Influence of the Sub-Peak of Secondary Surface Plasmon Resonance Onto the Sensing Performance of a D-Shaped Photonic Crystal Fibre Sensor

In this paper, we mainly investigate the sensing performance of a fabricable 6-fold D-shaped photonic crystal fibre sensor based on the surface plasmon resonance (SPR). Its resonance couplings between fundamental core mode and three surface plasmonic modes which have different electric filed distributions for analytes of various refractive indices have been studied in detail. We firstly observe two different types of SPRs, namely, `dielectric like' resonance with low-loss peak and `plasmon like' resonance with high-loss peak, by analysing the electric field distribution of the fibre modes. Then, we discuss the influence of the secondary SPR over the main SPR which is directly related to the limitation on the detection sensing range of the proposed sensor. In order to mitigate the adverse effect of the sub-peak of the secondary SPR on the sensor's sensing performance, we reduce the thickness of analyte's binding layer from 1500 nm to 500 nm. Thus, the sensing range of the proposed sensor can be tuned from 1.33 - 1.41 to 1.33 - 1.45 at the cost of a reduced maximum sensitivity from 7900 nm/RIU to 5300 nm/RIU. Owning to the simple structure design of the proposed sensor, we envisage that this highly sensitive D-shaped PCF-SPR sensor could be developed as a versatile and competitive instrument with a large and flexible refractive index detection range.