  Photonic molecules can be used to realize complex optical energy states and
modes, analogous to those found in molecules, with properties useful for
applications like spectral engineering and quantum optics. It is desirable to
implement photonic molecules using high quality factor photonic integrated ring
resonators due to their narrow atom-like spectral resonance, tunability, and
the ability to scale the number of resonators on a photonic circuit. However,
to take full advantage of molecule spectral complexity and tuning degree of
freedom, resonator structures should have full symmetry in terms of
inter-resonator coupling and resonator-waveguide coupling as well as
independent resonance tuning, and low power dissipation operation, in a
scalable integration platform. To date, photonic molecule symmetry has been
limited to dual- and triple-cavity geometries coupled to single- or
dual-busses, and resonance tuning limited to dual resonator molecules. In this
paper, we demonstrate a three-resonator photonic molecule, consisting of
symmetrically coupled 8.11 million intrinsic Q silicon nitride rings, where
each ring is coupled to the other two rings. The resonance of each ring, and
that of the collective molecule, is controlled using low power dissipation,
monolithically integrated thin-film lead zirconate titanate (PZT) actuators
that are integrated with the ultra-low loss silicon nitride resonators. This
performance is achieved without undercut waveguides, yielding the highest Q to
date for a PZT controlled resonator. This advance leads to full control of
complex photonic molecule resonance spectra and splitting in a wafer-scale
integration platform. The resulting six tunable supermodes can be fully
controlled, including degeneracy, location and splitting as well as designed by
a model that can accurately predict the energy modes and transmission spectrum
and tunable resonance splitting.
