Metasurfaces are moving beyond the era of simply bending light. A new review from Professor Din Ping Tsai’s group at City University of Hong Kong charts how these ultrathin optical platforms are evolving from broadband, non-resonant components into highly selective resonant devices capable of controlling color, polarization, imaging, and laser emission at the nanoscale. Published online on June 28, 2026, in Opto-Electronic Science, the review argues that the future of advanced optics will depend not on choosing between non-resonant and resonant technologies, but on combining their strengths in hybrid meta-devices.
Metasurfaces consist of carefully arranged structures smaller than the wavelength of visible or infrared light. These nanoscale elements can alter the phase, amplitude, polarization, and direction of incoming light, allowing a flat surface to perform functions traditionally associated with bulky lenses, filters, holograms, or optical assemblies. Non-resonant metasurfaces generally rely on geometric phase, propagation phase, group-delay engineering, or broadband band design. Because they do not depend strongly on a narrow optical resonance, they can operate efficiently across wide spectral ranges and are particularly attractive for achromatic metalenses, wavefront shaping, and full-color structural displays.
This broadband behavior has already enabled important advances in imaging and display technology. Achromatic metalenses, for example, are designed to focus different colors to the same position, overcoming the chromatic aberration that affects conventional lenses. Non-resonant structures can also generate structural colors without dyes or pigments by directing different wavelengths into selected viewing angles. However, the review points out a fundamental limitation: broadband operation often comes at the cost of spectral selectivity. Such devices may struggle to isolate extremely narrow wavelength bands or to provide independent control over multiple optical channels within the same pixel.
Resonant metasurfaces approach the problem differently. They use optical modes that temporarily trap light in nanoscale structures, increasing the interaction between electromagnetic fields and matter. Examples include localized surface plasmon resonances, surface lattice resonances, Mie resonances, bound states in the continuum, and local–nonlocal transitions. These mechanisms can produce sharply defined spectral responses, sometimes with very high quality factors, or Q-factors. A high Q-factor means that a device stores optical energy for many oscillation cycles and responds within a narrow wavelength range, making it useful for sensing, filtering, color generation, and spectrally selective wavefront control.
Among the most powerful concepts discussed in the review are bound states in the continuum, or BICs. In theory, a BIC is an optical state embedded within a range of radiation modes but unable to radiate because of symmetry or interference. In practical devices, slight structural asymmetry transforms the ideal BIC into a quasi-BIC, allowing controlled radiation while retaining a narrow linewidth and strong field enhancement. This combination gives designers a way to create bright, highly selective resonances suitable for visible, near-infrared, and terahertz applications. The review describes quasi-BIC metasurfaces used for single-pulse terahertz imaging, near-field sensing, and simultaneous spectral and polarization detection.
Resonant devices are also opening new routes to multifunctional imaging. The reviewed technologies include multilayer nonlocal metasurfaces that support several optical responses within a compact structure and nonlocal Huygens metalenses designed through the generalized Kerker effect. In these systems, electric and magnetic responses are balanced to control how light is scattered in the forward and backward directions. Reported Q-factors can reach approximately 10,000, allowing strong spectral confinement. Spin-multiplexed metasurfaces add another layer of functionality by encoding different operations for different circular polarization states, enabling bright-field and edge-enhanced images to be produced from the same optical element.
One especially striking example is a metalaser based on a local–nonlocal transition. According to the review, the device uses the transition to excite quasi-BIC resonances while combining the resonance with geometric phase control. It achieves a Q-factor of about 3,700 and emits narrowband laser beams with directly programmed wavefronts. Demonstrated outputs include focused spots, vortex beams carrying orbital angular momentum, and speckle-free holographic images. Unlike conventional laser systems that may require additional spatial light modulators, lenses, or beam-shaping optics, this approach integrates light generation and wavefront control into a single nanoscale platform. The result points toward compact programmable sources for imaging, sensing, optical communications, and augmented-reality systems.
Color technology is another area where resonant metasurfaces are making a visible impact. In high-contrast all-dielectric structures, a refractive-index-matching layer can suppress unwanted optical leakage and narrow the resonance linewidth. The resulting colors are more saturated and can cover a wider portion of the visible gamut than many conventional structural-color systems. Silicon nanoantennas based on quasi-BIC modes address a long-standing challenge sometimes called “Schrödinger’s red,” in which attempts to create a bright and pure red often produce competing higher-order resonances that contaminate the color. By suppressing those unwanted modes, researchers can produce cleaner and brighter red pixels.
Plasmonic metasurfaces provide a different strategy for high-resolution color display. Shallow nanocavities can be engineered to control hue, saturation, and brightness independently, rather than treating color as a single fixed optical response. The review highlights pixelated devices capable of reproducing full-color images at lithographic resolution, including famous paintings, while also embedding polarization-dependent information that remains hidden under ordinary viewing conditions. This combination of visual display and optical encryption could be valuable for anti-counterfeiting, secure labeling, artistic fabrication, and information storage.
The review concludes that the next generation of meta-optics will likely merge broadband non-resonant functions with narrowband resonant control. A hybrid device could, for example, provide achromatic imaging across a wide spectrum while selectively routing individual wavelengths or polarization channels at the pixel level. Adding tunable materials, electrical control, machine-learning-assisted inverse design, and CMOS-compatible fabrication could extend these capabilities to adaptive AR and VR displays, compact LiDAR systems, quantum photonics, and biosensors. Major obstacles remain, including the fabrication of large areas with nanometer-scale precision, accurate modeling of complex nonlocal interactions, optical losses, and integration with existing semiconductor manufacturing. Even so, the field is rapidly shifting from passive flat optics toward multifunctional photonic surfaces that can generate, shape, separate, and interpret light within structures only a fraction of a wavelength thick.
Subject of Research: Nanophotonics, metasurfaces, resonant and non-resonant meta-devices, optical imaging, color routing, displays, and wavefront control
Article Title: From non-resonance to resonant meta-devices: imaging, color routing, displaying, and beyond
News Publication Date: 28 June 2026
Web References: https://doi.org/10.29026/oes.2026.260016
References: Opto-Electronic Science, DOI: 10.29026/oes.2026.260016
Image Credits: Meta-devices Lab
Keywords: Metasurfaces, resonant meta-devices, non-resonant optics, bound states in the continuum, quasi-BIC, metalenses, metalasers, structural color, color routing, wavefront shaping, nanophotonics, AR/VR, optical imaging
