**3. Conclusion**

In this chapter, we presented a review work on the recent progress of PhC-based gas sensing research in the mid-infrared range. Various material structures including using porous silicon structure, photonic crystal waveguides, and hollow-core photonic crystal fibers, as well as both optical and electrical detection methods, have been thoroughly discussed. As mentioned, porous silicon structure enhanced sensing device achieved the highest sensitivity to detect NO2 at 1 ppm concentration level through measuring the conductance changes. But this method is restricted to a limited range of gases, and is unable to detect nonpolar gases such as CO, CH4, and alcohols. The other issue can be related to electrical components which are necessary for this method. These electrical components increase the risk of electrical discharge and augment the risk of explosion. Moreover, the electrical noises can strongly affect this kind of sensors. For the holey PCW, the sensor unit can deliver the measurement of Triethyl phosphate (TEP) with the concentration of 10 ppm. The small size (800 μm) of this PCW offers a great advantage which can potentially lead to the realization of SWaP sensors. The main drawback of this kind of sensors is that they are so sensitive to small fluctuation in the hole diameter. Thus, the fabrication process for this kind of sensors might be difficult and timeconsuming. However, the high energy overlap with gases within the holey PCWs, and its high power in slowing light and its small size make this sensor one of the

best candidates for gas sensing applications. While, compared to the PCW, the PBF shows more electrical overlap with gases and lower propagation loss. Generally speaking, the development of PhC-based mid-infrared gas sensing research is still in its early stage and not ready for commercialization. However, considering the strong demands from IoT infrastructure for modern sensor units with combined SWaP-C features, it is anticipated that the progress in the PhC-based mid-infrared gas sensing area will develop much faster in the coming years, and some of the discussed technical approaches could eventually advance to a practical level and make a significant impact to our daily life.
