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

A team of researchers from the University of Cologne, Hasselt University (Belgium) and the University of St Andrews (Scotland) has succeeded in using the quantum mechanical principle of strong light-matter coupling for a groundbreaking optical technology that overcomes the long-standing problem of angular dependence in optical systems. The study ‘Breaking the angular dispersion limit in thin film optics by ultra-strong light-matter coupling’ published in Nature Communications presents ultra-stable thin-film polariton filters that open new avenues in photonics, sensor technology, optical imaging and display technology. The study at the University of Cologne was led by Professor Dr Malte Gather, director of the Humboldt Centre for Nano- and Biophotonics at the Department of Chemistry and Biochemistry of the Faculty of Mathematics and Natural Sciences.

Optical filters are essential for many applications. However, their performance decreases considerably when light hits them at different angles – the colour of the transmitted light changes depending on the viewing angle. This reduction in performance is due to fundamental physical principles and can have a severe impact on the accuracy of optical sensors.

The solution developed by the international team utilizes a principle from quantum mechanics: When light particles are strongly coupled to the energy states of an organic material, so-called polaritons are created.

Conventional thin-film filters consist of many alternating transparent layers, often made of metal oxides. Light is partially reflected or transmitted by these individual layers. Their thickness then determines the colour of the transmitted light via constructive and destructive interference of the light waves, comparable to the shimmering colours of soap bubbles. The transmission and reflection properties of filters can be precisely adjusted through the controlled interaction of many such thin layers. However, this physical principle makes the filters fundamentally susceptible to so-called angular dispersion – a shift in spectral properties towards shorter wavelengths (blueshift) when the filter is tilted. In their new approach, the scientists integrate strongly absorbing organic dyes into optical filters, which leads to a strong coupling of the interfering light with the dyes.

“Usually, you want to avoid any kind of absorption in spectral filters in order not to compromise their optical quality. However, we specifically utilize the strong light absorption in organic materials to generate angularly stable polariton modes with excellent transmission properties,” said Dr Andreas Mischok from the University of Cologne, first author of the study.

The team was able to develop filters with exceptional angular stability, which showed a spectral shift of less than 15 nm even at extreme viewing angles of over 80°. Complex multilayer designs also showed a peak transmission of up to 98 percent – a value equivalent to the best conventional filters currently available.

In a collaborative research project with the group of Professor Dr Koen Vandewal from Hasselt University, the scientists integrated polariton filters into organic photodiodes to create narrowband photodetectors, paving the way for advances in hyperspectral imaging, e.g. for material characterization, and compact optical sensors.

The study indicates possibilities for applying the technology to polymers, perovskites, quantum dots and other materials and thus transferring the new filter principle to an even wider wavelength range. Possible areas of application for polariton filters include micro-optics, displays, sensor technologies and biophotonics. In all these areas, the angle independence of the new filters can drastically simplify the design of optical systems and extend their functionality. Professor Malte Gather, who is leading the reasearch at the University of Cologne, said: “This is a disruptive change in the way we design optical filters. By tackling the problem of angular dispersion with a fundamentally new approach, we are opening up completely new possibilities for optical systems.” 

The research team considers polariton filters as a cornerstone for the next generation of optical components with enormous scientific and economic potential. In addition to integrating the filters into sensors such as LiDAR (Light Detection and Ranging) and fluorescence microscopy, future work will focus on applications in display technology.