  We report on the modification of the optical and mechanical properties of a
silicon 1D optomechanical crystal cavity due to thermo-optic effects in a high
phonon/photon population regime. The cavity heats up due to light absorption in
a way that shifts the optical modes towards longer wavelengths and the
mechanical modes to lower frequencies. By combining the experimental optical
results with finite-difference time-domain simulations we establish a direct
relation between the observed wavelength drift and the actual effective
temperature increase of the cavity. By assuming that the Young's modulus
decreases accordingly to the temperature increase, we find a good agreement
between the mechanical mode drift predicted using a finite element method and
the experimental one.
