All-dielectric metasurfaces comprising arrays of nanostructured high-refractive-index materials are re-imagining what is achievable in terms of the manipulation of light. However, the functionality of conventional dielectric-based metasurfaces is fixed by design; thus, their optical response is locked in at the fabrication stage. A far wider range of applications could be addressed if dynamic and reconfigurable control were possible. We demonstrate this here via the novel concept of hybrid metasurfaces, in which reconfigurability is achieved by embedding sub-wavelength inclusions of chalcogenide phase-change materials within the body of silicon nanoresonators. By strategic placement of an ultra-thin G e 2 S b 2 T e 5 layer and reversible switching of its phase-state, we show individual, multilevel, and dynamic control of metasurface resonances. We showcase our concept via the design, fabrication, and characterization of metadevices capable of dynamically filtering and modulating light in the near infrared (O and C telecom bands), with modulation depths as high as 70% and multilevel tunability. Finally, we show numerically how the same approach can be re-scaled to shorter wavelengths via appropriate material selection, paving the way to additional applications, such as high-efficiency vivid structural color generators in the visible spectrum. We believe that the concept of hybrid all-dielectric/phase-change metasurfaces presented in this work could pave the way for a wide range of design possibilities in terms of multilevel, reconfigurable, and high-efficiency light manipulation.

difference in their electrical and optical properties between their amorphous and crystalline phases. Moreover, they can be switched (optically, electrically, or thermally) between phases reversibly (potentially >10 15 cycles) and quickly (nanoseconds or faster). [1][2][3] Both phases (and indeed intermediate phases between fully crystalline and fully amorphous) are also stable at room temperature for a time on the order of years. [4,5] All these properties have made phasechange materials extremely attractive for commercial data storage technologies, in the form of rewritable optical disks and nonvolatile electronic memories. [6][7][8] More recently, as a result of the rather unique properties that phase-change materials possesses, their use has been extended to a number of exciting emerging applications including neuromorphic computing, [9,10] integrated photonic memories [11,12] and, the focus of this work, reconfigurable optical metamaterials/metasurfaces, [13][14][15][16][17][18][19][20][21][22] which we here exploit for the realization of a new form of nonvolatile color display.Optical metasurfaces have great potential to generate color, and several different structures suited to this task have been suggested in the literature. [23][24][25][26][27][28][29][30][31] A common approach is to utilize metallic (or metal-dielectric) nanorods [26,27,30,31] or other lithographically patterned metal-dielectric nanostructures [24,28,29] that generate structural (i.e., noncolorant) color using plasmonic effects. Such approaches are in general though "fixed-by-design," meaning that colors and images are essentially written permanently into the metasurface by the specific nanostructures used. For display and electronic signage applications, however, the ability to change the displayed image or information in real time is required. Here we provide just such a capability by combining a metal-insulator-metal (MIM) resonant absorber type optical metasurface [32,33] with a thin layer of chalcogenide phase-change material (PCM), so providing the key attributes of nonvolatile color generation and dynamic reconfigurability, the latter achieved by turning the MIM resonance "on" and "off" by switching the PCM-layer between its crystalline and amorphous states. Nonvolatility is a particularly attractive feature offered by phase-change based displays, since no power is needed to retain an image once it is written into the phase-change layer/pixels. [34][35][36][37] Moreover, the displays can work using only ambient (natural or artificial) light, which can Chalcogenide phase-change materials, which exhibit a marked difference in their electrical and optical properties when in their amorphous and crystalline phases and can be switched between these phases quickly and repeatedly, are traditionally exploited to deliver nonvolatile data storage in the form of rewritable optical disks and electrical phase-change memories. However, exciting new potential applications are now emerging in areas such as integrated phase-change photonics, phase...

Metasurfaces and nanoantennas are redefining what can be achieved in terms of optical beam manipulation, as they provide a versatile design platform towards moulding the flow of light at will. Yet, once a conventional metasurface is designed and realised, its effect on optical beams is repeatable and stationary, thus its performance is 'locked-in' at the fabrication stage. A much wider range of applications, such as dynamic beam steering, reconfigurable and dynamic lensing, optical modulation and reconfigurable spectral filtering, could be achieved if real-time tuning of metasurface optical properties were possible. Chalcogenide phase-change materials, because of their rather unique ability to undergo abrupt, repeatable and non-volatile changes in optical properties when switched between their amorphous and crystalline phases, have in recent years been combined with metasurface architectures to provide a promising platform for the achievement of dynamic tunability. In this paper, the concept of dynamically tunable phase-change metasurfaces is introduced, and recent results spanning the electromagnetic spectrum from the visible right through to the THz regime are presented and discussed. The progress, potential applications, and possible future perspectives of phase-change metasurface technology are highlighted, and requirements for the successful implementation of real-world applications are discussed.

Advances in artificial intelligence have greatly increased demand for data-intensive computing. Integrated photonics is a promising approach to meet this demand in big-data processing due to its potential for wide bandwidth, high speed, low latency, and low-energy computing. Photonic computing using phase-change materials combines the benefits of integrated photonics and co-located data storage, which of late has evolved rapidly as an emerging area of interest. In spite of rapid advances of demonstrations in this field on both silicon and silicon nitride platforms, a clear pathway towards choosing between the two has been lacking. In this paper, we systematically evaluate and compare computation performance of phase-change photonics on a silicon platform and a silicon nitride platform. Our experimental results show that while silicon platforms are superior to silicon nitride in terms of potential for integration, modulation speed, and device footprint, they require trade-offs in terms of energy efficiency. We then successfully demonstrate single-pulse modulation using phase-change optical memory on silicon photonic waveguides and demonstrate efficient programming, memory retention, and readout of > 4 bits of data per cell. Our approach paves the way for in-memory computing on the silicon photonic platform. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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