Photonic Crystals with a Reversibly Inducible and Erasable Defect State Using External Stimuli

The controlled introduction of artificial extrinsic defects is critical to achieve the functions of photonic crystals. Smart defects capable of responding to external stimuli lead to more advanced applications. Here we report a microgel colloidal crystal with a defect state which could be induced and erased reversibly by external stimuli. The crystal was assembled from PNIPAM microgel and P(NIPAM-AAc) microgel of the same size. The resulting doped crystal does not exhibit a defect state in its stop band because of the similar optical properties of the dopant and the host. By increasing the pH value, however, the dopant P(NIPAM-AAc) spheres swell to a larger size and turn into real defects in the crystal, resulting in the appearance of defect state. Adjusting the pH value back restores the size of the dopant spheres, and thus erases the defect state. Temperature, a second external stimulus, could also be used to induce and erase defect states of the crystal.