We show that periodically structuring the metal cladding in surface-plasmon asymmetric waveguides for semiconductor lasers introduces new modes which exhibit low propagation losses. The onset of these modes is directly related to the conservation of charge in the metallic layer. Counter intuitively, for a 1st-order-grating the electric-field maxima of the low-loss mode are located below the metallic fingers, and not below the air-gaps. Field plots show that the mode is only weakly-bounded to the metal, explaining the loss reduction. We apply these theoretical findings to surface-plasmon quantum-cascade lasers emitting at λ≈7.5 micron. An important current threshold reduction is obtained, together with single-mode operation, thus confirming the theoretical predictions.

© 2008 Optical Society of America

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