SUB-PPM NANOMECHANICAL ABSORPTION SPECTROSCOPY OF SILICON

Abstract

Material absorption is a key limitation in nanophotonic systems; however, its characterization is often obscured by scattering and diffraction loss. Here we show that nanomechanical frequency spectroscopy can be used to characterize the absorption of a dielectric thin film at the parts-per-million (ppm) level, and use it to characterize the absorption of stoichiometric silicon nitride (Si3N4), a ubiquitous low-loss optomechanical material. Specifically, we track the frequency shift of a high-Q Si3N4 trampoline resonator in response to photothermal heating by a ~ 10 mW laser beam, and infer the absorption of the thin film from a model including thermal stress relaxation and both radiative and conductive heat transfer. A key insight is the presence of two thermalization timescales, a rapid (~ 0.1 sec) timescale due to radiative thermalization of the Si3N4 thin film, and a slow (~ 100 sec) timescale due to parasitic heating of the Si device chip. We infer the extinction coefficient of Si3N4 to be ~ 0.1 - 1 ppm in the 532 - 1550 nm wavelength range, comparable to bounds set by waveguide resonators and notably lower than estimates with membrane-in-the-middle cavity optomechanical systems. Our approach is applicable to a broad variety of nanophotonic materials and may offer new insights into their potential.