Unidirectional Giant Exciton Emission into a Photonic Waveguide

Efficient coupling of nanolight sources into photonic waveguides is crucial for integrated photonics, quantum technologies, and biosensing. Practical implementations require light sources with simultaneous high brightness and unidirectional emission. However, it is fundamentally incompatible between strong electromagnetic field confinement and directional radiation. Here, we demonstrate the simultaneous giant excitonic photoluminescence (PL) enhancement and unidirectional emission from a two-dimensional InSe film integrated with an asymmetric plasmonic nanocavity. A 3500-fold PL enhancement is achieved by engineering spatial, spectral, and orientational overlap between cavity modes and out-of-plane excitons in InSe. Symmetry breaking within the nanocavity ensures precise control of emission interference, yielding a record-high directivity exceeding 15 dB. The design achieves a high coupling efficiency of 24% and supports guided light propagation of 140 μm. Our results establish a scalable approach for the ultracompact integration of nanoscale light sources into monolithic photonic circuits and will advance the development of on-chip nanophotonics.