  We theoretically propose and numerically demonstrate that perfect feeding of
a polaritonic system with full electromagnetic energy under one-port beam
incidence, referred to as polaritonic critical coupling, can be achieved in a
hybrid dielectric metasurface-WS$_2$ monolayer structure. Polaritonic critical
coupling, where the critical coupling and strong coupling are simultaneously
attained, is determined by the relative damping rates of the cavity resonance,
$\rm \gamma_Q$, provided by a symmetry-protected quasi-bound states in the
continuum, and excitonic resonance of WS$_2$ monolayer, $\rm \gamma_X$. We
reveal that the population of the polariton states can be tuned by the
asymmetric parameter of the quasi-bound states in the continuum. Furthermore,
polaritonic critical coupling is achieved in the designed system while $\rm
\gamma_Q=\gamma_X$ and only strong coupling is achieved while $\rm
\gamma_Q\neq\gamma_X$. This work enriches the study of polaritonic physics with
controlled absorbance and may guide the design and application of efficient
polariton-based light-emitting or lasing devices.
