A new liquid crystal device lets scientists control how light twists and behaves using voltage, making it possible to reshape light patterns without changing the physical structure of the device.
(Nanowerk Spotlight) Light carries more than energy. It also carries structure, including patterns in its phase, polarization, and momentum. These internal features can encode information, guide interactions, and affect how light moves through materials. For researchers working on advanced optical technologies, controlling these properties is just as important as directing a beam or adjusting its brightness.
Some of the most intricate and revealing structures in light appear not in its brightness or color, but in the orientation of its oscillations. At certain points, this orientation behaves unpredictably. It can twist, rotate, or even vanish entirely. These points mark topological features—regions where light’s internal geometry breaks down in a precise and measurable way. They are not visual artifacts but physical phenomena that influence how light scatters, how it couples to matter, and how it evolves as it travels through space.
The ability to control these features is valuable. They can be used to steer beams, encode data, or shape how light interacts with atoms, molecules, or artificial materials. But most methods for producing them rely on fixed nanostructures that must be carefully designed and permanently etched into surfaces. Once fabricated, they cannot be changed. That inflexibility has created a bottleneck for researchers trying to explore new behaviors or develop devices that respond to external signals.
A recent study published in Advanced Science (“Electrically Tunable Momentum Space Polarization Singularities in Liquid Crystal Microcavities”) offers a way around this limitation. The researchers have developed a system that allows these internal structures in light to be created and tuned using electrical signals. Instead of relying on rigid patterned materials, they built a microscopic optical cavity filled with a type of liquid crystal whose optical properties can be changed with voltage. As the orientation of the liquid crystal shifts, so does the behavior of the light inside.
This makes it possible to generate complex internal patterns in the transmitted light field and to move or reshape them in real time. The result is a flexible platform for exploring the hidden structure of light and for potentially integrating this control into future optical devices.
At the center of this system is what the authors call a liquid crystal microcavity. It consists of a thin layer of nematic liquid crystal sandwiched between two highly reflective mirrors known as distributed Bragg reflectors. These mirrors trap light between them, creating resonances that are sensitive to the internal properties of the material in between. Transparent electrodes allow an external voltage to reorient the liquid crystal molecules. Because the material is birefringent, meaning that it slows down light differently depending on polarization, this reorientation shifts the relative energies of the optical modes supported by the cavity.
This energy difference, known as detuning, is crucial for creating the targeted light structures. When the horizontal and vertical polarization modes are tuned apart by a certain amount, the light that passes through the cavity begins to show structured behavior in momentum space. That is, its properties vary not just across space, but depending on its direction and angle of propagation.
A voltage-controlled liquid crystal device allows researchers to adjust how light behaves as it travels through the system. The panels show how the direction and polarization of light change with angle, revealing special points where the light becomes circularly polarized or behaves unusually. These features can be moved or switched on and off by changing the applied voltage, making the patterns in the light field fully tunable. (Image: Reprinted from DOI:10.1002/advs.202500060, CC BY) (click on image to enlarge)
Under the right conditions, the system produces C-points—locations where the light is purely circularly polarized, and around which the orientation of the linear polarization rotates in a quantized pattern.
To analyze how these features appear, the researchers used a combination of theoretical modeling and experimental measurements. Their first approach used a detailed computational method that accounts for how light moves through layered structures. While accurate, it does not provide much intuition. To gain insight, they also constructed a simplified model based on a two-mode Hamiltonian, a mathematical tool borrowed from quantum physics that describes how two energy states interact. In this case, the two states are the horizontal and vertical polarization modes of the cavity.
The model includes terms that account for both the energies of the modes and the rate at which light escapes from the cavity. These escape rates make the system non-Hermitian, a term used to describe systems where energy is not conserved. In such systems, the modes are not orthogonal and can overlap in unusual ways. This turns out to be essential for generating features like exceptional points, where two optical modes become mathematically degenerate in a way that leads to unique physical effects.
In their experiments, the authors used polarized white light and measured how it transmitted through the cavity at different angles. By rotating polarizers and wave plates and analyzing the transmitted light in various polarization states, they reconstructed the polarization field in momentum space. The results matched the theoretical predictions.
Under positive detuning, the upper optical branch contained two clear C-points, while the lower branch contained four. The positions and properties of these features could be shifted by changing the applied voltage. When the detuning was reversed, the singularities vanished entirely.
These polarization structures also correspond to known topological textures called merons. In the upper branch, the patterns matched Bloch-type merons, where the polarization twists tangentially. In the lower branch, they resembled Néel-type merons, where the twist is radial. These structures are known from magnetic systems and are of interest because of their stability and quantized behavior. In this optical system, switching between meron types was possible simply by adjusting the detuning, making the platform unusually versatile.
The experiment also revealed Möbius strip-like features in the polarization field, where the orientation of the polarization ellipse changes in a continuous but twisted way around the C-points. These structures are another consequence of the light’s internal topology and provide visual evidence of the complexity and richness of the polarization landscape in the cavity.
One limitation of the simplified model was its inability to capture all observed features. In particular, it failed to predict the full set of C-points in the lower optical branch. The more complete computational model showed that these features arise from interactions between the cavity modes and the optical modes supported by the mirrors. These interactions introduce additional terms that depend on the wavevector and cannot be ignored.
As a result, the authors suggest that future models should include these higher-order effects explicitly, especially when working with structures that have narrowband reflectors or closely spaced optical modes.
The study shows that optical microcavities filled with liquid crystal can serve as a flexible and tunable platform for generating topological features in light. Because the system operates at room temperature and uses standard optical components, it can be adapted to a wide range of applications. The ability to move, reshape, or eliminate polarization singularities with voltage opens the door to dynamic beam shaping, optical switching, and reconfigurable photonic systems.
This work also contributes to the study of non-Hermitian physics in optics, a growing area of research focused on systems that include loss and gain. In such systems, new kinds of topological features can emerge that are not possible in traditional settings. By providing a real system in which these features can be created and manipulated directly, the authors have opened a new direction for both fundamental study and technological development.
The liquid crystal microcavity offers a compact and adjustable way to explore how light’s internal structure can be engineered. It bridges a gap between theoretical models and experimental control, and shows how materials that respond to electric fields can be used to create optical systems with topological complexity that can be tuned on demand.
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