Plasmons are the collective oscillations of the electrons on a metal's surface. For this reason plasmons are highly localised to a metal surface. They can be coupled out of the near-field into far-field radiation/bulk waves through bumps in the metal surface (as an example). Since plasmons are highly localised, researchers have been looking at using plasmon waveguides to guide light on the sub-wavelength scale. Thus, metallic nano-structures can have applications as sub-wavelength waveguides for use in all-optical integrated circuits, which may one day replace the current electronic integrated circuits. For example, plasmonic waveguides made in the form of a metallic V-groove are characterized by a unique combination of strong sub-wavelength localization and relatively low dissipation, single-mode operation, possibility of nearly 100% transmission through sharp bends, and high tolerance to structural imperfections.
Kristy Vernon is part of the Applied Optics group at the Queensland University of Technology, a research group working on the development of sub-wavelength optical circuits. The group's work is currently concentrated on the waveguide component of the circuit, but future research will look at building the rest of the circuit.
One type of strongly localized plasmons the group is interested in are plasmons in narrow metallic gaps filled completely or partially with dielectric. These plasmons may have especially strong localization in the direction perpendicular to the gap. It is also possible to expect that they may experience high transmission through sharp bends, because leakage bend losses into the metal are not possible. If is for this reason that gap plasmons are also expected to be tolerant to structural imperfections, similar to channel plasmon-polariton (CPP) modes in sharp V-grooves. The fabrication of gap plasmon waveguides (GPWs) may also be simpler than that of V-grooves.
Figure 1 shows a GPW, consisting of two metal plates immersed in a dielectric medium (or placed on a substrate), separated by a nano-sized gap. A GPW designed for experimentation was constructed by evaporating a thin (~ 2.2 μm) film of silver onto a glass substrate, then etching rectangular ridges of varying widths into the film by means of focused ion beam lithography, so that the metal was completely removed from the valley regions. A thin film of silver was then evaporated onto the structure, so as to cover the valley regions by ~100 nm of silver. Figure 2 shows a scanning electron microscope image of the experimental structure1.
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| Figure 1: Gap plasmon waveguide - a narrow nano-sized gap between two metal plates (films) immersed in a dielectric medium, or placed on a substrate. | Figure 2: GPW consisting of silver film on glass substrate. The three ridges are of thickness (1) h ≈ 800 nm, (2) h ≈ 1200 nm and (3) h ≈ 1300 nm. Gap widths (w) range from 70 nm to 300 nm. |
Computational modelling of the system plays a very important role in the design of the waveguides. Kristy models the waveguides using a handwritten finite-difference time-domain MATLAB code on the Auriga supercomputer at QUT's HPC unit. These simulations typically run for 3-5 days, and are designed to show various properties of waveguides of various structures, such as localisation, how far waves propagate along the waveguide, how small the waveguides can be, and the amount of coupling between waveguides. Kristy is in the process of developing similar code in C.
Kristy is supported in her work by the Queensland Government in the form of a Smart State PhD Scholarship.
Contacts
Kristy Vernon,
Dr Dmitri K. Gramotnev
Applied Optics group, Queensland University of Technology
Publication
- D.F.P. Pile, T. Ogawa, D.K. Gramotnev, Y. Matsuzaki, K.C. Vernon, T. Okamoto, M. Haraguchi, M. Fukui, “Two-dimensionally Localised Modes of a Nano-scale Gap Plasmon Waveguide”, Applied Physics Letters 87, 261114 (2005)
Written by K. Vernon and T. Curtis, October 2006.