Unveiling Buried Air Gaps and Interface Inhomogeneities in van der Waals Stacks via Microreflectance Mapping for...

Van der Waals (vdW) heterostructures provide a versatile platform for multifunctional photonic and optoelectronic devices, yet their performance is often limited by interfacial inhomogeneities that perturb the local dielectric environment. This is particularly critical in polaritonic heterostructures, where the near-field optical response is extremely sensitive to buried defects. Here, we present a high-resolution microreflectance mapping technique for assessing interfacial quality in vdW heterostructures and enabling the fabrication of twisted polaritonic stacks. By combining hyperspectral imaging with transfer-matrix-method (TMM) fitting, we determine flake thickness directly on PDMS stamps and quantify buried air gaps and other interfacial nonuniformities in assembled stacks. Using h-BN and α-MoO₃ heterostructures as benchmark systems, we detect buried air gaps down to 7 nm with ∼1 μm lateral resolution. Correlation with scattering-type scanning near-field optical microscopy (s-SNOM) confirms the quantitative accuracy of the method and shows that nanometric air gaps already produce measurable changes in phonon-polariton (PhP) propagation. Our results establish microreflectance mapping as an accessible far-field approach for quantitative interfacial control in polaritonic heterostructures, a key requirement for twistoptics applications. To facilitate implementation, we provide the analysis in an open-source graphical user interface that can be integrated into standard optical-microscope setups.