Fig. 1. Schematic of three-arm windmill plasmonic nanoantenna, where the radius of the feed gap is r, the antenna arm is assumed as an ellipsoid with semiaxes a=b<c, the angles between adjacent arms are α,β,γ, respectively, and the azimuthal angle between the x axis and the polarization direction is θ.

Fig. 2. Comparison of the Cext between the C3v symmetric nanoantenna (λ=760  nm) and a single gold ellipsoid (λ=690  nm) with different polarization angles. (a) and (b) plot the Cext and the electric field distribution of the nanoantenna, respectively. (c) and (d) plot the Cext and the electric field distribution of the single gold ellipsoid, respectively.

Fig. 3. Dependence of volume average electric field enhancement Eavg on the polarization angle θ, where the maximum appears when the polarization direction is along the major axis of an arbitrary arm, and the minimum occurs when they are perpendicular.

Fig. 4. Variations of (a) the Cext and (b) the average electric field enhancement of three inner tips E¯tip as a function of the major semiaxis c with r=2  nm. Variations of (c) the Cext and (d) E¯tip as a function of the gap radius r with c=45  nm. Inset in (b) plots the distribution of Ey at c=60  nm with λ=640, 890 nm, respectively. Inset in (d) plots the distribution of Ey at r=1.5  nm with λ=640, 890 nm, respectively.

Fig. 5. (a) Extinction cross-section of the symmetry broken plasmonic nanoantenna with α=120° and β=30° under illumination of linear and circular polarizations; (b) normalized electric field distribution with circularly polarized illumination at resonant wavelengths of 736 and 966 nm, respectively.

Fig. 6. (a) Splitting and shifting of the resonant peak varying with β at α=120°; (b) normalized electric field distribution at resonant wavelength varying with β.

Fig. 7. Extinction cross-section of the symmetry broken three-arm windmill plasmonic nanoantenna for α=β=90° under (a) linearly and circularly, and (b) elliptically polarized incident light.