One possible way to achieve 100% reflection of a low order mode in an oversized waveguide is to usethe principle of transverse resonances. In a short resonant section a high order waveguide mode closeto cutoff is excited. If the coupling to the high order mode is equal to its losses and if the resonant phasecondition is fulfilled a 100% reflection of the low order mode is possible.
"Transverse resonances in oversized waveguides", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 21045Z (30 August 1993); doi: 10.1117/12.2298647; https://doi.org/10.1117/12.2298647