Absorption reduction of large Purcell enhancement enabled by topological state-led mode coupling

Abstract

Nanoscale single photon source is an important part of on-chip quantum information processing. Gap plasmon structures as a candidate of typical nanostructure, can provide large Purcell enhancement by nanoparticles owing to possessing ultrasmall optical mode volume, and effectively collecting scattering light by nanowire/nanofilm. However, their collecting efficiency is not very high. By introducing quantum Hall effect into optics, topological photonics becomes an important branch in nanophotonics. The edge states are characterized as nonscattering propagation of photons and immunity to a wide class of impurities and defects, i.e., topological protection. However, using topological protection into the Purcell enhancement has not been reported yet. We propose a specific topological photonic structure, i.e., a 1D topological PC containing a resonant nanoantenna. Under the condition of topological protection, we first reveal the edge state-led coupling mechanism; namely, surface plasmons of the antenna almost do not have any influence on the edge state, while the edge state greatly changes the pattern of the local field around the antenna. Based on this mechanism, an obvious absorption reduction in the spontaneous emission spectra is obtained due to the near-field deformation around the antenna induced by the edge state. By embedding an antenna into the topological PC, a strong local field near the antenna leads to a large Purcell enhancement, while the edge state can make almost all scattering photons propagate along some specific directions. As a result, total Purcell factors can reach more than 2×10^4 γ0 (γ0 is the spontaneous emission rate in vacuum), among which the propagating part along the edge state channel is more than 10^4 γ0. By reducing the photonic loss and guiding scattering photons into the edge state, this kind of Purcell enhancement will provide new sight for on-chip quantum light sources such as a single-photon source and nanolaser.