  Fizeau demonstrated in 1850 that the speed of light can be modified when it
is propagating in moving media. Can we achieve such control of the light speed
efficiently with a fast-moving electron media by passing electrical current?
Because the strong electromagnetic coupling between the electron and light
leads to the collective excitation of plasmon polaritons, it will manifest as
the plasmonic Doppler effect. Experimental observation of the plasmonic Doppler
effect in electronic system has been challenge because the plasmon propagation
speed is much faster than the electron drift velocity in conventional noble
metals. Here, we report direct observation of Fizeau drag of plasmon polaritons
in strongly biased graphene by exploiting the high electron mobility and the
slow plasmon propagation of massless Dirac electrons. The large bias current in
graphene creates a fast drifting Dirac electron medium hosting the plasmon
polariton. It results in nonreciprocal plasmon propagation, where plasmons
moving with the drifting electron media propagate at an enhanced speed. We
measure the Doppler-shifted plasmon wavelength using a cryogenic near-field
infrared nanoscopy, which directly images the plasmon polariton mode in the
biased graphene at low temperature. We observe a plasmon wavelength difference
up to 3.6% between plasmon moving along and against the drifting electron
media. Our findings on the plasmonic Doppler effect open new opportunities for
electrical control of non-reciprocal surface plasmon polaritons in
nonequilibrium systems.
