Amplification and spectral evidence of squeezing in the response of a strongly driven nanoresonator to a probe field

Because of their small decay rates, nanomechanical modes enable studying strongly nonlinear phenomena for a moderately strong resonant driving. Here we study the response of a driven resonator to an additional probe field. We experimentally demonstrate resonant amplification and resonant absorption of the probe field. The corresponding spectral peaks lie on the opposite sides of the strong-drive frequency. Even though the fluctuation-dissipation theorem does not apply, we show that the response to the probe field allows us to characterize the squeezing of fluctuations about the stable states of forced oscillations. Our two-tone experiment is done in the classical regime, but our findings should equally apply to quantum fluctuations as well. In quantum terms, the observed response is due to multiphoton processes. The squeezing parameter extracted from the spectra of the response is in excellent agreement with the calculated value with no free parameters.