Surface plasmon resonance based fiber optic refractive index sensor utilizing silicon layer: Effect of doping

Abstract

We present an experimental study on the surface plasmon resonance (SPR) based fiber optic refractive index sensor utilizing a high index silicon layer between a metal layer and sensing medium using the wavelength interrogation mode of operation. Both n- and p-type silicon have been used. For the metal layer, silver and gold have been used. For a given metal, experimental results predict higher sensitivity of the sensor for the n-type silicon than for the p-type silicon layer. Further, for a given type of silicon, the sensitivity for the gold coated probe is higher than that of the silver coated probe. Numerically, the sensitivity of the n-type silicon with silver as the metal layer is approximately 1.39 times higher than that of the p-type silicon. In the case of gold as the metal layer, the sensitivity of the n-type silicon is approximately 1.50 times that of the p-type silicon. Since the refractive index of both p-type and n-type silicon is the same it appears that the majority charge carriers in silicon play an important role in the sensitivity of the surface plasmon resonance based sensor. The charge carriers are either affecting the field in the analyte region or may be somehow affecting the propagation constant of the surface plasmon wave which is solely due to oscillation of free electrons in the metal layer. In addition, the sensitivity of only the metal coated probes is found to lie between their p-type and n-type silicon coated probes. This suggests that the effect of charge carriers on sensitivity is more than the refractive index of the silicon layer. The effects of charge carriers in silicon, electrons and holes appears to be opposite.