New route to produce dry colloidal crystals without cracks.

We report a new route for fabricating opal films in which the particles are bounded by polymer gel. A loosely packed colloidal crystal immobilized in a hydrogel is homogeneously shrunk and dried and converted into a crack-free opal film. The obtained film shows a uniform diffraction color and excellent transmission spectral properties inherited from the starting state. The optical stop band wavelength is adjustable by varying the amount of gel-polymer.