  Light concentration with strong temporal and spatial confinement is crucial
for tailoring light-matter interaction. Electromagnetic cavity modes in
photonic and plasmonic resonators provide platforms for optical field
localization. Here, we propose a concept of quasi-bound states in the continuum
gap cavity and reveal that ultra spatiotemporal confinements in free-space can
be realized in a dielectric nanocavity metasurface. By introducing an
asymmetric air slot in a nanodisk resonator, an ultra-high quality factor
$Q\sim 10^6$, accompanying an ultra-small effective mode volume, $V_m \sim
10^{-2}$ $(\lambda/n)^3$ are achieved resulting in a Purcell factor of
$10^6(\lambda/n)^{-3}$ in the visible wavelength range. The toroidal dipole
drives the electric and magnetic field concentration in the air gap with a
generated vortex polarizing electric field. As an alternative to plasmonic and
photonic crystal cavities, our study provides a more intriguing platform for
engineering light-matter interaction to advance a plethora of fundamental
studies and device applications, such as Purcell factor enhancement, room
temperature strong coupling and nonlinear nanophotoncis.
