  The optical properties of silicon can be greatly tuned by applying strain and
opening new perspectives, particularly in applications where infrared is key.
In this work, we use a recent model for the indirect light absorption of
silicon and include the effects of tensile and compressive uniaxial strains.
The model is based on material properties such as the bandgap, the conduction
and valence band density-of-states effective masses, and the phonon
frequencies, which are obtained from first principles including strain up to
+2% along the [110] and [111] directions. We show that the limit of absorption
can increase from 1.14 (1.09) to 1.35 $\mu$m (0.92 eV) under 2% strain and that
the absorption increases by a factor of 55 for the zero-strain cutoff
wavelength of 1.14 $\mu$m when a 2% compressive strain is applied in the [110]
direction. We demonstrate that this effect is mainly due to the impact of
strain on the electronic bandgaps of silicon, directly followed by the valence
band density-of-states effective mass.
