Chinese scientists have made a groundbreaking breakthrough in the field of optics, successfully breaking through the long-standing angle-wavelength locking limitation that has troubled the scientific community for decades. A joint research team led by Professor Dong Jianwen from Sun Yat-sen University and Professor Zhou Lei from Fudan University has achieved the first independent and precise control of both the angle and wavelength of light through an innovative double-layer metamaterial grating design. This discovery not only challenges traditional optical theory but also opens up new possibilities for technologies such as augmented reality, spectral imaging, and advanced semiconductor manufacturing.
In traditional optical systems, there is a natural relationship between the wavelength of light and its propagation angle, known as "angle-wavelength locking." This phenomenon arises from the dispersion effect in periodic structures, meaning that adjusting the incident angle of light typically causes a corresponding change in the device's filtering wavelength. This interrelated relationship has long been considered an unavoidable physical constraint, posing significant challenges for many high-precision optical applications.
The key insight of the theoretical breakthrough
The breakthrough of the research team lies in identifying the radiation directivity of optical modes as the key to overcoming this fundamental challenge. Through in-depth theoretical analysis, they established a complete phase diagram for engineering resonant spectra through radiation directivity. This theoretical framework revealed that spatial inversion symmetry and the highly directional radiation of optical modes are the basic physical conditions for breaking the angle-wavelength locking.
Schematic of customized resonance reflection using radiation directivity in misaligned metagratings. The new dual-layer metagrating selects a single angle and a single wavelength under broadband spectra and wide-angle incidence. This is achieved through a "directional eraser," which precisely suppresses the spectral features along the dispersion curve. Source: Zhuang Zepeng, Zhou Xin et al.
Based on these theoretical insights, the research team introduced a lateral displacement design in the double-layer metamaterial grating. This design maintains spatial inversion symmetry while breaking vertical mirror symmetry, thereby enabling precise angular control of radiation directivity. Theoretical predictions indicate that resonance reflection occurs only at normal incidence and near the central wavelength. The team also proposed a general design scheme for achieving spatial-spectral selectivity at any angle and wavelength.
Professor Dong Jianwen explained, "Radiation directivity acts like a 'magic eraser,' allowing us to precisely suppress the spectral characteristics of light along the dispersion curve. This capability makes it possible to achieve independent selectivity for angles and wavelengths, overcoming the limitations imposed by inherent dispersion."
Technical challenges in manufacturing processes
Converting theoretical designs into practical devices presents significant technical challenges. The experimental fabrication of double-layer metamaterial gratings requires both the flatness of ultra-thin spacer layers and the precise lateral displacement between layers, which demands complex nanofabrication techniques.
To address these challenges, the research team developed a novel manufacturing method involving multiple etching steps, indirect thickness measurement, and iterative deposition processes. This method combines high-precision double-layer alignment technology and successfully fabricated high-quality double-layer metamaterial gratings operating in the near-infrared band. The method provides excellent spacer layer flatness and adjustable thickness, with alignment accuracy reaching approximately 10 nanometers, and is compatible with various spacer materials, establishing a flexible experimental platform for studying double-layer photonic systems.
Theoretical design and experimental realization of the misaligned double-layer metagratings. a. A typical Fano resonance caused by guided resonance modes in the double-layer metagratings. b. The reflection angle and peak of the Fano resonance are related to the radiation directivity of guided modes with matching in-plane momentum k// and -k//. Red dots correspond to the case of mirror symmetry, and the dashed line represents P symmetry. c. Scanning electron microscopy image of the fabricated double-layer metagratings. d. Angle-resolved reflectance spectra in simulations and experiments. Source: Zhuang Zepeng, Zhou Xin et al.
Using this platform, the research team experimentally demonstrated a high reflectance phenomenon occurring at a single angle and a single wavelength. To confirm that this novel reflectance originates from radiation directivity, they also conducted angle-resolved optical microscopy measurements to characterize the radiation directivity of the samples. By combining time-coupled mode theory with cross-polarization measurement techniques, they quantitatively measured the unidirectional radiation characteristics of the resonant modes.
Broad application prospects
The practical value of this research has already begun to emerge. The research team developed a high-precision double-layer metamaterial grating on a millimeter scale and successfully achieved high-contrast imaging with simultaneous spatial and spectral frequency selectivity at 0° and 1342 nm. This opens up new opportunities for compact optical imaging and optical computing technologies.
In augmented reality (AR) and virtual reality (VR) display technologies, this breakthrough can eliminate the long-standing issue of rainbow color artifacts. Traditional AR displays exhibit color shifts at different viewing angles due to the angle-wavelength locking effect, affecting user experience. The application of this new technology will allow AR displays to maintain consistent color performance at any viewing angle.
In the field of spectral imaging, this technology can significantly improve the accuracy of detectors and reduce the impact of angular interference on spectral data. This is of great significance for applications such as remote sensing, medical imaging, and material analysis. Furthermore, in advanced semiconductor manufacturing, this technology can provide more precise light source control for lithography processes, improving manufacturing accuracy and efficiency.
Future development paths
Although this research has achieved major breakthroughs in both theory and experimentation, the research team believes there is still much work to be done. In particular, further optimization of the manufacturing process is needed to improve yield and reproducibility to meet the requirements of large-scale commercial applications.
The research team predicts, "This study not only provides an innovative solution to the fundamental challenge of independently controlling angle and wavelength but also offers new insights for technological applications such as AR/VR displays, spectral imaging, coherent thermal radiation, and advanced semiconductor manufacturing."
From a broader perspective, this research demonstrates the close connection between fundamental physics research and practical applications. By deeply understanding the basic principles of optical phenomena, scientists can develop revolutionary new technologies that drive the development of the entire industry.
As related technologies continue to mature, it is expected that commercial products based on this breakthrough will begin to appear in the coming years, offering consumers and industrial users unprecedented optical experiences.
Original article: https://www.toutiao.com/article/7528353477253284390/
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