  We report narrowband transmission filtering based on zero-contrast grating
(ZCG) reflectors at normal incidence. Computational results show that the
filtering is realized through symmetry-protected modes coupling. The guided
modes introduced by the slab layer make the filter frequencies flexible to
modify. The rectangular structure of the filter allows simple fabrication and
integration into optical systems. The quality factor of the filters could
exceed 106. Owing to the low refraction index dispersion of the semiconductor
and their scale-invariant operations, these filters can be applied in a broad
infrared range from near infrared to terahertz wavelengths.
