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.

中文翻译：

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时频域梯度超材料

近年来，大量的研究工作集中在使用超材料对电磁波（EM）进行灵活操纵的问题上。作为重要的一类，梯度超材料（具有梯度相的超材料）可以在电磁波传播过程中赋予亚波长尺度的空间变化相变，以形成EM辐射，其中具有特定相位梯度的元素阵列可以根据需要形成EM束传播。在此，提出了时频域梯度超材料的概念，其中通过向超材料引入频域相位梯度来获得新的自由度。超材料的频率梯度是通过相位梯度相对于频率的导数获得的，它可用于整形均匀分布的相位梯度，以在工作频带中根据需要而变化，而无需任何有源器件。1D / 2D中超材料中空间和频率梯度的组合为EM波的更强大控制开辟了新的可能性，从而产生了更复杂的功能，例如2D柔性光束扫描，涡旋模式的连续变换以及具有所需透射率的多束光束的产生方向和传输方式。