Waveguides and resonators are core components in electronics, photonics, and phononics, both in existing and future scenarios. In certain situations (space or frequency), critical coupling can occur between the two components, i.e., no energy passes through the waveguide after the incoming wave is coupled into the resonator. The transmission spectral characteristics resulting from this phenomenon are highly advantageous for signal filtering, switching, multiplexing, sensing, etc. However, under the existing mechanism, the occurrence of critical couplings always leads to increased reflection in the input channel due to the inevitable backscattering in practice. These reflection will further induce both intra- and interchannel crosstalk (noise) in an integrated system, whose accumulation will tend to generate large performance degradations, or even result in rapid failure of system functions. Unlike the electronic system, a passive integrated photonic or phononic diode has not been put into practical use thus far, although many notable attempts have been made. Therefore, avoiding input reflections, especially in spectral functional devices, poses a challenge for further development of integrated photonic or phononic circuitry.