Acoustically Mediated Microwave-to-Optical Conversion on Thin-Film Lithium Niobate

Taking advantage of the strong piezoelectricity of lithium niobate (LN) and the high-quality (
$Q$
) -factor acoustic resonator around 3 GHz, on-chip microwave-to-optical conversion is demonstrated by our integrated acousto-optic Mach-Zehnder modulator on suspended LN thin film. Our devices achieve microwave-to-acoustic coupling efficiencies of 90%, and the half-wave voltage of
$V_{\pi}=3.9\ \mathrm{V}$
for only a
$400-\mu \mathrm{m}$
modulation length at the telecommunication optical wavelength. This result gives the half-wave-voltage-length product
$V_{\pi}L$
, the figure of merit for optical modulator, as low as 0.15 V·cm, which is a 10-fold reduction over the start-of-the-art electro-optic modulators. In addition, our acoustic device demonstrates an acoustic
$Q$
factor of 3,600 at 3.3 GHz that results in a start-of-the-art acoustic frequency-quality product of
$fQ=10^{13}$
at room-temperature.