Silicon (Si) photonics is a highly promising platform for integrated optoelectronics. The ability to leverage existing CMOS infrastructure and the high index contrast between silicon and its native oxide have enabled compact, low-loss, and relatively low-cost photonic integrated circuits. Recently, there has been a drive towards integrating other CMOS-compatible materials in the silicon photonics platform, with silicon nitride (SiN) on the forefront. SiN offers a wider transparency range, lower temperature sensitivity and lower linear and nonlinear losses than Si. Many potential applications would benefit from having efficient, low-loss and fast phase modulators in these platforms. However, current solutions still lack in performance, plasma dispersion-based modulators in silicon are intrinsically lossy and suffer from spurious amplitude modulation. On SiN, simply no viable solutions with bandwidths beyond the MHz range exist. We tackle these issues by co-integration of thin films of lead zirconate titanate (PZT) onto integrated photonic structures. PZT is known to exhibit a strong Pockels effect, as well as related second order optical nonlinearities. This enables us to harness these effects for electro-optic modulation and other applications on existing Si and SiN platforms. In this work, we summarize our recent results. Including the demonstration of phase modulation using PZT-on-SiN waveguides in both the C- and the O-band, with bias-free operation, data rates of at least 40 Gbps and bandwidths beyond 33 GHz. We moreover demonstrate efficient phase modulation with a half-wave voltage length product of 3.2 Vcm and low propagation losses. Simulations indicate that further improvements are possible.