  We study optical properties of two dimensional silicene using density
functional theory based calculations. Our results on optical response property
calculations show that they strongly depend on direction of polarization of
light, hence the optical absorption spectra are different for light polarized
parallel and perpendicular to plane of silicence. The optical absorption
spectra of silicene possess two major peaks: (i) a sharp peak at 1.74 eV due to
transition from pi to pi* states and (ii) a broad peak in range of 4-10 eV due
to excitation of sigma states to conduction bands. We also investigate the
effect of external influences such as (a) transverse static electric field and
(b) doping of hydrogen atoms (hydrogenation) on optical properties of silicene.
Firstly, with electric field, it is observed that band gap can be opened up in
silicene at Fermi level by breaking the inversion symmetry. We see appreciable
changes in optical absorption due to band gap opening. Secondly, hydrogenation
in silicene strongly modifies the hybridization and our geometry analysis
indicates that the hybridization in silicene goes from mixture of sp^2 + sp^3
to purely sp^3. Therefore, there is no pi electron present in the system.
Consequently, the electronic structure and optical absorption spectra of
silicene get modified and it undergoes a transition from semi-metal to
semiconductor due to hydrogenation.
