METALLIC GRATING ASSISTED SURFACE PLASMON RESONANCE BASED SENSORS

Abstract

Surface plasmon resonance (SPR) is one of the widely used techniques for the accurate and rapid detection of various biomedical and biochemical reactions or parameters. Surface plasmon (SP) waves can be excited either by prism coupling or grating coupling. Prism coupled SPR sensors are highly sensitive, but these are bulky and require sophisticated instrumentation. As an alternative to prism based SPR sensors, grating coupled SPR sensors are the ones those have attracted attention of researchers the most. Generally, Au being a noble metal is preferred for grating based sensor though it does not give a narrow SPR curve. Al gives a narrower SPR curve but is more susceptible to being oxidized. Therefore, we worked towards the improvement of the sensing performance of conventional Au grating based SPR sensors by using various bimetallic (Au-Al or Au-Ag) configurations. In a conventional grating-based sensor, grating is engraved on metal. A study on the effect of grating parameters and operating wavelength on the performance of conventional sensor has been carried out and grating parameters have been optimized. We could achieve the typical sensitivity of 300°/RIU and quality factor of 106 RIU-1 with optimized parameters. To improve the performance, we proposed to use bimetallic configuration that consists of Ag grating over Au film and we could enhance the sensitivity from 300°/RIU to 315°/RIU. The quality factor of this configuration is 126 RIU-1. We have further extended our study on bimetallic configuration and carried out numerical simulations to investigate the effect of Al and Ag film underneath of Au grating. It is found that the FWHM of the SPR curve can be brought down by using Al or Ag film. We show that the bimetallic (Au grating over Al film) has better SPR response than that of the conventional Au grating-based sensor. Al undergoes oxidation and degrades the performance of the sensor. To address this issue, we propose to cover the exposed area of the Al by filling the grating space by Al2O3. The proposed sensor exhibits better SPR response and is chemically stable. Observed quality factor is more than 267.6 RIU-1. As an alternative solution for oxidation of Al we also proposed to use an ultrathin Au layer between the grating and the Al film. The sandwiched Au layer affects the sensitivity marginally but gives stability to the sensor. The quality factor of the sensor (Au grating over Au coated Al film) has been found to be 245.2 RIU-1. The sensor can be useful as a biosensor for determining the concentration of haemoglobin in human blood. The concept of bimetallic configuration is not only limited to Au-Al combination. It can be used to design a hydrogen sensor. Hydrogen is an ideal candidate for clean and green energy. But its physical properties such as high combustion energy, low ignition temperature make it hazardous. 4% hydrogen in air, is the lower explosion limit that need to be detected. In our design, we used a combination of Pd grating and metal (Au, Ag or Al) for the detection of hydrogen. It is shown that the bimetallic (Pd-Al) is the appropriate choice for hydrogen sensor. We have optimized Pd grating parameters while considering shift in the resonance angle, dip strength and FWHM of the SPR dip. The figure of merit (FOM) of the proposed sensor has been obtained to be 0.343. Recently, 2D materials, such as, graphene and TMDCs, have also been utilized for designing SPR sensor. It has been reported that the sensitivity of the prism-based sensor could be enhanced by using graphene layers. Although graphene is a good choice for enhancing the sensitivity, but it also broadens the SPR curve, which reduces the quality factor. To resolve this issue, we have proposed sensors based on nano-ribbons of graphene or WSe2. Introduction of nano-ribbons, reduces the FWHM of SPR curve and improves the quality factor. The quality factor of the sensor based on continuous film of graphene was obtained to be 164 RIU-1, which could be enhanced to 184 RIU-1 by using nano-ribbons of graphene.