Exotic light transition between superluminal and subliminal group velocity in a $\mathcal{P}\mathcal{T}$ coupled slab waveguide

We propose a rigorous full-wave modal analysis based on the eigenmodes approach (\textquotedblleft supermodes\textquotedblright), that allows the calculation of the transverse electric ($TE$) and transverse magnetic ($TM$) stationary propagation eigenmodes in a coupled $\mathcal{P}\mathcal{T}$-symmetric slab waveguide. Our findings show that the group velocity, for the fundamental $T{E}_0$ supermode, passes from superluminal to subluminal speed over both sides of a critical point (CP), located near to the exceptional point (EP). We also demonstrate that the first higher-order supermode, $T{E}_1$, exhibits a slow light phenomenon around the EP.