Nanophotonics has garnered intensive attention due to its unique capabilities in molding the flow of light in the subwavelength regime. Metasurfaces (MSs) and photonic integrated circuits (PICs) enable the realization of mass-producible, cost-effective, and highly efficient flat optical components for imaging, sensing, and communications. In order to enable nanophotonics with multi-purpose functionalities, chalcogenide phase-change materials (PCMs) have been introduced as a promising platform for tunable and reconfigurable nanophotonic frameworks. Integration of non-volatile chalcogenide PCMs with unique properties such as drastic optical contrasts, fast switching speeds, and long-term stability grants substantial reconfiguration to the more conventional static nanophotonic platforms. In this review, we discuss state-of-the-art developments as well as emerging trends in tunable MSs and PICs using chalcogenide PCMs. We outline the unique material properties, structural transformation, electro-optic, and thermo-optic effects of well-established classes of chalcogenide PCMs. The emerging deep learning-based approaches for the optimization of reconfigurable MSs and the analysis of light-matter interactions are also discussed. The review is concluded by discussing existing challenges in the realization of adjustable nanophotonics and a perspective on the possible developments in this promising area.

/pdf/tunable-nanophotonics-enabled-by-chalcogenide-phase-change-3q12l5zhtw.pdf

Tunable nanophotonics enabled by chalcogenide phase-change materials

Active metamaterials and metadevices: a review

Metasurfaces are engineered surfaces that consist of subwavelength periodic elements and can be used to manipulate electromagnetic waves. Multifunctional or reconfigurable electromagnetic meta-devices based on a direct-current biasing system can be built using lumped electronic components. However, such meta-devices require bulky power supplies, field-programmable gate arrays, electrical wires and complex control circuits. Here, we report a digital metasurface platform that can be programmed optically to implement electromagnetic functions. Our digital platform has 6 × 6 subarrays, each of which contains 4 × 4 metasurface elements based on electronic varactors integrated with an optical interrogation network based on photodiodes. The interrogation network can convert visible light illumination patterns to voltages and applies bias to the metasurface elements, generating specific microwave reflection phase distributions. To illustrate the capabilities of our approach, we use the optically driven digital metasurface for external cloaking, illusion and dynamic vortex beam generation. A digital platform that integrates a metasurface based on electronic varactors with an optical interrogation network based on photodiodes can be programmed by visible light to implement electromagnetic functions, including microwave cloaking, illusion and vortex beam generation.

An optically driven digital metasurface for programming electromagnetic functions

Metasurfaces have attracted increasing attention and provide promising solutions for cost‐effective and highly efficient optics due to their unprecedented capabilities in light manipulation. In recent years, phase‐change materials (PCMs), especially phase‐change chalcogenides, are integrated into metasurfaces to explore innovative configurations exhibiting remarkable tunability and reconfigurability due to the dramatic optical contrasts available in PCMs along with their high chemical and long‐term stability and high switching speeds. In this review, the current achievements and ongoing developments in dynamic metasurfaces based on phase‐change chalcogenides, an emerging field in nanophotonics and optoelectronics, are outlined. Starting with the basic material properties of phase‐change chalcogenides, the structural transformations and dielectric functions of two main chalcogenides are described. This is followed by an overview of the main progress in typical dynamic metasurfaces based on phase‐change chalcogenides enabling global and local amplitude/phase modulation. The review ends with a short conclusion and outlook on possible future developments.

https://findresearcher.sdu.dk:8443/ws/files/153582687/Dynamic_metasurfaces_using_phase_change_chalcogenides.pdf

Dynamic Metasurfaces Using Phase-Change Chalcogenides

Metasurfaces have attracted great attention due to their ability to manipulate the phase, amplitude, and polarization of light in a compact form. Tunable metasurfaces have been investigated recently through the integration with mechanically moving components and electrically tunable elements. Two interesting applications, in particular, are to vary the focal point of metalenses and to switch between holographic images. We present the recent progress on tunable metasurfaces focused on metalenses and metaholograms, including the basic working principles, advantages, and disadvantages of each working mechanism. We classify the tunable stimuli based on the light source and electrical bias, as well as others such as thermal and mechanical modulation. We conclude by summarizing the recent progress of metalenses and metaholograms, and providing our perspectives for the further development of tunable metasurfaces. 

Tunable metasurfaces towards versatile metalenses and metaholograms: a review

With inherent orthogonality, both the spin angular momentum (SAM) and orbital angular momentum (OAM) of photons have been utilized to expand the dimensions of quantum information, optical communications, and information processing, wherein simultaneous detection of SAMs and OAMs with a single element and a single-shot measurement is highly anticipated. Here, a single azimuthal-quadratic phase metasurface-based photonic momentum transformation (PMT) is illustrated and utilized for vortex recognition. Since different vortices are converted into focusing patterns with distinct azimuthal coordinates on a transverse plane through PMT, OAMs within a large mode space can be determined through a single-shot measurement. Moreover, spin-controlled dual-functional PMTs are proposed for simultaneous SAM and OAM sorting, which is implemented by a single spin-decoupled metasurface that merges both the geometric phase and dynamic phase. Interestingly, our proposed method can detect vectorial vortices with both phase and polarization singularities, as well as superimposed vortices with a certain interval step. Experimental results obtained at several wavelengths in the visible band exhibit good agreement with the numerical modeling. With the merits of ultracompact device size, simple optical configuration, and prominent vortex recognition ability, our approach may underpin the development of integrated and high-dimensional optical and quantum systems.

/pdf/spin-decoupled-metasurface-for-simultaneous-detection-of-3zb6ay1s00.pdf

Spin-decoupled metasurface for simultaneous detection of spin and orbital angular momenta via momentum transformation.

Imaging polarimetry is one of the most widely used analytical technologies for object detection and analysis. To date, most metasurface-based polarimetry techniques are severely limited by narrow operating bandwidths and inevitable crosstalk, leading to detrimental effects on imaging quality and measurement accuracy. Here, we propose a crosstalk-free broadband achromatic full Stokes imaging polarimeter consisting of polarization-sensitive dielectric metalenses, implemented by the principle of polarization-dependent phase optimization. Compared with the single-polarization optimization method, the average crosstalk has been reduced over three times under incident light with arbitrary polarization ranging from 9 μm to 12 μm, which guarantees the measurement of the polarization state more precisely. The experimental results indicate that the designed polarization-sensitive metalenses can effectively eliminate the chromatic aberration with polarization selectivity and negligible crosstalk. The measured average relative errors are 7.08%, 8.62%, 7.15%, and 7.59% at 9.3, 9.6, 10.3, and 10.6 μm, respectively. Simultaneously, the broadband full polarization imaging capability of the device is also verified. This work is expected to have potential applications in wavefront detection, remote sensing, light-field imaging, and so forth. 

https://www.oejournal.org/data/article/export-pdf?id=6336b17899d88145027d48be

Crosstalk-free achromatic full Stokes imaging polarimetry metasurface enabled by polarization-dependent phase optimization

Recently, disordered metasurface has attracted considerable interests due to the potential applications in imaging, holography, and wavefront shaping. However, how to emerge long-range ordered phase distribution in disordered metasurfaces remains an outstanding problem. Here, a general framework is proposed to generate spatially homogenous in-plane phase distribution from disordered metasurface, by engineering disorder parameters together with topology optimization. As a proof-of-concept demonstration, an all-dielectric disordered supercell metasurface with relatively homogeneous in-plane phase fluctuation is designed by disorder parameter engineering, manifesting as polarization conversion-dependent random scattering or unidirectional transmission. Then, topology optimization approach is utilized to overcome the lattice coupling effect and to further improve the homogeneity of complex electric field fluctuation. In comparison with initial supercell metasurface, the phase fluctuation range and relative efficiency of topology-optimized freeform metasurface are significantly improved, leading to long-range ordered electric field distribution. Moreover, three experimental realizations are performed, all of which agree well with theoretical results. This methodology may inspire more exotic optical phenomena and find more promising applications in disordered metasurfaces and disordered optics. This article is protected by copyright. All rights reserved.

Emerging Long-Range Order from Freeform Disordered Metasurface.

Metalenses have gained significant attention and have been widely utilized in optical systems for focusing and imaging, owing to their lightweight, high-integration, and exceptional-flexibility capabilities. Traditional design methods neglect the coupling effect between adjacent meta-atoms, thus harming the practical performance of meta-devices. The existing physical/data-driven optimization algorithms can solve the above problems, but bring significant time costs or require a large number of data-sets. Here, we propose a physics-data-driven method employing an “intelligent optimizer” that enables us to adaptively modify the sizes of the meta-atom according to the sizes of its surrounding ones. The implementation of such a scheme effectively mitigates the undesired impact of local lattice coupling, and the proposed network model works well on thousands of data-sets with a validation loss of 3×10−3. Based on the “intelligent optimizer”, a 1-cm-diameter metalens is designed within 3 hours, and the experimental results show that the 1-mm-diameter metalens has a relative focusing efficiency of 93.4% (compared to the ideal focusing efficiency) and a Strehl ratio of 0.94. Compared to previous inverse design method, our method significantly boosts designing efficiency with five orders of magnitude reduction in time. More generally, it may set a new paradigm for devising large-aperture meta-devices.

Physics-data-driven intelligent optimization for large-aperture metalenses

Full Stokes Polarimetry for Wide-Angle Incident Light

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