Flat optics has reshaped modern photonics by replacing bulky lenses, mirrors, and optical assemblies with ultrathin nanostructured surfaces. These planar optical elements can control light within a thickness much smaller than the wavelength, opening opportunities for compact imaging systems, integrated photonics, optical sensing, beam steering, and quantum light sources.
Among the many forms of flat optics, nonlocal flat optics occupies a particularly powerful regime. In these systems, including photonic crystal slabs, guided-resonance metasurfaces, and plasmonic lattices, light is not controlled by isolated meta-atoms alone. Instead, the optical response arises from collective modes extending across many unit cells. Such nonlocality enables sharp resonances, strong dispersion, high-Q optical states, and structured radiation that are difficult to achieve with purely local metasurfaces.
However, conventional nonlocal flat-optical devices are usually based on a single patterned layer. Once this layer is fabricated, its optical response is largely fixed by its in-plane geometry, lattice symmetry, and thickness. This creates a fundamental design bottleneck: many desirable properties, including narrow linewidth, directional radiation, polarization selectivity, chirality, and dynamic reconfigurability, are controlled by the same limited set of structural parameters and symmetry-imposed radiation channels. As a result, improving one function often comes at the expense of another.
In a new Review published in eLight , Professor Haoning Tang from the Massachusetts Institute of Technology, Professor Shanhui Fan from Stanford University, Professor Yao Jie from the University of California, Berkeley, Professor Hai Son Nguyen from Ecole Centrale de Lyon and CNRS, Professor Eric Mazur from Harvard University, Professor Yuan Cao from the University of California, Berkeley, and co-workers provide a comprehensive overview of an emerging solution: bilayer and multilayer nonlocal flat optics.
The Review shows that stacking two or more flat-optical layers does far more than simply duplicate a single metasurface. It creates a new design space in which the relative geometry between layers becomes an active optical degree of freedom. Interlayer spacing, lateral displacement, global twist, local twist, and lattice mismatch can all be used to control how optical modes couple, interfere, radiate, and exchange momentum. These interlayer parameters are independent of the in-plane pattern, and in many cases they can be tuned after fabrication.
The authors identify three key physical mechanisms that unify this rapidly growing field. The first is near-field mode hybridization, where resonant modes in different layers couple to form new collective optical states. The second is far-field radiation interference, where two layers radiate into shared external channels and their outgoing waves interfere. This mechanism can strongly modify linewidths and generate high-Q states, including bound states in the continuum. The third is momentum selection and mixing, where twisted or mismatched lattices create additional momentum channels and enable moiré-assisted optical responses.
In aligned bilayers, the interlayer distance serves as the simplest and most fundamental control knob. Changing the spacing modifies both the strength of near-field coupling and the radiative phase delay between the two layers. This allows resonant frequencies, mode splittings, and radiation losses to be tuned without changing the in-plane nanostructure. The Review highlights two distinct routes to high-Q resonances: symmetry-protected bound states in the continuum, which are relatively robust to spacing changes, and Fabry-Perot-type bound states in the continuum, which arise from destructive radiation interference and are highly sensitive to the interlayer phase.
A second level of control emerges when one layer is laterally displaced relative to the other. This sliding motion can break selected symmetries while preserving the overall lattice periodicity. As a result, dark optical states can become tunable quasi-bound states in the continuum, polarization singularities can move through momentum space, and radiation can become asymmetric between the two sides of the device. In this sense, lateral displacement acts as a synthetic dimension for controlling radiation topology, unidirectional guided resonances, and spatiotemporal optical vortices.
The design space becomes even richer when the two layers are mismatched or twisted. Lattice mismatch creates long-period moiré superlattices in which the interlayer coupling varies slowly across space. These moiré structures can generate minibands, flat-band localization, and moiré-scale optical cavities without the need for conventional point defects. Such effects provide new routes to slow light, enhanced density of optical states, and stronger light-matter and nonlinear optical interactions.
Global twist provides a particularly versatile form of momentum engineering. By rotating one lattice relative to the other, the system acquires new reciprocal-space matching conditions and becomes a multichannel moiré scattering platform. Twist can tune optical resonances, open or suppress radiation channels, generate moiré quasi-bound states in the continuum, and produce structured far-field radiation such as optical vortices and beam-steered emission. Remarkably, twisted bilayers can also exhibit optical chirality even when each individual layer is not chiral.
The Review further discusses locally twisted bilayers and multilayer stacks. In locally twisted structures, the lattice can remain periodic while the resonators inside each unit cell rotate relative to each other, forming compact three-dimensional chiral building blocks. These structures can support optical activity, circular dichroism, and polarization-selective high-Q resonances. In multilayer stacks, the system evolves into a coupled-layer optical network with many resonant pathways, enabling broadband spectral control, cascaded chirality, nonlinear phase matching, programmable nonlinear polarization, and hybrid moiré flat bands.
The scientists summarize the design principle:
“Bilayer and multilayer flat optics should not be viewed as simple stacks of planar metasurfaces. They are coupled-layer optical networks in which interference, symmetry, hybridization, and momentum selection can be engineered in a high-dimensional parameter space.”
They further point out:
“Interlayer spacing, translation, twist, and lattice mismatch provide new design knobs that can be tuned after fabrication. This makes bilayer nonlocal flat optics a natural platform for reconfigurable and programmable photonics.”
Looking forward, the field will benefit from advances in nanofabrication, heterogeneous integration, precision alignment, and dynamic actuation. Micro- and nano-electromechanical systems could provide in situ control of spacing, translation, and twist, allowing flat-optical devices to move from static, fabrication-defined components to real-time programmable optical systems. At the same time, high-Q resonances and moiré flat bands are sensitive to gap nonuniformity, tilt, twist-angle errors, finite-size effects, and material losses. Progress will therefore require improved fabrication, feedback control, and standardized characterization methods.
This Review establishes bilayer and multilayer nonlocal flat optics as a general framework for reconfigurable photonics. By extending flat optics from two-dimensional pattern design to three-dimensional interlayer coupling design, it points toward compact and programmable platforms for beam steering, chiral sensing, enhanced nonlinear optics, quantum light emission, and intelligent optical systems.
eLight
Bilayer nonlocal flat optics