  The optical properties of a hexagonal Boron Nitride (BN) monolayer across the
UV spectrum are studied by tuning its planar buckling. The strong
$\sigma\text{-}\sigma$ bond through sp$^2$ hybridization of a flat BN monolayer
can be changed to a stronger $\sigma\text{-}\pi$ bond through sp$^3$
hybridization by increasing the planar buckling. This gives rise to the $s$-
and $p$-orbital contributions to form a density of states around the Fermi
energy, and these states dislocate to a lower energy in the presence of an
increased planar buckling. Consequently, the wide band gap of a flat BN
monolayer is reduced to a smaller band gap in a buckled BN monolayer enhancing
its optical activity in the Deep-UV region. The optical properties such as the
dielectric function, the reflectivity, the absorption, and the optical
conductivity spectra are investigated. It is shown that the absorption rate can
be enhanced by $(12\text{-}15)\%$ for intermediate values of planar buckling in
the Deep-UV region, and $(15\text{-}20)\%$ at higher values of planar buckling
in the near-UV region. Furthermore, the optical conductivity is enhanced by
increased planar buckling in both the visible and the Deep-UV regions depending
on the direction of the polarization of the incoming light. Our results may be
useful for optoelectronic BN monolayer devices in the UV range including UV
spectroscopy, deep-UV communications, and UV photodetectors.
