  We present the results of variable-angle spectroscopic ellipsometry and
transmittance measurements to determine the variation of the complex refractive
index of ion implanted single-crystal diamond. An increase is found in both
real and imaginary parts at increasing damage densities. The index depth
variation is determined in the whole wavelength range between 250 and 1690 nm.
The dependence from the vacancy density is evaluated, highlighting a deviation
from linearity in the high-damage-density regime. A considerable increase (up
to 5%) in the real part of the index is observed, attributed to an increase in
polarizability, thus offering new microfabrication possibilities for waveguides
and other photonic structures in diamond.
