  Silicon nitride (SiN) is emerging as a competitive platform for
CMOS-compatible integrated photonics. However, active devices such as
modulators are scarce and still lack in performance. Ideally, such a modulator
should have a high bandwidth, good modulation efficiency, low loss, and cover a
wide wavelength range. Here, we demonstrate the first electro-optic modulators
based on ferroelectric lead zirconate titanate (PZT) films on SiN, in both the
O- and the C-band. Bias-free operation, bandwidths beyond 33 GHz and data rates
of 40 Gbps are shown, as well as low propagation losses ($\alpha\approx 1$
dB/cm). A $V_\pi L\approx$ 3.2 Vcm is measured. Simulations indicate that
values below 2 Vcm are achievable. This approach offers a much-anticipated
route towards high-performance phase modulators on SiN.
