Anisotropic Crystallographic Engineering of α-MoO₃

The α-phase molybdenum trioxide (α-MoO₃), a biaxial hyperbolic van der Waals (vdW) crystal, supports highly confined and anisotropic phonon polaritons (PhPs), positioning it as a superior platform for mid-infrared light manipulation. The performance of PhP-based devices critically depends on the properties of α-MoO₃ flakes, including their thickness, roughness, and pattern geometry. However, conventional patterning techniques, such as ion beam milling and plasma etching, often introduce doping artifacts and surface damage, resulting in high PhP losses. In this work, we develop a hot-water-based technique for the crystallographic engineering of α-MoO₃, leveraging its anisotropic etching properties for surface polishing and nanopatterning. This method exploits the notably higher etching rate along intralayer directions ([100], [001]) compared to the interlayer direction ([010]). Consequently, a 24% enhancement in PhP lifetime was observed in RIE-treated α-MoO₃ flakes after hot water polishing, with no measurable change in material thickness. To further validate this technology, we fabricated various two-dimensional PhP manipulation devices using standard nanopatterning and thinning processes, followed by chemical-free hot water anisotropic crystallographic etching. This approach enabled the creation of nanoresonators, lenses, nanocavities, and unidirectional emitters with sharp edges precisely aligned along the crystallographic planes. Our crystallographic engineering approach unlocks precise control of surface waves at the nanoscale, facilitating the development of photonic devices for cutting-edge nanophotonic and nanoscale sensing applications.