Nonlinearity-induced nonreciprocity for evanescent waves

The reciprocity principle constrains both propagating and evanescent wave regimes, yet magnet-free approaches to nonreciprocal transmission have so far been explored only for waves propagating along the transport direction. Here, we explore and experimentally demonstrate strong nonreciprocal responses for evanescent waves based on tailored asymmetric nonlinearities. By operating in the bandgap of a coupled-resonator system and exploiting engineered spectral singularities, we show that a saturable nonlinear defect enables pronounced nonreciprocal transmission under evanescent-wave excitation. Compared with the propagating regime, the evanescent-wave configuration allows a reduced local threshold intensity at the defect, while the required source intensity additionally depends on the evanescent decay between the source and the defect. Our results establish a distinct route to nonreciprocal wave control, extending nonlinear and non-Hermitian scattering physics to waves evanescent along the transport direction.