  Extensive research has explored the optical properties of topological
insulating materials, driven by their inherent stability and potential
applications. In this study, we unveil a novel functionality of
three-dimensional integer quantum Hall (3D IQH) states as broad-band filters
for circularly polarized light, particularly effective in the terahertz (THz)
frequency range under realistic system parameters. We also investigate the
impact of practical imperfections, demonstrating the resilience of this
filtering effect. Our findings reveal that this phenomenon is independent of
the microscopic origin of the 3D IQH state, prompting discussions on its
feasibility across diverse candidate materials. These results contribute to our
understanding of fundamental optical properties and hold promise for practical
applications in optical technologies.
