  Understanding optical absorption in silicon nitride is crucial for
cutting-edge technologies like photonic integrated circuits, nanomechanical
photothermal infrared sensing and spectroscopy, and cavity optomechanics. Yet,
the origin of its strong dependence on film deposition and fabrication process
is not fully understood. This Letter leverages nanomechanical photothermal
sensing to investigate optical extinction $\kappa_{\mathrm{ext}}$ at 632.8 nm
wavelength in LPCVD SiN strings across a wide range of deposition-related
tensile stresses ($200-850$ MPa). Measurements reveal a reduction in
$\kappa_{\mathrm{ext}}$ from 10$^3$ to 10$^1$ ppm with increasing stress,
correlated to variations in Si/N content ratio. Within the band-fluctuations
framework, this trend indicates an increase of the energy bandgap with the
stress, ultimately reducing absorption. Overall, this study showcases the power
and simplicity of nanomechanical photothermal sensing for low absorption
measurements, offering a sensitive, scattering-free platform for material
analysis in nanophotonics and nanomechanics.
