image: Fig. 1 | Schematic diagram of design concept for structured light beams. (a) The phase distribution at the input plane, (b) spatial transmission trace of cylindrical helical path, (c) polarization distributions on the incident plane, (d) the polarization states corresponding to different propagation distances z on the Poincaré sphere for linear longitudinal polarization variations, (e) polarization distributions on the incident plane, (f) the polarization states corresponding to different propagation distances z on the Poincaré sphere for nonlinear longitudinal polarization variations.
Credit: OES
A new publication from Opto-Electronic Science; DOI 10.29026/oes.2024.230052 discusses structured light beams with controllable polarization along arbitrary trajectories.
A structured light refers to a light field which is “customized” in both space and time, characterized by its unique distribution of amplitude, phase, and polarization state in both space and time. Polarization plays a crucial role in structured light beams. Besides polarization manipulation on a single transverse plane, controlling polarization along the propagation direction in longitudinal structured light beams is also an important dimension. Current research mainly focuses on controlling the polarization of beams along the optical axis. However, this greatly limits the spatial freedom of polarization for longitudinally structured light beams. It is worth exploring whether it is possible to break free from the constraints of the optical axis and manipulate polarization along any transmission trajectory in space. This could lead to the generation of structured light beams with polarization that varies along arbitrary trajectories, opening up new possibilities for creating customized spatial structured lights.
The authors of this article used tri-layer metallic metasurfaces to generate structured light beams with polarization variations along arbitrary spatial trajectories. In order to achieve these customized spatial structured lights, the work first designed a phase modulation function on the input plane, consisting of a series of expanding circular rings with moving centers, as shown in Fig. 1(a). The trajectory curve of propagation is formed by the convergence of conical beams emitted from these moving circular rings on the input plane. Secondly, by controlling the amplitude and phase difference of the orthogonally polarized beam components corresponding to the moving circular rings, polarization control at each point on the predetermined transmission trajectory can be achieved. Following the preset helical transmission trajectory, they designed continuous variations of structured light polarized from 15° to 75° along the equator line on the Poincaré sphere, as well as continuous variations of structured light polarized from 15° to circular polarization and then to 75° along the equator of the northern hemisphere, corresponding to Fig. 1(d) and 1(f), respectively.
A terahertz focal plane imaging system was used to obtain the intensity, amplitude, and phase difference of the electric field components Ex and Ey at different transmission distances z. Under metasurface modulation, a high-brightness main lobe appeared, surrounded by several side lobes, and the overall intensity distribution of the light field resembled a zero-order Bessel function. Observing the intensity distribution of the light field at different transmission distances z, they found that the main lobe of the beam rotated counterclockwise in a continuous range from 5 mm to 15 mm, with Ix gradually decreasing and Iy gradually increasing. The difference is that the phase difference of Ex and Ey between the two structured lights in the range of 5 mm to 15 mm gradually changes from 0 to -π, respectively, corresponding to the synthesized polarization states shown by the white solid lines in Fig. 2(d) and Fig. 3(a), which are in agreement with theoretical simulations.
The proposal for arbitrary trajectory structured light beams with longitudinally varying polarization offers a practical method for continuously regulating the characteristics of spatial structured light beams with non-axial transmission and opens up new possibilities for creating customized spatial structured light beams. This technique has potential uses in optical encryption, particle manipulation, and biomedical imaging.
Keywords: structured light beam / tri-layer metallic metasurface / longitudinal polarization / non-axial transmission
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Yan Zhang is a professor in the Department of Physics at Capital Normal University, a doctoral supervisor, director of the Beijing Key Laboratory of Metamaterials and Devices, and a fellow of the Optical Society of America. Professor Yan Zhang’s team is mainly engaged in research in terahertz optoelectronics, micro-nano optics, optical information processing, etc., and has published more than 340 SCI papers in internationally renowned journals such as Nature Photonics, Nature Communications, Advance Optical Materials, Opto-Electronic Advances, etc. He has been cited more than 8,000 times by SCI and has given more than 50 invited reports at international conferences.
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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.
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Nan T, Zhao H, Guo JY et al. Generation of structured light beams with polarization variation along arbitrary spatial trajectories using tri-layer metasurfaces. Opto-Electron Sci 3, 230052 (2024). doi: 10.29026/oes.2024.230052