  We propose a novel dielectric bow-tie nanocavity consisting of two tip-to-tip
opposite triangle semiconductor nanowires, whose end faces are coated by silver
nanofilms. Based on the advantages of the dielectric slot and tip structures,
and the high reflectivity from the silver mirror, light can be confined in this
nanocavity with low loss. We demonstrate that the mode excited in this
nanocavity has a deep subwavelength mode volume of 2.8*10^-4 um3 and a high
quality factor of 4.9*10^4 (401.3), consequently an ultrahigh Purcell factor of
1.6*10^7 (1.36*10^5), at 4.5 K (300 K) around the resonance wavelength of 1550
nm. This dielectric bow-tie nanocavity may find applications for integrated
nanophotonic circuits, such as high-efficiency single photon source,
thresholdless nanolaser, and cavity QED strong coupling experiments.
