Nonexponential Relaxation in the Rotating Frame of a Driven Nanomechanical Mode

We present direct observation of the ringdown dynamics in the rotating frame of a resonantly driven singlemode nonlinear nanomechanical resonator. An additional close-to-resonance harmonic force excites nonlinear oscillations about the fixed point in the rotating frame. When the secondary drive is removed, we measure decay of the in-phase and quadrature components toward this fixed point. We show that the decay of the in-phase signal is nonexponential, even though the vibration amplitude decays exponentially if both forces are switched off, revealing an unexpected interplay between conservative nonlinearity and dissipation. A minimalistic model captures these dynamics as well as the spectrum of the vibrations excited by the additional force, relating them to the dissipation-induced symmetry breaking of the dynamics in the rotating frame.