Space‐Frequency‐Domain Gradient Metamaterials

Abstract In recent years, an intense research effort has focused on the flexible manipulation of electromagnetic (EM) waves using metamaterials. As an important category, gradient metamaterials (metamaterials with gradient phases) can impart subwavelength‐scale space‐variant phase changes during the electromagnetic wave propagations to shape EM radiations in which arrays of elements with certain phase gradients can form EM beam propagations as desired. Here, the concept of space‐frequency‐domain gradient metamaterial is proposed, in which new degrees of freedom are attained by introducing frequency‐domain phase gradients to the metamaterial. The frequency gradients of the metamaterial are obtained by the derivative of phase gradients with respect to frequency, which can be used to shape the uniformly distributed phase gradients to vary as demand in the working frequency band without any active devices. The combinations of space and frequency gradients in metamaterials in 1D/2D open new possibilities for more powerful controls of EM waves, thereby yielding more complicated functions such as 2D flexible beam scanning, continuous transformations of vortex modes, and generation of multiple beams with desired transmission directions and transmission modes.