Mid-infrared plasmonic gas sensor

The Mid Infrared MIR wavelength range offers many advantages in different applications. Chemical and biological detection are one of these applications, as it contains the absorption fingerprints of many gases and molecules. In addition integrated plasmonics are suitable platform for high sensitivity on chip sensors. In this paper we propose plasmonic Mach-Zehnder Interferometer (MZI) working as a gas sensor near the absorption fingerprints of many gases in the mid-infrared region. The proposed MZI contains a vertically stacked metal-insulator-metal (MIM) and metalinsulator (MI) waveguide. The sensitivity of MI waveguide is lower at higher wavelengths and also lower for gaseous medium than for liquid medium. In addition the losses of the MIM waveguide with oxide layer as insulator are much larger than the losses of the MI waveguide with gas as insulator which will result in poor visibility interferometers. Using a high index layer above the metal of the MI waveguide the sensitivity of the waveguide to gaseous in the mid infrared has been significantly enhanced. This layer also balances the intrinsic losses of both MI and MIM waveguides. The thickness and the refractive index of this layer have been optimized using finite difference modal analysis. Using this layer high sensitivity and high figure of merit (FOM) have been achieved for our MZI. This structure offers simple fabrication and low cost sensor that is suitable for rapid, portable and high throughput optical detection using multiplexed array sensing technique.