Silver Nanowires as Surface Plasmon Resonators

We report on chemically prepared silver nanowires (diameters around 100 nm) sustaining surface plasmon modes with wavelengths shortened to about half the value of the exciting light. As we find by scattered light spectroscopy and near-field optical microscopy, the nonradiating character of these modes together with minimized damping due to the well developed wire crystal structure gives rise to large values of surface plasmon propagation length and nanowire end face reflectivity of about $10\mu \mathrm{m}$ and 25%, respectively. We demonstrate that these properties allow us to apply the nanowires as efficient surface plasmon Fabry-Perot resonators.

Figure 1

Scanning electron micrographs of a $18.6\mu \mathrm{m}$ long silver nanowire. The wire diameter of 120 nm was independently determined by measuring the height of the nanowire by atomic force microscopy. In the image, the wire diameter is larger, as the sample was sputtered with 30 nm gold to provide electric conductivity for electron microscopy imaging.

Figure 2

Surface plasmon propagation along the $18.6\mu \mathrm{m}$ long silver nanowire in Fig. 1. (a) Sketch of optical excitation; I is input and D is distal end of the wire. (b) Microscopic image—the bright spot to the left is the focused exciting light. The arrow indicates light scattered from the distal wire end. (c) SNOM image—the image area corresponds to the white box in (b). (d) $2\mu \mathrm{m}$ long cross-cut along the chain dotted line in (c).

Figure 3

Scattered light spectra of $3.3\mu \mathrm{m}$ long silver nanowires, diameter 90 nm. (a) Sketch of optical excitation. The exciting light propagation direction projected onto the substrate plane is parallel to the nanowire axis defining an input (I) and a distal end (D), and the polarization is fixed in the plane of incidence. (b),(c) Scanning electron micrographs of a chemically and an electron-beam lithographically fabricated silver nanowire, respectively. (d) Scattered light spectra from the distal nanowire end face of the chemically fabricated wire (single-crystalline, upper curve) and the lithographically fabricated wire (polycrystalline, lower curve).

Figure 4

Scattered light spectra of the nanowire distal and input end faces. (a) Spectra for a $8.3\mu \mathrm{m}$ long silver nanowire, diameter 75 nm. The vertical arrows highlight the spectral match between signal maxima and minima at the input and the distal end faces, respectively. (b) Spectra for a $18.6\mu \mathrm{m}$ long nanowire, diameter 120 nm. The arrows define $\Delta I$ and $I_{\min }$.

Figure 5

Spectral modulation depth $(\Delta I/I_{\min })$ around a light wavelength of 785 nm versus nanowire length. The squares give the experimetal values for 5 nanowires with diameter $110\pm 15\mathrm{nm}$. Line: Fit of the Fabry-Perot resonator model for determination of both the reflectivity of the wire end faces ($R$) and the propagation length of the plasmon mode ($l$).

Figure 6

Surface plasmon dispersion relation of a 120 nm diameter nanowire (NDR), as derived from the spectrum in Fig. 4b. The solid black line is the surface plasmon dispersion relation at a flat silver/air interface. The long- and the short-dashed curves define the light lines in air and the glass substrate, respectively. Figure and inset show the same curves over different axes ranges.