When an external magnetic field is applied perpendicularly to a magnetic fluid film, a self-assembly hexagonal ordered structure is formed in the film when the field exceeds a certain strength. The period of the ordered structure can be manipulated from several micrometers down to sub-micrometers by increasing the field strength or reducing the film thickness. By injecting a light propagating along the in-plane direction of the film, photonic-crystal band gaps can be observed. With the tunability of the ordered structure, the forbidden photonic bands can be adjusted by varying the external field strength. It was further found that resonant modes are generated when a point defect is artificially implanted into the self-assembly ordered structure in the magnetic fluid film. These results reveal that the self-assembly ordered structure in a magnetic fluid film under external magnetic fields acts as a photonic crystal.
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