Subminiature gas sensor based on the Photonic crystals

In this paper, we design a symmetric Mach-Zehnder interferometer (MZI) gas sensor based on two-dimensional Photonic crystals (Phc), which is filled with the gases of different refractive index in the holes of the silicon plate. Self-Collimation (SC) effect analyzed by equi-frequency contour (EFC) is used to route the propagation of light straightly; the bending and splitting of lights are realized by the line-defect structures. Finite Difference Time Domain (FDTD) simulations show that the outputs of the MZI vary with the gas in the test arm and a pi phase shift of the two output light beams is achieved as the gas refractive index n=1.24. By analyzing the corresponding relationship between the output results and the refractive index of gases in the test arm, it can act as a gas sensor. Finally, we conclude the mechanisms of the outputs and analyze the phase shift and the Effective Index (n_eff) in the SC direction which is calculated by the radio of vector-k As the wavelength lambda=632.8 nm (the wavelength of CO₂ laser), the size of the MZI is as small as 6745 nm times 6745 nm. Thus, we realize the micromation of the sensor, which may play an important role in high-density integration design.