Imagine a pair of augmented reality glasses that can filter out visual “noise,” letting only the light you want reach your eyes, not just by color, but by direction.
This kind of precision light control is now closer to reality, thanks to a new breakthrough from a team of physicists in China who have built a photonic device capable of selecting both the wavelength and direction of light with extreme accuracy.
In a new study published in the journal eLight, researchers from Sun Yat-sen University and Fudan University have experimentally demonstrated a bilayer nanostructure—or “metagrating”—that can pick out a specific light mode from a continuous spectrum, filtering out all others. This unprecedented “spatio-spectral selectivity” offers dramatic improvements in everything from AR/VR displays to thermal emitters, biosensing, and quantum photonics.
“Radiation directionality acts like a ‘magical eraser,’ allowing us to precisely suppress light’s spectral signature along a dispersion curve,” the researchers said in a press release issued by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics and Physics, CAS. “This capability allows for independent selectivity of angle and wavelength, overcoming the limitation imposed by intrinsic dispersion.”
Light travels as waves with different wavelengths (colors) and angles (directions). Traditional optical filters can block specific colors, while others can control the angle of incoming light. But doing both—selecting a single color at a particular angle — is like trying to hear just one voice in a crowded stadium filled with identical speakers.
That’s where the concept of spatio-spectral selectivity comes in. It allows optical systems to isolate a precise wavelength and angle, like filtering a single note from a single violin in an orchestra.
The problem? In typical photonic structures like diffraction gratings or photonic crystals, there’s an intrinsic “locking” between wavelength and angle, meaning you can’t freely isolate one without affecting the other.
However, the researchers say their new innovation circumvents this barrier.
The breakthrough hinges on a phenomenon known as Fano resonance. This interference effect creates sharp peaks and dips in light reflection. By designing their device to exploit radiation asymmetry—how light is radiated upward or downward at different angles — the team managed to break the rigid link between angle and wavelength.
Their tool of choice? A misaligned bilayer metagrating—two nanoscale silicon gratings stacked with a slight lateral offset and separated by a 35-nanometer-thin spacer. This misalignment breaks mirror symmetry but retains a crucial form of balance known as P-symmetry, which allows the system to manipulate light with greater precision.
Guided by phase diagrams and computer simulations, the researchers fabricated a version with a 37 nm offset, which they demonstrated could reflect light only when it hits at a zero-degree angle and at a particular wavelength, 1349 nanometers. The reflected band was extremely narrow, just ±5 degrees, and 14 nm wide.
In practical terms, the device acts as a razor-thin optical gatekeeper, ensuring only light in precisely the right color and direction gets through.
To prove their design’s utility, the team built a working prototype and used it in an imaging experiment. They placed the metagrating in front of a mask patterned with a cartoon-like image and illuminated it with a tunable laser.
When the light hit the metagrating at the perfect angle and wavelength, the image disappeared, blocked by the structure’s selective reflection. However, off-angle or off-wavelength, the image was clearly visible. This demonstration showcased real-world spatio-spectral filtering, akin to a lens that only sees what it’s designed to.
The intensity contrast between on-resonant and off-resonant states reached a factor of 6.2 in this setup, showing that the technology can offer sharp filtering without bulky optics or moving parts.
The implications of this advance are vast. In augmented reality (AR), for instance, future headsets could use these filters to block out stray reflections or selectively illuminate digital overlays, improving clarity and reducing eye strain. Because the devices are ultra-thin and passive (no power supply needed), they could be easily integrated into lenses or micro-optical systems, paving the way for a new generation of AR technology.
“Multifunctional glasses with narrow angular and spectral selectivity will contribute to additional detection of [a] human’s dynamic action with an unperturbed view of [the] real world in augmented reality,” researchers write.
In thermal engineering, the same tech could help develop directional thermal emitters — materials that only radiate heat in specific directions and wavelengths, which are key for energy efficiency and stealth. The structure could also be used as a selective mirror in laser systems, enabling finer control over laser emission and beam quality.
Beyond engineering, the ability to precisely control light direction and color at the nanoscale could improve biosensors and spectroscopy tools that rely on high-contrast light-matter interactions.
Additionally, researchers constructed a detailed phase diagram that maps how different structural parameters affect the device’s selectivity, offering a roadmap for other scientists to design similar or even better devices tailored to different wavelengths or materials.
“Our proposed phase diagram is general and can be extended into other spectral ranges with appropriate materials,” researchers note.
In an era where light is increasingly being used to communicate, compute, and sense the world around us, the ability to precisely control its properties could prove as transformative as the development of lenses or lasers.
Whether used to make AR glasses more intelligent, lasers sharper, or sensors more selective, this metagrating-based technology offers a glimpse into a future where light itself is sculpted with atomic precision.
“This research not only offers an innovative solution to address the fundamental challenge of independently controlling angle and wavelength,” the researchers conclude, “but also provides new insights for technological applications such as AR/VR displays, spectral imaging, coherent thermal radiation, and advanced semiconductor manufacturing.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com