Journal Article terahertz phase discontinuities Research Support, Non-U.S. Gov't coherent control diffraction metamaterials Metals Metasurfaces Pancharatnam-Berry phase optical element Review THz metasurface Vanadium dioxide asymmetric transmissions broadband chirality dielectric dynamic metasurface geometric phase metagratings metasurfaces near-field microscopy phase change material refraction surface waves terahertz metasurfaces