Large Purcell enhancement with nanoscale non-reciprocal photon transmission in chiral gap-plasmon-emitter systems

Chiral photon-emitter coupling has been extensively explored in its non-reciprocal property, which results from spin-locked photon transmission. It manifests the potential in on-chip non-reciprocal devices, such as optical isolators and photon routing in quantum networks. However, the enhancement of chiral coupling, which has been seldom studied, remains wanting. Here, we numerically propose a gap-plasmon-emitter system demonstrating large Purcell enhancement with effective nanoscale non-reciprocal photon transmission. Owing to the strong field enhancement and high transverse spin momentum (TSM) in gap plasmons, the Purcell factor reaches 10⁴. Simultaneously, the transmission in the nanowire is directional, in which 91% propagates in a single direction. The transmission confined around the nanowire also obtains a ∼700-fold enhancement compared with the vacuum decay rate of the emitter. Furthermore, the circularly polarized emitter couples preferentially to the opposite transmission direction in the two eigenmodes. This phenomenon is attributed to the special TSM profile of the two eigenmodes, that is, the transmission direction is locked to the opposite TSM in the two eigenmodes. Our proposed system offers an efficient way for photon routing in optical circuits and quantum networks and also extends methods for manipulating non-reciprocal devices.