  The rapid development of high-Q macroscopic mechanical resonators has enabled
great advances in optomechanics. Further improvements could allow for
quantum-limited or quantum-enhanced applications at ambient temperature. Some
of the remaining challenges include the integration of high-Q structures on a
chip, while simultaneously achieving large coupling strengths through an
optical read-out. Here, we present a versatile fabrication method, which allows
us to build fully integrated optomechanical structures. We place a photonic
crystal cavity directly above a mechanical resonator with high-Q fundamental
out-of-plane mode, separated by a small gap. The highly confined optical field
has a large overlap with the mechanical mode, enabling strong optomechanical
interaction strengths. Furthermore, we implement a novel photonic crystal
design, which allows for a very large cavity photon number, a highly important
feature for optomechanical experiments and sensor applications. Our versatile
approach is not limited to our particular design but allows for integrating an
out-of-plane optical read-out into almost any device layout. Additionally, it
can be scaled to large arrays and paves the way to realizing quantum
experiments and applications with mechanical resonators based on high-Q
out-of-plane modes alike.
