Optical neural networks (ONNs), enabling low latency and high parallel data processing without electromagnetic interference, have become a viable player for fast and energy-efficient processing and calculation to meet the increasing demand for hash rate. Photonic memories employing nonvolatile phase-change materials could achieve zero static power consumption, low thermal cross talk, large-scale, and high-energy-efficient photonic neural networks. Nevertheless, the switching speed and dynamic energy consumption of phase-change material-based photonic memories make them inapplicable for in situ training. Here, by integrating a patch of phase change thin film with a PIN-diode-embedded microring resonator, a bifunctional photonic memory enabling both 5-bit storage and nanoseconds volatile modulation was demonstrated. For the first time, a concept is presented for electrically programmable phase-change material-driven photonic memory integrated with nanosecond modulation to allow fast in situ training and zero static power consumption data processing in ONNs. ONNs with an optical convolution kernel constructed by our photonic memory theoretically achieved an accuracy of predictions higher than 95% when tested by the MNIST handwritten digit database. This provides a feasible solution to constructing large-scale nonvolatile ONNs with high-speed in situ training capability.

Metasurfaces are ultrathin metamaterials constructed by planar meta-atoms with tailored electromagnetic responses. They have attracted tremendous attention owing to their ability to freely control the propagation of electromagnetic waves. With active elements incorporated into metasurface designs, one can realize tunable and reconfigurable metadevices with functionalities controlled by external stimuli, opening up a new platform to dynamically manipulate electromagnetic waves. In this article, we review the recent progress on tunable and reconfigurable metasurfaces, focusing on their operation principles and practical applications. We describe the approaches to the engineering of reconfigurable metasurfaces categorized into different classes based on the available active materials or elements, which can offer uniform manipulations of electromagnetic waves. We further summarize the recent achievements on programmable metasurfaces with constitutional meta-atoms locally tuned by external stimuli, which can dynamically control the wavefronts of electromagnetic waves. Finally, we discuss time-modulated metasurfaces, which are meaningful to exploit the temporal dimension by applying a dynamic switching of the coding sequence. The review is concluded by our outlook on possible future directions and existing challenges in this fast developing field.