Optical sensing in single-mode filters base on surface plasmon H-shaped cavities

Three novel refractive index (RI) sensors based on one, two, and three H-shaped cavities (HCs) and metal- insulator-metal (MIM) waveguides are proposed in this paper. By coupling more than one HC (two and three), the Q-factor and then the FOM value of the designed RI sensors increase. A well-known method for increasing the sharpness transition between the passband and stopband of a spectrum is coupling cavities together. Fortunately, the proposed HC has such a property that increases the FOM value. The maximum obtained FOM of the proposed sensors using one, two, and three HCs reached 69.5, 100.19, and 108.36 RIU-1, respectively. Three proposed plasmonic structures which are used to design RI sensors have single-mode spectra in a wide wavelength range of 500 to 2000 nm. A single-mode spectrum is easier to cope with in circuits with higher complexity such as RI sensors. The metal and insulator materials that are used in the designed structures are silver and air, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed RI sensors. To verify FDTD simulations, the basic rectangular cavity which is used to design HC is analyzed using an analytical method. In summary, the benefits of the proposed RI sensors are having single-mode spectra in a wide wavelength range and also their simple structures. The proposed topology easily be redesigned and adapted with different numbers of HCs without disturbing its single-mode spectrum to achieve various FOM values. Furthermore, considering the fact that the proposed cavity is based on rectangular shapes, it has a simple fabrication process. Taking into account the mentioned advantages of the proposed structures, they can be used in integrated optical circuits for optical communication purposes.