Frequency Comb as a Probe of Fluctuation Mechanisms in Nanomechanical Systems

Frequency combs are indispensable in metrology. We show that a frequency comb also probes frequency fluctuations of the system. In our nanomechanical resonator, the comb arises from self-sustained vibrations of a resonantly driven nonlinear mode. The comb lineshapes reveal exponentially correlated (Ornstein--Uhlenbeck) frequency noise. Its correlation time agrees with the independently measured relaxation time of the vibrations. The noise leads to anomalous phase diffusion. Its mechanism is generic: like the comb itself, it stems from the strong nonlinearity of the self-sustained vibrations.