  We computationally study the effect of uniaxial strain in modulating the
spontaneous emission of photons in silicon nanowires. Our main finding is that
a one to two orders of magnitude change in spontaneous emission time occurs due
to two distinct mechanisms: (A) Change in wave function symmetry, where within
the direct bandgap regime, strain changes the symmetry of wave functions, which
in turn leads to a large change of optical dipole matrix element. (B) Direct to
indirect bandgap transition which makes the spontaneous photon emission to be
of a slow second order process mediated by phonons. This feature uniquely
occurs in silicon nanowires while in bulk silicon there is no change of optical
properties under any reasonable amount of strain. These results promise new
applications of silicon nanowires as optoelectronic devices including a
mechanism for lasing. Our results are verifiable using existing experimental
techniques of applying strain to nanowires.
