Longitudinal plasmon modes of Ag nanorod coupled with a pair of quantum dots.

The longitudinal plasmon modes of an elongated Ag nanorod induced by an obliquely incident plane wave were analyzed theoretically. Our results show that the proximities at the two apexes of the nanorod are the hotspots at the dipole (m = 1), quadrupole (m = 2), sextupole (m = 3), octupole (m = 4), decapole (m = 5) and duodecapole (m = 6) modes. This phenomenon implies that a pair of quantum dots (QDs) located at these hotspots might be excited simultaneously through these plasmon modes. Consequently, the coherent spontaneous emission of the paired QDs could be induced through these modes. Furthermore, the coherent emission of the pair of excited QDs was studied, where these QDs were modeled as two electric dipoles (bi-dipole) oscillating with anti-symmetric or symmetric configurations. The radiative and nonradiative powers show that the maximum Purcell factors occur at these modes; the odd modes enhance the emission of the anti-symmetric configuration, and the even modes the symmetric one. However, only those bi-dipoles emitting at the lower-order (e.g., dipole, quadrupole and sextupole) modes of Ag nanorod are with high apparent quantum yields. In addition, the correlation of these plasmon modes of Ag nanorod with the dispersion relation of Ag nanowire was discussed.