  Recent decades have seen significant advancements in integrated photonics,
driven by improvements in nanofabrication technology. This field has developed
from integrated semiconductor lasers and low-loss waveguides to optical
modulators, enabling the creation of sophisticated optical systems on a chip
scale capable of performing complex functions like optical sensing, signal
processing, and metrology. The tight confinement of optical modes in photonic
waveguides further enhances the optical nonlinearity, leading to a variety of
nonlinear optical phenomena such as optical frequency combs, second-harmonic
generation, and supercontinuum generation. Active tuning of photonic circuits
is crucial not only for offsetting variations caused by fabrication in
large-scale integration, but also serves as a fundamental component in
programmable photonic circuits. Piezoelectric actuation in photonic devices
offers a low-power, high-speed solution and is essential in the design of
future photonic circuits due to its compatibility with materials like Si and
Si3N4, which do not exhibit electro-optic effects. Here, we provide a detailed
review of the latest developments in piezoelectric tuning and modulation, by
examining various piezoelectric materials, actuator designs tailored to
specific applications, and the capabilities and limitations of current
technologies. Additionally, we explore the extensive applications enabled by
piezoelectric actuators, including tunable lasers, frequency combs, quantum
transducers, and optical isolators. These innovative ways of managing photon
propagation and frequency on-chip are expected to be highly sought after in the
future advancements of advanced photonic chips for both classical and quantum
optical information processing and computing.
