Low-loss Sb$_2$S$_3$ Optical Phase Shifter Enabled by Optimizing Sputtering Conditions

By quantitatively evaluating the atomic concentrations of sputtered Sb$_2$S$_3$ films with different sputtering powers and Ar flows, we reveal that a sputtered Sb$_2$S$_3$ film becomes close to the stoichiometric composition as the sputtering power and Ar flow decrease. We characterize the optical properties of Sb$_2$S$_3$ and show that the lower sputtering power leads to a better figure of merit of Sb$_2$S$_3$ as an optical phase shifter in the near infrared (NIR) range. Based on these results, we achieve a loss per phase shift of 0.33 dB/$\pi$ at a wavelength of 1.55 $\mu$m, one of the lowest losses among Sb$_2$S$_3$ phase shifters in the NIR range.