  A highly efficient on-chip acousto-optic modulator, as a key component,
occupies an exceptional position in microwave-to-optical conversion.
Homogeneous thin-film lithium niobate is preferentially employed to build the
suspended configuration forming the acoustic resonant cavity to improve the
modulation efficiency of the device. However, the limited cavity length and
complex fabrication recipe of the suspended prototype restrain further
breakthrough in the modulation efficiency and impose challenges for waveguide
fabrication. In this work, based on a nonsuspended thin-film lithium
niobate-chalcogenide glass hybrid Mach-Zehnder interferometer waveguide
platform, we propose and demonstrate a built-in push-pull acousto-optic
modulator with a half-wave-voltage-length product as low as 0.03 V cm,
presenting a modulation efficiency comparable to that of the state-of-the-art
suspended counterpart. Based on the advantage of low power consumption, a
microwave modulation link is demonstrated using our developed built-in
push-pull acousto-optic modulator. The nontrivial acousto-optic modulation
performance benefits from the superior photoelastic property of the
chalcogenide membrane and the completely bidirectional participation of the
antisymmetric Rayleigh surface acoustic wave mode excited by the
impedance-matched interdigital transducer, overcoming the issue of amplitude
differences of surface acoustic waves applied to the Mach-Zehnder
interferometer two arms in traditional push-pull acousto-optic modulators.
