Gas detection is of great importance in areas as diverse as industry, health or safety in domestic environments or public spaces, among others, and it is highly specific to each application. The detection method depends on factors such as the species of gas to be detected, concentration range, required resolution, sensitivity, specificity, response time, operating environment (temperature, humidity, interfering species, etc.), size and cost, among other considerations. Optical gas sensors are an attractive solution for gas detection. Most of them rely on molecular absorption and offer fast responses, minimal drift and are intrinsically reliable thanks to perform self-referenced measurements. Sensitivity and selectivity depend on the characteristics of the device. For example, laser-based gas sensors are highly selective with zero cross response to other gases and also with first-in-class sensitivity. The downside is that they are expensive. Non-dispersive infra-red (NDIR) sensors are a widespread alternative for cost-effective optical detection. They have inferior performances in terms of sensitivity and selectivity than laser-based sensors, but are two or three orders of magnitude less expensive.
This thesis is dedicated to improving the selectivity and sensitivity of NDIR devices through the use of macroporous silicon technology. More specifically, it studies how photonic crystals manufactured by electrochemical etching can be used as narrow mid-infrared filters for gas detection purposes. That is, the photonic crystals are designed in such a way that only a small range of frequencies from an external source are transmitted while the surroundings are blocked. These filters are narrower than those available on the market and can be used to improve the selectivity and the sensitivity of NDIR devices as well as to reduce cross detection with other gases. In addition, the study shows how macroporous silicon photonic crystals can be heated to work as selective emitters. This can be used to reduce the complexity of the NDIR device while maintaining similar optical characteristics. Furthermore, it is proven that photonic molecules can be employed to perform dual detection in both transmission and emission, giving a new approach to self-referenced measurements.
Conclusions of the work show that macroporous silicon technology is a versatile platform to provide solutions in the mid-infrared range for developing compact, sensitive and selective optical gas sensing.
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