The angular momentum possessed by a beam of light is a quantity of scientific and technological interest. Scientifically, the nature of light’s angular momentum in matter seems somewhat enigmatic; two rival formulations are available for the calculation of light’s angular momentum, named after their original proponents, Max Abraham and Hermann Minkowski.
Technologically, the prospect of harnessing the torque that angular-momentum-carrying light can exert on tiny objects is appealing for optical applications in micromanipulation, communications, and information encoding. Further technological avenues are opened up if the light in question happens to be a surface-plasmon-polariton (SPP) wave.
These are surface waves which are bound to the interface of a metal and a dielectric material. The tight confinement of these SPP waves to the metal/dielectric interface has enabled them to be successfully exploited in highly sensitive chemical and biochemical sensors, including single-molecule detectors, as well as in nanoantennas and integrated circuitry.
The angular momentum of SPP waves provides the setting for a theoretical study recently published in the Journal of Nanophotonics.
The authors, Xuerong Xiao, SPIE member Muhammad Faryad, and SPIE Fellow Akhlesh Lakhtakia from Pennsylvania State University (USA), take the novel step of considering a nanostructured thin film as the dielectric material to partner a metal to support the propagation of SPP waves.
Specifically, the nanostructured thin film they consider is a sculptured nematic thin film, which is periodically nonhomogeneous in the direction normal to the interface. This periodic nonhomogeneity is a key characteristic: an isotropic dielectric material partnered with a metal supports only a single SPP wave whereas a periodically-nonhomogeneous dielectric material partnered with a metal can support a multitude of SPP waves, each with their own phase speed, attenuation rate, and spatial field profile.
Journal authors building on previous SPP research
In fact, the Penn State group was first to report this multiplicity of SPP waves — both theoretically and experimentally — in a series of papers published earlier in the Journal of Nanophotonics.
Multiple SPP waves offer greater opportunities for optical sensing, in terms of multi-analyte detection and improved error reduction, as well as multi-channel optical communication.
In the present study, “Multiple trains of same-color surface-plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part VI: Spin and orbital angular momentums,” the authors calculated both the orbital and spin contributions to the SPP’s angular momentum, using both the Abraham and Minkowski formulations.
They demonstrated that the periodic nonhomogeneity of the sculptured nematic thin film affords a variety of directions for the orbital and spin contributions to the SPP angular momentum and can also significantly enhance the magnitudes of these momentum contributions.
Furthermore, as the optical properties of the sculptured nematic thin film can be tailored at the fabrication stage, the magnitudes and directions of the orbital and spin contributions to the angular momentum for a multiplicity of SPP waves can be engineered.
–SPIE Fellow Tom Mackay of University of Edinburgh (UK) is co-chair of the Nanostructured Thin Films conference at SPIE Optics + Photonics and a member of the Journal of Nanophotonics editorial board.
Special journal section on nanophotonic materials and devices
The Journal of Nanophotonics invites manuscripts for a special section on nanophotonic materials and devices.
The special section will be based on the Optics & Photonics Taiwan (OPTIC 2013) symposium, but all papers will be considered, whether presented at OPTIC 2013 or not.
SPIE was a co-sponsor of OPTIC 2013, held 5-7 December.
Topics for the special section will include fundamental nanophotonic properties, applications of nanomaterials and nanostructured materials, plasmonic sensing, E-beam lithography, nanoantennas, slow light, and laser ablation.
Chii-Chang Chen of National Central University (Taiwan) is guest editor.
Manuscripts are due 31 March.