Optical Modulation of Interfacial Coupling Drives a Transition from Free-Epitaxy to Locked-Epitaxy

Controlling the epitaxial state in quasi-van der Waals heterostructures remains challenging because weak interfacial guidance often favors free epitaxy. Here, we show that light irradiation during sputtering drives Fe₄N/mica from a free-epitaxy state to a locked-epitaxy state. Under dark growth, the film adopts the Fe₄N(001) with a bulk-like lattice constant and rotationally degenerate in-plane alignment, characteristic of free-epitaxy. Under illumination, the system follows a locked-epitaxy pathway and forms Fe₄N(111)/mica with a specific in-plane registry and finite in-plane strain. Timing-, wavelength-, and intensity-dependent experiments show that this transition is established during the earliest stage of growth. Density functional theory calculations further reveal that the Fe₄N(001)/mica interface is mainly vdW-dominated, whereas the Fe₄N(111)/mica interface is stabilized by a much stronger non-vdW contribution, dominated by chemically specific interaction. These results establish optical modulation of interfacial coupling as a practical route toward free-to-locked epitaxy in quasi-vdW heterostructures.