  Contactless characterization of mechanical resonances using Fabry-Perot
interferometry is a powerful tool to study the mechanical and dynamical
properties of atomically thin membranes. However, amplitude calibration is
often not performed, or only possible by making assumptions on the device
parameters such as its mass or the temperature. In this work, we demonstrate a
calibration technique that directly measures the oscillation amplitude by
detecting higher harmonics that arise from nonlinearities in the optical
transduction. Employing this technique, we calibrate the resonance amplitude of
two-dimensional nanomechanical resonators, without requiring knowledge of their
mechanical properties, actuation force, geometric distances or the laser
intensity.
