Recently, a team led by Professors Zhang Xinzheng, Chen Zhigang, and Xu Jingjun from the School of Physical Sciences at Nankai University, in collaboration with Professor Irena Drevenšek Olenik from the Stefan Institute in Slovenia, has demonstrated for the first time a circularly polarized flexible topological vertical-cavity surface-emitting laser (VCSEL) with high laser conversion efficiency. The research findings, titled "Soft-Matter-Based Topological Vertical Cavity Surface Emitting Lasers," were published in Light: Science & Applications and selected as the cover article for the second issue.
This topological VCSEL is based on a one-dimensional optical superlattice structure composed of a polymer cholesteric liquid crystal (PCLC) film spin-coated with a fluorescent gain medium and a commercial Mylar film. The superlattice features a modulated potential well that breaks inversion symmetry, analogous to Semenov insulators and the quantum valley Hall effect in two-dimensional synthetic parameter space. They demonstrate that this topological VCSEL maintains excellent single-mode laser emission at low pump powers. Notably, this thin-film topological VCSEL offers extremely low production costs, requires no complex fabrication techniques, can be easily integrated onto substrates of any shape, and retains its desired laser characteristics and beam steering capability even after multiple bends.
With the continuous advancement of optical information technology and the growing demand for miniaturization, lightweight design, and integration in photonic chips, the development of on-chip lasers has garnered increasing attention. However, current on-chip lasers still face significant challenges, including low output power and poor stability. Topologically protected lasers offer an effective design approach to achieve high stability and low threshold on-chip lasers. However, constrained by semiconductor materials, the fabrication of most topological VCSELs currently requires complex and precise processes. Their lasing wavelengths are primarily confined to the near-infrared band, and they possess fixed geometries incapable of translational beam steering. Therefore, the low-cost development of mechanically flexible, polarization-maintaining, and lightweight on-chip topological VCSELs using soft matter materials and novel principles holds significant theoretical and practical value.
Fundamental Construction Principle and Synthesis Parameter Space of One-Dimensional Flexible Optical Superlattices
During the research, the team first stacked Mylar films of two different thicknesses with PCLC thin films to form a one-dimensional binary optical superlattice structure that breaks spatial inversion symmetry. By introducing coupling coefficient modulation to construct a synthetic parameter space, they elucidated the difference in topological properties between AB and BA superlattices with hetero-site potentials, analogous to the quantum valley Hall effect in two-dimensional hexagonal lattices. Subsequently, they joined AB and BA superlattices with different topological properties-one featuring equal coupling and the other with hetero-site potentials-to generate topologically protected interface states. These interface states exhibit high localization within the bandgap and demonstrate robustness against structural disorder, providing an ideal optical cavity mode for laser oscillation. Experimentally, the team fabricated a flexible laser device with 17 layers by spin-coating the gain dye PM597 onto a PCLC thin film surface. Under 532 nm pulsed laser pumping, the device achieved left-handed circularly polarized topological laser output at 575.4 nm with a threshold as low as 0.47 μJ (1.5 mW·cm⁻²), exhibiting a slope efficiency of 4.0%. The laser beam exhibits excellent directionality, with its spatial distribution highly consistent with the pump spot shape, demonstrating potential applications in image transmission and display. Furthermore, by fixing the pump laser direction while bending and moving the film from bottom to top, the pump light illuminated five distinct regions (I-V) of the sample, causing the emitted laser to move sequentially from bottom to top on the optical screen. Adjusting the curvature radius of the VCSEL film further controls beam steering angle. This property enables the topological VCSEL to emit laser light at multiple angles without rotating the laser device. Notably, the topological VCSEL exhibits thermal stability, maintaining its original laser performance even after prolonged pumping. These characteristics enable integration with various wearable photonic devices for applications such as wearable displays, electronic anti-counterfeiting, and laser scanning.
Application demonstration of flexible topological VCSEL
This work was primarily conducted at Nankai University. Postdoctoral researcher Wang Yu and Professor Xia Shiqi of Nankai University are the co-first authors, while Professors Zhang Xinzheng, Chen Zhigang, and Xu Jingjun are the corresponding authors. The research received support from the National Key R&D Program of China, the National Natural Science Foundation of China, the Key Project of the Tianjin Natural Science Foundation, the China Postdoctoral Science Foundation, and the Slovenian Research Agency.
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