Chiral Surface Plasmon Polaritons on Metallic Nanowires

Chiral surface plasmon polaritons (SPPs) can be generated by linearly polarized light incident at the end of a nanowire, exciting a coherent superposition of three specific nanowire waveguide modes. Images of chiral SPPs on individual nanowires obtained from quantum dot fluorescence excited by the SPP evanescent field reveal the chirality predicted in our theoretical model. The handedness and spatial extent of the helical periods of the chiral SPPs depend on the input polarization angle and nanowire diameter as well as the dielectric environment. Chirality is preserved in the free-space output wave, making a metallic nanowire a broad bandwidth subwavelength source of circular polarized photons.

Figure 1

(a) Schematic illustration of the nanowire in homogeneous background. The incident Gaussian beam with an electric field $\mathbf{E}$ and a polarization angle $\theta$ is focused normally on the left nanowire end. ${\epsilon }^M$ and $L$ are the permittivity and the length of the Ag nanowire, respectively. Excitation of the (b) $m=0$ (${\mathrm{TM}}_0$) and (c) $m=-1$ (${\mathrm{HE}}_{-1}$) mode at an incident phase $\phi =0$ and $\phi =\pi /2$, respectively, for $\theta =0°$. Excitation of the (d) $m=1$ (${\mathrm{HE}}_1$) and (e) $m=2$ (${\mathrm{HE}}_2$) mode at $\phi =0$ and $\phi =\pi /2$, respectively, for $\theta =90°$. The electric field $|\mathbf{E}|$ profile of each excited SPP mode is shown on the right in panels (b)–(e). The radius $R$ of the simulated Ag nanowire is 60 nm in (b-d) and 250 nm in (e). The permittivity of the embedding medium ${\epsilon }^D$ is 2.25 and the incident wavelength ${\lambda }_0$ is 632.8 nm in vacuum, same as in the following figures.

Figure 2

Chiral SPPs. (a) Surface charge density plot on a Ag nanowire. The maximal value of surface charge was truncated at the incident end to better show the plasmon modes on the wire. (b) Time-averaged power flow in the $yz$ plane at different positions along the nanowire, where $x=2.0$ to $3.8\mu \mathrm{m}$ (i-x) in steps of $0.2\mu \mathrm{m}$, indicated by the blue frames in (a). The white arrows highlight the rotation of electromagnetic energy as a function of position along metal nanowire, showing right-handed chiral SPPs. (c) Periods of the plasmon helix, $\Lambda$, as a function of nanowire radius. The blue region denotes the single mode-dominant regime and the magenta region denotes the multimode regime. The length of the simulated nanowire ($R=60\mathrm{nm}$) is $L=5.0\mu \mathrm{m}$ and the incident polarization angle is $\theta =45°$ in (a),(b).

Figure 3

(i) Optical image of a Ag nanowire. The scale bar is $5\mu \mathrm{m}$. (ii),(iii) Fluorescence images of right-handed (ii) and left-handed (iii) SPPs, respectively. The white helical arrows highlight the handedness of the plasmon helix. (iv),(v) SPPs launched with incident polarization (iv) parallel and (v) perpendicular to the nanowire axis show suppression of the plasmon helix. All images (ii-v) are obtained by quantum dot imaging, with He-Ne laser excitation (632.8 nm).

Figure 4

Maps of degree of circular polarization $C$ (a) and figure of merit $f$ (b) in a vertical plane 200 nm beyond the distal end of the nanowire in Fig. 2. The black dotted circles indicate the cross section of the nanowire. $C$ at the center of (a) on the symmetric axis of the nanowire as a function of $\theta$ (c) and ${\lambda }_0$ (d). The transmission spectrum $I({\lambda }_0)$ (green) is also shown in (d).