A photodetector including a photodiode formed in a semiconductor substrate and a waveguide element formed of a block of a high-index material extending above the photodiode in a thick layer of a dielectric superposed to the substrate, the thick layer being at least as a majority formed of silicon oxide and the block being formed of a polymer of the general formula R.sub.1R.sub.2R.sub.3SiOSiR.sub.1R.sub.2R.sub.3 where R.sub.1, R.sub.2, and R.sub.3 are any carbonaceous or metal substituents and where one of R.sub.1, R.sub.2, or R.sub.3 is a carbonaceous substituent having at least four carbon atoms and/or at least one oxygen atom.