These are artificially fabricated two dimensional surfaces which are capable of radiating due to its leaky wave nature. Professors like Dr. Maci, Dr. Sievenpiper, Dr. Grbic and Dr. Constantine balanis are working on this topic. You can read their papers on further understanding.
any material that obtains its electromagnetic properties from its structure rather than from its chemical composition; especially a material engineered to have features of a size less than that of the wavelength of a class of electromagnetic radiation