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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. 

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. 

We investigate a convolutional neural network with the first layer implemented by specially designed plasmonic metasurface photodetectors, showing an order of magnitude decrease in computational complexity for the accurate classification of transparent biological cells. 

We demonstrate the use of plasmonic gradient metasurfaces to tailor the angular response of generic planar photodetectors. The resulting devices are promising for a wide range of computational imaging applications with enhanced miniaturization and functionality. 

We use a generative deep learning method based on denoising diffusion probabilistic model to design plasmonic phase-imaging sensors for broadband operation. This flexible method enables optimized inverse design for a wide range of nanophotonic devices. 

The development of photonic technologies for machine learning is a promising avenue toward reducing the computational cost of image classification tasks. Here we investigate a convolutional neural network (CNN) where the first layer is replaced by an image sensor array consisting of recently developed angle-sensitive metasurface photodetectors. This array can visualize transparent phase objects directly by recording multiple anisotropic edge-enhanced images, analogous to the feature maps computed by the first convolutional layer of a CNN. The resulting classification performance is evaluated for a realistic task (the identification of transparent cancer cells from seven different lines) through computational-imaging simulations based on the measured angular characteristics of prototype devices. Our results show that this hybrid optoelectronic network can provide accurate classification (>90%) similar to its fully digital baseline CNN but with an order-of-magnitude reduction in the number of calculations. 

Angle-sensitive photodetectors are a promising device technology for many advanced imaging functionalities, including lensless compound-eye vision, lightfield sensing, optical spatial filtering, and phase imaging. Here we demonstrate the use of plasmonic gradient metasurfaces to tailor the angular response of generic planar photodetectors. The resulting devices rely on the phase-matched coupling of light incident at select geometrically tunable angles into guided plasmonic modes, which are then scattered and absorbed in the underlying photodetector active layer. This approach naturally introduces sharp peaks in the angular response, with smaller footprint and reduced guided-mode radiative losses (and therefore improved spatial resolution and sensitivity) compared to analogous devices based on diffractive coupling. More broadly, these results highlight a promising new application space of flat optics, where gradient metasurfaces are integrated within image sensors to enable unconventional capabilities with enhanced system miniaturization and design flexibility. 

We report a new technique for single-shot quantitative phase retrieval from transparent objects, based on plasmonic metasurface photodetectors featuring an asymmetric angular response around normal incidence combined with a particularly simple optical setup. 

Abstract The visualization of pure phase objects by wavefront sensing has important applications ranging from surface profiling to biomedical microscopy, and generally requires bulky and complicated setups involving optical spatial filtering, interferometry, or structured illumination. Here we introduce a new type of image sensors that are uniquely sensitive to the local direction of light propagation, based on standard photodetectors coated with a specially designed plasmonic metasurface that creates an asymmetric dependence of responsivity on angle of incidence around the surface normal. The metasurface design, fabrication, and angle-sensitive operation are demonstrated using a simple photoconductive detector platform. The measurement results, combined with computational imaging calculations, are then used to show that a standard camera or microscope based on these metasurface pixels can directly visualize phase objects without any additional optical elements, with state-of-the-art minimum detectable phase contrasts below 10 mrad. Furthermore, the combination of sensors with equal and opposite angular response on the same pixel array can be used to perform quantitative phase imaging in a single shot, with a customized reconstruction algorithm which is also developed in this work. By virtue of its system miniaturization and measurement simplicity, the phase imaging approach enabled by these devices is particularly significant for applications involving space-constrained and portable setups (such as point-of-care imaging and endoscopy) and measurements involving freely moving objects.