Toward developing chip-integrated and miniaturized nanophotonic devices, the incorporation of metasurfaces onto optical waveguides has attracted widespread attention as a compact platform for manipulating in-plane lightwaves. In previous studies, on-chip integrated metasurfaces have enabled precise control of the amplitude, phase, and polarization of the out-coupled light, yet they offer limited control over its spectral properties, making practical wavelength-selective extraction challenging. Typically, guided waves perturbed by nanostructures produce broadband, low-selectivity outputs, constraining applications such as wavelength-division multiplexing and color routing. Conventional free-space metasurfaces or multilayer thin films can achieve color filtering and routing via spatial multiplexing, but non-target wavelengths are often reflected or absorbed, causing substantial energy utilization efficiency (EUE) loss. Thus, achieving efficient, narrowband, and controllable wavelength-selective extraction on a compact on-chip platform without the intrinsic EUE loss from spatial multiplexing, remains a critical challenge.
In a new paper published in Light: Science & Applications , a team of researchers, led by Professor Zhongyang Li from Wuhan University, reports a novel strategy to create on-chip nonlocal metasurface color routers by exploiting symmetry-broken quasi-bound states in the continuum (q-BICs). By precisely engineering meta-diatom pairs atop the waveguide with tailored scaling and asymmetry, this approach simultaneously modulates both the extraction intensity and the narrowband (~20 nm) spectral output of the out-coupled light. As a proof of concept, the team spatially mapped and horizontally cascaded distinct q-BIC-assisted meta-diatom pixels, demonstrating multicolor routing along the waveguide. The stronger the asymmetry (tilt angle), the higher the intensity of light extracted into free space. Meanwhile, at a fixed tilt angle, the peak wavelength of the extracted light can be tuned by adjusting the geometric size, with larger structures causing a redshift in the spectral peak. Furthermore, the researchers implemented spatial mapping and horizontally cascaded distinct q-BIC-assisted pixels to demonstrate multicolor routing display, encoding both color and intensity information into meta-diatom pairs with varying geometric dimensions.
Crucially, on-chip metasurfaces possess a natural advantage for horizontal cascading, enabling efficient multi-wavelength channel multiplexing. Beyond conventional free-space spatial-multiplexed routers, non-target wavelengths are typically reflected or absorbed, which leads to substantial energy loss and limits the theoretical EUE to only one-third in a three-wavelength channel incident system. The on-chip cascaded architecture effectively overcomes this issue. In this scheme, each meta-diatom pixel selectively extracts only its designated wavelength from the guided light, while the remaining components continue to propagate along the waveguide to subsequent functional units. As a result, the theoretical EUE can approach unity. This mechanism is similar to water flowing through a pipeline, where each branch takes only the amount it needs and the rest continues downstream without waste.
In summary, this work represents an original demonstration of nonlocal on-chip metasurface color routing on a waveguide, enabled by symmetry-broken q-BICs. The approach allows simultaneous control over both the extraction intensity and the primary wavelength of the out-coupled lightwaves. Notably, the q-BIC-assisted routers support cascaded multiplexing while delivering a substantial improvement in EUE compared with conventional free-space architectures. The researchers anticipate that this strategy will open new avenues for next-generation wearable meta-display devices, multiplexing information routing, and intelligent integrated photonic systems.
On-chip nonlocal metasurface for color router: conquering efficiency-loss from spatial-multiplexing