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.
D. Wagner, D. Wagner,
"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