Normal Doppler frequency shift in negative refractive-index systems

TL;DR

This paper reveals that in negative refractive-index materials, the Doppler effect can be normal rather than inverse inside the backward Cherenkov cone when the source moves at twice the phase velocity of light, supported by a metal-insulator-metal structure.

Contribution

It introduces the novel concept that the Doppler effect can be normal in negative refractive-index systems within the backward Cherenkov cone, contrary to previous assumptions.

Findings

Normal Doppler shift occurs inside the backward Cherenkov cone at high source velocities.

A metal-insulator-metal structure can realize the conditions for this phenomenon.

The Doppler effect is not always inversed in negative refractive-index materials.

Abstract

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 we combine these two features and discuss the Doppler effect inside the backward Cherenkov cone. We reveal 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.