The performance of surface plasmon resonance (SPR) sensors can be different depending on which characteristic of the SPR sensor the amplitude or the phase is monitored. The phase sensitivity strongly depends on the geometry and the optical properties of the system. The existence of sharp changes in the phase spectrum variations is found as the thickness of SPR-supporting gold (d g ) varies around a critical thickness (d c ). In addition, the simulation results indicate that the phase sensitivity is divided into two regions so that phase sensitivity S j for region d d g c is greater than region d g >d c . It is demonstrated in condition of d d g c the phase sensitivity has a strong jump when the sensing medium refractive index lies within a certain interval, while the amplitude sensitivity has a monotonic shape. The phase analysis from another aspect exhibits that the phase maximum difference Δj max towards the blank sample is sensitive to refractive index in a continuous interval. As a result, the phase detection interval is tunable by varying the gold thickness, which potentially is important for medical, biology and chemistry applications.
The amplitude and phase sensitivities of a sensor with a multiple nanolayers consisting of gold-silicon dioxide-gold (Au-SiO2-Au) are investigated. The proposed sensor has the ability to limit to the certain regions of the sensing medium refractive index n s . The number of these regions can be increased by increasing the thickness of the silicon dioxide layer ( d SiO 2 ). The results show that for the d SiO 2 = 200 nm the sensitivity has a nearly Gaussian in shape which its position depends on the refractive index of the prism n p . This type of sensor is introduced as a singlet refractive index limited sensor (S-RILS) that is useful tool in the detection of the target sample optical properties with certain refractive indices. It is found that two nearly Gaussian curves are generated in the amplitude and phase sensitivities for the d SiO 2 = 450 nm , which is introduced as a doublet refractive index limited sensor (D-RILS). The optical properties of the mixture samples can be detected by D-RILS, in while the S-RILS is used to detect the optical properties of single samples. The efficiency of the sensors can be improved by adding the titanium or titanium-graphene layers. The results exhibit that the optimal amplitude and phase sensitivities occur for multiple nanolayers (Ti-Au-SiO2-Ti-Au).
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