  We investigate optical Tamm states supported by a dielectric grating placed
on top of a distributed Bragg reflector. It is found that under certain
conditions the Tamm state may become a bound state in the continuum. The bound
state, in its turn, induces the effect of critical coupling with reflectance
amplitude reaching an exact zero. We demonstrate that the critical coupling
point is located in the core of a vortex of the reflection amplitude gradient
in space of wavelength and angle of incidence. The emergence of the vortex is
explained by the coupled mode theory.
