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Gradient metasurfaces provide a well-established platform for the wavefront and polarization control of light propagating in free space. More recent studies have also focused on their integration with plasmonic and dielectric waveguides and optoelectronic devices such as light emitters and photodetectors. In this context, metasurfaces can be used to interface guided modes with free-space radiation, for example, to enforce the selective input coupling or detection of light with prescribed characteristics (in terms of direction of propagation, state of polarization, orbital angular momentum, etc.) or to manipulate the properties of the out-coupled or emitted light according to a desired system operation. These functionalities, which normally involve complex combinations of bulky optical elements, have important applications in multiple areas of high technological significance, from optical interconnects to microdisplays and multifunctional image sensors. Here, we provide a tutorial overview of this field of research, including a description of the basic building blocks (meta-atoms) used in integrated gradient metasurfaces, a review of the main capabilities reported to date, and an outlook on future directions of study and technological prospects. 

Abstract Light waves possess multiple degrees of freedom besides intensity, including phase and polarization, that often contain important information but require complex and bulky systems for their measurement. Here we report a pair of compact multifunctional photodetectors that can selectively measure the local phase gradient of, respectively, the right and left circular-polarization component of any incident wave. These devices employ a chiral pair of integrated plasmonic metasurfaces to introduce a sharp dependence of responsivity on local direction of propagation of the desired polarization component. An order-of-magnitude polarization selectivity with respect to phase gradient is demonstrated with both devices. Using the measured device characteristics, we also describe computationally a pixel array that allows for the simultaneous separate mapping of the right and left circularly-polarized incident wavefronts in a particularly simple imaging setup. These unique capabilities may be exploited to enable new functionalities for applications in chemical sensing, biomedical microscopy, and machine vision.