Deep-subwavelength waveguide taper with adiabatic-like performance based on epsilon-near-zero materials

We propose an ultracompact class of waveguide tapers based on epsilon-near-zero (ENZ) media, enabling adiabatic-like fundamental-mode evolution at deep-subwavelength scales. A unified design framework is established for coupling dissimilar waveguides: a passive ENZ section ensures efficient matching when characteristic impedances are matched, while mismatched cases are addressed using either a non-Hermitian design for unidirectional reflectionless transport or a cascaded passive configuration for bidirectional coupling. Full-wave simulations, including a practical 3D cutoff-waveguide model, confirm reflectionless performance with extreme miniaturization. The approach combines low-loss transmission, practically viable structures, and deep subwavelength footprints, offering a scalable solution for high-density photonic integration.