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Tailoring the interaction between light and sound has opened new possibilities in photonic integrated circuits (PICs) that range 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 interdigital transducers (IDTs) on the PZT film with a contact photolithography and electron-beam lithography to generate the acoustic waves in MHz and GHz ranges, respectively. We obtain a V π L ∼ 3.35 V•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 π L ∼ 3.60 V•cm at 2 GHz from a 100 nm thick aluminum (Al) IDT consisting of only four 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 a broadband and efficient acousto-optic modulator for future integration.

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Second-order nonlinear optical processes enable a wide range of applications used in research and industry. The majority of available second-order nonlinear devices however relies on bulk nonlinear crystals with low second-order nonlinearity. By exploiting the advancements made in integrated optics, materials with large second-order nonlinearity could enable efficient and small-sized on-chip nonlinear devices at low cost. Unfortunately, silicon and silicon nitride, mostly used for photonics integrated circuits exhibit negligible second-order nonlinearity (χ (2) )and alternate materials have to be investigated. PZT thin films with high second-order nonlinearity stand as a good candidate for on-chip nonlinearity. We demonstrate here an electric-field induced tuning of χ (2) in PZT thin films grown on glass substrates with a tuning efficiency of 3.35 pmV -2 . We record strong second-harmonic generation and report a very high dominant tensor component ( ) of 128 pmV -1 . The χ (2) of our PZT thin films can be reversed by poling with a DC electric field at room temperature. This opens avenues for highly efficient and tunable on-chip nonlinear devices.

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