  Hybrid photonic-plasmonic cavities can be tailored to display high Q-factors
and extremely small mode volumes simultaneously, which results in large values
of the Purcell factor, FP. Amongst the different hybrid configurations, those
based on a nanoparticle-on-a-mirror (NPoM) plasmonic cavity provide one of the
lowest mode volumes, though so far their operation has been constrained to
wavelengths below 1 {\mu}m. Here, we propose a hybrid configuration consisting
of a silicon photonic crystal cavity with a slot at its center in which a gold
nanoparticle is introduced. This hybrid system operates at telecom wavelengths
and provides high Q-factor values (Q ${\approx} 10^{5}$) and small normalized
mode volumes (Vm ${\approx} 10^{-4}$), leading to extremely large Purcell
factor values, FP ${\approx} 10^{7}$ - ${10^{8}}$. The proposed cavity could be
used in different applications such as molecular optomechanics, bio- and
chemo-sensing, all-optical signal processing or enhanced Raman spectroscopy in
the relevant telecom wavelength regime.
