Polariton manipulation via boundary engineering.

The pursuit of nanoscale light manipulation represents a fundamental challenge in nanophotonics, where overcoming the diffraction limit is essential for developing next-generation optoelectronic devices. Polariton boundary engineering has emerged as a transformative approach that enables unprecedented control over light confinement and propagation at subwavelength scales. However, the field currently lacks a systematic framework to integrate the diverse manifestations of boundary effects across different material systems and polariton types. This comprehensive review addresses this critical gap by providing a unified perspective on how engineered interfaces-including edges, heterojunctions, and metallic structures-govern polariton behaviors such as focusing, stealth transmission, and negative refraction. The analysis encompasses seminal work on van der Waals materials (h-BN, α-MoO3) and phase-change compounds, showcasing advanced devices ranging from hyperbolic nanoresonators to reconfigurable waveguides and super-resolution lenses. By examining the synergy between boundary geometry and material properties alongside advanced nanofabrication techniques, this review not only catalogs state-of-the-art achievements but also establishes design principles for dynamic boundaries using stimuli-responsive materials and machine-learning-driven inverse design. This synthesis provides both a foundational framework for researchers and a roadmap toward developing programmable photonic circuits and multifunctional optical platforms for quantum information technology applications.