Waveguide-coupled surface plasmon resonance sensor structures: Fano lineshape engineering for ultrahigh-resolution sensing

We analyze in detail the plasmon-induced transparency and Fano resonance exhibited by a waveguide-coupled surface plasmon resonance sensor structure. It is shown that the results of electromagnetic calculations made for the structure agree very well with those of mechanical calculations made for two coupled harmonic oscillators. This implies that an analogy holds between the present electromagnetic system and the coupled-oscillator system. The analogy established allows us to conclude that the plasmon-induced transparency and Fano resonance are caused by the coupling between a surface plasmon polariton and a planar waveguide mode. Sensing action of the Fano resonance is also analyzed in detail. From the calculation of the figure of merit for the sensitivity by intensity, it is shown that there is an optimum condition for the coupling of the modes to achieve a maximum sensitivity. Under the optimum condition, the figure of merit is found to be three orders of magnitude higher than that of a conventional surface plasmon sensor.