  We report experimental studies of a driven spin-mechanical system, in which a
nitrogen vacancy (NV) center couples to out-of-plane vibrations of a diamond
cantilever through the excited-state deformation potential. Photoluminescence
excitation studies show that in the unresolved sideband regime and under strong
resonant mechanical driving, the excitation spectra of a NV optical transition
feature two spectrally sharp peaks, corresponding to the two turning points of
the oscillating cantilever. In the limit that the strain-induced frequency
separation between the two peaks far exceeds the NV zero-phonon linewidth, the
spectral position of the individual peak becomes sensitive to minute detuning
between the mechanical resonance and the external driving force. For a fixed
optical excitation frequency near the NV transition, NV fluorescence as a
function of mechanical detuning features resonances with a linewidth that can
be orders of magnitude smaller than the intrinsic linewidth of the mechanical
mode. This enhanced sensitivity to mechanical detuning can potentially provide
an effective mechanism for mechanical sensing, for example, mass sensing via
measurements of induced changes in the mechanical oscillator frequency.
