Metal-coated silicon nanowire plasmonic waveguides Yongsop Hwang1,*, Min-Soo Hwang1, Won Woo Lee2, Won Il Park2, and Hong-Gyu Park1, 1

Department of Physics, Korea University, Seoul 136-701, Republic of Korea; 2Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea; *[email protected]

A plasmonic waveguide is an essential component in ultracompact photonic integrated circuits, where surface-plasmon-polaritons (SPPs) are strongly confined within the cross-section and propagate along the waveguide axis. We propose and demonstrate a new subwavelength plasmonic waveguide that takes full advantage of the unique properties of plasmonic waveguides [1]. Our plasmonic waveguide consists of a bottom-up synthesized silicon nanowire (Si NW) coated with silver (Fig. 1(a)). Since the waveguide structure has a smooth surface, SPP guided modes can experience low scattering loss at the interface between the Si NW and the metal, thus enabling the measurement of long propagation length. In addition, SPP guided modes can be confined strongly in the Si NW surrounded by metal. We measure the propagation of SPPs on the subwavelength-scale silver-coated Si NW plasmonic waveguide, and estimate the propagation lengths for different polarization directions of an input beam. Numerical analysis is also performed for deep understanding of these experimental results. Si NWs with a diameter of ~80 nm were synthesized by chemical vapor deposition and dispersed onto a SiO2/Si wafer. Electron-beam lithography and lift-off process after the deposition of silver and gold on the Si NWs were then performed. Figures 1(b) and 1(c) show representative scanning electron microscope (SEM) images of a fabricated waveguide structure. The Si NW was covered with silver (Fig. 1(c)), while its two ends are exposed either to couple a vertically incident beam to an SPP waveguide mode or to measure the output light ejected from the waveguide, as shown in the schematic diagram of Fig. 1(a). We measured the propagation of SPPs in the fabricated plasmonic waveguides. A continuous-wave laser with a wavelength of 1310 nm was used as an incident beam to inject light into one end of the waveguide. The scattered light from the other end of the waveguide was collected by a x60 objective lens and focused onto an infrared (IR) camera. The output intensity of the guided SPPs was obtained by integrating the intensity at each pixel of the IR camera within the output emission spot. The measurement was performed for plasmonic waveguides with different lengths but similar NW diameters. The average diameter of the Si NWs was 83.2 nm. The measured propagation lengths for TM and TE SPP guided modes are estimated to be 8.05±1.57 μm and 6.61±1.73 μm, respectively. The

propagation length of the TM SPP guided mode is longer than that of the TE mode. We note that this propagation length is sufficient for chip-scale communication. In addition, we performed numerical simulations using the finite-difference time-domain (FDTD) method. The propagation lengths were calculated as 17.52 and 3.33 m for the TM and TE SPP guided modes, respectively. This tendency is similar to that of the measured propagation length. The simulation also shows that the SPP guided modes are confined significantly in the silver-coated Si NW waveguide of subwavelength scale. In summary, the propagation characteristics of the SPP guided modes in a silver-coated Si NW plasmonic waveguide were measured in experiment and analyzed systematically using FDTD simulation. Subwavelength plasmonic waveguides were fabricated using a chemically synthesized Si NW with a diameter of ~80 nm, and the propagation lengths of ~8.05 and ~6.61 m were measured for the TM and TE SPP guided modes, respectively. We believe that these waveguides will thus be highly useful for the demonstration of ultracompact multifunctional plasmonic integrated circuits.

Fig. 1: (a) Schematic of a metal-coated Si NW plasmonic waveguide. (b) SEM image of a fabricated plasmonic waveguide. The scale bar is 5 m. (c) Magnified SEM image of one end of the plasmonic waveguide. The scale bar is 200 nm. References: [1] Y. Hwang, M.-S. Hwang, W. W. Lee, W. I. Park, and H.-G. Park, submitted