Design of Compact Mach–Zehnder Interferometer-Based Slow-Light-Enhanced Plasmonic Waveguide Sensors

We investigate slow-light-enhanced ultracompact plasmonic Mach–Zehnder interferometer sensors. The sensing arm consists of either a conventional metal–dielectric–metal plasmonic waveguide or a waveguide system based on a plasmonic analogue of electromagnetically induced transparency. Such plasmonic electromagnetically induced transparency waveguide systems can be engineered to support slow-light modes. We find that, as the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of the material filling the structures increases. Such slow-light enhancements of the sensitivity to refractive index variations lead to enhanced sensing performance. We show that the Mach–Zehnder interferometer sensor using the plasmonic electromagnetically induced transparency waveguide system leads to approximately an order of magnitude enhancement in the refractive index sensitivity, and therefore, in the minimum detectable refractive index change, compared to the sensor using a conventional metal–dielectric–metal plasmonic waveguide. We also find that the refractive index sensitivity for such a plasmonic Mach–Zehnder interferometer sensor is approximately two times larger than the sensitivity of a waveguide-cavity sensor in which the plasmonic electromagnetically induced transparency waveguide system is sandwiched between two conventional metal–dielectric–metal waveguides.