We present an experimental study of the bending waves of freestanding
\ce{Si3N4} nanomembranes using optical profilometry in varying environments
such as pressure and temperature. We introduce a method, named Vibrometry in
Continuous Light (VICL) that enables us to disentangle the response of the
membrane from the one of the excitation system, thereby giving access to the
eigenfrequency and the quality (Q) factor of the membrane by fitting a model
of a damped driven harmonic oscillator to the experimental data. The validity
of particular assumptions or aspects of the model such as damping mechanisms,
can be tested by imposing additional constraints on the fitting procedure. We
verify the performance of the method by studying two modes of a
478Â nm thick \ce{Si3N4} freestanding membrane and find Q factors
of 2×104 for both modes at room temperature. Finally, we observe a
linear increase of the resonance frequency of the ground mode with temperature
which amounts to 550 Hz/∘C for a ground mode
frequency of 0.447Â MHz. This makes the nanomembrane resonators
suitable as high-sensitive temperature sensors