Light-Driven Phase Engineering in 2D Materials From Mechanistic Assignment to Ambient Near-Field Control.

Light-driven phase transitions and transformations provide a route for post-synthesis control of structure, charge order, ferroic polarization, chemistry, and local transport in 2D materials. Optical excitation can activate carrier dynamics, phonon coupling, heating, strain redistribution, vacancy formation, surface reactions, and substrate-mediated effects during one experiment. Mechanistic assignment consequently requires separate identification of the process initiating conversion and the mechanism preserving the modified material. We organize representative experiments into seven practical classes based on the dominant experimentally supported mechanism and outline experimental criteria for distinguishing competing contributions. A quantitative workflow combines independent material verification, controlled perturbations, temporal measurements, and material-specific descriptors of conversion and stabilization. Far-field laser processing remains valuable for patterning and device fabrication, although diffraction-limited illumination averages over nanoscale heterogeneity. Tip-enhanced femtosecond excitation in scattering-type scanning near-field optical microscopy provides a complementary route by localizing excitation and linking nanoscale writing with local optical, structural, and chemical verification. Localized excitation can support reproducible ambient nanoscale phase control when excitation, phase identity, and retention are verified within one region.