and SiN:C films of high refractive index in the case of nitrogen. The coatings deposited from TMDSN were characterized by refractive index magnitudes of 1.99 and 2.31 for the SiOC and for the SiNC materials, respectively. Similar data for the films obtained from HMDSN amounted to 1.65 and to 2.22. On the basis of the above results, HMDSN was selected as the more promising precursor of the gradient coatings, characterized by both chemical composition and refractive index varying with their thickness. Those gradients were acquired by an application of the variable composition of reaction atmosphere. Sinusoidal alterations of that composition resulted in a deposition of the coatings with sinusoidal changes in their optical properties. The method developed was used to manufacture a "cold mirror" type of interference filter. The filter is characterized by a good agreemen<sup>t</sup> between its real parameters and those assumed in the model, which reveals a good stability of the deposition process established.
