On-chip lasers are becoming important for coherent communication, advanced displays, precision sensing, imaging and laser arrays. For these applications, a laser should be compact, stable and efficient. It is also highly useful if the laser can directly emit high-purity linearly polarized light, because this can reduce the need for external polarizers and simplify optical systems.
Bound-state-in-the-continuum, or BIC, photonic crystal lasers are promising because their high quality factors can support low-threshold, single-mode surface emission. However, BIC modes usually contain a polarization vortex in momentum space. In a real finite-size laser, the far-field beam covers a range of momenta, so different parts of the beam can carry different polarization states. As shown in Figure 1, a regular quasi-BIC laser can therefore show non-uniform polarization across its emitted light spot, limiting the final polarization purity.
In a new paper published in Light: Science & Applications, a team of scientists from Wuhan University, Jinan University, China Information Communication Technologies Group Corporation and co-workers proposed a dispersion-assisted polarization engineering strategy to address this problem. The work was published under the title High-purity linearly polarized emission from a compact BIC laser.
The core concept of this work lies in far-field beam-polarization matching. For finite-size BIC lasers, high-purity polarized emission cannot be solely determined by the polarization state at a single momentum-space point. Instead, it relies on the spatial overlap between the far-field beam profile and the extended momentum region with uniform linear polarization. The team summarize the underlying design principle as: "Symmetry perturbation shapes the beam profile, while anisotropic dispersion unifies the global polarization state." Benefiting from this synergistic mechanism, the laser output achieves consistent linear polarization across the entire beam cross-section.
For experimental validation, the team fabricated a compact quasi-BIC laser consisting of a 20 × 20 unit-cell array, with a device footprint of 16.4 μm × 13.4 μm. The laser yields stable single-mode lasing near 1590 nm, with a measured polarization extinction ratio as high as 298:1. This result demonstrates that compact quasi-BIC lasers can realize ultrahigh polarization purity without compromising beam quality or single-mode performance.
Furthermore, the proposed design strategy enables low-threshold operation for large-area BIC lasers. As the device dimension increases, the dispersion-engineered anisotropic quasi-BIC system maintains high polarization purity and excellent beam quality while achieving a substantially higher Q factor than conventional isotropic quasi-BIC counterparts. A boosted Q factor suppresses radiative loss and enhances cavity optical feedback, thereby effectively lowering the lasing threshold of large-scale surface-emitting lasers.
Overall, dispersion-assisted polarization engineering provides a simple way to build miniaturized polarized light sources for coherent optical communication, advanced displays, precision measurement, high-contrast imaging, laser arrays and integrated photonic systems.
