Spectropolarimetric Surface Plasmon Resonance Sensor and the Selection of the Best Polarimetric Function

Intensity based spectral surface plasmon resonance (SPR) sensors are becoming more and more used due to the availability of miniature and low-cost spectrometers, however, they still lack the low limit of detection, because the SPR dip spectral width is around 50 nm even though the spectral sensitivity is considered relatively high. Profiting from the sharp jump in phase under SPR, several works suggested the extraction of the phase of the reflected wave. In this work, we propose a simple spectral polarimetric setup, which involves the addition of an analyzer at three different orientations and performing measurements of the intensity spectrum. Measurements at different concentrations of ethylene glycol in water were carried out using this technique and showed a comparable performance to some of the best spectral interrogation techniques. Theoretical simulations showed that the derivative of the phase difference between the TM and TE waves gives the highest resolution, although practically the more directly measured cosine function of the phase difference is the best one to use. Another important result is that the tolerance on the metal layer thickness is improved significantly when the spectropolarimetric method is used.