  We present a design and a top-down fabrication method for realizing diamond
nanowires in both bulk single crystal and polycrystalline diamond. Numerical
modeling was used to study coupling between a Nitrogen Vacancy (NV) color
center and optical modes of a nanowire, and to find an optimal range of
nanowire diameters that allows for large collection efficiency of emitted
photons. Inductively coupled plasma (ICP) reactive ion etching (RIE) with
oxygen is used to fabricate the nanowires. Drop-casted nanoparticles (including
$\mathrm{Au}$, $\mathrm{SiO_{2}}$ and $\mathrm{Al_2O_3}$) as well as electron
beam lithography defined spin-on glass and evaporated $\mathrm{Au}$ have been
used as an etch mask. We found $\mathrm{Al_2O_3}$ nanoparticles to be the most
etch resistant. At the same time FOx e-beam resist (spin-on glass) proved to be
a suitable etch mask for fabrication of ordered arrays of diamond nanowires. We
were able to obtain nanowires with near vertical sidewalls in both
polycrystalline and single crystal diamond. The heights and diameters of the
polycrystalline nanowires presented in this paper are $\unit[\approx1]{\mu m}$
and $\unit[120-340]{nm}$, respectively, having a $\unit[200]{nm/min}$ etch
rate. In the case of single crystal diamond (types Ib and IIa) nanowires the
height and diameter for different diamonds and masks shown in this paper were
$\unit[1-2.4]{\mu m}$ and $\unit[120-490]{nm}$ with etch rates between
$\unit[190-240]{nm/min}$.
