  Tailoring the interaction between light and sound has opened new
possibilities in photonic integrated circuits (PICs) that ranges from achieving
quantum control of light to high-speed information processing. However, the
actuation of sound waves in Si PICs usually requires integration of a
piezoelectric thin film. Lead Zirconate Titanate (PZT) is a promising material
due to its strong piezoelectric and electromechanical coupling coefficient.
Unfortunately, the traditional methods to grow PZT on Silicon are detrimental
for photonic applications due to the presence of an optical lossy intermediate
layer. In this work, we report integration of a high quality PZT thin film on a
Silicon-on-insulator (SOI) photonic chip using an optically transparent buffer
layer. We demonstrate acousto-optic modulation in Silicon waveguides with the
PZT actuated acoustic waves. We fabricate inter digital transducers (IDTs) on
the PZT film with a contact photo-lithography and electron-beam lithography to
generate the acoustic waves in MHz and GHz range respectively. We obtain a
V$_{\pi}$L $\sim$ 3.35 V$\cdot$cm at 576 MHz from a 350 nm thick gold (Au) IDT
with 20 finger-pairs. After taking the effect of mass-loading and grating
reflection into account, we measured a V$_{\pi}$L $\sim$ 3.60 V$\cdot$cm at 2
GHz from a 100 nm thick Aluminum (Al) IDT consisting of only 4 finger-pairs.
Thus, without patterning the PZT film nor suspending the device, we obtained
figures-of-merit comparable to state-of-the-art modulators based on SOI, making
it a promising candidate for broadband and efficient acousto-optic modulator
for future integration.
