Plasmonic propagation and spectral splitting in nanostructured metal wires
Authors: Xu Di-Hu  , Hu Qing  , Peng Ru-Wen  , Zhou Yu  , Wang Mu  ,   National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China;,   Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Published: December 2020

Due to the coupling of photons with the electrons at a metal-dielectric interface, surface plasmons (SPs) can achieve extreflely small wavelengths and highly localized electromagnetic fields. Hence, plasmonics with subwavelength characteristics can break the diffraction limit of light, and thus has aroused great interest for decades. The SP-inspired reflearch, in the application respect, includes extraordinary optical transmission, surface enhanced Raman spectroscopy, sub-wavelength imaging, electromagnetic induced transparency, perfect absorbers, polarization switches, etc.; and in the fundamental respect, includes plasmon-mediated light-matter interaction, such as plasmonic lasing, plasmon-exciton strong coupling, etc.#br#Recently a series of studies has been performed to push the dimensions of plasmonic devices into deep subwavelength by using nanowires. The chemically synthesized metallic nanowires have good plasmonic properties such as low damping. The reported silver nanowire structures show great potential as plasmonic devices for communication and computation. Now we develop the nanostructured metal wires for plasmonic splitters based on the following considerations. One is that we introduce cascade nano-gratings on a metallic nanowire, enabling a single nanowire to act as a spectral splitting device at subwavelength; and the other is that we use silicon as a substrate for the metallic nanowire, making the plasmonic nanowire device compatible with silicon based technologies.#br#In this paper, we continue and develop our previous work on position-sensitive spectral splitting with a plasmonic nanowire on silicon chip (see Scientific Reports (2013) 3 3095). The three parts are organized as follows. In the first part, we derive analytically the dispersion relation of the SPs in a suspended silver nanowire based on Maxwell equations. In the second part, we placed a silver nanowire in the silicon substrate, and use the finite-element method (FEM) to obtain the dispersion relation of the SPs for the practical applications. The calculations show that the SP mode can be confined better in this system, howbeit with larger loss. Starting from the dispersion relation, we then calculate the mode area, the propagation length and the effective index of the SP modes, with respect to the nanowire dimension and the substrate materials. It is shown that a thinner nanowire has smaller mode area and a higher-index substrate induces larger loss. We also perform the finite-difference time-domain (FDTD) simulation to investigate the electromagnetic field distribution in this system. We find that the SP mode is mainly confined around the top surface of the nanowire, and in the crescent gap between the nanowire and the substrate. In the third part, we demonstrate both experimentally and theoretically that the silver nanowire with two cascaded gratings can act as a spectral splitter for sorting/demultiplexing photons at different spacial locations. The geometry of the grating is optimized by rigorous coupled wave analysis (RCWA) calculation. The carefully designed gratings allow the SPs with the frequencies in the plasmonic band and prohibit the SPs with the frequencies in the plasmonics bandgap. Those prohibited SPs areflemitted out through a single groove in front of each grating. Both the detected images and the measured optical spectra demonstrate that the SPs with different colors can be emitted at different grooves along a single nanowire. Thus the structured metal nanowire shows potential applications in position-sensitive spectral splitting and optical signal processing on a nanoscale, and provides a unique approach to integrating nanophotonics with microelectronics.