A Global Phase‐Modulation Mechanism for Flat‐Lens Design

Abstract

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

Metasurfaces toward Optical Manipulation Technologies for Quantum Precision Measurement

“…Propagation phasetype metasurfaces tend to keep the structure thickness constant and change the phase by adjusting the equivalent refractive index or the propagation constant. The equivalent refractive index theory is usually composed of periodic arrays of high-refractiveindex nanopillars and low-refractive index dielectric bases, and the equivalent refractive index is achieved by controlling the duty cycle of the structural unit as a way to control the phase of electromagnetic waves, for example, the change in equivalent refractive index is usually achieved by changing the diameter [110,111] and height [112] of the high-refractive index dielectric material. Different from the equivalent refractive index theory, the surface plasmon waveguide theory uses the property that the propagation constant varies with the width of the slit in the surface plasmon waveguide to regulate the refractive index and the local phase, [113][114][115] as shown in Figure 4a.…”

A review of light‐controlled programmable metasurfaces for remote microwave control and hybrid signal processing

“…Guided by the generalized Snell's law, numerous metasurfaces have been constructed to control amplitude, phase, polarization, frequency, and more other features of EM waves. With these ability and advantages, metasurfaces show great potential for applications in antennas, [16][17][18][19] polarization converters, [20][21][22] absorbers, 23,24 cloaks 25,26 and so on.…”

“…To solve these problems, metamaterials are developed towards two‐dimensional (2D) and planar version, and thus metasurface emerge as the times require 6 . Metasurface consists of many subwavelength artificial elements arranged on a surface, which not only can control accurately electromagnetic (EM) waves, but also provides more opportunities for engineering applications due to 2D properties, and has aroused enormous interest and extensive attention in many fields 6‐26 . In 2011, the generalized Snell's law was proposed by introducing abrupt changes at metasurface interface, 10 and provides an effective method to design metasurfaces, and in turn shows the strong manipulation capabilities of metasurfaces.…”