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.