‘Strong’ filters: Innovative technology for better displays and optical sensors

A research team from the Universities of Cologne, Hasselt (Belgium) and the University of St. Andrews (Scotland) has successfully exploited the quantum mechanical principles of strong light-matter coupling to develop a breakthrough optical technology that overcomes long-standing problems in optical systems angle dependence. The research “Breaking the angular dispersion limit in thin film optics through ultra-intense light-matter coupling” was published in nature communications Introducing ultra-stable thin film polarizing filters that open new avenues in photonics, sensor technology, optical imaging and display technology. The research at the University of Cologne was led by Professor Malte Gather, Director of the Humboldt Center for Nano and Biophotonics at the Department of Chemistry and Biochemistry at the Faculty of Mathematics and Natural Sciences.

Optical filters are essential for many applications. However, their performance degrades significantly when light hits them at different angles – the color of the transmitted light changes depending on the viewing angle. This performance degradation is due to fundamental physical principles and can have a serious impact on the accuracy of optical sensors.

The solution developed by the international team exploits the principles of quantum mechanics: when light particles strongly couple with the energy states of organic materials, so-called polaritons are created.

Traditional thin film filters consist of many alternating transparent layers, usually made of metal oxides. Light is partially reflected or transmitted by these individual layers. Their thickness then determines the color of the transmitted light through constructive and destructive interference of light waves, similar to the shimmering color of a soap bubble. The transmission and reflection properties of the filter can be precisely tuned through the controlled interaction of many such thin layers. However, this physics makes filters fundamentally susceptible to what is known as angular dispersion—the shift (blue shift) of spectral characteristics toward shorter wavelengths when the filter is tilted. In their new method, the scientists integrated strongly absorbing organic dyes into optical filters, resulting in strong coupling of interfering light with the dye.

“Typically, you want to avoid any form of absorption in a spectral filter that would compromise its optical quality. However, we specifically exploit the strong light absorption of organic materials to produce angularly stable polariton modes with excellent transmission properties,” the said the study’s first author, Dr. Andreas Mischok of the University of Cologne.

The team was able to develop filters with excellent angular stability, with spectral shifts of less than 15 nm even at extreme viewing angles of over 80°. The complex multi-layer design also exhibits peak transmission of up to 98%, a value equivalent to the best conventional optical filters currently available.

In a collaborative research project with Professor Koen Vandewal’s group at the University of Hasselt, scientists integrated polariton filters into organic photodiodes to create narrow-band photodetectors for hyperspectral imaging such as materials characterization and Compact optical sensors) paved the way.

The research shows the possibility of applying the technology to polymers, perovskites, quantum dots and other materials, thereby transferring new filter principles to a wider wavelength range. Possible areas of application for polarizing filters include micro-optics, displays, sensor technology and biophotonics. In all of these areas, the angle-independence of new filters can greatly simplify the design of optical systems and expand their functionality. Professor Malte Gather, who led the research at the University of Cologne, said: “This is a game-changing change in the way we design optical filters. By solving the angular dispersion problem in a completely new way, we open up completely new possibilities for optical systems.”

The research team believes that polarizing filters are the cornerstone of next-generation optical components with huge scientific and economic potential. In addition to integrating filters into sensors such as lidar (light detection and ranging) and fluorescence microscopy, future work will focus on applications in display technology.