Macroporous silicon photonic crystals for gas sensing

In this paper we study a compact gas sensor based on a photonic crystal built from macroporous silicon. Its sensing mechanism is based in the absorption of infrared light by a gas. Photonic crystals are structured materials which can be engineered to have photonic bandgaps. They also can be tailored to create localized states inside the bandgaps. We exploit the possibility to confine light inside a cavity with very high-Q, which allows for long interaction time between the gas and light. Simulation of different 2-D and 3-D structures have been done to extract the appropriate dimensions for gas detection, and their optical behaviour. Resonant cavities were created by adding defects in the ordered geometrical structure, thus creating a single state and confining the trapped light in a crystal bandgap. The structures were tested by simulating the presence of ethanol inside the structures. Gas is to be detected by a noticeable change in the resonance peak both in amplitude and spread, caused by the gas detuning the cavity. Macroporous silicon samples of the investigated structures with defects were fabricated and measured by IR spectrography. Cavity resonances can be clearly seen in the samples, though we need to improve fabrication to adjust the theoretically calculated dimensions.