  Strong coupling between the electronic states of monolayer transition metal
dichalcogenides (TMDC) such as MoS$_2$, MoSe$_2$, WS$_2$, or WSe$_2$, and a
two-dimensional (2D) photonic cavity gives rise to several exotic effects. The
Dirac type Hamiltonian for a 2D gapped semiconductor with large spin-orbit
coupling facilitates pure Jaynes-Cummings type coupling in the presence of a
single mode electric field. The presence of an additional circularly polarized
beam of light gives rise to valley and spin dependent cavity-QED properties.
The cavity causes the TMDC monolayer to act as an on-chip coherent light source
and a spontaneous spin-oscillator. In addition, a TMDC monolayer in a cavity is
a sensitive magnetic field sensor for an in-plane magnetic field.
