Coupled acousto-optical phonons in 2D and bulk materials*

In piezoelectric materials acoustic and optical phonons are generally coupled. We present a detailed discussion of symmetry effects in zincblende materials (GaAs as an example) and 2D inversion-asymmetric structures such as molybdenum disulfide. We predict that acousto-optical phonons cannot exist at the LO phonon frequency where the dielectric constant vanishes. We show that confined optical phonon modes exist for the group-IV materials but not for molybdenum disulfide.

We also discuss the influence of crystal growth orientation effects and a simple Drude model for the permittivity in discussing excitations of coupled acousto-optical phonons in piezoelectric structures. It is verified that piezoelectric effects lead to a drastically enhanced coupling of acoustic and optical phonon modes and increase in the local phonon density of states near the plasma frequency where the permittivity approaches zero.

[1] M. Willatzen, L. C. Lew Yan Voon, A. N. Gandi, and U. Schwingenschlogl, Beilstein Journal of Nanotechnology, 8, 1345 (2017).
[2] M. Willatzen and Z. L. Wang, Phys. Rev. B 92, 224101 (2015).
[3] L. Duggen and M. Willatzen, Phys. Rev. B 95, 035310 (2017).