Cyclotron resonance overtones and near-field magnetoabsorption via terahertz Bernstein modes in graphene

Two-dimensional electron systems subjected to a perpendicular magnetic field absorb electromagnetic radiation via the cyclotron resonance (CR). Here we report a qualitative breach of this well-known behaviour in graphene. Our study of the terahertz photoresponse reveals a resonant burst at the main overtone of the CR, drastically exceeding the signal detected at the position of the ordinary CR. In accordance with the developed theory, the photoresponse dependencies 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, ultra-slow magnetoplasmonic excitations reshaped by nonlocal electron dynamics. Close to the CR harmonics, these modes are characterized by a flat dispersion and a diverging plasmonic density of states that strongly amplifies the radiation absorption. Besides fundamental interest, our experimental results and developed theory show that the radiation absorption via nonlocal collective modes can facilitate a strong photoresponse, a behaviour potentially useful for infrared and terahertz technology.