Excitation of surface plasmons in a single silver nanowire using higher-order-mode light

Excitation of surface plasmons in a single silver nanowire using higher-order-mode light shows that nanowire waveguide has no request on the spatial mode of the input light, which is determined by its orbital angular momentums (OAM) in the experiment. The excitation efficiency can be controlled by adjusting the light polarization. Experimental result also indicates that the propagating modes of surface plasmons in nanowire are not the OAM eignenstates. & 2010 Elsevier B.V. All rights reserved. Today, the major problem to increase the speed of microprocessors is how to carry digital information from one end to the other faster. Optical interconnectors can carry much more digital data than that of electronic interconnectors, while fiber optical cables cannot be minimized to nanoscale due to the optical diffraction limit. To solve this size-incompatibility problem, we may need to integrate the optical elements on chip and fabricate them at the nanoscale. One such proposal is surface plasmons, which are electromagnetic waves that propagate along the surface of a conductor [1]. Plasmonics, surface plasmon-based optics, has been demonstrated and investigated intensively in nanoscale metallic hole arrays [2–4], metallic waveguides [5–7], and metallic nanowires [8–16] in recent years. Among the various kinds of plasmonics waveguides, silver nanowires have some unique properties that make them particularly attractive, such as low propagation loss due to their smooth surface and scattering of plasmons to photons only at their sharp ends. Since the momentums of the photons and plasmons are different, it is a challenge to couple free-space light into plasmon waveguides efficiently. The typical methods for plasmon excitation include grating coupling, prism coupling and focusing of light onto one end of the nanowire with a microscope objective. Nanoparticle antenna-based approach is also proved as an effective way for direct coupling into straight, continuous ll rights reserved. x: 865513606828. [email protected] (S.-H. Yu). nanowires [13]. Recently, polymer waveguides are used to couple light into several nanowires simultaneously [15] as well. Because the former researches about the nanowires always concentrate on using Gaussian mode light to excite surface plasmons, here we discuss whether surface plasmons can be launched by other higher-order-mode light. The higher-ordermode light is produced by changing the orbital angular momentum (OAM) of photons, since photons have different orbital angular momentum corresponding to light with different spatial energy distribution. We focus higher-order-mode laser beam on one end of a nanowire and observe scattering light from the other end. Surface plasmons are launched not only by Gaussian mode light but also by higher-order-mode light. The coupling strength over light polarization is also studied for higher-order-mode light and gives the similar results with the case of Gaussian mode. The output intensity increases linearly with the input intensity even for higher-order-mode light. Ag nanowires were synthesized through a polyol process in a mixture of ethylene glycol (EG) and poly (vinyl pyrrolidone) (PVP) at a certain temperature, which was very similar as the previous report [17–19]. They have well-defined crystal structure [20–22] and their smooth surface help to minimize surface plasmons damping due to scattering at roughness, domain boundaries or defects. Scanning electron micrograph (SEM) image in Fig. 1(a) shows that all the nanowires are straight and have uniform diameters that vary from 60 to 100nm and lengths from 10 to 40mm. A typical nanowire with diameter of 60nm and a tapered end is shown in Fig. 1(b). High resolution TEM image in Fig. 1(c)