Extraction of Elastooptic Coefficient of Thin-Film Arsenic Trisulfide Using a Mach–Zehnder Acoustooptic Modulator on Lithium Niobate

An acousto-Optic Modulator (AOM) formed by an Arsenic Trisulfide (As
$_2$
S
$_3$
) Mach–Zehnder interferometer (MZI), operating in a push-pull configuration and placed inside a surface acoustic wave (SAW) cavity on a Y-cut Lithium Niobate (LN) wafer is demonstrated. This is the first demonstration of such acousto-optic (AO) modulator in this As
$_2$
S
$_3$
-LN hybrid platform. In this approach, the high index contrast of As
$_2$
S
$_3$
waveguides is exploited to attain a high optical confinement. Additionally, the placement of the photonic MZI inside the SAW cavity enhances the AO interaction due to the high quality factor (Q) of the SAW resonator. An analytical expression that describes such enhancement as a function of the elasto-optic (EO) coefficient of As
$_2$
S
$_3$
is derived in conjunction with COMSOL finite element methods to describe strain in the SAW cavity. By fitting this expression to experimental data, the EO coefficient (
$p_{11}$
and
$p_{12}$
, where
$p_{11} \approx p_{12}$
) for As
$_2$
S
$_3$
is extracted to be 0.29 at the wavelength of 1550 nm. This is the first time that the EO coefficient of thin film As
$_2$
S
$_3$
is experimentally derived.