Ultra-Sensitive Extinction Measurements of Optically Active Defects in Monolayer MoS${}_2$

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.