Sub-ppm Nanomechanical Absorption Spectroscopy of Silicon Nitride

Area of Science:

• Nanophotonics
• Materials Science
• Mechanical Engineering

Background:

• Material absorption is a critical challenge in nanophotonic systems.
• Characterizing absorption is difficult due to scattering and diffraction.
• Accurate measurement of optical properties like extinction coefficient is essential.

Purpose of the Study:

• To develop a novel method for characterizing material absorption at the parts-per-million level.
• To accurately determine the extinction coefficient (κ) of stoichiometric silicon nitride (Si3N4).
• To demonstrate the applicability of nanomechanical frequency spectroscopy for material characterization.

Main Methods:

• Utilized nanomechanical frequency spectroscopy to monitor high-Q silicon nitride (Si3N4) trampolines.
• Tracked frequency shifts induced by laser photothermal heating.
• Developed a model incorporating stress relaxation and heat transfer (conductive and radiative) to infer extinction coefficient.

Main Results:

• Successfully characterized material absorption at the parts-per-million level.
• Inferred an extinction coefficient (κ) of approximately 0.1-1 ppm for Si3N4 in the 532-1550 nm range.
• Identified two distinct thermalization time scales: rapid radiative cooling of the Si3N4 film and slow parasitic heating of the Si chip.

Conclusions:

• Nanomechanical frequency spectroscopy is a powerful tool for precise material absorption characterization.
• The developed method provides accurate extinction coefficient values for nanophotonic materials.
• This approach is versatile and can be applied to various photonic materials, offering new insights.