Near-infrared (NIR) cut-off filters play a critical role in modern imaging systems by blocking NIR interference on CMOS sensors, thereby preventing image ghosting and distortion. Currently, the dominant NIR cut-off filter products are fabricated by depositing NIR-reflective films on highly transparent quartz glass. However, multiple reflections and refractions of incident light among optical lenses can generate ghost images and light spots.
As an alternative, absorption-type NIR cut-off filters, represented by specially designed transition-metal-doped glasses, have rapidly emerged as promising solutions because of their low cost, simple processing, and their ability to address the limitations of traditional filters. Among them, Cu 2+ -doped fluorophosphate glasses, known as blue glasses, have attracted considerable research interest. In general, the Cu 2+ : 3d → 3d transition gives rise to NIR absorption, whereas the associated Cu + : 3d 10 → 3d 9 4s 1 transition produces violet-blue absorption, which markedly reduces visible-light transmittance. Current research has mainly focused on the changes in the glass network structure induced by Cu ion introduction and the regulation of absorption behavior by crystal field strength. However, the NIR absorption of Cu 2+ -doped fluorophosphate glass is also affected by thickness. To date, no studies have systematically explored how to achieve better performance at low thickness (0.21 mm, compatible with most commercial lenses).
To address this challenge, a team led by Prof. Daqin Chen from Fujian Normal University turned to glass network engineering. “Achieving both high visible transmittance and efficient NIR cut-off requires a high Cu 2+ /Cu + ratio and a suitable crystal field that enables strong NIR absorption by Cu 2+ ,” explained Prof. Chen. “We hypothesized that regulating the glass network through alkali-metal doping could simultaneously optimize both factors.”
The team designed fluorophosphate glasses with nominal compositions of 55NH 4 H 2 PO 4 -10M 2 CO 3 -20NH 4 F-10CuO-1Al 2 O 3 -1Y 2 O 3 -3BaF 2 (M = Li, Na, K, Rb, Cs) via a melt-quenching method. By systematically varying the AM ion, they investigated the effects of ionic radius on the glass network structure, Cu valence state and optical performance.
The team published their work in Journal of Advanced Ceramics on October 13, 2026.
The results were remarkable: as the AM ion radius increased from Li + to Cs + , the glass color gradually changed from emerald green to sky blue, T 400 nm increased from 43.3% to 85.4%, λ T50 redshifted from 631.2 nm to 668.9 nm and T 1100 nm decreased from 2.2% to 0.8%. The Cs + -doped glass achieved the best overall performance with T 400 nm = 85.4%, λ T50 = 669 nm and T 1100 nm = 0.8%, significantly outperforming commercial CXA700, CXB700 and BG47 blue glasses.
XPS analysis revealed that the Cu 2+ /Cu + ratio increased from 6.21 (Li) to 29.87 (Cs), far exceeding the 3.61 of commercial CXA700. O 1s XPS, FTIR and Raman spectroscopy confirmed that the NBO/BO ratio increased from 1.06 to 1.60, indicating progressive depolymerization of the glass network. EXAFS fitting showed that Cu 2+ exists in a six-coordinate form as [CuO 4 F 2 ] 8- units, with Cu-O bond length increasing from 1.94 Å to 1.97 Å and Cu-F bond length decreasing from 2.18 Å to 2.15 Å. EPR results demonstrated that the g ∥ /A ∥ ratio decreased from 278 to 252, confirming reduced octahedral distortion.
“These structural changes originate from the weaker polarizing power of larger AM ions, which reduces their attraction to O 2- electron clouds,” noted Prof. Chen. “This promotes the conversion of bridging oxygen to non-bridging oxygen and increases optical basicity, creating a more oxidizing environment that stabilizes Cu 2+ .”
The mechanism can be summarized as follows: large-radius AM ions provide higher optical basicity, which suppresses Cu + formation and increases the Cu 2+ proportion, enhancing NIR absorption. Meanwhile, they reduce the octahedral distortion of the [CuO 4 F 2 ] 8- unit, weakening the Jahn-Teller effect, decreasing energy-level splitting, and redshifting Cu 2+ NIR absorption. Consequently, only Cs + -doped blue glass can simultaneously achieve high visible transmittance and deep NIR cut-off.
In imaging applications, the as-prepared blue glass demonstrated superior performance. Under identical exposure, ISO, aperture, white balance and image processing parameters, images captured with the present blue glass showed higher saturation and more realistic color reproduction than those captured with commercial CXA700, without NIR light interference. Color-fidelity tests at different times of day further confirmed its advantages under various ambient-lighting conditions.
“This work provides a new strategy for addressing key bottlenecks in commercial blue glasses and offers an alternative material for advanced imaging,” Prof. Chen concluded. “The glass network engineering approach can be extended to other optical glass systems, paving the way for next-generation high-performance imaging technologies.”
Other contributors include Ziyi Hu, Shuangyin Zhu, Shisheng Lin and Feng Huang from the School of College of Physics and Energy at Fujian Normal University in Fujian, China; Tao Pang from the School of College of Science at Huzhou University in Zhejiang, China; Lingwei Zeng from the School of Chemistry and Chemical Engineering at Hunan University of Science and Technology in Hunan, China.
The research was supported by the National Natural Science Foundation of China (52572155, 52272141) and the Natural Science Foundation of Fujian Province (2024J02014).
About Author
Daxin Chen is a professor and doctoral supervisor at Fujian Normal University. In 2008, he obtained a doctoral degree in condensed matter physics from the Fujian Institute of Physical Science, Chinese Academy of Sciences. In 2012, he was promoted to researcher with exceptional qualifications. From 2014 to 2018, he was a distinguished professor at Hangzhou Dianzi University. In 2018, he joined Fujian Normal University as a high-level introduced talent. His main research focus is on luminescent materials and their applications. He has been the principal investigator of key research projects, sub-projects, and various national natural science foundation projects. He has published over 200 SCI papers as the first or corresponding author in journals such as Chem. Soc. Rev., Sci. Adv, Adv. Mater, Adv. Funct. Mater, ACS Energy Lett, and JACS. His h-index is 84, and he has been included in the ESI highly cited list more than 30 times.
DOI LINK: 10.26599/JAC.2026.9221398
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC’s 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in “Materials Science, Ceramics” category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
Journal of Advanced Ceramics
Glass network engineering enables efficient nir cut-off in Cu2+ blue glasses
14-Oct-2026