Silver nanowires as surface plasmon resonators.

The integration of optics with nanotechnology is hin- dered by the lack of subwavelength photonic elements. Surface plasmons—coupled excitations of light and elec- trons at a metal surface—are a potential solution to this problem, as they allow the concentration of light to sub- wavelength volumes (1). Recent advances in plasmonics have demonstrated surface plasmon waveguiding and op- tical addressing and, thus, the feasibility of integrated plasmon optics. Waveguiding inm-wide metal thin films (2-4) and nanowires (5-9) and passive (10) and dynamic control (11) thereof has been shown. Here we report the experimental realization of Fabry-Perot-type plasmon res- onators by chemically prepared silver wires with 100 nm cross-section diameters and lengths up to about 20 � m. Our resonators rely on specific plasmon modes with wave- lengths considerably shorter than the exciting light wave- length. These modes are not radiation damped and lead, thus, to unexpectedly large propagation lengths. Besides laying the foundation for wavelength selective devices, nanowire resonators might, therefore, enable improved spatial resolution in plasmon-based photonic circuitry. Provided that the wire end faces reflect an incident surface plasmon, a nanowire can be turned into a surface plasmon resonator. Then resonator modes, i.e., standing surface plasmon waves along the nanowire axis, exist whenever an integer of half the surface plasmon wave- length equals the wire length. The maximum achievable resonator length is, however, limited by the metallic damp- ing of the surface plasmon mode (12). We investigate chemically prepared silver nanowires with a well defined crystal and surface structure, thereby minimizing surface plasmon damping due to scattering at roughness, domain boundaries, or defects. The nanowires are produced by a chemical reduction method of silver ions in an aqueous electrolyte solution. The fabrication process yields nano- wires with cross-section diameters of 13-130 nm and lengths up to 70 � m (13). High resolution transmission electron microscopy reveals the nanowires to consist of a lattice aligned bundle of five monocrystalline rods of a triangular cross section forming an almost regular pentago- nal cross section (13). Casting the purified electrolyte on a glass slide and letting it dry under ambient conditions yields well separated individual wires on the slide. One such wire is shown in Fig. 1. Surface plasmon propagation along a nanowire can be straightforwardly demonstrated by local optical excitation (7). We focus a laser beam under normal incidence with respect to the substrate plane with a microscope objective (60 , numerical aperture of 1.4) onto one end face (input end) of a 18:6 � m long nanowire with a diameter of 120 nm; see Fig. 2(a). The laser wavelength is 785 nm, and the polarization is oriented along the nanowire axis.