Abstract: An ultragrating is a nanometer-period optical grating that is fabricated from a horizontal superlattice. A superlattice is a material structure grown on a substrate by molecular-beam epitaxy or metal-organic chemical vapor deposition and having periodic compositional variations. A horizontal superlattice is one in which the compositional variations are in a direction parallel to the substrate surface. By the selective removal of one of the superlattice materials, an ultragrating is obtained. The smallest grating periods possible before this discovery were those made by electron-beam lithographic techniques which are limited to values greater than 100 nanometers. Thus, the ultragrating with grating periods ranging from one to a hundred nanometers represents an order of magnitude advancement in the state of the art of making optical gratings. The ultragrating will fine utility in the design of advanced electronic devices and for general scientific and engineering purposes.