  Using an optical probe along with single photon detection we have performed
effective phonon counting measurements of the acoustic emission and absorption
processes in a nanomechanical resonator. Applying these measurements in a
Hanbury Brown and Twiss set-up, phonon correlations of the nanomechanical
resonator are explored from below to above threshold of a parametric
instability leading to self-oscillation of the resonator. Discussion of the
results in terms of a "phonon laser", and analysis of the sensitivity of the
phonon counting technique are presented.
