  Axion insulators are 3D magnetic higher-order topological insulators
protected by inversion-symmetry that exhibit hinge-localized chiral channels
and induce quantized topological magnetoelectric effects. Recent research has
suggested that axion insulators may be capable of detecting dark-matter
axion-like particles by coupling to their axionic excitations. Beyond its
fundamental theoretical interest, designing a photonic AXI offers the potential
to enable the development of magnetically-tunable photonic switch devices
through the manipulation of the axionic modes and their chiral propagation
using external magnetic fields. Motivated by these facts, in this work, we
propose a novel approach to induce axionic band topology in gyrotropic 3D Weyl
photonic crystals gapped by supercell modulation. To quantize an axionic angle,
we create domain-walls across inversion-symmetric photonic crystals,
incorporating a phase-obstruction in the supercell modulation of their
dielectric elements. This allows us to bind chiral channels on
inversion-related hinges, ultimately leading to the realization of an axionic
chiral channel of light. Moreover, by controlling the material gyrotropic
response, we demonstrate a physically accessible way of manipulating the
axionic modes through a small external magnetic bias, which provides an
effective topological switch between different 1D chiral photonic fiber
configurations. Remarkably, the unidirectional axionic hinge states supported
by the photonic axion insulator are buried in a fully connected 3D dielectric
structure, thereby being protected from radiation through the electromagnetic
continuum. As a result, they are highly suitable for applications in
guided-light communication, where the preservation and non-reciprocal
propagation of photonic signals are of paramount importance.
