Abstract
The effects of air gaps in metal stripe waveguides supporting long-range surface plasmon polaritons have been determined theoretically. The study is motivated by a recently adopted fabrication approach based on direct bonding, where various kinds of air gaps may form near the metal stripe due to fabrication imperfections. Specifically, “Air wings,” a top air gap, and side air gaps have been considered as possible perturbations. The main effects of the air gaps on the propagation of the long-range surface plasmon-polariton wave are that its attenuation and confinement decrease as the gaps become more invasive, and that its mode fields become strongly perturbed. Taken together, these effects are deleterious, so air gaps do not appear suitable for range extension. In general, very small air gaps can only be tolerated before confinement is completely lost. A lower-index planarizing layer could be used to help eliminate air gaps but the waveguide design space becomes constrained. The air gaps perturb the long-range mode such that its main transverse electric-field component (Ey) develops a maximum within the gaps and some localization therein. This feature could be interesting in applications where high-intensity fields in nanometric air gaps are sought, but only if coupling and radiation losses are not much of a concern.
Published Version
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