Optimization of 2D slab photonic crystal geometry for gas sensing

The two-dimensional (2D) slab photonic crystal (PhC) geometry is optimized for the application of the PhC as a near-infrared spectroscopic gas sensor´s active element. Sensor operation in the slow-light regime is assumed. The 2D theoretical model is based on the finite-difference time-domain (FDTD) propagation of the electromagnetic field. Linear-defect 2D photonic crystal structures are considered. Optimization is performed according to the following criteria: maximum light coupling efficiency, maximum light extraction efficiency, and maximum light-gas overlap. In the simulations, output signal collection is performed by means of either an output strip waveguide. Antireflection sections suited for individual PhC geometry types, are applied to prevent excessive reflections resulting from the high group velocity of light propagating in the slow-light regime within the PhC. Configurations with and without antireflection sections are compared. Triangular-lattice 2D PhC types are: line-defect crystals and line-defect crystals featuring an additional row of holes that improve the light-gas overlap.