New research has revealed that the quantum mechanical concept of strong coupling offers unprecedented opportunities for crafting optical filters.
A collaborative team from the University of Cologne, Hasselt University in Belgium, and the University of St Andrews in Scotland has utilized the quantum principle of strong light-matter coupling to create an innovative optical technology. This breakthrough addresses the long-standing issue of angular dependence in optical systems. Their study, titled ‘Breaking the angular dispersion limit in thin film optics by ultra-strong light-matter coupling,’ was published in Nature Communications and introduces ultra-stable thin-film polariton filters that promise to revolutionize fields such as photonics, sensor technology, optical imaging, and display applications. The research was led by Professor Dr. Malte Gather, who directs the Humboldt Centre for Nano- and Biophotonics within the Department of Chemistry and Biochemistry at the Faculty of Mathematics and Natural Sciences in the University of Cologne.
Optical filters play a crucial role in numerous applications. However, their efficiency significantly decreases when light hits them at varying angles; this results in changes to the color of the light that passes through, depending on the viewing position. This performance drop is due to fundamental physical laws, which can negatively impact the accuracy of optical sensors.
The solution devised by this international research team employs a principle from quantum mechanics: when light particles engage in strong coupling with the energy states of an organic material, they generate structures known as polaritons.
Traditional thin-film filters are composed of multiple alternating transparent layers, typically made from metal oxides. Light interacts with each layer by being partially reflected or transmitted. The thickness of these layers determines the resulting color through constructive and destructive interference of the light waves, similar to the shimmering surfaces of soap bubbles. The optical properties of these filters can be finely tuned by the deliberate interaction of these thin layers. Nevertheless, this approach inherently makes the filters vulnerable to angular dispersion—when the filter is tilted, there can be a shift in spectral properties towards shorter wavelengths (known as a blueshift). In their novel method, the researchers incorporated strongly absorbing organic dyes into optical filters, resulting in strong coupling between the interfering light and these dyes.
“Typically, one aims to minimize absorption in spectral filters to maintain their optical quality. However, we intentionally exploit the strong light absorption capabilities of organic materials to create angularly stable polariton modes with outstanding transmission properties,” explained Dr. Andreas Mischok from the University of Cologne, the study’s lead author.
The team has successfully developed filters that exhibit remarkable angular stability, showing a spectral shift of less than 15 nm even at extreme viewing angles exceeding 80°. Complex multilayer designs achieved peak transmission rates as high as 98 percent, rivaling the best existing conventional filters.
In a joint research effort with Professor Dr. Koen Vandewal’s group at Hasselt University, the researchers have embedded polariton filters into organic photodiodes, leading to the development of narrowband photodetectors that pave the way for advancements in hyperspectral imaging, such as material characterization, as well as compact optical sensors.
The study also suggests the potential for applying this technology to materials like polymers, perovskites, quantum dots, and others, thereby expanding the new filter principle across an even broader range of wavelengths. Possible applications for polariton filters include micro-optics, display technologies, sensor systems, and biophotonics. In all these areas, the angle-independent nature of the newly developed filters can simplify the design of optical systems while enhancing their functionalities. Professor Malte Gather, who is spearheading the research at the University of Cologne, stated: “This marks a transformative shift in how we design optical filters. By addressing the challenge of angular dispersion using a fundamentally novel approach, we are unlocking entirely new possibilities for optical systems.”
The research team views polariton filters as foundational for the next generation of optical components, poised with significant scientific and economic potential. Future efforts will focus on incorporating the filters into sensors like LiDAR (Light Detection and Ranging) and fluorescence microscopy, as well as exploring applications in display technology.
