Sub-ppm Nanomechanical Absorption Spectroscopy of Silicon Nitride.

Material absorption is a key limitation in nanophotonic systems; however, its characterization is often obscured by scattering and diffraction. Here we show that nanomechanical frequency spectroscopy can be used to characterize material absorption at the parts per million level and use it to characterize the extinction coefficient κ of stoichiometric silicon nitride (SiN). Specifically, we track the frequency shift of a high- SiN trampoline in response to laser photothermal heating and infer κ from a model including stress relaxation and both conductive and radiative heat transfer.