{HEAD ~ The photonic band gap material} may be characterized by periodicity of dielectric properties in two or three dimensions, and may comprise holes, defects, cavities, or other elements in layers of dielectric materials that give {TAIL ~ rise} to the band gap behavior 
{ HEAD ~ The photonic band gap material } may be characterized by periodicity of dielectric properties in two or three dimensions , and may comprise holes , defects , cavities , or other elements in layers of dielectric materials that give { TAIL ~ rise } to the band gap behavior
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