Description:Title: Exploiting orbital angular momenta and nonlinear frequency conversion in plasmonic devices
Surface plasmons (SPs) are evanescent waves generated throughthe collective oscillations of electrons at a metal/dielectric interface underoptical excitation. Due to the strong light-electron coupling and near-fieldnature, surface plasmons offer: (1) the opportunity for sub-wavelength spatialconfinement of optical waves is enabled; and (2) giant local field enhancementof optical waves is permitted. These unique attributes lead to the long-envisagedoptical 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 plasmonictwo-wire transmission-line (TWTL) [6-8] that have all enriched the scientificworld.
Optical vortices are waves carrying orbital angular momentum andexhibit helical phase fronts. Helical phase front leads to discontinuousazimuthal phase jumps and the number of phase discontinuities (abrupt phasejumps from -p to p) within a 2p range is referred to as the topologicalcharge of an optical vortex. Generation of optical beams carrying orbitalangular momentum has received increasing attentions recently, both in thefar-field and in the near-field. Near-field vortices are typically generatedthrough the excitation of SPs. In this talk, I will first introduce our recentprogress on applying surface plasmon vortex for selectable particle trappingand rotation. The ability to spatially shape the near-field spatial patterns ofsurface plasmon vortices will be addressed as well. Moreover, in all paststudies, SP vortices were excited by far-field circularly polarized light. Thismeans the functionality of the SP devices were merely converting the far-fieldspin angular momentum to orbital angular momentum in the near-field. Next, Iwill focus on the creation of surface plasmon vortex using non-angular momentumexcitation.
Nonlinear optical frequency conversion in plasmonics has attractedimmense research attention recently. Through various device geometries,harmonic generations of various orders have been reported. However, in thesedemonstrations, the near-field effect utilized has been limited to localizedsurface plasmons. It was not until last year that nonlinear frequencyconversion utilizing propagating surface plasmon polaritons being reported in asingle plasmonic nanowire. A plasmonic TWTL is comprised of two metallicnanowires with a nano-gap between these two nanowires. Depending on the laserexcitation polarization, TE or TM mode could be selectively excited within aplasmonic TWTL. It has been demonstrated the two modes could be freelyconverted through the geometrical design to the TWTL [6,7]. Here I will presentour most recent experimental data on ultrafast second-harmonic generations(SHG) in a plasmonic TWTL. We also show that regardless of the fundamentalharmonic mode excited by the laser beam, the SHG signals are always the TM modeif no special treatment to the laser beam or the TWTL is provided. In thesecond part of this talk, the functionality of a plasmonic two-wiretransmission-line will be introduced. I will also extend the studies into thenonlinear optical regime, where interesting modal behaviors are observed.
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