Bistable Physical Geometries for Terahertz Plasmonic Structures Using Shape Memory Alloys

Shape memory alloy foils that are appropriately patterned are cycled between two different metal foil geometries resulting in two different terahertz (THz) plasmonic responses. This is accomplished by using patterned foils of a nickel–titanium alloy (Nitinol) that switches between the martensite phase below 31 °C, yielding one physical geometry, and the austenite phase, when the foil is heated above 51 °C, yielding a second physical geometry. In order to enable this reproducible switching, the sample is initially put through a two‐way training procedure, through which the two different desired physical geometries are imprinted. Specifically, the metal foils are trained to switch between a sinusoidal corrugation, either 1D or 2D, at close to room temperature and a flat metal sheet above the austenite phase transition temperature. The foils are found to switch reproducibly between geometries over at least 100 thermal cycles. Using THz time‐domain spectroscopy, the transmission properties of the foils are measured as a function of incident polarization and foil geometry. The changes in spectrum are explained qualitatively and through numerical simulation.