Broadband optical characterization of silicon nitride membranes

Determine the optical properties of the silicon nitride membrane used to support graphene over a sufficiently broad frequency range to obtain a Kramers–Kronig-consistent dielectric response for reliable Casimir-force calculations.

Background

The Casimir-force calculations for graphene-covered suspended silicon nitride rely on incomplete optical data and modeling assumptions. In particular, the available dielectric-function measurements do not span a sufficiently broad frequency range for the Kramers–Kronig integration to reproduce the known low-frequency dielectric response consistently. The uncertainty is consequential because the optical properties may vary with silicon nitride composition and deposition conditions, contributing to discrepancies between calculated and measured force gradients and preventing a clear experimental resolution of thermal Casimir effects.

The paper identifies direct broadband measurement of the optical properties of the silicon nitride layer used in the experiment as a concrete route for resolving this uncertainty and improving the comparison between theory and experiment.

References

First, as described in earlier sections, the dielectric function of silicon nitride is not known over a sufficiently large frequency range for the Kramers-Kronig integration to give consistent values with the dielectric function at low, real frequencies.

— Casimir forces on suspended thin dielectric membranes covered with graphene  (2610.21437 - Ngai et al., 15 Oct 2026) in Section 3, “Measurement results”