Laser‐Written Ruby Emitters in MXenes: Transforming Residual Impurities Into Integrated Narrowband Luminescent Probes

MXenes, a class of 2D materials that have emerged recently, are known for their unique properties and broad applicability. They are traditionally synthesized from MAX phases by etching A‐group atoms, usually Al. Here, the controlled transformation of residual aluminum is demonstrated, as both MAX‐phase component and pervasive impurity in MXene synthesis, into a functional diagnostic asset. Through laser irradiation, Al is photooxidized in situ, forming embedded Al 2 O 3 :Cr 3 ⁺ centers with ruby line luminescence with a peak width of about 1 nm by laser irradiation of individual grains of Cr 2 AlC. The luminescence emerges due to Al oxidation with Cr doping. It is also demonstrated that the ruby luminescence is an unexpectedly universal phenomenon which occurs even in Cr‐free MAX‐phases and MXenes, owing to the incomplete Al etching and slight Cr doping of natural origin. Discovered sensitivity of ruby luminescence intensity to residual Al enables rapid, minimally destructive, and spatially resolved control of etching quality, essential for the industrial maturation of MXenes. Furthermore, the capacity to locally generate integrated optical probes opens new pathways for in operando diagnostics in MXene‐based devices.