Magnetically tunable colloidal photonic crystals

Nanostructured superparamagnetic particles can be self-assembled to form colloidal photonic crystals whose photonic properties can be tuned widely, rapidly, and reversibly in the visible spectrum. The successful assembly is achieved by establishing a balance between interparticle repulsive and attractive forces. While the attractive force can be induced through the application of external magnetic fields, the repulsive forces need to be created by engineering the particle surface in accordance to the polarity of the solvents. The dynamic photonic structures can be rapidly fixed in a polymer matrix through instant assembly of superparamagnetic colloidal particles in UV curable resin followed by an immediate UV curing process. Combining with an emulsification procedure, the fixation process can be used to produce magnetochromatic microspheres that can change their colors through the manipulation of their relative orientation using external magnetic fields. By using hygroscopic salt solutions as inks, we also demonstrate the use of the polymerized photonic structures in applications such as rewritable photonic papers.