  When high-frequency radiation is incident upon graphene subjected to a
perpendicular magnetic field, graphene absorbs incident photons by allowing
transitions between nearest LLs that follow strict selection rules dictated by
angular momentum conservation. Here we show a qualitative deviation from this
behavior in high-quality graphene devices exposed to terahertz (THz) radiation.
We demonstrate the emergence of a pronounced THz-driven photoresponse, which
exhibits low-field magnetooscillations governed by the ratio of the frequency
of the incoming radiation and the quasiclassical cyclotron frequency. We
analyze the modifications of generated photovoltage with the radiation
frequency and carrier density and demonstrate that the observed photoresponse
shares a common origin with microwave-induced resistance oscillations
previously observed in GaAs-based heterostructures, yet in graphene, it appears
at much higher frequencies and persists above liquid nitrogen temperatures. Our
observations expand the family of radiation-driven phenomena in graphene and
offer potential for the development of novel optoelectronic devices.
