Sometimes the most interesting physics emerges not from a perfectly designed structure but from a small experimental imperfection. Optical metasurfaces have emerged as a powerful platform for controlling light at the nanoscale and have recently attracted growing interest for nonlinear optics. A key advantage of metasurfaces is their subwavelength thickness, which largely relaxes the phase-matching requirements that typically constrain nonlinear processes in bulk materials. At the same time, their limited physical thickness restricts the interaction length between light and the nonlinear medium, which reduces conversion efficiency. Bulk nonlinear structures offer the opposite situation: they provide longer interaction lengths but require strict phase matching to maintain efficient energy transfer. We were interested in whether stacked metasurfaces could combine the advantages of both approaches by increasing the effective interaction length while preserving the resonant field confinement that metasurfaces provide.
Stacking metasurfaces introduces additional structural parameters that do not exist in single-layer designs. In particular, the vertical spacing between layers and their relative lateral displacement can significantly modify how optical modes interact. Motivated by this idea, we began investigating bilayer dielectric metasurfaces to understand how interlayer coupling and symmetry breaking could be used to enhance nonlinear optical responses. The basic concept of the stacked metasurfaces platform and the resulting optical mechanisms are illustrated in Fig. 1.
Our initial prediction was relatively straightforward. Numerical simulations suggested that two perfectly aligned metasurfaces layers would interact to form a transmission gap at certain interlayer separations. Near the edge of this gap, electromagnetic fields become strongly localized, which should enhance nonlinear optical processes such as third-harmonic generation. These simulations indicated that even a simple bilayer configuration could already provide a significant improvement compared with a single metasurfaces. However, when we fabricated the structures and measured their transmission spectra, we observed something unexpected. A new spectral dip appeared that was not present in our original simulations. At first we assumed it might be a measurement artifact, but repeated measurements consistently revealed the same feature. Even more intriguingly, the electromagnetic field associated with this new resonance was more strongly localized than the resonance we had initially designed.
To understand the origin of this feature, we examined the fabricated structures more carefully. Using focused ion beam milling we cut through the multilayer samples and inspected their cross sections. The images immediately revealed an important clue: the top and bottom metasurfaces layers were not perfectly aligned. Although alignment marks were used during fabrication, small lateral displacements between the layers inevitably occurred, as shown in the cross-sectional SEM image in Fig. 2. When we repeated the simulations with a slight lateral displacement between the two metasurfaces, a very sharp transmission dip appeared at a shorter wavelength that matched the previously unexplained experimental observation.
To investigate this phenomenon further, we fabricated additional samples with different controlled displacements between the layers. These experiments showed that the resonance is extremely sensitive to the relative alignment of the metasurfaces. Numerical simulations revealed that the displacement simultaneously breaks both in-plane and out-of-plane symmetries and enables strong interlayer coupling that produces a guided resonance with strong electromagnetic field confinement. The nonlinear response was particularly striking: while stacking perfectly aligned metasurfaces already enhanced the third-harmonic signal compared with a single layer, introducing lateral displacement created an even stronger resonance that boosted the nonlinear signal by several orders of magnitude. After extensive numerical studies and a careful search through the literature, we realized that the behavior could be described using a model based on a non-Hermitian Hamiltonian, which captures the coupling between resonant modes and radiation channels. In the end, what initially appeared to be a small fabrication imperfection turned out to reveal a new physical mechanism for engineering resonances in multilayer metasurfaces and suggests that controlled symmetry breaking may become a powerful design tool for future nonlinear photonic devices.
Figure 1. Concept of stacked metasurfaces for enhancing nonlinear optical response. A single-layer metasurfaces supports a Mie-type resonance that provides the basic building block for nonlinear enhancement. When two identical metasurfaces are stacked, their interaction forms a transmission gap that increases field confinement. A small lateral displacement between the layers breaks symmetry and creates a high-Q guided resonance, leading to dramatically enhanced third-harmonic generation.
Figure 2. Fabrication of bilayer metasurfaces. Tilted-view SEM image of a single-layer grating (left). Cross-sectional view of the fabricated bilayer structure, where a small lateral displacement between the two metasurfaces layers plays a key role in creating the unexpected resonance discussed in the study (right).
Read the full paper here:
https://www.nature.com/articles/s44455-025-00016-3