A team including researchers from the University of Warsaw, the Military University of Technology in Poland, and the Institut Pascal at Université Clermont Auvergne demonstrated that tunable photonic crystals can be achieved by embedding a self-organizing cholesteric liquid crystal (CLC) in an optical microcavity. This discovery could support advances in lasing and topological photonic states. The researchers found that when a CLC is embedded in a planar microcavity, it spontaneously forms a uniform lying helix. This helix structure creates a 1D, polarization-dependent photonic crystal that can be tuned by applying an external electric field. The uniform lying helix is a spiral structure comprising layers of molecules. The orientation of the molecules is slightly twisted from layer to layer to form the helical structure. The pitch of the helix (that is, the distance over which the molecules complete a 360° twist) determines many of the molecules’ optical properties. Researchers created a 1D photonic crystal with strong polarization dependence that is tunable using an applied electric field by embedding a cholesteric liquid crystal (CLC) in a planar microcavity. Courtesy of University of Warsaw. “A uniform lying helix (ULH) structure of a cholesteric phase liquid crystal is arranged in the optical cavity,” professor Jacek Szczytko said. “The self-organized helix structure, with the axis lying in the plane of the cavity, acts as a one-dimensional periodic photonic lattice. This is possible due to the unique properties of liquid crystals.” The axis of the helix is determined by the direction that is perpendicular to the molecular layers of the uniform lying helix. Under appropriate illumination, the researchers studied the structure in the direction perpendicular to the axis, and noted distinct stripes with a width equal to the helix pitch. “The use of liquid crystals that respond to an electric field enables precise control of this pitch, and thus of the structure of the photonic bands, opening up new perspectives in photonic engineering,” Szczytko said. The optical microcavity enables control of the helix pitch by restricting the movement of light in 1D, giving the photons properties similar to those found in particles with mass. In the microcavity, photons that have no rest mass start to behave like massive particles. Adding a photonic potential in space, with a period associated with a helix jump, allows further manipulation of these properties. The voltage that is applied to the photonic crystal controls the orientation of the CLC molecules and, consequently, the strength of the polarization-dependent periodic potential. The researchers demonstrated that the uniform lying helix can induce an in-plane periodic potential acting on the photonic modes, and resulting in the formation of a spin-polarized lattice band structure that can be tuned by the external voltage controlling the CLC molecular orientation. When the ULH is tilted, the tilt induces a spin-orbit coupling between the lattice bands. This interband, spin-orbit coupling is analogous to optical activity and can be considered a synthetic, non-Abelian gauge potential. The researchers showed that when two cavity modes are brought in resonance, photonic spin orbit coupling efficiently couples the periodic potential bands of different parity. By introducing a dye within the microcavity, the researchers demonstrated the capability of their platform to achieve dual and circularly-polarized lasing with all the tunable properties of the CLC microcavity. “For years, scientists have been developing nano- and microstructures that modulate the properties of the light that interacts with them,” researcher Marcin Muszynski said. “However, typical technologies for producing photonic crystals have several drawbacks. Their manufacture is technologically complex and therefore expensive and time-consuming. “Our work solves these problems,” he said. “The structures created by self-organization have a surface area in the order of hundreds of square micrometers, and thanks to the reorientation of the liquid crystal molecules in an electric field, we can dynamically control the band structure of the light trapped in the microcavity.” The results of the research could help lay the groundwork for advances in topological photonics and laser technologies. “The goal of our research is to discover how light can acquire properties normally attributed to matter while retaining its unique characteristics,” Szczytko said. The research was published in Laser & Photonics Reviews (www.doi.org/10.1002/lpor.202400794).
