  A fine control of the variation of the refractive index as a function of
structural damage is essential in the fabrication of diamond-based optical and
photonic devices. We report here about the variation of the real part of the
refractive index at lambda=632.8 nm in high quality single crystal diamond
damaged with 2 and 3 MeV protons at low-medium fluences (10^13 - 10^17 ions
cm^-2). After implanting the samples in 125x125 um^2 areas with a raster
scanning ion microbeam, the variation of optical thickness of the implanted
regions was measured with laser interferometric microscopy. The results were
analyzed with a model based on the specific damage profile. The technique
allows the direct fabrication of optical structures in bulk diamond based on
the localized variation of the refractive index, which will be explored in
future works.
