Excitation of Dark Plasmons in Metal Nanoparticles by a Localized Emitter

We study theoretically a dipole emitter placed near a metal nanoparticle and near small chains of two and three nanoparticles. The emitter can efficiently excite dark, or nonradiative, surface-plasmon modes in the nanostructures, in addition to the well-known bright modes. In the case of coupled nanoparticles, the origins of the bright and dark modes can be understood in the context of plasmon hybridization. Excitation of dark modes in nanoparticle chains allows for subwavelength guiding of optical energy with no radiative losses and thus with improved propagation lengths.

Figure 1

(a) Radiative enhancement ${\Gamma }_r/{\Gamma }_0$ (solid black line) and nonradiative enhancement ${\Gamma }_{\mathrm{nr}}/{\Gamma }_0$ (dashed red line) of a point dipole by a gold bipyramid. The configuration of the system is shown in the inset. Also shown is the optical absorption cross section, $C_{\mathrm{abs}}$, of the gold bipyramid (dashed black line). (b) The $z$ component of the electric field on the $x\mathrm{\text{−}}z$ plane (through the center of the bipyramid) containing a dipole radiating at 1.5 eV, coupled to the dipolar mode of the nanoparticle. The emitting dipole is located at $x=60\mathrm{nm}$, $z=45\mathrm{nm}$. The solid white line indicates the boundary of the nanoparticle. (c) The same plot as (b), but for emission at 1.75 eV, coupled to the quadrupolar mode of the nanoparticle.

Figure 2

(a) Optical absorption spectra of a single silver ellipsoid (thin dashed line) and a pair of ellipsoids (thick dashed line). The polarization of light is along the major axes of the ellipsoids. (b) Radiative enhancement ${\Gamma }_r/{\Gamma }_0$ (solid line) and nonradiative enhancement ${\Gamma }_{\mathrm{nr}}/{\Gamma }_0$ (dashed line) of a point dipole by a pair of silver ellipsoids in its vicinity. The system configuration is shown in the inset. The arrows show the resonant energies from electrostatic models. ${\omega }_1$ and ${\omega }_2$ correspond to the dipolar and quadrupolar modes, respectively, and are obtained from the quasistatic approximation. ${\omega }_{1\pm }$ and ${\omega }_{2\pm }$ are the corresponding bonding and antibonding modes, and are obtained from the plasmon-hybridization model.

Figure 3

(a) Radiative enhancement ${\Gamma }_r/{\Gamma }_0$ (solid black line) and nonradiative enhancement ${\Gamma }_{\mathrm{nr}}/{\Gamma }_0$ (dashed red line) of a point dipole by a chain of three gold bipyramids in its vicinity. Also shown is the optical absorption cross section, $C_{\mathrm{abs}}$, of the gold bipyramid chain (dashed black line). (b) The $z$ component of the electric field on the $x\mathrm{\text{−}}z$ plane containing the dipole radiating at 1.59 eV, coupled to the antibonding dipolar mode of the chain. The radiative dipole is located at $x=60\mathrm{nm}$, $z=62\mathrm{nm}$. The solid white lines indicate the boundaries of the nanoparticles. (c) The same plot as (b), but for radiation at 1.75 eV, coupled to the quadrupolar mode.