  Cavity-enhanced radiation-pressure coupling of optical and mechanical degrees
of freedom gives rise to a range of optomechanical phenomena, in particular
providing a route to the quantum regime of mesoscopic mechanical oscillators. A
prime challenge in cavity optomechanics has however been to realize systems
which simultaneously maximize optical finesse and mechanical quality. Here we
demonstrate for the first time independent control over both mechanical and
optical degree of freedom within one and the same on-chip resonator. The first
direct observation of mechanical normal mode coupling in a micromechanical
system allows for a quantitative understanding of mechanical dissipation.
Subsequent optimization of the resonator geometry enables intrinsic material
loss limited mechanical Q-factors, rivalling the best values reported in the
high MHz frequency range, while simultaneously preserving the resonators'
ultra-high optical finesse. Besides manifesting a complete understanding of
mechanical dissipation in microresonator based optomechanical systems, our
results provide an ideal setting for cavity optomechanics.
