Silver Nanowires as Surface Plasmon Resonators Harald Ditlbacher,1 Andreas Hohenau,1 Dieter Wagner,2 Uwe Kreibig,2 Michael Rogers,3 Ferdinand Hofer,3 Franz R. Aussenegg,1 and Joachim R. Krenn1 1Institute of Physics, Karl-Franzens-University, 8010 Graz, Austria 2Physikalisches Institut I.A, RWTH, 52074 Aachen, Germany 3Austrian Centre for Electron Microscopy and Nanoanalysis, 8010 Graz, Austria (Received 6 September 2005 published 16 December 2005) 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 m and 25%, respectively. We demonstrate that these properties allow us to apply the nanowires as efficient surface plasmon Fabry-Perot resonators. DOI: 10.1103/PhysRevLett.95.257403 PACS numbers: 78.67.Lt, 73.20.Mf, 73.22.Lp, 78.66. w 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 in m-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. FIG. 1 (color online). Scanning electron micrographs of a 18:6 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 imag- ing. PRL 95, 257403 (2005) P H Y S I C A L R E V I E W L E T T E R S week ending 16 DECEMBER 2005 0031-9007=05=95(25)=257403(4)$23.00 257403-1 �� 2005 The American Physical Society

Part of the incident laser intensity is scattered into a surface plasmon mode, which propagates towards the distal end of the wire. There, part of the plasmon intensity is scattered to light, which can be detected with a conventional optical microscope, as shown in the image acquired with a charge- coupled-device (CCD) camera in Fig. 2(b). For light field polarization normal to the wire axis, the distal end remains dark. Light emission from the nanowire in Fig. 2(b) is con- stricted to the distal end face due to the strongly bound character of the surface plasmon field, which couples to far field light only at wire discontinuities. Direct imaging of the surface plasmon field along the nanowire can, thus, be accomplished only by a near-field technique such as scan- ning near-field optical microscopy (SNOM) [14,15]. While maintaining the same excitation scheme as above, the optical image is now acquired by a sharp glass fiber tip raster scanned over the sample in a distance of a few nanometers. This distance is monitored by measuring the shear force between tip and sample [16] and maintained by a piezoelectric actuator. A SNOM image over the sample area defined by the box in Fig. 2(b) is shown in Fig. 2(c). The image reveals the modulation of the surface plasmon near field along the nanowire due to plasmon reflection at the distal wire end face. Similar patterns have been ob- served before on metal stripes with widths ranging from a few m down to 200 nm [8���10]. In all these cases, surface plasmon wavelengths (2 times the observed modulation pitch) closely matching those expected for a flat extended surface were found. For the present case of a silver nano- wire with a diameter of 120 nm, however, the surface plasmon wavelength is 414 nm, which is considerably shorter than the exciting light wavelength of 785 nm. The ratio of these two wavelengths shows that the surface plasmon mode cannot directly couple to far field light neither in air nor in the glass substrate (refractive index 1.5). This finding implies that plasmon propagation along the wire is not radiation damped. For closer analysis, we turn to spectroscopy, analyzing light scattered from both nanowire end faces as a function of wavelength. Therefore, we replace the laser by a halo- gen lamp as a white light source and extend the CCD camera setup for acquiring optical spectra in combination with a monochromator. The scattered light is collected by a microscope objective (50 , numerical aperture of 0.85). For suppression of excitation light, we use a dark field technique, illuminating the sample under total internal reflection through a glass prism optically coupled to the substrate glass slide [Fig. 3(a)]. Since white light cannot be focused as tightly as a laser beam, we illuminate the nano- wire uniformly (focus diameter 1 mm). For two reasons, FIG. 3 (color online). Scattered light spectra of 3:3 m long silver nanowires, diameter 90 nm. (a) Sketch of optical excita- tion. 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 chemi- cally 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). FIG. 2 (color online). Surface plasmon propagation along the 18:6 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 m long cross-cut along the chain dotted line in (c). PRL 95, 257403 (2005) P H Y S I C A L R E V I E W L E T T E R S week ending 16 DECEMBER 2005 257403-2