Chirality is a property of some objects by which it is not possible to superimpose them to their mirror image by translations and/or rotations — a well-known example of chiral objects are the left and right hands, which are mirror images of each other but it is not possible to superimpose them. This property is also present in materials and molecules, leading to very different responses depending on their left- or right-handed types. One of these responses is the emission of light. Molecules with different chirality emit light with different polarization. This may open exciting prospects in different technologies, such as displays, sensors, data storage, and optical communication. However, most natural materials have weak chiral effects, which limits their practical application in these technologies.
To address this challenge, we need to strengthen and control the chiral effects in materials. One way to do this is by using specially engineered optical structures called "metasurfaces" that are designed to enhance and control the chiral response of nearby molecules. These metasurfaces can interact with matter and produce light that regular materials cannot, allowing us to enhance the chiral emission and direct it in defined directions. In this work, we used such a metasurface formed by an array of Si dimers with nanodisks of different sizes to create directional chiral light emission from achiral molecules. This was possible by generating special light modes in the metasurface called "quasi bound states in the continuum" (Q-BICs). The molecules emit light into these modes that are coupled out by the metasurface with defined polarization and in certain directions. This process has been verified using experimental and theoretical methods. Relevant research results were recently published in Photonics Research, Volume 12, Issue 11, 2024. [Minpeng Liang, Lucio Claudio Andreani, Anton Matthijs Berghuis, José Luis Pura, Shunsuke Murai, Hongguang Dong, José A. Sánchez-Gil, Jaime Gómez Rivas, "Tailoring directional chiral emission from molecules coupled to extrinsic chiral quasi-bound states in the continuum," Photonics Res. 12, 2462 (2024)]
With this metasurface, we achieved a high degree of circularly polarized emission. We were also able to enhance the light emission by more than ten times in defined directions compared to the case without metasurface. Both the degree of polarization and the direction of the emission can be controlled by adjusting the structural parameters of the metasurface, offering an unprecedented control of the emission of polarized light. These findings are promising for future chiral optical devices. Our study provides insights into how these chiral effects work on metasurfaces and suggests new opportunities for technology that relies on highly directional and polarized light.
The corresponding author Prof. Jaime Gómez Rivas noted: "The manuscript represents a significant advancement in the field of chiral photonics. By harnessing the potential of quasi-bound states in the continuum (Q-BICs) for light-matter interaction, the study effectively demonstrates a means of controlling the direction and polarization of the light emission from achiral molecules. This achievement is particularly notable because it addresses the inherent limitations of natural materials, which typically exhibit weak chiral responses. One of the key contributions of this work is the enhanced emission of circularly polarized light in very narrow angles, which may find potential applications in areas such as displays, projection, and optical communication. Future research should focus on determining the parameters that can further enhanced the emission of polarized light and on the dynamical control of this emission."
