  Driven non-linear resonators can display sharp resonances or even multistable
behaviours amenable to induce strong enhancements of weak signals. Such
enhancements can make use of the phenomenon of vibrational resonance whereby a
weak low-frequency signal applied to a bistable resonator can be amplified by
driving the non-linear oscillator with another appropriately-adjusted
non-resonant high-frequency field. Here we demonstrate the resonant enhancement
of a weak signal by use of a vibrational force yet in a monostable system
consisting of a driven nano-electromechanical nonlinear resonator. The
oscillator is subjected to a strong quasi-resonant drive and to two additional
tones: a weak signal at lower frequency and a non-resonant driving at an
intermediate frequency. We show experimentally and theoretically a significant
enhancement of the weak signal thanks to the occurence of vibrational resonance
enabled by the presence of the intermediate frequency driving. We analyse this
phenomenon in terms of coherent nonlinear resonance manipulation. Our results
illustrate a general mechanism which may have applications in the fields of
radio-frequency signal processing or sensing for instance.
