Surface-plasmon waveguide permits easy coupling

At the California Institute of Technology (Caltech; Pasadena, CA), researchers have demonstrated a new two-dimensional waveguide that transports energy in the form of surface plasmons.

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At the California Institute of Technology (Caltech; Pasadena, CA), researchers have demonstrated a new two-dimensional waveguide that transports energy in the form of surface plasmons. Unlike other devices built using similar principles, it has two major advantages: the new structure confines the input energy more effectively and highly efficient (more than 90%) coupling is possible using a simple tapered fiber—one in which the cladding has been stripped away to make the fiber's electric field more accessible. Waveguides with these properties may find application in telecommunications and sensing because the interface of the devices are accessible to the outside world.

Surface plasmons occur at a metal/dielectric interface where a group of electrons is collectively moving back and forth between states, which sets up an electron-density oscillation perpendicular to the plane of the surface. Therefore, the oscillations can be caused by optical input and produce optical output. To provide lateral confinement of this energy, Stefan Maier and his colleagues at Caltech have used a structure similar to what others in the field use, in which nanoscale metal dots (in this case, gold) are patterned on the dielectric (silicon) to define the path that the plasmon should take.1 These dots have a spacing that is a fraction of the photon wavelength, creating a diffusing pattern that helps shape the electric field.

Although conventional in these respects, there are some important differences between the Caltech device and others (see figure). First, the waveguide structure is not uniform across its width; the size of the metal dots, and thus the oscillation that can be supported, reduces from 80 × 80 nm at the center to 50 × 50 nm at the edges. This has the effect of confining the energy more tightly to the middle of the guide. As a result, the main source of loss is the resistive heating of the metal structures. Because the metal dots are small, the losses are also very small.

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A plasmon waveguide consists of nanoscale gold dots on a silicon-on-insulator surface, with the metal exposed to the air (bottom left). An end mirror built into the structure contrasts with the main waveguide, showing how, in the latter, the dots get smaller the farther they are from the center (bottom right). The electric field produced as the plasmons propagate is well confined in all directions and protrudes above the level of the silicon surface and metal features (top).
Click here to enlarge image

A second major difference is that the periodicity and design of the Caltech device force the electric field to protrude from the surface above the level of the metal features. While the silicon/metal/air structure remains intact, the energy is still well confined. However, the intense—and accessible—electric field presents other opportunities. For instance, biological or chemical species can be attracted by the field. Upon reaching the surface, or getting sufficiently close to it, the cell or molecule will absorb energy and/or couple it out of the waveguide. A receiver down the line will be able to detect the drop in optical power, and therefore act as a chemical or biological sensor.

The protruding field can also be used as a means of coupling a signal into the waveguide in the first place. A tapered fiber can be put in contact with the waveguide so that the two electric fields overlap. Therefore, the energy can be easily coupled out of the fiber and into the waveguide. In their recent paper, the Caltech researchers point out that, theoretically, the process could be efficient even in the presence of fabrication errors. The researchers have since been able to show this works in practice; in a recent experiment, Maier says, they demonstrated such coupling with 75% efficiency.

REFERENCE

  1. S.A. Maier, et al., Appl. Phys. Lett. 84, 20, (May 17, 2004).

SUNNY BAINS is a scientist and journalist based at Imperial College London; e-mail: [email protected]

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