Slow-light enhanced integrated plasmo-photonic index sensors: possibilities and performance limits

This is the authors' abstract. We don't add key points for this paper.

Slowing down the light leads to increased interaction time with matter. In photonic sensors, this promises higher sensitivity in smaller device footprints. A robust theoretical understanding, however, of such approach in a specific example of integrated photonic sensors, addressing design parameters, compromises, and performance limits, is lacking. Here, we theoretically investigate the operation and performance limits of an integrated plasmo-photonic Mach–Zehnder interferometer enhanced by the slow-light effects of Bragg gratings. To systematically explore the full parameter design space of the Bragg-defined coupled cavity structures decorating the plasmonic strip sensor we develop and validate a convenient one-dimensional effective model of the full structure. Considering photonic band structure, slow-light properties, and overall sensitivity enhancement, we report extreme sensitivity improvements up to 1800% compared to bare plasmonic systems of equal length, easily exceeding 10 5 nm RIU −1 at the photonic bandgap edges and at the coupled cavity bands. We also report the remarkable existence of a critical plasmonic length, across which the critical parameter for enhancing sensitivity reverses from being the number of Bragg periods defining a cavity to being the number of cascaded cavities. Results are validated by full finite-difference time-domain simulations while discussing practical considerations such as free-space scattering and fabrication constraints. The theory and results presented here provide for the first time a clear perspective on utilizing slow light effects for index sensing and a comprehensive framework for optimizing slow-light plasmo-photonic sensors for biological and chemical sensing applications.