Exploiting Orbital Angular Momenta and Nonlinear Frequency Conversion in Plasmonic Devices
Surface plasmons (SPs) are evanescent waves generated through the collective oscillations of electrons at a metal/dielectric interface under optical excitation. Due to the strong light-electron coupling and near-field nature, surface plasmons offer: (1) the opportunity for sub-wavelength spatial confinement of optical waves is enabled; and (2) giant local field enhancement of optical waves is permitted. These unique attributes lead to the long-envisaged optical circuits, and allowed breakthroughs in the generally termed “plasmonics” . For example, in the realizations of optical nano-antennas , surface plasmon vortices [3-5], and sub-wavelength guiding in plasmonic two-wire transmission-line (TWTL) [6-8] that have all enriched the scientific world. Optical vortices are waves carrying orbital angular momentum and exhibit helical phase fronts. Helical phase front leads to discontinuous azimuthal phase jumps and the number of phase discontinuities within a 2p range is referred to as the topological charge of an optical vortex. Generation of optical beams carrying orbital angular momentum has received increasing attentions recently, both in the far- field and in the near-field. Near-field vortices are typically generated through the excitation of SPs. In this talk, I will first introduce our recent progress on applying surface plasmon vortex for selectable particle trapping and rotation. The ability to spatially shape the near-field spatial patterns of surface plasmon vortices will be addressed as well. Moreover, in all past studies, SP vortices were excited by far-field circularly polarized light. This means the functionality of the SP devices were merely converting the far-field spin angular momentum to orbital angular momentum in the near-field. Next, I will focus on the creation of surface plasmon vortex using non-angular momentum excitation. Nonlinear optical frequency conversion in plasmonics has attracted immense research attention recently. Through various device geometries, harmonic generations of various orders have been reported. However, in these demonstrations, the near-field effect utilized has been limited to localized surface plasmons. It was not until last year that nonlinear frequency conversion utilizing propagating surface plasmon polaritons being reported in a single plasmonic nanowire. A plasmonic TWTL is comprised of two metallic nanowires with a nano-gap between these two nanowires. Depending on the laser excitation polarization, TE or TM mode could be selectively excited within a plasmonic TWTL. It has been demonstrated the two modes could be freely converted through the geometrical design to the TWTL [6,7]. Here I will present our most recent experimental data on ultrafast second-harmonic generations (SHG) in a plasmonic TWTL. We also show that regardless of the fundamental harmonic mode excited by the laser beam, the SHG signals are always the TM mode if no special treatment to the laser beam or the TWTL is provided. In the second part of this talk, the functionality of a plasmonic two-wire transmission-line will be introduced. I will also extend the studies into the nonlinear optical regime, where interesting modal behaviors are observed.
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