  An accurate control of the optical properties of single crystal diamond
during microfabrication processes such as ion implantation plays a crucial role
in the engineering of integrated photonic devices. In this work we present a
systematic study of the variation of both real and imaginary parts of the
refractive index of single crystal diamond, when damaged with 2 and 3 MeV
protons at low-medium fluences (range: 10^15 - 10^17 cm^-2). After implanting
in 125x125 um^2 areas with a scanning ion microbeam, the variation of optical
pathlength of the implanted regions was measured with laser interferometric
microscopy, while their optical transmission was studied using a spectrometric
set-up with micrometric spatial resolution. On the basis of a model taking into
account the strongly non-uniform damage profile in the bulk sample, the
variation of the complex refractive index as a function of damage density was
evaluated.
