  TiO$_2$ anatase has its significant importance in energy and environmental
research. However, the major drawback of this immensely popular semi-conductor
is its large bandgap of 3.2 eV. Several non-metals have been doped
experimentally for extending the TiO$_2$ photo-absorption to the visible
region. Providing in-depth theoretical guidance to the experimentalists to
understand the optical properties of the doped system is therefore extremely
important. We report here using state-of-the-art hybrid density functional
approach and many body perturbation theory (within the frame work of GW and
BSE) the optical properties of p-type (S and Se doped) and n-type (N and C
doped) TiO$_2$ anatase. The anisotropy present in non-metal doped TiO$_2$ plays
a significant role in the optical spectra. The p-type dopants are optically
active only for light polarized along xy direction, whereas the n-type dopants
are optically active when light is polarized along xy and z direction in low
energy region. We have found that, in all the doped systems optically allowed
transitions are introduced well below 3 eV (i.e. visible spectra region). This
helps to improve its opto-electronic and solar absorption properties. All the
calculations are well validated with respect to the available experimental
observation on pristine TiO$_2$ anatase.

Materials Science