Two-plasmon spontaneous emission from a nonlocal epsilon-near-zero material

Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for enhancing the spontaneous emission process towards both classical and quantum optical applications. Conventionally, light cannot be directly emitted from the plasmonic metal itself. Here, we explore the large field confinement and slow-light effect near the epsilon-near-zero (ENZ) frequency of the light-emitting material itself, to greatly enhance the "forbidden" two-plasmon spontaneous emission (2PSE) process. Using degenerately-doped InSb as the plasmonic material and emitter simultaneously, we theoretically show that the 2PSE lifetime can be reduced from tens of milliseconds to several nanoseconds, comparable to the one-photon emission rate. Furthermore, we show that the optical nonlocality may largely govern the optical response of the ultrathin ENZ film. Efficient 2PSE from a doped semiconductor film may provide a pathway towards on-chip entangled light sources, with an emission wavelength and bandwidth widely tunable in the mid-infrared. Plasmonics involves the engineering of light-matter interactions and has range of possible uses in the study of quantum phenomenon, but efficient plasmonic sources are required. The authors report a method to achieve efficient two-plasmon spontaneous emission using an epsilon-near-zero material with highly-confined surface plasmon polaritons to simultaneously serve as a two-plasmon emitter for emission acceleration.