Broadband Enhancement of the Spontaneous Emission by a Split-Ring Nanoantenna: Impact of Azimuthally Propagating Surface Plasmon Polaritons

We propose a split-ring optical nanoantenna which can achieve enhancement factors of total and radiative emission rates up to high values of 7411 and 1941, respectively, with a tunable bandwidth of enhanced emission (Purcell factor over 1000) up to 221 nm at visible wavelengths. To clarify the underlying physical mechanism, we build up a semi-analytical model by considering an intuitive multiple-scattering process of surface plasmon polaritons (SPPs) that propagate azimuthally along the curved antenna arms. The SPP model provides an intuitive picture that under two phase matching conditions, the SPPs form paired and tunable whispering-gallery-like resonances and are then coupled to the emitter in the nano-gap or scattered into free space, which results in an enhancement of the total or radiative emission rate, respectively. Based on the SPP model, we further build up a semi-analytical model for the quasi-normal modes (QNMs) supported by the antenna, which provides a frequency analyticity and indicates that the broadband enhancement of the spontaneous emission rate results from an excitation of paired symmetric and anti-symmetric QNMs with different eigenfrequencies. The proposed models clarify the impact of azimuthally propagating SPPs on the antenna radiation and may be extended to other optical antennas with curved arms.