  The magnetic circular dichroism and the Faraday rotation are the fundamental
phenomena of great practical importance arising from the breaking of the time
reversal symmetry by a magnetic field. In most materials the strength and the
sign of these effects can be only controlled by the field value and its
orientation. Furthermore, the terahertz range is lacking materials having the
ability to affect the polarisation state of the light in a non-reciprocal
manner. Here we demonstrate, using broadband terahertz magneto-electro-optical
spectroscopy, that in graphene both the magnetic circular dichroism and the
Faraday rotation can be modulated in intensity, tuned in frequency and,
importantly, inverted using only electrostatic doping at a fixed magnetic
field. In addition, we observe strong magneto-plasmonic resonances in a
patterned array of graphene antidots, which potentially allows exploiting these
magneto-optical phenomena in a broad THz range.
