Abstract
Two-dimensional electron systems subjected to a perpendicular magnetic field absorb electromagnetic radiation via cyclotron resonance (CR). Here we report a qualitative deviation from this well-known behaviour in graphene. Our measurements of the terahertz photoresponse reveal a resonant burst at the main overtone of the CR that exceeds the signal detected at the position of the ordinary CR. The dependencies of photoresponse on the magnetic field, doping level and sample geometry suggest that the origin of this anomaly lies in the near-field magnetoabsorption facilitated by the Bernstein modes—ultraslow magnetoplasmonic excitations reshaped by non-local electron dynamics. Close to CR harmonics, these modes are characterized by a flat dispersion and diverging plasmonic density of states that amplify radiation absorption. Besides carrying fundamental interest, our results show that radiation absorption via non-local collective modes can facilitate a strong photoresponse—a behaviour potentially useful for infrared and terahertz technology. Electrons in an external magnetic field absorb electromagnetic radiation via cyclotron resonance. Deviations from this behaviour in the form of overtone resonances due to ultraslow magnetoplasmonic excitations are now reported for graphene.
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