A diagram illustrating the branched flow of light in an NLC film. The random orientation of NLC molecules in a glass cell can be adjusted using electrical voltage bias. Credit: Chang, Ss., Wu, KH., Liu, Sj. et al/ Natural communications. 10.1038/s41467-023-44500-8.
A new study in Natural communications studies the electrical tuning of branched light flux in nematic liquid crystal (NLC) films, revealing controlled patterns and statistical features with potential applications in optics and photonics.
Branching light flow manifests as complex patterns of light waves navigating through a disordered medium, forming multiple branching pathways.
Situated between ballistic and diffusive transport phenomena – where ballistic implies unimpeded straight-line movement similar to a laser beam, and diffusive implies dispersed, chaotic behavior – the phenomenon is gaining importance for its potential to control physical processes, in particularly optics and photonics.
Acting as a transition state between ordered and disordered propagation of light, it provides a platform for controlled and complex steering of light.
This manipulation becomes the focus of a study led by Dr. Jin-hui Chen of Xiamen University in China and Dr. Jian-Hua Jiang of the University of Science and Technology of China, where they specifically explore tuning electrical of the branched luminous flux. in NLC films.
“Due to their erratic nature and rich behaviors, manipulating branched flows in a controllable manner has never been achieved experimentally. We find that electro-optically effected disordered liquid crystal films provide an excellent platform for generation and regulation of branched flows of light,” Dr. Chen told Phys.org.
“When I visited Professor Chen at Xiamen University, he was studying the branched flow of light in liquid crystals. Recognizing the importance of topological defects in this context, I understood that their stability under electric fields contributes to the stability of the system, allowing the repeated on/off switching of the branched light flow,” added Dr. Jiang.
Topological defects in NLCs
Liquid crystals exhibit the characteristics of both fluid and solid states. Their molecules can flow like a liquid while maintaining a certain degree of order similar to that of a solid. This distinctive behavior results from the delicate balance between intermolecular forces and thermal energy.
Researchers have particularly focused on the behavior of NLCs. Nematic liquid crystals are characterized by the alignment of their molecules in a specific direction, creating a distinct order within the material. This alignment is sensitive to external factors, such as electric fields.
Electrical tuning of branched light flux in NLC films involves manipulation of the orientation of these liquid crystal molecules. When an electric field is applied, it induces a reorientation of the molecules, changing the properties of the NLC film. This process is crucial for generating and regulating the complex patterns of branching light fluxes.
The topological defects of the NLC film play a dual role in the phenomenon.
Dr. Chen explained: “First, they contribute to the spontaneous formation of structured patterns called schlieren textures, resulting from disordered orientations of NLC molecules and uneven dielectric anisotropy. This acts as a low disordered potential for light propagation. »
“Second, under low electrical voltage, the reorientation of liquid crystal molecules occurs without disturbing the schlieren textures. The robustness of the topological defects, possibly blocked by surface forces at the interface, ensures good recovery of the generated branched flow by light waves in the system.”
Observation of branched light flux in NLC films
The researchers used a meticulous experimental setup to study the electrical tuning of branched light flux in NLC films. A high-precision three-dimensional translation step enabled fine tuning of the light coupling in the NLC film.
This involved manipulating the polarized field of a 532 nm laser with a polarizer and a half-wave plate. Light flux observations were facilitated by a microscope with a 10x objective and an optical camera collecting the intrinsic light scattering of the NLC film.
Additionally, the researchers used simulations to explore the orientations of the liquid crystals in response to the triggering (controlling) electric field.
One of the researchers’ most surprising discoveries was the robustness of the topological defects that fixed schlieren textures in liquid crystals and, therefore, light scattering patterns.
Dr. Jiang explained: “Even with a noticeable electrical voltage that significantly tilts the orientation of liquid crystal molecules, after turning off the electrical voltage, topological defects are recovered, as are schlieren textures. »
“This allows electrical adjustment (on and off) of the scattering potentials, and the branched light flow can be repeated several times. This is really beyond our expectations. This tells us the stability of topological defects in liquid crystals.”
A notable observation was the variation in the scintillation index, a crucial statistical property of branched flow, with changes in the polarization of the input light, Dr. Chen noted. This reliance on polarization, previously unachievable on other platforms, has added an additional layer of complexity and control to the branched light output generated in NLC film.
Besides topological defects and the relationship between scintillation index and polarization, a third factor was important: the correlation length of the disordered potential, a measure of how disorder is structured or ordered in the material, relating to the wavelength of light propagation.
The correlation length of the disordered potential must be greater than the wavelength of the propagating light for the appearance of a branched flow. A larger correlation length implies a larger and more consistent pattern of disorder.
“Due to the robustness of the topological defects, the schlieren textures and the scattering potential are quite consistent. These factors make everything controllable and allow us to demonstrate the beautiful tuning of the branched light flow,” explained Dr. Jiang.
Optical neural networks and sensors
Explaining potential applications and future work, Dr Chen said: “Liquid crystals can create programmable hierarchical superstructures for light-matter interactions, showing high sensitivity to external fields. »
“Our group’s future research will look at the interaction of light with disordered liquid crystal systems, exploring in-plane and out-of-plane transport configurations with potential applications such as optical neural networks.”
From a technological point of view, Dr. Jiang pointed out that this phenomenon could be improved when manipulating light beams. “Electrical adjustment holds great promise for the operation of devices. For example, it can be used as a switch for sensors or detectors when connected to the liquid crystal film,” he concluded.
More information:
Shan-shan Chang et al, Electrical adjustment of branched light flux, Natural communications (2024). DOI: 10.1038/s41467-023-44500-8
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