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Higgs mode in superconductors, i.e. the collective amplitude mode of theorder parameter does not associate with charge nor spin fluctuations, thereforeit does not couple to the electromagnetic field in the linear response regime.On the contrary to this common understanding, here, we demonstrate that, if thedc supercurrent is introduced into the superconductor, the Higgs mode becomesinfrared active and is directly observed in the linear optical conductivitymeasurement. We observed a sharp resonant peak at $\omega=2\Delta$ in theoptical conductivity spectrum of a thin-film NbN in the presence of dcsupercurrent, showing a reasonable agreement with the recent theoreticalprediction. The method as proven by this work opens a new pathway to study theHiggs mode in a wide variety of superconductors.

Infrared activation of the Higgs mode by supercurrent injection in superconducting NbN