Designable vectorial lasing via quasi-BIC Möbius loop

Driven by ongoing progress in integrated photonics, compact on-chip light sources are expected not only to lase efficiently, but also to emit structured vector beams with nontrivial polarization textures and topological charges. Such topological vectorial lasing enriches the degrees of freedom of light, enabling applications ranging from precision metrology and sensing to optical communications and light–matter interaction. Yet generating vectorial lasing with designable topological charge from a single compact photonic structure is highly desired but remains challenging. Although several microlaser platforms—such as Dirac-vortex cavities, disclination cavities, and bound states in the continuum (BICs)—have demonstrated compact vectorial lasing, their ability to tailor the emitted polarization topology is often constrained, offering limited design freedom and leaving only a narrow range of accessible charges.

 

In a paper published in Light: Science & Applications, a team led by Prof. Jian Zi and Prof. Lei Shi and Prof. Jiajun Wang from Fudan University reports a new design paradigm for vectorial microlasers with designable topological charges. Their approach is enabled by an uncovered quasi-BIC Möbius-like correspondence in symmetry-broken photonic-crystal (PhC) slabs, which establishes a clear and explicit mapping between real-space structures and the radiated polarization states. Guided by this mapping, quasi-BIC PhC slabs with different symmetry-breaking parameters can be purposefully arranged to form a compound microcavity, thereby generating vectorial lasing with desired topological charge.

 

At the core of the work is a novel and powerful observation: slight symmetry breaking in a PhC slab can convert a high-order BIC into a quasi-BIC, whose eigen polarization evolves continuously with the symmetry-breaking parameter and sweeps across all linear polarization states. This continuous, robust evolution follows a Möbius-strip topology in parameter space, providing a clear guideline for polarization engineering in lasing systems.

 

“Conventional BIC-based microlasers host polarization vortices protected by real-space symmetry, which also constrains the accessible topological charges—typically −2, −1, or +1,” the researchers explain. “Through symmetry breaking, richer polarization-topological properties of BIC systems can be uncovered, establishing the basis for on-demand topological-charge design in vectorial microlasers.”

 

With the Möbius-like correspondence as guidance, the team proposes a universal topological cavity construction approach. It allows PhC structures to be spliced in a goal-driven way for a desired lasing topological charge. Specifically, for any target far-field polarization vortex, the required polarization winding can be viewed as a repetition of basic polarization-evolution units along the azimuth. The Möbius-like correspondence then converts each unit into a real-space structural “building block.” As a result, to obtain a desired charge, one only needs to choose the repetition number of the building block (setting the charge magnitude) and the assembly direction (setting the sign), and then construct the compound microcavity. Simply changing the repetition number of the same building block yields a full series of vectorial microlasers with charges spanning −5 to +5. This capability offers a practical route to compact, programmable structured-light sources for integrated photonic circuits and multi-dimensional optical information encoding.

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