Plasmonic Coupled Nanowire: Light Localization, Propagation, and Beam Splitter

Propagation and localization of light at sub-wavelength scale is an important question in nanophotonics and a reliable solution has great relevance in realizing the leading nanophotonic applications of light. In dielectric optics, diffraction of light has been a major hindrance to control light at nanoscale, but in recent years metallic nanostructures that support surface plasmons have opened up new avenues to propagate and localize light at subwavelength regime. This has led to tremendous interest in interaction of light with nanoscale plasmonic geometries such as silver nanowires (Ag-NW) that can be harnessed as circuit-element in a nanophotonic circuit. For such circuits to emerge, it is necessary to further develop and understand capability of plasmonic nanowires to perform various logical functions similar to circuit elements in electronics. The following details have directly taken from research article titled 'Propagation of light in serially coupled plasmonic nanowire dimer: Geometry dependence and polarization control'

 Fabrication of Serially-coupled Ag Nanowires

The chemically synthesized nanowire geometry were developed using polyol seed-mediated method approach. Using such method, the single crysalline silver nanowire can be synthesized. The synthesized nanowire can be washed using centrifugation to remove unwanted particles. Later, the nanowire sonicated to form the coupled form. In this unique geometry, the nanowire is formed by two arms where the lattices are attached with each other without any external material interface. During the whole synthesis methods, it provides the various geometries with varying angle between the two arms from acute to obtuse. The diameter of nanowire was obtained from 150nm to 250nm and length was from 10um to 50um, which is highly depends on the synthesis parameters. Such single crystal nanowire geometry provides an important platform to study the light manipulation and control, beyond the diffraction limit. 

Controlled light propagation

One can transmit the light beyond diffraction limit using a unique phenomena called surface plasmons polaritons (SPPs). Most commonly used materials to observe SPPs are Silver (Ag) and gold (Au). The guided modes of light supported by nanowires using such materials shows new ways to transmit light signals at low dimensions. The light propagation and its control using geometry is possible using a unique geometry of Ag NWs called serially coupled nanowires. Beyond the propagation, one can also split the light with specific polarization. Such nano polarizing beam splitting is vital platform for signal transmitting in photonic chips. The experimentally studied plasmon-polariton-assisted light propagation in serially coupled silver nanowire (Ag-NW) dimers and probed their dependence on bending-angle between the nanowires and polarization of incident light. From the angle-dependence study, we observed that obtuse angles between the nanowires resulted in better transmission than acute angles. From the polarization studies, we inferred that light emission from junction and distal ends of Ag-NW dimers can be systematically controlled. Further, we applied this property to show light routing and polarization beam splitting in obtuse-angled Ag-NW dimer. The studied geometry can be an excellent test-bed for plasmonic circuitry. 

Plasmonic Polarization Beam Splitter (PBS)

Can this geometry of coupled-nanowires meniaturize the polarization dependence to be used to route light? To answer this question, Ag NW dimers were tested for light routing capability of obtuse angled Ag NW dimer. First fig. shows the experimental schematic to test the light routing capability in Ag NW dimer. The location of illumination was at the Ag NW junction; and by varying the polarization of the incident light at the NW junction, we observed the emission of light at two ends of the Ag NW. Second figure shows the optical image of Ag NW dimer. An observed emission at both ends of the Ag NW when the polarization of the incident beam had projection on both of the nanowire axis. However, when the polarization was oriented such that the electric field was along one of the axis of nanowires (see the black arrow for incident polarization) shows the emission of light from only one end of the Ag NW dimer.  Such nano plasmonic polarization beam splitter will extend the boundaries of application in photonic devices.