  The absorption spectra of silicon nanowires (SiNW) are calculated using
semi-empirical $sp^{3}d^{5}s^{*}$ tight binding (TB) and Density Functional
Theory (DFT) methods. The role of diameter, wave function symmetry, strain and
crystallographic direction in determining the absorption are discussed. We find
that compressive strain can change the band edge absorption by more than one
order of magnitude due to change in wave function symmetry. In addition, photon
polarization with respect to the nanowire axis significantly alters the band
edge absorption. Overall, the band edge absorption of [110] and [100] silicon
nanowires can differ by as much as three orders of magnitude. We find that
compared to bulk Silicon, a strained Silicon nanowire array can absorb infrared
photons (1.1 eV) approximately one hundred times better. Finally, we compare a
fully numerical and a computationally efficient semi-analytical method, and
find that they both yield satisfactory values of the band edge absorption.
