Enhanced Phase Sensitivity in Plasmonic Refractive Index Sensor Based on Slow Light

Abstract

Fano spectrum exhibits an intriguing feature of narrow asymmetrical resonance profile, which can be used in designing an ultra sensitive refractive index sensor. Here, in this letter, an ultra compact surface plasmon sensor is investigated. The sensor is based on Metal-Dielectric-Metal (MDM) waveguide geometry and connected to a pair of stub resonators. The distance between resonators is adjusted in such a manner that their increased coupling leads to an asymmetrical profile resonance, which is known as Fano resonance. The Fano resonance is very sensitive to any change in refractive index of the material. To measure the sensitivity of the device, stubs are filled with liquid/gaseous material under test. The structure is numerically simulated by the Finite difference time-domain method and the value of sensitivity is obtained as high as S = 510 nm/refractive index unit with a narrow line width of 19 nm and large quality factor Q ≈ 80. Fano resonance is generally accompanied with sharp dispersion, which leads to the generation of slow light. To account an effect of slow light in the waveguide phase, sensitivity is also analyzed as a function of a group delay. The large value of phase sensitivity is reported, which is equal to S O = 41.9 rad for per unit change in refractive index unit. The phase sensitivity rises linearly with the increasing value of group delay or decreasing value of group velocity. Thus, the device is well suited for designing on-chip optical sensors, optical buffers, switches, modulators, and so on.