Scientists have overcome a fundamental limitation in optics by using innovative bilayer metagratings to separate wavelength and angle—two key properties that define light’s behavior. Precise control over both is crucial for many advanced optical technologies.

In periodic systems with repeating structures, a natural dispersion effect causes wavelength and angle to be linked—a phenomenon known as angle-wavelength locking. This means that changing the angle of incoming light usually alters the device’s filtering wavelength. The new approach breaks this constraint, opening the door to more flexible and powerful optical devices.

Figure 1. Resonant Reflection Control Using Directional Eraser in Misaligned Metagratings

This tight link between angle and wavelength has long been considered unavoidable, posing serious challenges for technologies that need to adjust each property independently. It creates technical issues such as rainbow-like artifacts in augmented reality (AR) displays, color blurring in wide-field imaging, reduced spectral accuracy in photodetectors due to angular interference, and design limitations for efficient, directionally tuned light sources. Figure 1 shows Resonant Reflection Control Using Directional Eraser in Misaligned Metagratings.

A Breakthrough in Controlling Light Direction

In a recent paper published in eLight, a research team led by Professors Jian-Wen Dong (Sun Yat-sen University) and Lei Zhou (Fudan University) has uncovered a key to breaking the angle-wavelength coupling in optics: the directionality of optical mode radiation. Through detailed theoretical analysis, they created a complete phase diagram for engineering resonant spectra based on radiation directionality, showing that spatial inversion symmetry and highly directional radiation are essential for overcoming angle-wavelength locking.

To achieve this, the team introduced lateral displacement in bilayer metagratings—a design that maintains spatial inversion symmetry but breaks vertical mirror symmetry. This structure enables precise angular control of radiation directionality. Their models predict that resonant reflection only occurs at normal incidence near the central wavelength. They also proposed versatile designs capable of achieving angle and wavelength selectivity independently.

“Radiation directionality acts like a ‘magical eraser,’ letting us suppress light’s spectral features along the dispersion curve,” the researchers explained. “This makes it possible to control angle and wavelength separately, bypassing the constraints of intrinsic dispersion.”

Tackling Fabrication Hurdles

The researchers acknowledged that fabricating bilayer metagratings posed significant challenges, particularly in achieving ultra-flat spacer layers and precise lateral alignment between layers—requirements that demand advanced nanofabrication techniques. To overcome this, they developed a new fabrication method that includes multiple etching steps, indirect thickness measurements, and iterative deposition. A high-precision alignment process enabled the successful creation of high-quality bilayer metagratings operating in the near-infrared range. This technique ensures excellent flatness, tunable thickness, and ~10 nm alignment precision, making it adaptable to different spacer materials and establishing a robust platform for exploring bilayer photonic systems.

With this platform, the team demonstrated that high reflectance occurred only at a specific angle and wavelength. To confirm that this unique behavior resulted from radiation directionality, they conducted angle-resolved optical microscopy and used temporal coupled-mode theory alongside cross-polarization techniques to measure the unidirectional radiation of the resonant modes.

Additionally, the researchers developed millimeter-scale, high-precision bilayer metagratings and achieved high-contrast imaging with simultaneous spatial and spectral selectivity at 0° and 1342 nm. This breakthrough paves the way for compact optical imaging systems and optical computing [1]. The team believes their work not only solves a long-standing optical challenge but also lays the groundwork for future technologies, including AR/VR displays, spectral imaging, thermal emission control, and next-generation semiconductor processes.

References:

1. https://scitechdaily.com/one-tiny-structure-just-broke-a-fundamental-rule-of-optics/

Cite this article:

Janani R (2025), Tiny Structure Defies a Core Principle of Optics, AnaTechMaz, pp. 243