  We utilize cavity-enhanced extinction spectroscopy to directly quantify the
optical absorption of defects in MoS$_2$ generated by helium ion bombardment.
We achieve hyperspectral imaging of specific defect patterns with a detection
limit below 0.01% extinction, corresponding to a detectable defect density
below $10^{11}$ cm$^{-2}$. The corresponding spectra reveal a broad sub-gap
absorption, being consistent with theoretical predictions related to sulfur
vacancy-bound excitons in MoS$_2$. Our results highlight cavity-enhanced
extinction spectroscopy as efficient means for the detection of optical
transitions in nanoscale thin films with weak absorption, applicable to a broad
range of materials.
