relies on optical path inside the refractive material. As a result, it usually tends to be bulky and heavy.In the past decade, planar-structured interface, named as metasurface, [1][2][3][4] emerged as a new and intriguing concept in optics, providing a platform to realize the flat lens (also named as metalens). [5] Contrary to bulky element, the metasurface refers to a 2D metamaterial element with typical thickness of about (or less than) one wavelength. It is comprised of an array of deep subwavelength scatters (antennas) and its working principle strongly relies on local interaction between light and the scatters. Therefore, the light-matter interaction in the subwavelength scale provides a local phase modulation approach for designing a functional metasurface. For example, one can design a metasurface to obtain a spherical wavefront, so that the incoming light can be focused to a diffraction-limit hotspot. [5] Since then, metalens has gained much attention and numerous kinds of lenses including the ultrahigh numerical aperture lens, [6][7][8][9] achromatic lens, [10][11][12][13] and the flat lens performed at the visible, [11,12] infrared [14,15] and terahertz [16] wavelengths have been recently demonstrated.However, the design of the metalens is still limited to the local phase-modulation mechanism, which relies on either the resonant or non-resonant effect (or both). The resonant effects of the nanoscatters such as the Mie resonance and the plasmonic resonance give rise to an abrupt phase change at the local position; [17][18][19][20] however, these effects are wavelengthdependent. The resultant metalens only works at an extremely narrow range of wavelength. The non-resonance approach is on the basis of either the geometric phase or the propagation phase (or both). While the geometric phase metalenses can extend the bandwidth of those resonance ones, [10][11][12] they depend on precise rotation of the antennas as well as polarization state of the incident beam. The propagation phase metalens, [7,13,14] however, requires an ultrahigh aspect ratio that leads to severe difficulties in fabrications. All these working principles mentioned above, needless to say, allow for important investigations in different fields of optics including spinorbit interactions, [21][22][23] light beam shaping, [24,25] high-resolution imaging, [7] holography, [26,27] optical cloaking, [28] etc. Regarding the flat lens, however, there are important problems as follows: First, owing to the deeply subwavelength feature size, the local