| Summary: | Besides the well‐known negative refraction, a negative refractive‐index material can exhibit another two hallmark features, which are the inverse Doppler effect and backward Cherenkov radiation. The former is known as the motion‐induced frequency shift that is contrary to the normal Doppler effect, and the latter refers to the Cherenkov radiation whose cone direction is opposite to the source's motion. Here these two features are combined and the Doppler effect inside the backward Cherenkov cone is discussed. It is revealed that the Doppler effect is not always inversed but can be normal in negative refractive‐index systems. A previously un‐reported phenomenon of normal Doppler frequency shift is proposed in a regime inside the backward Cherenkov cone, when the source's velocity is two times faster than the phase velocity of light. A realistic metal–insulator–metal structure, which supports metal plasmons with an effective negative refractive index, is adopted to demonstrate the potential realization of this phenomenon.
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