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The Doppler effect is a fundamental frequency shift phenomenon that occurs whenever a wave source and an observer are moving with respect to one another. It has well-established applications in astrophotonics, biological diagnostics, weather and aircraft radar systems, velocimetry and vibrometry. The counterintuitive inverse Doppler effect was theoretically predicted in 1968 by Veselago in negative-index materials. However, because of the tremendous challenges of frequency shift measurements inside such materials, most investigations of the inverse Doppler effect have been limited to theoretical predictions and numerical simulations^,,,,. Indirect experimental measurements have been conducted only in nonlinear transmission lines at ∼1–2 GHz (ref. ) and in acoustic media at 1–3 kHz (ref. ). The inverse Doppler shift at optical frequencies was demonstrated in an acousto-optic modulator using a fibre Bragg …