Exceptional points in lossy media enable decay-free wave propagation

Abstract

Waves entering a spatially uniform lossy medium typically undergo exponential decay, arising from either the energy loss of the Beer-Lambert-Bouguer transmission law or the evanescent penetration during reflection. Recently, exceptional point singularities in non-Hermitian systems have been linked to unconventional wave propagation, such as the predicted extremely spatially broad constant-intensity guided modes. Despite such promises, the possibility of decay-free wave propagation in a purely lossy medium has been neither theoretically suggested nor experimentally realized until now. Here we discover and experimentally demonstrate decay-free wave propagation accompanied by a striking uniformly distributed energy loss across arbitrary thicknesses of a homogeneous periodically nanostructured waveguiding medium with exceptional points. Predicted by coupled-mode theory and supported by fully vectorial electromagnetic simulations, hundreds-of-waves deep penetration manifesting spatially constant radiation losses are experimentally observed in photonic slab waveguides. The uniform, decay-free radiative energy loss is measured across the entire structured waveguide region, regardless of its length. While the demonstrated constant-intensity radiation finds an immediate application for generating large, uniform and surface-normal free-space plane waves directly from the photonic chip surface, the uncovered decay-free wave phenomenon is universal and holds true across all domains supporting physical waves, opening new horizons for dispersion-engineered materials empowered by exceptional point physics.