A chemical reactor includes two substrates that are joined along a surface and a chemical reaction chamber formed between the substrates. The chemical reaction chamber has a hollow interior and one or more light reflectors located along walls of the hollow interior. The chemical reaction chamber has one or more inputs to transport fluid into the hollow interior and an output to transport fluid out of the hollow interior. The one or more light reflectors cause light rays to make multiple crossings of the hollow interior as a result of reflections off the one or more reflectors.