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 1 × 10 11  cm -2 . The corresponding spectra reveal a broad subgap 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.