A research team from the Faculty of Physics at the University of Warsaw, the Military University of Technology, and the Institut Pascal at Université Clermont Auvergne has developed a novel method that uses cholesteric liquid crystals inside optical microcavities. The team created a platform capable of forming and dynamically tuning photonic crystals that integrate spin-orbit coupling (SOC) with controlled laser emission. The results were published in Laser & Photonics Reviews. Inside the optical microcavity, a uniformly lying helix (ULH) arrangement of cholesteric liquid crystals self-organizes into a one-dimensional periodic photonic lattice with its helical axis lying in the plane of the cavity.

"We have designed a uniform helical structure formed by self-organization of elongated molecules that resemble pencils," explained Professor Jacek Szczytko from the University of Warsaw's Faculty of Physics. The cholesteric helix consists of molecular layers aligned parallel to each other, with the molecular orientation twisting layer by layer to form a DNA-like spiral. Under specific illumination, clear stripes with a width equal to the helical pitch can be observed when viewed perpendicular to the helix axis. "Thanks to the liquid crystal's response to an electric field, we can precisely control the pitch and thereby tune the photonic band structure, opening new perspectives for photon engineering," added Professor Szczytko.

Optical microcavities confine light in one dimension, causing photons to behave like particles with mass. Using liquid-crystal optical microcavities developed in collaboration with the Military University of Technology, the team explored how light can acquire material-like properties while retaining its unique characteristics. The group led by Professor Wiktor Piecek at the Military University of Technology fabricated the optical microcavities, employed the helical structure designed by Professor Eva Oton, and performed cavity assembly under Dr. Przemysław Morawiak and Dr. Rafał Mazur. "Developing suitable liquid-crystal mixtures and achieving an ordered uniform helix inside an optical cavity is a complex materials engineering challenge," emphasized Professor Piecek.

The system features self-organized structures with surface areas reaching hundreds of square micrometres, and the liquid-crystal molecules can reorient under an electric field, enabling dynamic tuning of the photonic band structure inside the microcavity. "After applying voltage, we can observe the structural evolution in real time with a camera while maintaining periodic order," noted Dr. Marcin Muszyński, first author of the paper. The study also introduced organic dyes, leading to observations of dual-wavelength laser emission as well as linear and circularly polarized lasing. "These results demonstrate that our research has both fundamental and applied significance," concluded co-author Dr. Piotr Kapuściński.