Nanowire Plasmon Excitation by Adiabatic Mode Transformation

We show with both experiment and calculation that highly confined surface plasmon polaritons can be efficiently excited on metallic nanowires through the process of mode transformation. One specific mode in a metallic waveguide is identified that adiabatically transforms to the confined nanowire mode as the waveguide width is reduced. Phase- and polarization-sensitive near-field investigation reveals the characteristic antisymmetric polarization nature of the mode and explains the coupling mechanism.

Figure 1

Normalized wave vector of the SPP modes guided by a Au waveguide on glass as a function of waveguide width. The dots between 200 and 550 nm widths for 1550 nm are a cubic spline interpolation [23]. For shorter wavelengths, only the fundamental mode is indicated. Plotted also are the cross sections of the electric field amplitude of the fundamental mode for 60 nm and $5\mu \mathrm{m}$ widths at a free-space wavelength of 1550 nm. The arrows schematically depict the direction of the transverse electric field.

Figure 2

Near-field imaging of SPP excitation and propagation on a nanowire. (a) Optical microscope image of the structure. (b) Collected near-field SPP amplitude and (c) distribution of $|A|\cos \varphi$. SPPs excited on the Au-glass surface in the left are converted to a mode guided along a 150 nm wide nanowire. The excitation wavelength is 1550 nm. (d),(e) Measured $|A|\cos \varphi$ on a small section of the nanowire for $x$ and $y$ polarizations. The dotted lines indicate the height step that the near-field probe makes due to the presence of the nanowire. (f)–(h) Calculated field components $E_x$, $E_y$, and $E_z$ at a height of 20 nm above the sample.

Figure 3

(a) Measured dispersion curves for three Au nanowire widths. The dashed line denotes the dispersion of light in the substrate. The error bars represent the experimental uncertainty. (b) Measured wave vector and propagation length as a function of waveguide width (symbols), compared to calculations (curves) for a free-space wavelength of 1550 nm.

Figure 4

(a) Secondary electron micrograph of a $2\mu \mathrm{m}$ long nanowire connected by tapered waveguide sections for input and output coupling. (b) Near-field amplitude of forward-propagating waves in the structure at $\lambda =1550\mathrm{nm}$. The intensity transmission of the complete structure is $20\pm 6\%$.