High-Purity Linearly Polarized Light Achieved On-Chip with Compact BIC Lasers
This new method harnesses compact BIC lasers to produce high-purity linearly polarized light, potentially revolutionizing a variety of fields with its combination of small footprint and superior polarization performance.
The ability to access linearly polarized emission through miniaturized light sources is of increasing importance for coherent communications, advanced displays, and many other applications. Linearly polarized light that is also high-purity is particularly valuable, because it can reduce the need for external polarizers and simplify optical systems.
BIC lasers have high quality factors (Q-factors) that support low-threshold, single-mode surface emission, making them promising candidates for delivering high-purity, linearly polarized light. However, achieving high-purity polarized emission, while preserving the excellent BIC lasing features, is challenging for finite-sized photonic crystal laser systems, especially if they are micro- or nanoscale size.
Evolution of bound-state-in-the-continuum (BIC) laser emission toward beam-polarization matching. The image shows how a compact BIC laser evolves from a polarization-vortex emission pattern to a single-lobed beam with uniform linear polarization. By combining symmetry perturbation with anisotropic dispersion, the dispersion-assisted quasi-BIC design matches the main beam with a uniform polarization region.
Usually, BIC modes contain a polarization vortex in momentum space. In a finite-sized laser, the far-field beam of the laser covers a range of momenta so different parts of the beam can carry distinct polarization states, causing a quasi-BIC laser to show nonuniform polarization across its emitted light spot. This limits the purity of the final polarization.
A team comprising researchers from Wuhan University, Jinan University, and China Information Communication Technologies Group Corporation found a way to achieve a high polarization purity in photonic crystal BIC laser emission without degrading other performance metrics. The researchers developed dispersion-assisted polarization engineering — an approach that uses beam-polarization matching to control polarization.
In this synergistic approach, symmetry perturbation shapes the beam profile, while anisotropic dispersion unifies the global polarization state. The researchers use displacement perturbation with broken inversion symmetry to achieve anisotropic polarization control. To enhance polarization purity, they use periodicity deformation with anisotropic lattice dispersion.
To experimentally validate this new approach to high-purity, linearly polarized emission, the researchers 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 yielded 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, single-mode performance, or device compactness. Although linearly polarized light emission can be achieved either by breaking symmetry or by engineering the dispersion, using these two strategies in combination can enable consistent linear polarization across the entire beam cross-section and substantially increase polarization purity.
As a further benefit, the new design strategy enables low-threshold operation for large-area BIC lasers. When the device dimension increases, the dispersion-engineered, anisotropic quasi-BIC system continues to provide high polarization purity and excellent beam quality, while achieving a significantly higher Q-factor than conventional isotropic quasi-BIC counterparts. A boosted Q-factor suppresses radiative loss and enhances cavity optical feedback, lowering the lasing threshold of large-scale, surface-emitting lasers.
Because the new strategy relies solely on symmetry modulations for polarization control, it can be extended to other wavelength ranges and diverse photonic platforms.
Dispersion-assisted polarization engineering thus offers the potential to provide miniaturized polarization light sources across a range of applications, including coherent optical communications, advanced displays, precision measurement, high-contrast imaging, laser arrays, and integrated photonic systems.
Source: photonics
