  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.
