Waveguiding and plasmon resonances in two-dimensional photonic lattices of gold and silver nanowires

We report the fabrication of triangular lattices of parallel gold and silver nanowires of high optical quality, with diameters down to $500\mathrm{nm}$ and length-to-diameter ratios as high as 100 000. The nanowires are supported by a silica glass matrix and are disposed around a central solid glass core, i.e., a missing nanowire. These centimeter-long structures make it possible to trap light within an array of nanowires and characterize the plasmon resonances that form at specific optical frequencies. Such nanowire arrays have many potential applications, e.g., imaging on the subwavelength scale.

Figure 1

(Color online) (a) SEM of the end face of a cleaved metal-filled PCF, polished by focused ion beam etching (hole diameter of $1.52\mu \mathrm{m}$, hole spacing of $2.9\mu \mathrm{m}$). The wires undergo ductile thinning during cleaving, breaking at random positions. Those that protrude from the end face are polished and appear as bright disks in the SEM; the rest (six in total) are dull in appearance. (b) SEM image of the unpolished end face of a PCF with two metal nanowires located next to the solid core. (c) Schematic of a metal nanowire array; SPRs are created by surface plasmons spiraling around the nanowires.

Figure 2

(Color online) SPR wavelength as a function of nanowire diameter. The white dots on the solid lines are the numerically simulated positions of the $m=2$ and $m=3$ SPRs, and the isolated points (triangles: silver; circles: gold) are experimental measurements. The points labeled 1 and 2 are for PCFs completely filled with metal, whereas point 3 is for a PCF in which only three holes were filled (two in the first ring and one in the second). The inset shows the typical transmission $\left(T\right)$ spectra of PCFs filled with gold (blue curve) and silver (red curve). The wire diameter was $980\mathrm{nm}$ in each case, and the transmission was referenced to that of an unfilled PCF.

Figure 3

(Color online) Loss spectra in the visible for two different PCFs (hole diameters of 980 and $1520\mathrm{nm}$) filled with gold and silver. Also plotted is the imaginary part of the dielectric constant for gold and silver (Ref. 17). The inset is a near-field optical micrograph of the end face of a gold-filled PCF when white light is launched into it; the core light is green and the nanowire array is dark, with a bright orange ring surrounding it. The distortions in the picture are caused by irregularities in the cleaved surface.

Figure 4

(Color online) Dispersion and loss characteristics of the main modes in the system. (a) Wavelength dependence of effective axial refractive index and loss (dark blue curve) for a silver-filled PCF ($d=1000\mathrm{nm}$, $\Lambda =2.9\mu \mathrm{m}$, with three rings of holes). The green curves are the $m=2$ and $m=3$ SPRs for a single wire, the light blue dashed curve represents the guided mode in an unfilled PCF, the dashed black curve is for silica glass, and the red curves are for the guided mode in the nanowire array. The array has photonic band gaps in the gray shaded regions. (b) Close-up view of the $m=3$ anticrossing point. (c) Dispersion of the $m=2$ mode in the regime of the nanowire cutoff. (d) Loss characteristics of the fundamental core mode of the silver-filled PCF compared with the loss of a silver-clad cylindrical silica rod-waveguide of the same diameter $\left(5.3\mu \mathrm{m}\right)$.