Array of gold L's fabricated by electron beam lithography. Bar = 5µm Credit: Phys. Rev. X 4, 021026.
Metamaterials hold great promise for manipulating light in ways not found in nature, but these tiny structures usually work over a relatively narrow spectral window. Researchers at Nanjing University in China have built a metal-and-dielectric metastructure that cancels out dispersion and controls the polarization of broadband light in the terahertz region (Phys. Rev. X 4, 021026).
The Lorentz resonance inherent in metallic patterned metamaterials creates a large amount of dispersion, but an adjacent dielectric layer cancels out that effect, according to the team led by Mu Wang, a physicist with Nanjing's National Laboratory of Solid State Microstructures.
The researchers sandwiched the metallic metastructure—a gold sheet with tiny L-shaped cutouts—with a 100-nm-thick silver mirror and a 510-nm-thick dielectric layer of silicon dioxide. Alone, the L-patterned layer, with a lattice constant of 1,550 nm, “acts as a highly dispersive anisotropy resonator,” according to the authors of the paper.
Changing the structural parameters of the metastructure's components—for example, reducing the lattice constant to 1,400 nm or reducing the thickness of the dielectric layer to 420 nm—also changes the polarization of the reflected light. The team built broadband quarter-wave and half-wave plates out of the metamaterial components to demonstrate the concept.