Silicon nitride (SiNx) is a promising material for photonic integrated circuits due to its higher power-handling capability compared to silicon, owing to the absence of two-photon absorption in the telecom regime. This makes SiNx particularly suitable for nonlinear processes like optomechanics. The interactions between photons and phonons enable for instance cooling, quantum information storage, and RF-to-optical transduction.
To design SiNx-based optomechanical devices with GHz-level mechanical modes, knowledge of the full photoelastic tensor is essential. For amporphous SiNx, the tensor is uniquely characterized by p11 and p12. So far, Gyger et al. have measured |p12|=0.047 [1]. In this work, we determine both p11 and p12 by using a silicon nitride microgear cavity [2], enabling independent interaction of TE-like and TM-like whispering gallery modes with a 1.82 GHz mechanical breathing mode at the microgear’s edge. The device was fabricated by Ligentec, a Swiss silicon nitride foundry. We used buffered HF to under-etch the device that was embedded in silica. Optical quality factors of 105 were measured through optical transmission spectroscopy.
We use optomechanically induced absorption (OMIA) to determine the vacuum optomechanical coupling rate g0. Comparing the experimental data together with numerical results, with the previously obtained results from Gyger et al. [1], we find the values of p11 and p12, which have opposite signs, indicating that the photoelasticity of SiNx is closer to silicon than to silica, yet stronger. We anticipate that this work will serve as a crucial milestone in advancing SiNx optomechanics.
[1] F. Gyger, J. Liu, F. Yang, J. He, A.S. Raja, R. N. Wang, S.A. Bhave, T.J. Kippenberg, L. Thévenaz, Phys. Rev. Lett. 124, 013902, (2020).
[2] R.O. Zurita, C.M. Kersul, N.J. Schilder, G.S. Wiederhecker, and T.P.M. Alegre, J. Opt. Soc. Am. B 42, 1-6 (2025)