Plasma-enhanced chemical vapor deposition (PECVD) of dimethylsilane and hexamethyldisilane produced thin films with a refractive index of 1.56 +/- 0.01 at 633 nm. A decrease in the refractive index of approximately 3% was observed after irradiation with UV light from an ArF laser operating at 193 nm. Lower-intensity UV light from a Hg arc lamp induced a slower and controllable decrease in the refractive index. Top-side prism coupling showed the as-deposited organosilicon films to be multimode at 633 nm and single mode at 1550 nm. A model predicted that 30 s of UV irradiation with the Hg arc lamp would decrease the refractive index of the light-guiding film by approximately 0.01, converting the waveguide into single-mode operation across the spectrum of essential wavelengths for microphotonics. Irradiation followed by further coupling experiments confirmed this tunability. Trimming the refractive index of patternable organosilicon polymeric films presents a method of optimizing the coupling performance of PECVD microphotonic interconnect layers postdeposition.