We explore the dynamics of a graphene nanomechanical oscillator coupled to a
reference oscillator. Circular graphene drums are forced into self-oscillation,
at a frequency fosc, by means of photothermal feedback induced by illuminating
the drum with a continuous-wave red laser beam. Synchronization to a reference
signal, at a frequency fsync, is achieved by shining a power-modulated blue
laser onto the structure. We investigate two regimes of synchronization as a
function of both detuning and signal strength for direct (fsync = fosc) and
parametric locking (fsync = 2fosc). We detect a regime of phase resonance,
where the phase of the oscillator behaves as an underdamped second-order
system, with the natural frequency of the phase resonance showing a clear
power-law dependence on the locking signal strength. The phase resonance is
qualitatively reproduced using a forced van der Pol-Duffing-Mathieu equation.Comment: 11 pages, 4 figures, 25 reference