  The band structure and the optical conductivity of an ABA (Bernal-type)
stacked graphene trilayer are calculated. It is shown that, under appropriate
doping, a strong resonant peak develops in the optical conductivity, located at
the frequency corresponding to approximately 1.4 times the interlayer hopping
energy and caused by the 'nesting' of two nearly parabolic bands in the
electronic spectrum. The intensity of this resonant absorption can be
controlled by adjusting the gate voltage. The effect is robust with respect to
increasing temperature.
