Tunable nanowires: An additional degree of freedom in plasmonics

We report on the excitation of surface plasmon polaritons (SPP’s) on a periodic arrangement of metallic ribbons with tunable periodicity. The ability to vary its lattice constant results in an additional degree of freedom, permitting excitation of SPP’s for any combination of wavelength and angle of incidence within the tuning range of the system. Moreover, the designed smart material allows fundamental questions to be answered regarding SPP propagation and localization. Finally, the concept opens vistas in the development of other methodologies in spectroscopy including plasmonic sensors and chips.

Figure 1

Scanning electron micrograph of a silver coated periodic polymer structure with average silver thickness of $50\mathrm{nm}$: (a) unstretched, periodicity $750\mathrm{nm}$; (b) 20% stretched, periodicity $900\mathrm{nm}$.

Figure 2

(Color online) (a) Scan of the intensity of the $-1$ diffraction order versus angle of incidence of $s$-polarized and $p$-polarized light. Lattice constant is fixed at $780\mathrm{nm}$. The experimental setup does not allow measurements in the shaded area. (b) $\mathbf{k}$-vector diagram of the SPP excitation: 1st order (left) and $-2\text{nd}$ order (right).

Figure 3

(Color online) (a) Angular scan of the reflectivity of the tunable system for six different lattice constants, recorded for the $-1$ diffractive order. (b) Development for the $(-2)$ plasmonic order. The solid line represents a prediction of the plasmon excitation, which proves to be accurate within the limits of the experimental errors; squares show data taken from (a), circles from additional measurements.

Figure 4

(Color online) Simulation of the specular reflection of a tunable PDMS based grating with initial periodicity of $640\mathrm{nm}$ and $40\mathrm{nm}$ silver thickness. The excitation wavelength is $632.8\mathrm{nm}$. The dots indicate the experimentally measured SPP excitations from three different samples.