  Efficient transmissive pure-phase resonances are highly desirable for optical
modulation and wavefront engineering. Here, we propose a novel principle to
realize a pure-phase resonance in an extremely broad transmission band, as
opposed to previous approaches restricted to operating in reflection mode or
over a narrow spectral band. We show that a glide-symmetric bilayer metasurface
mathematically mimicking a two-dimensional Dirac semimetal induces
unidirectional guided-mode excitation and perfect leakage-radiation blazing at
the transmission channel. These effects create a peculiar resonant-scattering
configuration, similar to the classical reflective Gires-Tournois etalon, but
in transmission, providing full 2pi phase modulation with constant
transmittance near 100%. Most importantly, this effect persists over an
extremely wide band, associated with topological effects. Hence, our proposed
approach produces a spectrally and parametrically robust pure-phase resonance
effect in transmission, which is highly beneficial for practical applications.
