  Nanowires are versatile nanostructures, which allow an exquisite control over
bandgap energies and charge carrier dynamics making them highly attractive as
building blocks for a broad range of photonic devices. For optimal solutions
concerning device performance and cost, a crucial element is the selection of a
suitable material system which could enable a large wavelength tunability,
strong light interaction and simple integration with the mainstream silicon
technologies. The emerging GaBiAs alloys offer such promising features and may
lead to a new era of technologies. Here, we apply million-atom atomistic
simulations to design GaBiAs/GaAs core-shell nanowires suitable for low-loss
telecom-wavelength photonic devices. The effects of internal strain, Bi
Composition (x), random alloy configuration, and core-to-shell diameter ratio
($\rm \rho_D$) are analysed and delineated by systematically varying these
attributes and studying their impact on the absorption wavelength and charge
carrier confinement. The complex interplay between x and $\rm \rho_D$ results
in two distinct pathways to accomplish 1.55 um optical transitions: either
fabricate nanowires with $\rm \rho_D \geq$ 0.8 and $x \sim$15\%, or increase
$x$ to $\sim$30\% with $\rm \rho_D \leq$ 0.4. Upon further analysis of the
electron hole wave functions, inhomogeneous broadening and optical transition
strengths, the nanowires with $\rm \rho_D \leq$ 0.4 are unveiled to render
favourable properties for the design of photonic devices. Another important
outcome of our study is to demonstrate the possibility of modulating the strain
character from a compressive to a tensile regime by simply engineering the
thickness of the core region. The availability of such a straightforward knob
for strain manipulation would be highly desirable for devices involving
polarisation-sensitive light interactions.
